Contamination of the Hydrogeological System: A Primer

Chapter 3

Contamination of theHydrogeological System:

A Primer

CHAPTER HIGHLIGHTS● Movement of chemicals is directly linked to water movement, over and through the soil.● Natural factors affecting potential for agrichemical contamination of groundwater are

complex, interactive, and not enough is known about them to specify solutions for mostlocations.

● Diffuse sites and diverse modes of entry, and multiple agrichemical transportmechanisms render agrichemical contamination of groundwater true nonpoint sourcepollution.

. Natural factors associated with suspected groundwater vulnerability are widespread andsupport national concern. Federal and State data collection and information managementactivities to identify and understand these natural factors are underway, but national-levelefforts to synthesize this information to assist decisionmaking are still evolving.

● Long periods of time elapse between changes in surface activities and impacts ongroundwater contamination, and contamination is extremely costly to reverse, such thatprevention is preferable to redemption.

● Reduction of agrichemical contamination of groundwater requires that the entireagroecosystem be managed to minimize waste and leaching*

CONTENTSPage

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Groundwater and the Hydrologic Cycle ● . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Contamination of the Hydrogeologic System . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . 43Natural Factors Affecting Leaching of Agrichemicals to Groundwater . . . . . . . . . . . . . . 47Characteristics of Underlying Geological Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Aquifer Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Putting It All Back Together . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

HUMAN MODIFICATIONS OF THE HYDROGEOLOGIC SYSTEM . . . . . . . . . . . . . . . 57Preferred Pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Changing Groundwater/Surface-Water Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

AGRICHEMICAL CHARACTERISTICS RELATED TO LEACHING ...............60SUMMARY AND POLICY IMPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63

Improving Data Collection and Management for Groundwater Protection . . . . . . . . . . . 63Coordinating Agroecosystem Simulation Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Developing Geographic Information Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

CHAPTER 3 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

BoxesBox Page3-A. Groundwater Contamination From Natural Sources: Kesterson Wildlife Refuge . . . 603-B. Using Hydrogeologic Information To Predict Sites Vulnerable to Groundwater

Contamination: Minnesota’s Groundwater Contamination Experience . . . . . . . . . . . . 64

FiguresFigure Page

3-1. Hydrologic Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443-2. Zones of Subsurface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .453-3. Perched Water Tables in Relation to the Main Water Table . . . . . . . . . . . . . . . . . . . . . 453-4. Surface Water and Groundwater Relationships . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . 463-5. Horizons of a Typical Soil Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .513-6. Microfauna and Macrofauna Open Conduits and Create Pore Spaces in Soils . . . . . 523-7. General Direction and Rate of Groundwater Movement . . . . . . . . . . . . . . . . . . . . . . . . 543-8. Extent of Pleistocene Continental Glaciation in the United States . . . . . . . . . . . . . . . . 563-9. Schematic Diagram of Agricultural Drainage Well ● . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3-10. Cones of Depression Resulting From Water Withdrawal May Result inContamination of Water Supplies Near Non-contaminated, Well-constructedWells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3-11. Status of State Soil Geographic Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663-12. Thickness of Pleistocene Deposits in Illinois ● . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

TablesTable Page3-1. Estimated Permeability of Typical Geologic Materials in Illinois . . . . . . . . . . . . . . . . . 543-2. Key Chemical and Mineralogical Factors Commonly Considered When Assessing

a Pesticide’s Behavior and Ability To Leach From Agricultural Lands . . . . . . . . . . . . 623-3. Pesticides With High Potential for Leaching to Groundwater . . . . . . . . . . . . . . . . . . . . . 62

Chapter 3

Contamination of the Hydrogeological System: A Primer

INTRODUCTIONGroundwater has represented a vast and seem-

ingly inexhaustible resource for years, and hasbecome an indispensable source of freshwater. Evenuntil the 1970s, the soil was believed to be a‘ ‘livingfilter’ preventing groundwater contamination fromchemicals applied to the land (74). Today, however,a growing body of information tells us that agrichem-icals (pesticides and nitrate) have moved through thesoil cover to contaminate groundwater. Contamin-ated well-water in many U.S. agricultural areas isevidence that groundwater is ultimately affected byman’s aboveground activities. Clearly, environ-mental contamination from agrichemicals requires athree-dimensional view of agriculture and its im-pacts rather than the two-dimensional view held bymany in the past,

Three categories of factors largely determine thepotential for agrichemical leaching to groundwater:

1.

2.

3.

natural characteristics of the site of agrichemi-cal use that affect leaching of water and thustransport of agrichemicals,nature and extent of human modification tothose natural characteristics that may affectleaching patterns, andcharacteristics of the agrichemicals used thatdetermine their environmental fate.

To understand how the problem originated and howit might be solved requires a basic understanding ofhow water moves through the atmosphere, over theland surface, and below the ground-the hydrologiccycle.

Groundwater and the Hydrologic Cycle

The hydrologic cycle begins with the evaporationof water from oceans and other open bodies of water,vegetation, and land surfaces (figure 3-l). Themoisture from evaporation forms clouds, and fallsback onto the Earth’s surface as rain or snow. Whenit rains, some of the rainfall is taken up byvegetation, some returns to the atmosphere byevaporation and through transpiration by plants, and

some water runs off the land to lakes and rivers andon to the sea.

Part of the rainfall falling directly on the land orcollected in surface water bodies seeps downwardthrough the Earth’s surface. Water moves throughthe interconnected spaces among individual parti-cles of soils and geologic materials, along cracks andfissures in these materials, or through openingswhere worms have burrowed or roots have decayed.These spaces may become temporarily saturatedwith water after a heavy rain, but near the surface, inthe ‘‘vadose zone,”2 open spaces normally containair as well as water. With increased depth, water fillsall available pore space in the Earth’s sediments androck formations. This fully saturated zone is wheregroundwater is stored; the upper surface of thissaturated zone defines the water table (figure 3-2).

Although groundwater is ubiquitous, only certaingeologic formations (aquifers) have an extractablequantity of water sufficient for human use. Aquifersmay reach hundreds of feet in thickness and mayextend laterally for hundreds of miles. The Ogallalaaquifer, for example, underlies parts of eight GreatPlains States (6,18) and is vital to agriculture over alarge region. Other groundwater aquifers are thinand of small areal extent and, thus, only a few wellscan draw from them. The smallest aquifers—perched water tables—sit on small impermeablelayers of geologic material above the region’sgeneral water table (figure 3-3).

Water moves continuously below the Earth’ssurface, much as surface water flows from higherregions towards the sea. Many aquifers contribute tosurface water bodies, such as springs, wetlands,rivers, and lakes, and others flow directly into theocean. Some deep aquifers, however, contain ‘fossilwater’ sequestered under the soil thousands of yearsago.

Contamination of the Hydrogeologic System

Water reaches the groundwater table through twoprimary natural pathways in the course of the

lcont~m(lon here ~efers t. tie m~~umble ~rcsence of an a~chefi~ or its br~do~ products, and docs not necessarily imply the eXiStenCe

or absence of a threat to human health or the environment.

‘This zone also may be referred to as the unsaturated zone or the zone of aeration.

43–

44 ● Beneath the Bottom Line: Agricultural Approaches TO Reduce Agrichemical Contamination of Groundwater

Figure 3-1—Hydrologic Cycle

Saline groundwater

SOURCE: Adapted from B.J. Sldnner and S.C. Porter, The Dynamic Earth (New York, NY: John Wiley& Sons, 1989).

hydrologic cycle: direct leaching through soils androck formations, and via recharge from surfacewaters. Although the waters leaching through farm-lands to groundwater may pickup agrichemical andnatural contaminants as they move through thesystem, contaminants also may derive from atmos-pheric deposition or contaminated surface waters.

Atmospheric Deposition

Agrichemicals can be transported and dispersed inthe atmosphere, eventually returning to lands andsurface waters. With spraying from airplanes, inparticular, pesticides aimed at a specific field arelikely to drift beyond its boundaries and settle ondistant land areas, lakes, and streams.

Contamin ation of rainfall has been documentedfor certain organochlorinated pesticides. Studiesshow that the pesticide toxaphene (now banned bythe Environmental Protection Agency (EPA)) wascarried long distances from its use site and depositedthrough rainfall in concentrations high enough todamage fisheries (1 1). Similarly, in a pilot study of

atmospheric dispersal of pesticides in the Northeast-ern United States, rainwater samples were analyzedfor 19 commonly used pesticides and 11 were foundin detectable levels (62).

The detected compounds showed strong seasonalvariation consistent with application times andchemical stability and, thus, are thought to haveoriginated mostly from local sources (62). However,wind also can transport agrichemical particles andvapors hundreds or thousands of miles before theyfall back to Earth. In 1980, an insecticide used tocontrol boll weevils in cotton fields in the SouthernUnited States was discovered in fish in the waters ofLake Superior. The global scope of atmospherictransport became apparent when insecticides used inAsia and southern Europe appeared in Arctic andAntarctic waters.

Recharge by Contaminated Surface Waters

Readily soluble agrichemicals maybe carried offfields with runoff. Some agrichemicals have atendency to attach themselves to certain soil parti-

Chapter 3-Contamination of the Hydrogeological System: A Primer ● 45

Surface water

Rivers and lakes

Figure 3-2—Zones of Subsurface Water

——

.——.

——

— —.—————— — ——— ————— —-

—— ————— ———— ————————.

-—————.CAPILLARY FRINGE

—— — — ————— ——— — —— —-/

Water table

Well/

-—

, Water level

——

SOURCE: Adapted from R. C. Heath, Ground- Wafer Regions of the United States, Geological Survey Water-Supply Paper No. 2242 (Washington, DC: U.S.Government Printing Office, 1984).

Figure 3-3-Perched Water Tables in Relation to the Main Water Table

Perchedwater bodies

Dryhole I

I I

SandMain water table

. ., ,, :,.

SOURCE: B.J. Skinner and S.C. Porter, The Dynamic Earth (New York, NY: John Wiley& Sons, 1989).


Figure 3-4-Surface Water and Groundwater Relationships

Rainwater entersground by infiltration

———— —

Water leaches downwardin zone of aeration

Ground water percolatesvia curved paths andemerges in nearest stream

A. Generalized movement of groundwater in uniformly permeablerock in a humid region, or during high precipitation conditionsin an arid/semiarid region

/

B. Generalized movement of groundwater in uniformly permeablerock in arid/semiarid regions-or during long-lasting drought in humid regions

SOURCE: B.J. Skinner and S.C. Porter, The Dynamic Earth (New York, NY: John Wiley& Sons, 1989).

cles; the impact of raindrops can erode soils andassociated agrichemicals from the field. In suchcases, agrichemicals may end up in surface waterbodies. From there, agrichemicals or their break-down products may leach into groundwater.

Groundwater and surface waters are closelylinked, with the flow of one to the other dependingon the relative altitudes of the surface water and thegroundwater table (figures 3-4a and b). For example,in humid regions, the flow of groundwater generallyis toward surface water bodies because the ground-water table in the surrounding land is higher than thesurface water body. In arid/semiarid regions, how-ever, the flow direction is reversed because thealtitude of streams tends to be higher than thegroundwater table (75,18,66). Under conditions ofabnormally high rainfall in arid/semiarid regions orabnormally low rainfall in humid regions, thepredominant direction of water flow may change

accordingly. Thus, in any region of the country,potential exists for climatic factors to promoterecharge of groundwater by surface waters. Inaddition, pumping of high-capacity municipal wellscan draw surface water into the aquifer.

The absence of oxygen below the water tableprecludes most reactions that degrade contaminantsin the vadose zone (40). Contaminants that reach andmove with groundwater are therefore likely toremain chemically intact for long periods.

Once contaminants reach groundwater they mayspread laterally to a greater extent than they mayhave in the vadose zone. In certain instances, a largeaquifer may be encountered through which contami-nants can disperse regionally (e.g., Ogallala aquifer)(25). While it might be years before contaminantsreach the deeper parts of a very thick aquifer, deepgroundwater may act as a long-term reservoir forcontaminants. Thus, contaminants in groundwater

Chapter 3---Contamination of the Hydrogeological System: A Primer ● 47

Photo credit: U.S. Fish and Wildlife Service-L. Childers

Groundwater and surface water, such as the wetlands shown here, are intimately connected. Contamination of groundwater cantherefore result in contamination of surface-water bodies, and vice versa

may be discharged to a stream decades, or centuries,after percolating rainwater introduced pollutants inthe first place (86).

Natural Factors Affecting Leaching ofAgrichemicals to Groundwater

The potential for agrichemicals to leach directlythrough soils and rock to groundwater depends onnumerous factors. Natural site characteristics canenhance or reduce the potential for a given agrichem-ical to leach and to contaminate groundwater. Localtopography and landforms can favor surface runoffover downward soil seepage or vice versa. Vegeta-tion and climatic parameters (temperature, precipita-tion, air movement, and solar radiation levels) affectthe environmental fate of contaminants as well (14).Roots and sunlight can interact directly with thecontaminant (e.g., photochemical degradation ofchemicals exposed to sunlight, root uptake ofnutrients and pesticides); vegetation and climatealso have impacts on soil properties. Other variables

such as the depth to the water table, characteristicsof the unsaturated zone, and the presence anddistribution of low-permeability layers also canaffect contaminated water flow. Pesticide degrada-tion may occur via one or a combination of severalbiological and chemical pathways, and the operativepathways may vary from site to site (58).

Certain soils may have direct physical or chemicalinteractions with agrichemicals. Some chemicalreactions, relating to the presence or absence ofoxygen or to the hydrolysis of a chemical, may serveto detoxify contaminants in the soil. Sometimes,though, the pesticide breakdown products may bemore toxic than the parent compound (14).

Topography and the Soil Surface

Topography of the land and the roughness of thesoil surface can affect the movement and fate ofagrichemicals applied to agricultural lands. Slopingagricultural lands tend to be more prone to watererosion than are flat lands. On flatter agricultural

48 ● Beneath the Bottom Line: Agricultural Approaches To Reduce Agrichemical Contamination of Groundwater

lands, water erosion is less of a problem and thelikelihood for infiltration of the agrichemical-bearing water into the soil and into groundwater maybe enhanced. On flat land, wind is likely to be theagent that erodes soil and carries agrichemicals fromagricultural lands. Strong winds can remove fine soilparticles and lightweight organic matter from drysoils. These airborne materials may end up in distantwater bodies; any attached agrichernicals may ulti-mately move into the groundwater. Rougher soilsurfaces, such as those produced by leaving cropstubble on the field, tend to reduce runoff and thushold agrichemicals and soil particles on site, afford-ing time for agrichemical degradation.

Some pesticides will break down when exposed todirect sunlight, a process called photochemicaldegradation. The longer pesticides are exposed tosunlight, the more likely it is that photosensitivechemicals will break down. Topography obviouslyaffects length of exposure to sunlight (e.g., north-v.south-facing slopes); it also affects soil temperatureand microbiota, which in turn affect pesticidedegradation.

Vegetation

The presence and type of vegetation-forests,grasslands, or agricultural crops—strongly affect themovement of water and water-borne solutes withinthe vadose zone. Crops such as alfalfa with roots upto 20 feet deep and high water demand, andsunflowers and safflowers with roots penetrating toat least 6 feet, have impacts far different from thoseof shallow-rooted crops with lower water demand.Agrichemicals are less likely to pass beyond deep-rooted crops to contaminate groundwater than totravel beyond the much shallower root zone of cropslike corn (17,64). Once agrichemicals pass the rootzone there is little to stop them from movingdownward to the groundwater.

The closer the spacing between individual plantsthe less potential there is for soil erosion and theinadvertent movement of agrichemicals to off-sitelocations and potential groundwater contamination.Close-grown crops such as grasses or small grains,are more likely to intercept raindrops and shield thesoil from wind than widely spaced crops such ascorn, soybeans, or cotton. Moreover, the denser theroot system the less likely it is that soluble nutrientswill pass the root zone and move into groundwater.This is particularly true when the nutrients areapplied at that time during the growth period when

Photo credit: U.S. Department of Agriculture,Agricultural Research Service

USDA researchers take groundwater samples from atest site next to a cornfield in Beltsville, Maryland.

Groundwater will be tested for pesticides in a study on theeffects of different tillage methods on pesticide movement.

the plants have the most demand for them. Thoseareas having the longest growing seasons provide forthe maximum nutrient uptake.

When annual crop plants die, nutrient and wateruptake by the plants ceases, thus providing a periodwhen water, agrichemicals remaining in the soil, andnutrients from decomposition of crop residue canmove downward. Some nutrients may be seques-tered by soil organic matter; others are subject toleaching and may contaminate groundwater. Conse-quently, the removal or harvest of annual crops andits timing plays an important role in the fate ofagrichemicals (64).

Water Table

The movement of water into and through the soilis very complex, and there are seasonal and regionalvariations in the amount of water that enters the soiland eventually recharges groundwater (25,57). The

Chapter 3--Contamination of the Hydrogeological System: A Primer ● 49

amount of recharge, depth to the water table, andfluctuations in depth to water table vary withclimate, soils, topography, and geology.

The water table tends to be shallower and morereadily recharged in the Eastern United States whereprecipitation normally exceeds evaporation, than inthe arid/semi-arid regions of the Western UnitedStates, where the reverse is true. Streams supplied bywater sources originating in distant mountains arefor the most part the only significant source ofgroundwater recharge in some arid regions of theWestern United States (75). With little rainfall overlong periods of time, the groundwater table inarid/semiarid regions may be as much as 1,500 feetbelow the land’s surface (6).

In humid regions, the likelihood of contaminatinggroundwater with agrichemicals is higher than in dryregions where water is scarce, because of the shorterdistance between the land surface and the ground-water table. Longer transit time in dry regions thanin humid areas may afford greater opportunity forthe natural breakdown of pesticides. However, forthose pesticides requiring moisture for degradation,this condition may lead to a persistence in the soil.

The water table fluctuates seasonally, typicallyrising during the winter and early spring rains, andfalling during drier months. Under drought condi-tions, the water table will continue to fall. Streamsand ponds that once served as outlets for ground-water may begin to dry up as their waters follow thefalling water table. In normal times, the water tablemay rise to the plant root zone during the “springflush’ ’-when snows melt and rains are morefrequent or intense-minimizing potentially mediat-ing soil effects, Spring also tends to be the period ofheaviest plant nutrient application.

Soil Characteristics

Soil characteristics are determined by the inter-action of soil-forming factors such as the soil’sgeologic parent material, the climate under whichthe soil formed, its topographic position, the natureof the vegetative cover, the kinds and abundance ofsoil organisms, and the amount of time the soil hasbeen forming. The resulting soil properties in turnhave a direct influence on how rapidly or slowlyagrichemicals move through the soil into ground-water. Therefore, in a country as large as the UnitedStates where significant variation exists in soil,geology, climate, and topography, it is natural to

expect large variations in soil properties verticallyand horizontally in different areas. It would benecessary to have site-specific data on the soil typeto indicate soil structure, mineralogy, chemistry, andtexture before making detailed predictions on thepotential for contaminating groundwater with agri-chemicals.

Soils exist in a water-saturated or unsaturatedstate. Plants growing in ponds and marshes havetheir roots in water-saturated soils. Most agriculturalcrops, however, grow on unsaturated soils compris-ing the top few feet of the vadose zone. The soilfactors that affect leaching and degradation proc-esses through unsaturated soils include organiccarbon, clay and moisture content, pH, temperature,texture and structure, nutrient status, and microbialactivity (14).

Physical and Chemical Soil Characteristics—The texture of soil relates to the size and shape of itsconstituents, and extent of particle aggregation (56),all of which affect the volume of air or water a soilcan hold or transmit. Soil texture exerts substantialcontrol over the movement of water and associatedagrichemicals.

Soils have many open spaces between constituentparticles that can hold and transmit water. This openspace in a soil is called porosity. However, if theopen spaces or pores are not interconnected, watercannot flow through the soil rapidly. Such soils aresaid to lack permeability even though they areporous. Clean sand (sands containing little silt orclay or other fine-grained materials) and gravel soilsare porous and permeable but as the content of finesilt and clay particles increases, the pores becomeplugged and the rate at which water moves throughsuch soils decreases. Therefore, it is important toknow how porous a soil is, how large the pores are,and to what degree the pores are interconnectedbefore predicting the fate of agrichemicals applied tothat soil.

Some of the best agricultural soils are calledloams, i.e., those containing about 5 to 25 percentclay with approximately equal parts of silt and sandconstituting the remainder. Such soils commonlyremain well-aerated throughout the year and draineffectively. Loam soils are better than either coarse-grained soils or fine-grained, poorly drained soils infaltering out and arresting downward percolatingcontaminants (45).


Pore size is an important characteristic to considerwhen evaluating the likely movement of contamin-ated water. A thin film of water is held tightly onthe mineral particles making up soil by forces ofmolecular attraction. This film of water (adsorbedwater) does not behave like the water in the center oflarge pores. The adsorbed water will not flow out ofa soil’s pores as will the water in the center of a largepore (absorbed water). Consequently, a soil com-posed of fine-grained materials may have a highporosity and the pores may be interconnected, butbecause the pores are so small most of the water isadsorbed and little will be able to flow through thesoil (66). Such soils give farmers problems bemusethey are slow to dry out, waterlog easily, are difficultto cultivate, and do not crumble but form clods (53).The oxygen content of the soil can be reduced insuch situations to the point where plants are ad-versely affected.

Soil particles tend to be spherical in the large grainsizes (e.g., sand) but more plate-like in the freerfractions (e.g., clay). Fine clay particles can bearranged in two general forms, one like a deck ofcards and the other like a house-of-cards. Theadsorbed water is continuous between parallel clayparticles and, therefore, essentially is immobile.Little pore space exists in the “deck-of-cards”arrangement. The house-of-cards clay arrangementhas a high porosity and may have interconnectedpores, but because the clay sheets are so small, thelayer of adsorbed water on each sheet overlaps withthat of adjacent sheets, also restricting water flow.Clay-rich soils and rocks thus transmit water poorlyand, therefore, retard agrichemical movement intogroundwater.

Clay minerals have other important properties forretarding the movement of certain agrichemicals,heavy metals (toxic constituents of sewage sludgecontaining industrial wastes), and bacteria intogroundwater. Many U.S. soils contain several com-mon types of clay minerals that can trap fertilizernutrients on their outer surfaces as well as betweenmineral layers. The clays can incorporate nutrientsimportant to plants such as potassium, calcium, ormagnesium, hold them in an exchangeable form, andrelease them later to plant roots or the soil solution.The movement of nutrients to and from clay surfacesis called ‘‘ion exchange. ’

Some pesticides and heavy metals also can betrapped by appropriate kinds of clay minerals. In

addition, some bacteria that might originate insewage sludge, manure, or even dead farm animalscan be filtered out of soil water or groundwater andtrapped by clays and even fine-grained sands (66).Viruses, being much smaller than bacteria, are noteasily faltered out but their properties are such thatthey are likely to adhere to clay mineral surfaces.

Another important component of soils is thehumus that gives the uppermost part of soils theirdark color (figure 3-5). Humus is a breakdownproduct of plant and animal organic matter and, likeclays, has the ability to filter out and capture bacteriaand many chemical contaminants. Organic mattercan hold water, heavy metals, and some organicchemicals and it promotes the retention of solubleplant nutrients that otherwise would tend to leachdownward with percolating waters. Pesticide ad-sorption in soils in many studies has been found tocorrelate with the soil organic-matter content (14).

Soil organic matter plays a key role in successfulagriculture, imparting benefits to soils that, for themost part, cannot be obtained by merely addingchemicals. Soil organic matter promotes soil particleaggregation, which in turn improves soil tilth andsoil percolation (74). Thus, soil organic matterrelates directly to the capacity of the soil to hold airand moisture, and promote more extensive, deepercrop root systems. The latter is important in theoverall water use efficiency of the crop.

Further, organic matter ultimately is biologicallydegraded to release the ‘‘macronutrients’ (nitrogen,potassium, and phosphorus) most essential to plantgrowth. The main natural source of nitrogen forplant growth is soil organic matter, however, most ofthe nitrogen is unavailable to plants until it isconverted to ammonia and nitrate by microorga-nisms. Soil organic matter also helps control potas-sium supply for plant growth. As soil reservoirs ofavailable potassium are depleted, they are replen-ished by potassium released from organic residues,fertilizer, living organisms, and soil minerals (47).

The mineral part of soils ordinarily contains about400 to 6,000 lb. per acre foot of nitrogen in the plowlayer. Somewhat lesser amounts are found in sub-soils (3). Nitrate levels in range and wheat fallowsoils of central and south central Nebraska wereestimated up to 150 pounds per acre foot at depths of30 to 40 feet. These high natural volumes of nitrateexceed the amount applied as fertilizer in the State,

Chapter 3---Contamination of the Hydrogeological System: A Primer Ž 51

—

—

SOIL HORIZONS

Humus just below surface

A Horizon(Zone of Ieaching)

B Horizon(Zone of accumulation)

—C. Horizon(Slighty weatheredparent material)

SOURCE: B.J. Skinner and S.C. Porter, T,+A Dynamic Earth (New York,NY: John Wiley & Sons, 1989).

and constitute a considerable threat to groundwatershould they leach (13).

Soil inorganic matter may contain from 15 to 80percent of the total soil phosphorus, an importantplant nutrient (3). Mycorrhizal fungi are active incollecting phosphorus for plant use. As the phospho-rus is slowly released during weathering of certainsoil minerals, it is moved to plant roots by the fungi(76).

Characterizing the amounts and types of clayminerals, organic matter, and other soil componentsis complex, yet such information is fundamental toassessing the fate of commercial fertilizers, pesti-cides, and the heavy metals in sewage sludge thatmight be applied to agricultural land. Increasedregional and soil series data are needed.

Biological Characteristics

Biological agents also affect the movement ofwater and water-borne substances within the vadosezone. Organic compounds break down most readilywithin the uppermost ‘‘bioactive” soil layers, al-though microbial populations are present and can besignificant in deeper unsaturated zones (58). Thesoils most reactive with agrichemicals possess

substantial water-holding and ion-exchange capaci-ties, an open physical structure, and thriving popula-tions of beneficial bacteria, fungi, and invertebrates(figure 3-6).

However, burrowing animals and decaying plantroots may create vertical ‘‘macropores’ that permitthe rapid passage of water (41,55). Rapid, channeledflow, as opposed to dispersed, slow seepage leavesless room for soil reactions to cleanse water physi-cally or chemically, and increases the potential forthe movement of soil nutrients and other contamin-ants into groundwater.

Microorganisms-Most soil microorganisms aremicroscopic or barely visible to the naked eye. Soilmicroorganisms (bacteria, fungi, actinomycetes, andprotozoa) serve a critical function in that theymetabolize extant organic matter to release thenutrients essential for plant growth. Microbial de-composition of organic matter also releases ele-ments not used directly as plant nutrients. Some ofthese elements may be converted to gaseous form(e.g., carbon dioxide and nitrous oxides). By suchconversions, microorganisms in part regulate thechemistry of the Earth’s surface and atmosphere.

Microorganisms comprise the sole or chief naturalmeans for converting organic forms of nitrogen,sulfur, phosphorus, and other elements to plant-available forms. In the final stages of biochemicaldecomposition of organic matter, nutrients arerecycled, humus forms, and soil particle aggregationis fostered (21). Any actions or agrichemicalsdeleterious to these microbial processes ultimatelywould have adverse consequences on crops.

Potential groundwater pollutants can be degraded(converted to a non-toxic form) or created bybiological agents. Certain “nitrifying” soil mi-crobes convert organic compounds of nitrogen intonitrate useful to plants and potentially available forleaching to groundwater. In the absence of highlevels of commercial nitrogen fertilizers, the rate atwhich microorganisms convert nitrogen to productsuseful to plants largely determines the rate of plantgrowth. Leaching of microbially produced nitrate-not of fertilizer nitrate-is thought by some Britishscientists to be the primary source of nitrate detectedin some of their water supplies (l).

Further, soil microorganisms are responsible fordecomposing a wide array of synthetic organicchemicals in agricultural soils and water, including


Figure 3-6—Microfauna and Macrofauna Open Conduits and Create Pore Spaces in Soils

SOURCE: P.H. Raven, R.F. Evert, and S.E. Eichhorn, BiohgyotPLants (New York, NY: %rth Publishers, Inc., 1986).


pesticides, industrial wastes, and precipitated airpollutants, converting them to inorganic products.The breakdown process may lead to detoxificationof toxic chemicals, the formation of short- orlong-lived toxicants, or the synthesis of nontoxicproducts. Scientists have investigated only a few ofthe multitude of chemicals to determine whatbreakdown products are formed when microorga-nisms encounter chemicals in natural systems (2).

Soil Invertebrates and Vertebrates-Most soilsare inhabited by a diversity of life forms. The soilbiota includes, in addition to numerous microbes, awide variety of invertebrate animals and a fewvertebrates. Some of these larger soil invertebratessuch as earthworms, ants, other soil insects, and landsnails and slugs are important to agrichemicalleaching or degradation processes. Small mammalsare the dominant vertebrate animals found belowground, but some amphibians, reptiles, and even afew birds live at least a part of their lives within soils.

Soil “macro-organisms” often modify and en-hance the soil by their activities, carrying out theearly stages of the physical and chemical decompo-sition of all types of organic debris in or on the soil.They are vital to the formation and maintenance ofthe natural soil system and perform functionsessential for plant growth. Annually, earthworms inone hectare of land can produce as much as 500metric tons of castings, the soil material passingthrough their gut. The castings are enriched innutrients compared to the adjacent soil: 5 times asmuch nitrogen, 7 times as much phosphorus, 11times as much potassium, 3 times as much magne-sium, and 2 times as much calcium (61). Before thewidespread availability of commercial fertilizers,nutrients recycled by the biota were recognized as amajor component of soil fertility and so soil biologyranked high among the agricultural sciences. Inrecent decades, however, there has been much lessemphasis on soil biology as increased soil fertilityhas been achieved through use of commercialfertilizers.

Despite the lack of quantitative data on the impactof farming practices on invertebrates in most U.S.soils, some qualitative information does exist. Thesituation is not the same for soil vertebrates, whichinclude such animals as moles, gophers, mice, otherburrowing mammals, and some reptiles and amphibi-ans. Even though some people worry that agrichem-icals may harm beneficial soil invertebrates, the

activities of soil vertebrates are commonly andnarrowly viewed as negative: for example, makingburrows in which farm machinery can becomeentrapped, consuming valuable grain or forage, orproviding pathways for agrichemicals to reach thegroundwater table. Some studies of soil vertebratessuggest that they may also have beneficial impacts,such as breaking up hardpan a foot or more below thesurface, thus improving drainage and increasingrooting depth. Unfortunately, such ecological stud-ies typically are conducted on virgin land and aredifficult to relate to agricultural lands (63).

No economically feasible substitutes exist for thesignificant functions of organic matter and soil biota,so their maintenance in croplands and rangelands iscritical. Soil invertebrates and microorganisms as-sist in breaking down plant remains, producing neworganic compounds that promote good soil structure,and convert soil nutrients to forms usable by plants.Microbes also break down pesticides and other toxicchemicals. Without the soil biota, the organic matterfrom plant residues and manure would be of littleuse. Consequently, care is needed to assure thatagrichemicals moving through the soil and ground-water do not adversely affect the soil biota.

Characteristics of UnderlyingGeological Materials

In situations where soils lie directly over bedrockit is generally easier to predict the likelihood foragrichemical leaching to underlying aquifers than ininstances where unconsolidated sediments separatethe soil from the bedrock. In this latter situation, thecharacteristics of the intervening materials play animportant role in determiningg the fate of agrichemi-cals.

Bedrock Characteristics

Accumulations of unconsolidated materials andvarious kinds of bedrock may lie beneath the soilsurface. Whatever its name and origin, it is largelythe chemical and physical nature of bedrock thatgoverns water flow and pollutant dispersal. Eventhough the permeability of some types of bedrock isvery low (table 3-1; figure 3-7), most types ofbedrock are criss-crossed with hairline cracks andfractures, and larger cracks or “joints” providepathways through which water can flow. Some rockslike sandstones and conglomerates may be highlypermeable even where joints are scarce.


Table 3-l—Estimated Permeability of Typical Geologic Materials in Illinois

Geologic material Flow rate Comments

Clean sand and gravel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 ft/yr May be highly permeableFine sand and silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ft/yr -100 ft/yrSilt (Ioess, colluvium, etc.) . . . . . . . . . . . . . . . . . . . . . . . . . .

—10 ft/yr -1 ft/10 yr

Gravelly till, less than 10% clay . . . . . . . . . . . . . . . . . . . . . ..

1 ft/yr -1 ft/100 yr Often contains gravel/sand lenses or zonesTill, less than 25% clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ft/10 yr -1 ft/1 ,000 yrClayey tills, greater than 25% day . . . . . . . . . . . . . . . . . . . 1 ft/100 yr -1 ft/10,000 yr Often contains gravel/sand lenses or zonesSandstone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ft/yrCemented fine sandstone . . . . . . . . . . . . . . . . . . . . . . . . . . .

—10 ft/yr -1 ft/100 yr Frequently fractured

Fractured rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 ft/yr May be extremely permeableShale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1ft/100 yr -1 ft/1,000,000 yr Often fracturedDense limestone/dolomite (unfractured) . . . . . . . . . . . . . . 1ft/1000 yr -1ft/1,000,000 yr —

SOURCE: Adapted from R.C. Berg, J.P. Kempton, and K. CartWright, “Potential for Contamination of Shallow Aquifers in Illinois,” Illinois State GeologicalSurvey, 1984.

Figure 3-7-General Direction and Rate ofGroundwater Movement

I / \ ,\ Decades / / ‘\ ‘ . 1

1 , \Centuries

/\ / \

‘ - F l o w l i n e s -~ — —— .- \— — — — — — — - - Mlllennla

\\

— — — — — — —— — — — —— — — — —— — — — — —l— — — — — — . — — — I

SOURCE: R.M. Wailer, “Groundwater and the Rural Homeowner, ”(Reston, VA: U.S. Geological Survey, 1988),

Generally, fractures and joints in bedrock becomeless common with increasing depth and groundwatermovement and storage volume decreases. At leastone-half of all groundwater, including most of theusable groundwater, occurs within the upper 2,500feet of the land’s surface (66).

Bedrock commonly shows evidence of distortionand folding and faulting. The variation of bedrocktypes and properties, and the different geologicstructures present beneath the land’s surface, allaffect the flow of water and, hence, complicatepredictions of contaminant movement in surface andgroundwater. Groundwater follows an erratic pathrather than a straight, vertical line and contaminantsmay be carried considerable horizontal distancesaway from the original site of surface application.Where water encounters solution cavities and chan-nels in an area of carbonate bedrock, it may moverapidly downward as if through an open well.Without detailed subsurface geological data, it isnearly impossible to predict precisely where ground-

water and its pollutants are likely to move oraccumulate in the subsurface.

Solution Cavities in Carbonate Rocks

Limestone, dolomite, and marble are commonrocks that can dissolve slowly as water comes incontact with them. Over centuries, rainwater andgroundwater can dissolve a considerable volume ofthese rocks leaving behind a variety of solutionfeatures (cf: 66). Regions where limestone is com-mon at, or very near, the land surface and wheresolution of this rock is at an advanced stage, arecharacterized by sinkholes, caves, and streams thatseem to disappear into the ground. These featurestypfiy what geologists call karst topography.

If agrichemicals are used in karst regions there ishigh probability that groundwater will be contami-nated. Once such chemicals move into the ground-water in such a setting, they can move rapidly overlarge distances diluting to lower concentrations orcausing contamination in unexpected places. Wellsin karst regions, therefore, are highly susceptible tocontamination from agricultural activities.

In certain cases, limestone karst topography isburied far below the land surface. Overlying sedi-ments may have low permeabilities and conse-quently downward moving agrichemicals may notreach the water-filled limestone cavities. In suchcases, well-water pumped from the limestone aqui-fer may be uncontaminated, However, in caseswhere the limestone beds are tilted and crop out atthe land surface, the entire aquifer may becomecontaminated as agrichemical-laden groundwaterflows laterally from its shallow to its deepest parts.Wells miles from the source of contamination can beadversely affected. Thus, groundwater contamina-


tion that begins as a local problem can, under certainconditions, become regional in nature.

Unconsolidated Materials

Unconsolidated materials commonly underlie soilsin many parts of the United States. For example,extensive unconsolidated glacial deposits separatethe soil from bedrock across much of the farmlandsof the northern part of the United States fromMontana to Maine. These and other unconsolidatedmaterials affect how slowly or quickly contaminatedwater will reach groundwater in confined andunconfined aquifers. Geologists can assist withassessing the subsurface character of these sedi-ments where concerns exist about agrichemicalcontamination of subsurface waters.

Unconsolidated sediments deposited alongstreams and rivers (alluvium) can cover bedrock andcan vary greatly in thickness. Similarly, sedimentsthat move downhill and accumulate at the foot ofslopes (colluvium) also can cover bedrock to varyingdepths. Other unconsolidated material form in placefrom weathering of underlying bedrock. These typesof sediments can vary in composition vertically andlaterally over short distances, thus directly affectingthe downward flow of water.

The porosity and permeability of the unconsoli-dated materials relate to the sediment’s sourcematerial, the degree of weathering, whether or notthe unconsolidated material has been transported,and the mode of transportation. Where unconsoli-dated materials are thick, porous and permeable,they commonly are filled with water in their lowerparts if rainfall is sufficient, and they are used asunconfined aquifers by farmers and others. Ofcourse, where they have a high degree of porosityand permeability and underlie agricultural sites, theyare likely to be contaminated easily where agrichem-icals are applied to the land surface.

Glacial Geology and US. Midwest Agriculture

Glaciers moving south from what today is Canadaonce covered large parts of the United States fromMontana to Maine and as far south as southernIllinois (figure 3-8). The last glaciers melted orretreated about 10,000 years ago leaving behind avariety of sediments of varying thicknesses, fillingin old river valleys and giving the land a muchsmoother topography than before. Today, rivershave cut through these glacial sediments in some

places but much of the flat land of this region still hasa glacial sediment cover.

This glaciated region—nearly one-quarter thearea of the lower 48 States--contains 40 percent ofthe U.S. population and some of the best agriculturalland in the world, including the “Corn Belt. ” Thisalso is the region of the United States where theapplication of agrichemicals is highest.

The geology of the glacial deposits is complicatedbecause the sediments had different origins; thecomposition of this sedimentary veneer varies later-ally and vertically. Some of the sediments weredeposited directly by moving ice and are clay richand relatively impermeable (glacial tills). These tilldeposits are likely to contain intermixed sand,cobbles, and boulders. Trapped beneath tills in somelocalities are the compressed remains of forests andother vegetation that may assist in agrichemicalbreakdown. Some sediments were derived fromglacial melt-water and consist of permeable, cleansands and gravels. Still other deposits are composedof the fine silts from stream valleys blown across theland during dry periods (loess).

Each of these sediment types transmits water at adifferent rate. Wind-blown loess deposits, for exam-ple, drain more slowly than gravels and sands butmuch more rapidly than clay-rich tills. Conse-quently, knowledge of the origin, distribution, andcomposition of these glacial sediments verticallyand horizontally is key to understanding whereagrichemical-bearing water from agricultural opera-tions may have potential to reach groundwater.

Aquifer Configuration

Below the groundwater table, pores of the rocksand sediments are filled with water. However, thisdoes not imply necessarily that the water is availableto a well in sufficient amounts to satisfy humanneeds (an aquifer). For example, a completelysaturated fine-grained sediment or rock would yieldwater to a well too slowly to be considered anaquifer. (Many mines exist below the groundwatertable but because the tunnels are in rock having littlepermeability, the mines stay quite dry and have fewwater problems.) Therefore, downward-moving watercontaining agrichemical contaminants could in factcontaminate groundwater but not necessarily anaquifer.


Figure 3-8-Extent of Pleistocene Continental Glaciation in the United States

SOURCE: Adapted from R.E. Snead, 14WdAtk?s of Georrrorphic Features (Huntingdon, NY: R.E. Kreiger Publishing Co., Inc., 1980).

Aquifers are classified as being “unconfined’ or‘‘confined. ’ Unconfined aquifers are those in whichthe water table is the top of the aquifer. A confinedaquifer (or artesian aquifer) is separated from thegroundwater table above by a layer of relativelyimpermeable sediment or rock and is sealed at itsbase by another layer of materials having lowpermeability. The water in the aquifer is underpressure and, therefore, rises above the top of theaquifer in a well. A greater potential for agrichemi-cal contamination of well-water exists in unconfinedaquifers than in confined aquifers that may haverelatively small recharge areas.

Putting It All Back Together

The hydrogeologic cycle is a complex system ofinteractive components and processes, driven by theSun and modified by local variations in climate,topography, vegetation, soils and bedrock, andhuman activity. Groundwater problems and solu-

tions, therefore, cannot be addressed without refer-ence to the atmosphere, surface waters, the soils andbedrock that overlie and contain groundwater, andhuman activity at the Earth’s surface.

Changes affecting any one component of thehydrological cycle are likely to be felt by othercomponents, or throughout the system. Over thelong term, changes in regional climates affect howrocks weather and, hence, influence soil develop-ment and soil thickness. Soils, in turn, help deter-mine what kinds of agriculture are possible in aregion, and the extent to which agricultural activitiesand different cropping and tillage systems mightaffect groundwater.

Because water on and below the ground’s surfaceis part of the same integrated system, what happensto groundwater, through human use, ultimatelyaffects water resources on the land’s surface and viceversa. Due to changes in rainfall patterns andagricultural activities, infiltration rates may vary


over time in a given area, leading to fluctuations inaquifer levels (groundwater storage), and affectingthe dynamics of surface and groundwater exchange,and sometimes water quality.

Because of the many different factors that affectgroundwater storage and quality, groundwater man-agement poses complex challenges. In assessingknown or potential groundwater qualityall components of the hydrologic cycleman’s ability to modify them should beaccount.

problems,as well astaken into

HUMAN MODIFICATIONS OF THEHYDROGEOLOGIC SYSTEM

Agriculture, by definition, continually modifiesthe landscape and its vegetative cover throughoutthe year and over the years. Application of chemicalsto agricultural fields is but one possible source ofgroundwater and surface water contamination prob-lems related to agriculture. Two additional pathwaysexist for agrichemicals to reach groundwater, bothrelated to changing the nature of the hydrogeologicalsystem itself. The first way is through openings inthe soil or exposed bedrock that circumvent soilfiltration processes (preferred pathways), and thesecond way is through land-use practices thatchange the groundwater/surface water relationships.

Humans have dug and drilled holes in the groundfor many purposes over time, inadvertently provid-ing pathways for agrichemicals to reach ground-water. These include, for instance, water-wells, drillholes for mineral exploration, seismic shot-holes,test drilling for foundations, injection wells, tile-drainage wells, missile silos, and mines. On a muchsmaller scale, plant roots and burrowing animalsmay create vertical charnels allowing for rapidinfiltration of water.

Similarly, land-use changes also can affect theflow of surface water and groundwater therebymoving agrichemicals to unwanted sites. For exam-ple, changing dry-land agriculture to irrigated (andperhaps chemigated) agriculture, construction ofponds for groundwater recharge, construction ofdams and reservoirs, and channeling and dikingstreams can cause such changes. The followingsection describes a few of these land-use examplesand relates them to possible movement of agrichem-icals beneath the land surface.

Photo credit: U.S. Department of Agriculture,Agricultural Research Service

Since climate affects pest outbreaks, weather balloons arereleased near the Mexico-U.S. border to study migratorybehavior of can and cotton pests. Better information onpest populations can help farmers be more selective on

when and where to apply pesticides.

Preferred Pathways

Water will flow along the path of least resistance.Even though a soil maybe fine-grained and have lowpermeability, if it is pierced by small, naturalchannels (macropores) or larger manmade conduits(megamacropores), water contaminated with agrichem-icals can move rapidly through these toward thegroundwater table rather than slowly through thesoil matrix where most contaminants are trapped orbroken down. Although the amount of agrichemicalsmoving downward through such openings may besmall for any single opening, the total that can bemoved during a growing season could significantlyand adversely affect water quality.

The most common natural macropores derivefrom earthworm channels, decayed plant roots, or

58 ● Beneath the Bottom Line: Agricultural Approachees TO Reduce Agrichemical Contamination of Groundwater

cracks from soil drying. Freezing and thawing willcollapse some of these conduits. Nevertheless,during the warm spring and summer months, agrichem-ical contaminated water can move easily downward.Similarly, the burrows of larger vertebrate animalsprovide pathways deep into the soil. Such conduitswill not extend below the groundwater table unlessthe water table rises.

Megamacropores can be natural, such as sink-holes where the land’s surface has collapsed intounderground caves eroded from carbonate rocks(“solution cavities”), or manmade conduits likeabandoned wells and drill holes. The latter may beseveral inches to several feet in diameter, whilesinkholes may be hundreds of feet across.

Poorly constructed water-wells can lead to ground-water contamination problems. Water-wells havingcontinuous steel casing from the land surface downinto the aquifer can eliminate the possibility ofdegrading the drinking-water source with contami-nated water from shallower aquifers. Completion ofsuch wells so that contaminated surface runoffcannot enter the well head is essential to keepagrichemicals from contaminating the well-water. Ifactive or abandoned wells are only partly cased or ifcasings corrode or crack, a potential will exist forcontaminants to reach the well’s aquifer.

Abandoned Drill Holes and Wells

Drilling holes in the ground for oil, water, mineralexploration, foundation testing, and other uses hasbeen a common practice in the United States formany years. The first productive oil well wascompleted in Titusville, Pennsylvania in 1859 (66),but water-wells predated oil exploration by manyyears. Only recently have States developed regula-tions about the proper sealing of abandoned wellsand other such holes. Quantitative data on thenumber of wells and drill holes is sparse and thenumber of improperly sealed abandoned holes ineach State probably will never be known.

Minnesota is one State where some quantitativeinformation exists, although estimates are based onextrapolation of certain field sites. The MinnesotaDepartment of Health (MDH) estimates that some700,000 to 1.2 million abandoned water-wells inMinnesota have a potential to endanger groundwaterquality (88). Today, Minnesota has roughly 500,000producing water-wells, and some 10,000 new water-wells are drilled annually. By a conservative esti-

mate, about 10 percent of these are replacementwells. Therefore, at least 1,000 additional water-wells are abandoned each year. At the present rate ofsealing (2,500 in 1988 at an average cost of $500each), the MDH estimates that it will take 480 yearsto seal already abandoned wells. If 1,000 additionalwater-wells are abandoned each year, sealing thecombined backlog of abandoned wells will take 800years.

Minnesota is not an oil- or gas-producing State, sothe number of abandoned wells there probably is farbelow the total number of wells and exploratory drillholes and seismic shot-holes scattered over Statessuch as Texas and Oklahoma. Some abandonedwells and holes may have collapsed so that they nolonger present avenues through which agrichemicalsmight move to contaminate groundwater. Further,water flowing down the walls of an open holethrough the unsaturated zone are subject to strongwithdrawal into the unsaturated zone. Contami-nants, therefore, may not reach the water table if thecontaminated supply of water is small (5). Yet otherabandoned holes and wells probably are still openand may present a serious threat to States’ ground-water resources.

Agricultural Drainage Wells

Agricultural drainage wells are structures de-signed expressly to provide access to undergroundstrata for disposal of water drained from saturatedsoils or from irrigation systems. Farmland drainage,the primary agricultural water management and farmreclamation activity in this country, occurred through-out the last century, peaking in the 1930s (74).Nearly 75 percent (77 million acres) of the croplandon which wetness is a dominant constraint onproduction (105 million acres; (77)) have manmadesurface or subsurface drainage systems (79). Thereare indications that many of the drainage systemsconstructed in the early 1900s, particularly in theMidwest, are now obsolete and in need of repair; intheir current state, they promote leaching (74).

Drainage outflows can be directed through drain-age wells and sinkholes into subsurface strata (figure3-9). If outflow waters are directed into sinkholes fordisposal, the relatively rapid movement of ground-water through karst may provide relatively rapiddilution of the soluble chemicals carried, However,in areas with fractured bedrock or slow-movinggroundwater, chemicals may remain concentrated inthe subsurface.


Figure 3-9-Schematic Diagram of AgriculturalDrainage Well

Drainage wellGround surface

Surface water intake

Confining layerpreventing infiltration

SOURCE: Adapted fromlowa Department of Natural Resources, Envi-ronmental Protection Commission, Iowa Grourdwater Pro-tection Strategy, 1987

Drainage outflows and irrigation tailwaters com-monly carry agrichemicals and naturally occurringsoluble soil minerals, such as nitrate and seleniuminto surface- and groundwaters (see box 3-A).Unless properly processed or diluted, concentrationsof natural and introduced chemicals can contaminategroundwater or aquifers posing environmental andhealth hazards.

Changing Groundwater/Surface-WaterRelationships

Certain human activities can alter the naturalrelationship of surface waters and groundwater and,hence, how easily and in which directions contami-nants are likely to move. Some common examplesinclude darn construction, stream diversion, drain-age and irrigation, and over-pumping of water-wells.These can either promote contamination, or dilutegroundwater contaminated from other sources.

Dam Construction and Stream Diversion

Construction of a dam can greatly reduce thenatural rate and volume of groundwater rechargedownstream of the dam. Consequently, the ground-water table may drop to such an extent thatcontaminated surface- water bodies disappear as theydrain into the falling groundwater table. Conversely,the water reservoir that forms behind the dam canraise the area’s water table bringing the groundwatertable close to or above the land surface. In suchcases, the near-surface and surface water can pick up

agrichemicals as contaminants. Previously contami-nated groundwater may also be diluted.

Streams sometimes are diverted from their naturalchannels to new charnels to irrigate farmland, todivert water around developments, or to redirectwater to water-poor areas. The groundwater impactsalong the old charnel are similar to those that occurdownstream of a new dam, and those along thediversion channel will parallel those occurringbehind the dam.

Irrigation

Used on some 55 million acres of U.S. crops (75),irrigation is essential for crop production in aridareas, will increase crop yield or quality every yearin semiarid areas, and ensures consistent crop yieldand quality in subhumid and humid areas. However,irrigation has the potential to hasten leaching ofapplied and natural chemicals if excessive deeppercolation occurs.

Irrigation systems commonly are established onagricultural lands with excessive soil drainagewhere they provide water for leaching. Irrigationwater may release naturally occurring water contam-inants including nitrate from certain mineral-bearingformations, Leaching of naturally occurring nitratehas been documented in several areas in the GreatPlains and the Southwest (73).

In arid parts of Western States rainfall may not besufficient to leach excessive soil salts below the rootzone, requiring periodic ‘‘soil flushing’ with largeamounts of water to allow continued agriculturalproduction. This will also transport chemicals otherthan salts into the deeper soil profiles and potentiallyto groundwater. In arid areas where the contamin-a t e d ‘ ‘outflow’ ‘ waters from soil flushing aredirected into surface waters, they can seep directlybelow the water table to recharge groundwater (box3-A).

Over-pumping Water-wells

When water is pumped from a well the water tableis drawn down in the area adjacent to the wellformin g what is called a “cone of depression. ’ Thesize of the cone of depression and how quickly thedepression disappears after pumping ceases dependson the rate of water withdrawal from the well and thepermeability of the surrounding rocks or sediments.If the cone of depression becomes large enough itcan change the slope of the groundwater table. In


Box 3-A-Groundwater Contamination From Natural Sources: Kesterson National Wildlife Refuge

Kesterson National Wildlife Refuge was established from ponds built in 1971 for disposal of agriculturaldrainage water and also to provide wildlife habitat. Agricultural drainage water became the only source of inflowto the ponds by 1981, and by 1982 problems were first observed. Large-mouth bass and striped bass and carpdisappeared from the ponds. In 1983, investigations of declining waterbird births showed deformities in embryosthat were blamed on selenium (22,23,49).

Irrigated agriculture depends on the flushing of salts that accumulate in the rooting zone in order to maintainproductivity; tailgaters thus have high salt content. Normally, the oceans are the ultimate sink for dissolved salts,however, depending on the drainage system these waters may or may not reach the ocean and drainage intocontained basins may create a highly saline water body (e.g., Salton Sea, Dead Sea, Great Salt Lake).

Generally, trace elements (e.g., arsenic, selenium, molybdenum) are not contained in tailwaters, however, thesoils in the San Joaquin contain naturally elevated levels of selenium and the hydrologic conditions promoted themovement of soluble selenium into irrigation tailwaters. The damage has been attributed to a combination of factors,including: 1) the high soluble-selenium content of soils, 2) increased irrigation development and installation ofsubsurface drains, and 3) lack of understanding of the potential adverse impacts from the method of disposal (49).Irrigated agriculture can clearly create adverse offsite effects over time. Irrigation management then must includeadequate treatment and disposal plans for tailwaters.

A survey of 20 sites conducted by the Department of the Interior in Western States shows that at least four(Stillwater Wildlife Management Area, NV; Salton Sea, CA; Kendrick Reclamation Project, WY; Middle GreenRiver Basin, UT) show potential trace-metal levels (boron, arsenic, molybdenum, and selenium) similar to thoseat Kesterson (22,49).

Technical options for remediation of the Kesterson refuge have been examined, including:. transport and disposal of drainage water (ocean disposal, and deep-well injection);● source control (retirement of land from irrigation, irrigation management, evaporation ponds); and. water treatment (desalinization, chemical and biological removal of contaminants) (49).The Bureau of Reclamation, the Fish and Wildlife Service, and the California Department of Fish and Game

have developed a plan to offset the loss of the nearly 1,283 acres of wetlands destroyed. The plan calls for acquisitionand management of 23,000 acres in the San Joaquin Drainage Basin to replenish the wetland acreage. Water neededto maintain the wetland will come from the Bureau’s Central Valley Project (27).

some cases contaminated water from another well icals vary in chemical structure, behavior andcan flow downslope along the cone of depression ofthe uncontaminated well, degrading its water supply(figure 3-10). Each water-well produces its owncone of depression and where many wells exist, theirintersecting cones of depression create complicatedpatterns in the surface of the groundwater table andaffect normal flow patterns. In such cases, a properlymaintained and constructed farm well still maybecome contaminated with agrichemicals even ifnone percolate downward directly from farm opera-tions.

AGRICHEMICALCHARACTERISTICS RELATED

TO LEACHINGCharacteristics of agrichemicals may be as impor-

tant as site hydrogeological characteristics in pre-dicting groundwater contamination potential. Agrichem-

stability and, hence, in the extent to which theyvolatize into the air, are taken up by plants, dispersethrough the soil, degrade through chemical, bio-chemical, or photochemical action, or remain availa-ble for leaching through the soil (28).

Determining the probable fate of an agrichemical(it’s “partitioning” among a variety of sequestrationand degradation processes) is a complex process, butdetermination of certain key chemical characteris-tics helps scientists make such analyses (see table3-2). In general, however, agrichemicals that aremobile and persistent, if used in hydrogeologicallysensitive areas in sufficient quantities, have thehighest probability of leaching to groundwater (16).Nitrate and certain pesticides have these characteris-tics (table 3-3).

Some studies suggest that nitrate might be used asa ‘‘mwker’ for potential vulnerability to pesticide


Figure 3-10-Cones of Depression Resulting From Water Withdrawal May Result in Contamination of WaterSupplies Near Non-contaminated, Well-constructed Wells

No, 1 No. 2Uncontaminated well Contaminated well

Situation A:Both wells havemoderate withdrawalrates

..’

Situation BWell No 1 significantlyIncreased the rate ofwithdrawal with respectto welt No 2 causingcontaminated water fromwell No 2 to flow towell No 1

SOURCE: Office of Technology Assessment, 1990.

contamination, although no study has shown a clearone-to-one link between the presence of nitratecontamination and pesticide contamination. In areasof Nebraska, at least, occurrence of high nitrateconcentrations has been shown to be correlated withtriazine-herbicide concentrations (71 ,84). Likewise,LeMasters and Doyle (38) found a significantassociation between wells in Wisconsin containinggreater than 10 ppm nitrate and detectable levels of

Groundwater table

.,4———— Agrichemical contaminant

\\

Groundwater table

pesticides. However, the same researchers did notfind a quantitative relationship between pesticideconcentrations and nitrate concentrations. Similarly,the correlation was very weak in one two-countyarea of Iowa (39). Thus, in areas where herbicidesare known to be used, nitrate might serve as aninexpensive test to identify areas potentially con-taminated by herbicides (84), but more extensivedata are needed for a broader correlation analysis.


Table 3-2—Some Chemical and MineralogicalFactors Commonly Considered When Assessing

a Pesticide’s Behavior and Ability To Leacha

From Agricultural Lands

Volubility in water The amount of pesticide that will dissolve inwater in part will relate to the pH of the water (pH is a measureof acidity).

Half-life/persistence: The time needed in the field for 50 percentof the pesticide molecules to degrade.

Stability in wafer The degree to which a pesticide resistshydrolysis (breakdown in water).

Volatility from the soil: A measure of how easily a liquid pesticideapplied to the soil is able to change to a gas.

Octanol-water partition coefficient A laboratory test to deter-mine the preference a pesticide has for fats versus water.

Photolysis The breakdown process of a pesticide when exposedto sunlight.

Ability to ionize: Whether the pesticide behaves as a cation (+),anion (-), zwitter ion (+ and -), or is neutral at various pH valuesin water.

Nature and amount of soil organic matter The biologicalbreakdown products in a soil’s A horizon (uppermost layer ofa soil).

Clay mineralogy of the soil and underlying geological materi-als: The nature of the fine-grained minerals, some of which canbind pesticides tightly.

aAbility to le~h (leachability) refers to the fOllOWing pesticide Property:

when used in a normal agricultural manner under cmditions conducive tomovement, the pesticide moves down through the soil in quantitiessufficient to b detected in nearby wells of proper construction.

SOURCES: D. Gustafson, “1989 Ground Water Ubiquity Sawe: A SimpleMethod for Assessing Pesticide bachability,” Journal ofErrvironmenfai Toxiwbgical Chemistry, pp. 339-357, unpub-lished paper; A. Moye, pestiade c+emist, personal communi-cation, December 1989.

A mobile agrichemical tends to move in the waterphase without tightly adhering to soil. A pesticidewould be considered mobile if its soil/water partitioncoefficient is 1 in a soil with 1 percent organiccarbon (15). Pesticides vary widely in mobility. Thepesticide paraquat, for example, is attracted to claysurfaces where it is held tightly whereas pesticideslike picloram are repelled by the clay surfaces andcan move freely through the soil (53). Atrazine, oneof the most widely used agricultural pesticides, isonly weakly held by the soil (30), and has appearedin the groundwater of at least 13 States (82).

Volubility can also affect a pesticide’s mobilityand fate. Highly soluble pesticides are more likely tobe mobile and can move long distances with thenatural flow of surface or groundwater. Plants cancapture water-soluble pesticides along with soilmoisture, potentially sequestering them in planttissues. Pesticides that are not degraded by the plantsmay be re-released to the environment through cropresidues remaining after harvest (14).

Table 3-3-Pesticides With High Potential forLeaching to Groundwater

AcifluorfenAlachlorAIdicarbAldicarb sulfoneAldicarb sulfoxideAmetrynAtrazineBaygonBentazonBromacilButylateCarbarylCarbofuranCarboxinChlorambenChlordane-alphaChlordane-gammaChlorothalonilCyanazine2,4-DDalaponDBCPDiazinonDicamba1,2-Dichloropropanecis-1 ,3-Dichloropropenetram-l ,3- DichloropropeneDieldrinDinosebDiphenamid

DisulfotonDiuronEDBEndrinETUFenamiphosFluometuronHeptachlorHeptachlor epoxideHexachlorobenzeneHexazinoneMethomylMethoxychlorMetolachlorMetribuzinNitrate/nitriteOxamylPicloramPrometonPronamidePropachlorPropazineProphamSimazine2,4,5-TTebuthiuronTerbacilTerbufos2,4,5-TPTrifluralin

apriority pesti~des imlud~ in the EPA National Pestiade survey ofDrinking Water Wells, which includes testing for over 100 pestiddes(general use, restricted use, or banned) or their breakdown products.

SOURCE: U.S. Environmental Protection Agency, “Pesticide Included inthe EPA National Pesticide Survey,” Apr. 14, 1988.

A persistent pesticide tends to degrade veryslowly in the soil-water matrix. A pesticide with asoil-degradation half-life of 100 days would beconsidered persistent. Certain pesticides, such asDDT, can persist unchanged for long periods of timein the soil, and will accumulate over time if usedregularly.

All else being equal, if agrichemicals resistant todegradation and only weakly interactive with soilparticles are applied to widely-spaced, shallow-rooted row crops, where the water table is near thesurface, there is great potential for groundwatercontamination. If the same chemical is used onclose-grown crops with deeply penetrating roots, theunderlying aquifer may not be affected, particularlyif it is confined. Chemicals that are more easilydegraded in, or retained by soil materials, have lesspotential to reach groundwater than persistent chem-icals that interact poorly with the soil.


SUMMARY AND POLICYIMPLICATIONS

Groundwater is one of the key components of theglobal hydrogeological cycle, as well as being animportant resource. Whether pure or contaminated,groundwater can reside in some aquifers for thou-sands of years. Still, groundwater discharge (e.g., atsurface springs, or into lakes, rivers, or the ocean)and recharge through rainfall eventually cycleswater, and any contaminants it may hold, throughmost aquifers (14), Because groundwater recycles soslowly, over decades, centuries, or even millennia,and because the aquifers in which groundwater iscontained lack the cleansing mechanisms of surfacewatersheds, a degraded aquifer may not recover at allin human time frame, The surest way of protectinggroundwater is to prevent contamination at thesource.

In areas characterized by many different soils androcks it is extremely difficult to predict where, orhow fast water-soluble pollutants will spread oncethey are underground and out of sight (40). Predict-ing the patterns of contaminant dispersal below thewater table can be nearly impossible, particularly ingeologically complex regions. Understanding thehydrogeology of a site is integral to determining thepotential for leaching agrichemicals to groundwater(box 3-B), and therefore is imperative in identifyingtechnologies that may reduce potential contamina-tion.

Because of its close link to surface conditions andactivities, groundwater must be considered a part ofany agroecosystem. Agrichemical contaminants caninvade groundwater as a result of a farmer’s agrichem-ical handling or agricultural management practices,changes in land uses, or through poorly constructedor abandoned manmade holes or wells. Whetheragrichemical contamination actually occurs dependson a large number of interactive physical, chemical,and biological factors. A systems approach tomitigate or eliminate such problems today is essen-tial.

Different agricultural chemicals move through theenvironment at different rates. In some cases, lowlevels of detection may simply represent the forwardedge of a contamination pulse that is working its waythrough the soil profile (35). Without expandedresearch efforts on the fate and transport of thesechemicals, we will not know if these low levels

indicate that there is nothing to worry about, or thatthe worst is just now coming (54). Clearly, repeatedsampling of each aquifer, and testing for everyagrichemical, would be impractical. Systematicprocedures for monitoring, sampling, testing, andfor data collection and management are necessary toidentify critical site/agrichemical combinations(33).

Improving Data Collection and Managementfor Groundwater Protection

Numerous Federal agencies collect natural re-source and land-use information relevant to predic-tion of potential agrichemical contamination ofgroundwater. An evaluation of the data collection,management, and coordination systems within Fed-eral agencies is beyond the scope of this assessment.However, prediction of potential vulnerability, de-sign of site-specific agricultural practices to mitigatethat potential, and implementation of programs toreduce adverse impacts of agricultural practices willrequire extensive, detailed data and comprehensive,readily accessible information derived from thatdata.

It would clearly be advantageous for agriculturaland groundwater scientists and policymakers tohave access to relevant databases, including:

●

●

●

●

●

●

●

●

●

climate data (National Oceanic and Atmos-pheric Administration and Agricultural Experi-ment Stations);topographical, hydrological, and aquifer map-ping data (USGS);surface water quality and associated data (EPA;USGS);soil data (USDA/SCS);cropping patterns data (USDA/ASCS);nitrogen use data (TVA/NFERC);pesticide use data (USDA/NASS, USDA/ERS,EPA, and Resources for the Future);groundwater quality monitoring data (EPA,USGS); anddata on hydrogeological vulnerability (USDA/ERS).

Other data not currently available in national-leveldatabases, such as extent of tillage patterns ordistribution of and waste production from livestockconfinement facilities, would also improve decision-making.


Box 3-B—Using Hydrogeologic Information To Predict Sites Vulnerable to GroundwaterContamination: Minnesota’s Groundwater Contamination Experience

Recent baseline field and laboratory research by Minnesota’s Departments of Health and Agriculture (36,37)illustrates how hydrogeological information can be put to use in making a first approximation of the nature andmagnitude of agrichemical contamination of groundwater resources. Researchers tested well water in two differentsettings: 1) where coarse-grained soils overlie either sands and gravels or limestone bedrock having well-developedsolution channels and cavities, conditions thought to promote movement of contaminants to groundwater; and 2)where clay-rich glacial tills overlie sand/gravel aquifers, conditions thought to retard movement of contaminantsto groundwater. Depth to bedrock generally was 25 feet or less in most wells but in some it was 50 feet. Most sampleswere taken intentionally from wells in geological setting number one, therefore the percentage of wells foundcontaminated with agrichemicals probably is higher than if samples had been taken randomly from both settings.

The assumption that “confined aquifers” underlying the clay-rich tills would be less likely to showcontamination from agrichemicals than the groundwater in shallow, karst limestone environments and/or overlainonly by coarse-grained soils and glacial sands and gravels (’‘unconfined aquifers”) seems borne out by the fieldand laboratory work. The researchers found that, in general, pesticide contamination was higher in private wells thanin public wells. The former normally are shallower and nearer to fields where pesticides are applied than wells usedfor public water supplies.

Pesticide contamination was common in the karst limestone region of southeastern Minnesota; mostcontaminated wells were not associatedcontamination occurred where a thickcontaminants fkom the aquifer.

Geological setting No. 1 (unconfined aquifer)high probability of agrichemical

contamination of well-water

Waterwell

. .. .

. .

Soil

Sand and

,+

1

I I I

with obvious point sources of pollution. The fewest detections of aquiferlayer of clay-rich till or other fine-grained materials separate surface

Geological setting No. 2 (confined aquifer)low probability of agrichemical

contamination of well-water

Waterwell

Groundwater table andwell-water level

Sand and gravel aquifer

or

Karst limestone aquifer

SOURCE: Office of Technology Assessment, 1990.

Soil

%Groundwater table

% Clay-rich glacial

$$-~ Well-water level

Sand and gravel aquifer

Also playing important roles in whether a particular well showed contamination were the contaminationsource, the properties of the agrichemicals, local agrichemical practices, and well construction. These factors variedfrom well to well. However, the local hydrogeology seems to have played a lead role. Such determinations are likelyto be repeated as further data on other sites become available.


Data Adequacy for Prediction of AgrichemicalContamination of Groundwater

Producing maps and developing three-dimen-sional displays to show where agrichemical contam-ination of aquifers is likely to occur in the absenceof detailed data on soils, unconsolidated sediments,bedrock geology, and subsurface waters can lead toincorrect interpretations. It seems clear that thesynthesis of such information is critical for assess-ment of where and when possible adverse impactsfrom agrichemicals might affect groundwater re-sources. Increased State and Federal activities inproducing and presenting information depicting theEarth in three dimensions is highly important tounderstanding the nature of agriculture’s impact ongroundwater quality.

Status of Major Hydrogeologic Data CollectionEfforts-The natural earth materials-soils, uncon-solidated sediments, and bedrock-that containgroundwater are sometimes referred to as the “con-tainer” for groundwater. Characteristics of thiscontainer will determine the groundwater’s directionof flow, its chemical purity, its residence time in theEarth, and a host of other variables. Therefore, it isimportant to know the status of the information basethat currently exists to describe the “container.”Data on topography, soils, and bedrock geology arefairly comprehensive, but detailed knowledge of theintervening unconsolidated sediments is less certain.Additional data continuously are being gathered atthe State and Federal level to add to this knowledgebase, but as yet may not exist in a published form.Synthesis of the major databases described below isstarting to occur, but certain gaps still need to befilled.

Soils—The Soil Conservation Service has longstriven to develop detailed maps of soils, topogra-phy, other site characteristics, especially as theyrelate to capability to support conventional agricul-ture. Today, soil maps for most States have beencompiled. Soil data for some States have beendigitized to allow for computer manipulation, andthe other States are moving in that direction (figure3-1 1). Digitized soil databases include SOILS-5 andSOILS-6 that describe soil characteristics and suita-bility for uses such as cropping, woodlot manage-ment, and certain types of development. SCSdatabases also include the progressively freer-scaleSoil Geographic Data Bases, including National SoilGeography database (NATSGO) of soils data related

to the major land resource areas (1 :7,500,000 scale),State Soil Geographic database (STATSGO) for‘‘general’ soils mapping (1:250,000 scale), and SoilSurvey Geographic database (SSURGO) presentingdetailed soils data (1:15,840 to 1:31,680 scale) (8).

Geology and Topography-Each of the 50 Stateshas produced a map showing the bedrock geology.The oldest State map is Ohio’s, published in 1920;most other States have published maps producedbetween 1970 and 1980. A provisional bedrockgeological map was prepared for Puerto Rico in1964; few other U.S.-afffiated islands have beenmapped. Most of these maps were published at ascale of 1:500,000; some at 1:100,000; Wisconsinand Nebraska at 1: 1,000,000; and Alaska at 1 :2,500,000.Even though some of these maps are old, detailedrelated information is continually collected andevaluated by each of the State geological surveys aswell as the USGS (85).

Topographic maps are important to geologicalmapping and all aspects of land-use evaluation orplanning. The Defense Mapping Agency will, in1990, complete and publish the last 7½ minute scaletopographic maps for all States except Alaska.Alaska is completely mapped in 15-minute quadran-gles and, at this time, no plans to map at the 7½minute scale have been made (85).

Unconsolidated Materials—Even though localsoil and geologic maps showing the hard, subsurfacebedrock may exist, little is known in detail of themakeup of the unconsolidated sediments lyingbetween soil and bedrock in many States. Thishinders efforts to collate information and predictvulnerable sites. Illinois is a notable exception. TheIllinois State Geological Survey has developed mapsshowing the thickness of unconsolidated glacialsediments throughout the State (figure 3-12), anddetailed lithological and mineralogical data exist formany glacial deposits there. Data are sufficient overmuch of this area to permit detailed, three-dimensional analyses of variations of the glaciallithologies. With this information at hand, Illinois isin the position to make reasonably sound estimatesof where its groundwater and its aquifers might bevulnerable to agrichemical contamination.

The U.S. Geological Survey (USGS) is preparinga map based on data assembled from 850 sourcesthat will show the extent, thickness, and grosslithology of glacial deposits in 28 glaciated Stateseast of the Rockies (70). The map combines soil


Figure 3-n-Status of State Soil Geographic Databases

. . . . . . . . .I ~ Compilation complete. . . . . . . .

SOURCES: Adapted from U.S. Department of Agriculture, Soil Conservation Service, “Status of State Soil Geographic Databases (STATSGO),” mapcompiled using automated map construction with the FOCAS equipment, National Cartographic Center, Fort Worth, TX, revised November 1989;D. Goss, soil scientist, National Water Quality Technology Development Staff, South National Technical Center, Soil Chservation Sergvice, U.S.Department of Agriculture, personal communication, February, 1990.

data, glacial sediment data, and subsurface bedrockgeological data into a three-dimensional geologicalpicture, but published in a two-dimensional mapcalled a ‘‘stacked map. ” Such three-dimensionaldepictions are useful for analysis of where potentialagrichemical groundwater problems might exist.This new map will show for the first time the generalnature of the glacial sediments covering this largeregion (69). The map shows that the thickness of theglacial deposits is 50 feet or less over much of theregion but that broad areas exist that have at least200 feet of sediment; in some cases, thicknesses mayreach 1,000 feet or more. The thickest section ofglacial sediments (1,200 feet) occurs in the lowerpeninsular of Michigan (68). Acceleration andexpansion of efforts to produce maps showing

information on unconsolidated sediments in greaterdetail is integral to predicting the fate of agrichemi-cals applied to the land, and to assuring thatgroundwater contamination is minimized.

Water Quality-EPA and USGS maintain waterquality databases. EPA’s REACH file is a digitized,graphical database of surface water attributes cover-ing three-quarters of a million miles of the Nation’srivers, streams, lakes, bays, and estuaries. It wasdesigned primarily to analyze pollutant movementin surface water bodies, and would require consider-able expansion to include movement in ground-water. Associated with the REACH files are the EPAand USGS Water Quality Databases, which include


0 20 40 60miL I

1 1

0 40 80km

SOURCE: R.C. Berg, J.P. Kempton, and K. Cartwright, “Potential for Contamination of Shallow Aquifers in Illinois,” Illinois State Geological Survey, Circular

No. 532, 1984.


Photo credit: U.S. Geological Survey

U.S. Geological Survey personnel have routinely collectedgroundwater data on water levels, total dissolved solids,

and many inorganic chemicals in monitoring wellsthroughout the country. However, information has notbeen collected routinely on organic substances and

other key chemical parameters.

approximately 40 million observations of chemicaland natural attributes.

The USGS has a recently developed NationalWater Quality Assessment Program designed toassess water quality on a regional watershed/aquiferbasis through joint monitoring of surface- andgroundwater. The information collected includes: 1)source of agrichemicals, 2) rate of loading, and 3)where and how they are moving. Seven 2-year pilotstudies based on the initial program proposal arenearing completion, and followup monitoring isplanned to occur in 5 years. Further, the datacollection program is based on drainage systems, notpolitical boundaries. A pilot study just completed inKansas and Nebraska provides a common data setfor both States, and indicates that some agrichemi-cals are moving from Nebraska into Kansas surfacewaters (31). Full implementation of the Programwould involve work at about 120 aquifer systemsand river basins nationwide, covering about 80percent of the water currently used in the UnitedStates.

Aquifers-The USGS also has had a RegionalAquifer-System Analysis (RASA) Program, in oper-

ation since 1978, to study the 28 major regional U.S.aquifer systems USGS has identified. To date, 14studies have been completed (42). Objectives of theRASA programs are to ‘‘define the regional hydrol-ogy and geology, and to establish a framework forbackground information-geologic, hydrologic, andgeochemical-that can be used for assessment oflocal and regional groundwater resources” (83,42).The RASA studies use computer simulation to assistin the understanding of groundwater flow patterns,recharge and discharge characteristics, and effects ofdevelopment on aquifer systems. The Programalready has helped improve the matching of geo-logic and hydrologic data at State boundaries, andhas developed numerous groundwater flow-modelsfor regional use (83).

Integrated Natural Resource Information Data-bases—By congressional mandate, the SCS main-tains a comprehensive survey of agricultural andrelated natural resources on 1.5 billion acres ofnon-Federal rural lands. Surveys have been con-ducted six times in the past 30 years, including theextensively detailed 1982 National Resources In-ventory (NRI). The 1982 NRI consists of datacollected from roughly 1 million individually in-spected locations. Attributes evaluated includednearly 200 variables, such as land use and cover,conservation needs and practices, and irrigationwater source. The NRI sample points (inspectedlocations) also are directly linked to the SOILS-5databases described above (44). Because the data onmultiple attributes were collected simultaneouslyfor each sample point, this database allows analysisof associations between specific land use andresource conditions, whereas combined use of non-integrated databases using data generalized to anarea (e.g., county) cannot.

Status of Agrichemical Use Data-CollectionEfforts-Groundwater contamination potential isbased on the combination of natural factors and typeand intensity of agrichemical use or livestock wasteapplication. While NRI data is collected to evaluatesoil conservation efforts, no comparable informationgathering process currently exists related to otherresource conservation concerns (e.g., agrichemicallosses to the atmosphere and groundwater) (19).

As a result of special appropriations in 1964, ERSprovided a great deal of information on pesticide andfertilizer use from the mid-1960s up until the early1970s, in order to provide a basis for determining


costs and benefits of pesticides and to determinetrends in pesticide use. However, the U.S. Govern-ment has drastically reduced its surveys of pesticide-use patterns in the last nine years: publishedinformation for the early 1980s is sparse andpublished pesticide-use data for the mid-1980s isalmost nonexistent. Resources for the Future, anongovernmental organization, has developed anational pesticide use database by compiling State-and county-level use data, but these data are basedon average use estimates (26). Hence, we now haveless specific knowledge of how farmers and otherpesticide users are actually using materials than inthe 1960s and 1970s (87).

USDA’s Water Quality Program Plan developedin response to the President’s Water Quality Initia-tive identifies the need for comprehensive nationaldata on agricultural chemical use, related farmingpractices, and the links with the agroecosystem toassist Federal and State governments to ‘‘assess thebenefits, costs, and other effects of current agricul-tural practices and to evaluate consequences ofalternative policies and practices for reducing anyadverse effects of agricultural production on waterquality” (78).

The Economic Research Service (ERS) and theNational Agricultural Statistics Service (NASS) arecharged with the design of a continuous cycle ofnational surveys. The NASS plans to collect data onfarm use of pesticides and certain other chemicals,and type of production practices. Farm survey effortswill cover field crops in major producing States aswell as a range of vegetables in five large producingStates (9). Statistical analyses are to be conducted byNASS and results summarized and disseminated byERS. The first pilot test of this survey process isplanned for a single crop in 1990 and will beexpanded over a 3- to 4-year period to cover theother major commodities (78).

Rationalization of Data Collectionand Management

Although many pertinent databases exist, in mostcases they were created autonomously to addressdifferent fundamental questions. This hinders theiruse in predicting potential groundwater or aquifervulnerability to agrichemical contamination. Myriadnatural resource, land-use, and agrichemical-usefactors combine to determine vulnerability to ground-water contamination, however, preliminary identifi-cation of regions exhibiting high association with

agrichemical contamination of groundwater can bemade.

Congress could direct USDA to correlate agrichem-ical-use data contained in the planned NASSAgrichemical-Use Survey and the National Agri-cultural Census with EPA and USGS data onidentified groundwater contamination problemsto identify areas or regions with high apparentvulnerability to groundwater contamination. Re-gions showing a high correlation between incidenceof agrichemical contamination and intensity or typeof agrichemical use could be designated target areasfor intensified monitoring, and hydrogeologic re-search efforts. As data and data integration proce-dures improve, definition of highly vulnerableregion can be refined.

Baseline information on current nutrient and pestmanagement practices and continued information onchanging agricultural practices will help policymakersassess the impacts of policy changes on groundwaterquality, agricultural productivity, and the farmeconomy. Understanding of how and where thechemicals with greatest contamin ation potential arebeing used could assist in identifying pest control ornutrient problems that are in the greatest need ofresearch and extension of alternate products orpractices. Without such a clear link, research andextension may remain focused on issues unrelated togroundwater protection and associated environ-mental issues,

Although established, many extant natural re-source databases are not readily accessible for usersoutside each agency, and may be of unusable formatfor integrated or geographically specific analyses.Moreover, no clearly defined Federal commitmenthas been made for provision of multi-use, national-scale maps and related geographic information forpublic and private users (50). Provision of informa-tion derived from these data probably would be ofmore use to agriculture and water quality decision-makers than the raw data.

Most legislation has dealt with parts of the totalhydrogeologic system; only in the last several yearshave studies of how agrichemicals move through thelarger environment been initiated. EPA is organizedto address different components rather than the totalecosystem; its offices address air or water orgroundwater rather than attempting to follow move-ment of particular contaminants through the entireenvironment. USDA and TVA have historically


focused on the effect of agrichemicals on cropgrowth. Thus, they have studied the movement ofthese chemicals from site of application through theplant root zone, which usually is considered to be 6feet deep (20). USGS traditionally has focused onmovement of contaminants within the saturatedzone, from the groundwater table down (60). Littleresearch by these agencies has focused on themovement of contaminants between the root zoneand the saturated zone. A Memorandum of Under-standing between USGS and USDA has definedrelative responsibilities of these agencies regardingsuch research, but few cooperative efforts have beeninitiated (54). Were this separation of research anddata collection focus to continue, it would impededevelopment of agricultural practices to reduceagrichemical contamination of groundwater in vul-nerable areas, and would likely result in duplicationof effort.

For example, a group of hydrologists might createa database that includes information on the move-ment of herbicides through the soil profile. Theymight measure parameters relevant to the chemistryand physics of chemical transport through the soil,but as hydrologists they may need to consult withsoil scientists, and cropping system specialists toinclude measurements describing influences of till-age practice or crop types, information that would becritical to an agronomist trying to develop newcultural practices to minimize the movement of anherbicide out of the root zone (54). Preliminaryconsultation with potential database users couldsave substantial money and effort by adding meas-urements of a few extra attributes to the database.

Further, only some of the databases have beenautomated (entered into a computerized data man-agement system), or “digitized” (entered into aspatial or geographically registered database ingeneric format) to allow ready transmission to users,easy manipulation of data for different decisionmak-ing efforts, or integration of different data sets toallow for more comprehensive analysis. In addition,the systems of information search and retrieval(manual or computerized) commonly are unique toeach database system. Consequently, many datahave been collected relevant to groundwater protec-

tion, but much is inaccessible or of unusable formatfor scientists from other agencies. Efforts are under-way to define data-entry protocols and standardformats such that future databases might be moreintegrable (cf: 82,48).

Congress could undertake a number of mutu-ally beneficial options to rationalize naturalresource data collection and management efforts.Such efforts might include:

●

●

●

●

●

accelerating extant hydrogeologic and agricul-tural land-use data collection efforts (e.g., SCSsoils surveys, USGS RASA analyses);initiating additional data collection efforts toensure comprehensive provision of information(e.g., used and abandoned well locations, State-level groundwater monitoring);accelerating digitization of data already col-lected by Federal agencies;mandating digital storage of all new, relevantland-use and natural resource attribute datacollected by the Federal agencies; andrequiring regular data updating, maintenance ofdatabases, and cost-effective provision of datato users.

Furthermore, in order to ensure that the necessaryinformation is collected for accurate Federal, State,and local decisionmaking to reduce agrichemicalcontamination of groundwater, Congress could en-courage establishment of an interagency TechnicalInformation Integration Group3 that will determinewhat data is necessary, what data is available, whomight collect data not presently available, and howdata might be integrated to support non-technicaldecisionmakers and how data might be sharedamong public user groups.

Although the efforts listed above could be under-taken simultaneously and immediately, the costs ofdata collection and digitization can be enormous.Many data collected thus far are available only‘‘manually, ‘‘ on maps or in tables, and thus must betransferred into computerized databases. Digitizingdata is an expensive process. For example, the SCSestimates the cost of updating and digitizing soilsdata for the Nation at $200 million (72). Therefore,Congress might initially require the General Ac-

3A T&~c~ ~temtion Group ~G) is ~ ~teragency orga~tio~ s~c~e d~igned to p~rnotc Coordinaam and standardization at a tt%bkdlevel. At preseq the only extant TIG is a the-tiered structure sponsored by the USGS including technical and administrative representatives of severalFederal agencies, States, and academic research organizations. The tiers include four Strategy Teams comprised of researchers in certain topical areas,the Technical Integration Group of technical program managers, and a Headquarters Team of research administrators with authority to allocate researchresources (59) ~one, personaI communication Mar. 1990].

Chapter contamination of the Hydrogeological System: A Primer . 71

counting Office or a Federal interagency group (e.g.,the Technical Information Integration Group) toevaluate the status and needs of data collection andmanagement efforts specifically related to agrichem-ical contamination of groundwater, and to recom-mend specific steps to achieve a comprehensive,integrated system in the most cost-effective manner.

Coordination of Data Collection and Storage—Most Federal agencies have means to internallycoordinate the information collected by that organi-zation. Other systems have been developed tocoordinate data acquisition and sharing of certaintypes of information. For example, the U.S. Geolog-ical Survey coordinates water resources data acqui-sition and data sharing activities among Federalorganizations through its Office of Water DataCoordination (42). Coordination is accomplishedamong Federal agencies through a Federal Inter-agency Advisory Committee on Water Data andbetween Federal agencies and the States and privatesector through a non-Federal Advisory Committeeon Water Data for Public Use. While of immense useto those seeking specific information, such systemsdo little to improve integration of different types ofdata (e.g., integrating water data with soils andvegetation data) without specifications describingdata detail, content, and accuracy.

Congress could require the creation of acoordinated database network, to ensure that therelevant agencies develop rational interfaces be-tween extant databases and follow standardizeddata entry, format, and search protocols. Alter-natively, Congress could aggregate all of therelevant databases into a single national databaseclearinghouse .4 The Federal Interagency Coor-dinating Committee on Digital Cartography(FICCDC), was established in 1983 by the Office ofManagement and Budget (OMB) to facilitate coordi-nation of 30 participating Federal agencies’ digitalinformation system activities and geographic infor-mation system activities and to establish standardsfor production of digital cartographic data (24).However, FICCDC has no authority to require thatFederal agencies follow data protocols.

Further, the FICCDC has focused on thematicdata collection, for example by recommending that

the Soil Conservation Service be the lead agency oncollection and management of digital soils informa-tion. It was not structured to assist in development ofintegrated databases, nor to coordinate data collec-tion and management among Federal agencies andState and local information management systems orusers (43). Any data management system also willneed to be designed to accept data ‘uploaded’ fromregions and States that likely will be collecting moredetailed data related to crops, cropping systems, andhydrogeologic vulnerability than national efforts.Such a system would have capacity to aggregateinformation “upwards” to evaluate national trendsand needs, providing a more accurate nationalpicture than a random sample of a few points, as wellas allowing resolution “down” to the local deci-sionmaking scale.

If all national-level natural resource and relevantland-use databases were transferable to a centralizedorganization, standardization of protocols and coor-dination among Federal, State, local, and privateusers might be simplified. Each Federal agencycould maintain its own system, following formatsfor specific sets of environmental information set bythe clearinghouse, but would periodically move theirdata to the clearinghouse.

However, some agencies may resist changingtheir own systems to accommodate outside users.Further, agencies may be reluctant to house all oftheir information within a separate organization,especially if it is part of an established agency.

If Congress wishes to focus solely on groundwaterand agricultural production, the central databaseclearinghouse might be located within the SoilConservation Service or at the National AgriculturalLibrary. But if Congress prefers to address databaseintegration for a broader array of agricultural/environmental issues, it may be preferable to createa separate office for environmental data acquisition,integration, and management (54). Such an officecould be established with a ‘‘neutral’ data collec-tion agency (e.g., USGS), within a central govern-mental unit such as the Council on EnvironmentalQuality, or as a new part of the Department ofEnvironmental Protection. Wherever located, theagency components of the system could remain

4A v~at;on on &his concept would be &he creation of a ‘‘universal COMpUter SWCh prOgr~. fi~er ~ le~g the computer language or searchprotocol of each database, or wait until the information is transferred to a national clearinghouse, individual inquirers could access an interactive searchprogram that would a..k them a series of questions. On the basis of the answers, the program would ‘‘dial-out’ to the appropriate databases and retrievethe relevant information (54).

72 Ž Beneath the Bottom Line: Agricultural Approaches To Reduce Agrichemical Contamination of Groundwater

housed within the agencies, but the central officewould provide the integrating structure and man-date.

Coordinating Agroecosystem SimulationModeling

Data are collected and managed to help makedecisions. A working model of the world—whethera formal computer paradigm or an informal set ofassumptions—is used when decisions are made.

In the case of groundwater management, as wellas other environmental issues, the number of varia-bles and parameters of concern are so numerous andthe interactions between these factors so complexthat there is an increasing reliance on computermodels 5 (cf: 52). Computer modelers, in turn, arediscovering that environmental modeling has be-come a large and complex undertaking. Conse-quently, discussions are underway regarding thedevelopment of “modular modeling. ’ Individualresearchers and teams develop the particular modelsfor which they have interest and expertise, but buildthe “input” and “output” components of theirmodels according to agreed upon standards so thatother scientists can incorporate models withouthaving to repeat work that has already been done byothers.

For example, one scientist could develop a modelof nitrogen movement through the soil, anothercould develop a model of how plants absorb nitrogenfrom the soil, another could develop a model onnitrogen volatilization, and yet another could de-velop a model of how nitrogen leaches through thesoil profile to groundwater. Left as individualprojects, these models would not be able to helpanswer questions on how to balance nitrogen fertil-izer applications so as to ensure healthy plant growthwhile protecting groundwater resources. However,if the models are developed according to agreed onstandards, an integration team could concentrate onthe interactions of the models and put them togetherinto a comprehensive nitrogen management model.

Congress could require that the U.S. Depart-ment of Agriculture, perhaps jointly with otheragencies, evaluate current simulation modelingefforts related to the environmental fate ofagrichemicals. Based upon this analysis, a Technical

Agroecosystem Modeling Integration Group (TAMIG)could be established to coordinate research anddevelopment of computer simulation models relatedto the environmental fate of agrichemicals in farm-ing systems. Such a TAMIG should include thetechnical program managers from relevant Federalagencies undertaking such modeling efforts (e.g.,USDA, EPA), State government and academicspecialists, and might include members of theenvironmental and agricultural research community.

One goal of such a group might be to ensuredevelopment of simulation models that can begeneralized, through agreed upon means, to allowprediction of environmental fate on sites withdifferent hydrogeology or agricultural systems. An-other goal might be to coordinate development ofdetailed simulation models of certain parts ofhydrogeologic systems (e.g., the Pesticide RootZone Model developed by EPA or the GroundwaterLoading Effects of Agricultural Management Sys-tems model developed by USDA/ARS) so that theymay be “hooked” to simulation models of otherparts of hydrogeologic systems to allow morecomprehensive analysis. USDA/ARS has used thisapproach in developing NTRM, a Soil-Crop Simula-tion Model for Nitrogen, Tillage, and Crop-ResidueManagement (65).

Developing Geographic Information Systems

Geographic Information Systems (GISs) are com-puter-based technologies including hardware, soft-ware, and graphics capabilities. More than auto-mated mapping systems, GIS can encode, analyze,and display the natural and built environment inmultiple ‘‘layers’ that are geographically registeredto unique locations on the Earth’s surface. Results ofGIS analyses can be described in reports, tables, andmost importantly, in maps at any scale.

Relationships between data can be used to depictcomplex variables such as hydrogeologic vulner-ability to agrichemical contamination as well asspatial displays of component simple variables suchas average depth to water table. Further, GISs arecapable of displaying “option” variables, such asthe percentage of lands eligible for the ConservationReserve land-retirement program that coincide withareas containing hydrogeologically vulnerable crop-land. By using GIS, the decisionmaker can alter

5For de~~ discussion of ~~d~ater m~e~, se Natio~ R~~h (~o~cil, Water Science and TdMIoIogy Bored, committcc on ti~d Wat=Modeling Assessment, Ground Warer ~odeZs: Scientific and Reguk.voryAp placations (Washington DC: National Academy Press, 1990).


variable components and test the impacts of decisionalternatives before enacting new provisions. Givenadequate and reliable data, and a sufficient under-standing of the pertinent variables and their interac-tions, GISs provide a rapid means to assess whereefforts might be allocated to have the greatestbeneficial impact, or whether proposed policy op-tions have potential to solve problems.

Databases and Systems

The first requirement for a GIS is spatiallycoordinated, geographically registered, digitizeddata: data transferred into a computer so that it iselectronically associated with known geographiccoordinates (unique locations on the Earth’s sur-face). Then, using those coordinated layers, othergeographic information can be added and attributesor characteristics of those geographically referencedlocations can be described by the computer ingraphic colors, textures, and shapes as well asnumbers. For example, a county might have attri-butes including 1990 population, amount of agrichem-icals used in a year, or wheat production in bushelsper acre. A well shown as a point on the map mayhave attributes including depth to bedrock, nitrateconcentration, or yield of water in gallons perminute. A stream shown as a curved line at aparticular location may have a known flow rate,sediment loading at certain times, or average num-bers of bass.

Some of the most important databases for assess-ing potential groundwater vulnerability to agrichem-ical contamination are digitized soil and geologicdata at National, State, and local levels. SCS is in theprocess of digitizing soil surveys; however, digitiz-ing all soils data, collected at the county level, for theNation will cost nearly $200 million. This estimateincludes $100 million for updating, recompiling,and establishing the geographic referencing systemfor soil survey data, and $100 million for digitizing(72).

Dearth of Federal funding has led a number ofStates to proceed with digitization on their own;however, some are not using the protocols proposedby SCS or USGS so that State-level ‘pieces’ are notlikely to be easily assembled into a national system(54). On the other hand, EPA has moved to providedigital surface-water networks—another importantdata layer—based on USGS hydrography data at arelatively detailed, but still national scope. How andwhether this database, known as the ‘‘Reach File, ’

together with associated Water Quality Assessmentdata and systems will be freely available for GISusers outside the agency is not yet clear (43).

Approaches to using GIS to describe vulnerabilityof surface- and groundwater should:

●

●

●

Ž

●

GIS

Integrate gee-referenced overlays of naturalresource information such as geology, subsur-face hydrology, and terrain from USGS; soilsfrom SCS; and surface hydrography fromUSGS and EPA.Incorporate agricultural land use variables foragricultural vulnerability assessments, includ-ing cropland and individual crops and croppingsystems, vegetative cover, climate, pesticideand chemical use, and irrigation practices.Incorporate derived variables such as: 1) mean-ingful hydrogeological units, 2) watershedunits based on elevation and terrain data, and 3)surface stream and river networks that routewater-borne contaminants through watersheds(this information should include associatedwater quality information including well andwater samples, and the location of water intakesites for community water supplies).Develop or use existing GIS capabilities tomanage and display the information, includingmaps of hydrogeologic parameters of particularconcern to groundwater management.Identify needed information and databases thatdo not yet exist (54,43).

Users

GIS for surface- and groundwater assessmentshave been developed and used for some time bycertain Federal agencies, such as USGS, privateorganizations concerned with natural resources suchas the newly established National Center for Re-source Innovations, many State agencies, and someAgricultural Experiment Stations and Land GrantUniversities such as Minnesota (7).

USDA, EPA, and others are showing increasinginterest in these systems and have included proposeduses of such systems in their planning documents(cf: 78). A survey of Federal organizations using orintending to use GISs found at least 37 used GIS in1988, 20 plan to have an operational GIS by 1990,and 10 others have developed policies related to GIS(24). For example, the NASS is planning to developa GIS to support USDA’s water quality programplan. The proposed system will link nationwide data


and statistical information on agricultural productiv-ity, cropping practices, land use, agrichemical use,physical attributes of the land and surroundingwatersheds, climate and water quality (9).

Perhaps the greatest need for GIS development,however, lies at the local level where detailedinformation is most extensive. A 1985 surveyconducted by the American Farmland Trust foundthat approximately 22,000 non-metropolitan ruralgovernments have authority to allocate 1.5 billionacres of non-Federal rural lands and resources in3,041 counties, 16,000 townships, 6,000 naturalresources special district governments. Only 25 ofthese governments had operating GISs (4). Thatnumber is rapidly increasing; today it is estimatedthat approximately 1,000 urban and rural localgovernments use GISs.

Current Programs for GIS Support and Delivery

To assist with GIS research and development, theNational Science Foundation recently establishedthe National Center for Geographic Information andAnalysis to: 1) serve as a clearinghouse on GISresearch, teaching, and application; 2) promote useof GIS analysis and train users; and 3) study thelegal, social, and institutional aspects of GIS (12).To assist with GIS technology delivery to primarilynon-technical local, regional, and national decision-makers, Congress has funded the National Center forResource Innovations (NCRI). NCRI is a consor-tium of regionally distributed GIS technology trans-fer centers whose objectives include: 1) encouragingthe use of established specifications and standardsfor data development, quality, and applications; 2)coordinating technical assistance from public re-source specialists in interpretation and use of infor-mation in GIS systems; 3) developing trainingprograms; 4) delivering GIS technology research; 5)supporting and identifying needed GIS developmentin the applications and decisionmaking environ-ments; and 6) developing GIS into education toolsfor public decisionmakers and the public.

Approaches to GIS Assessment of GroundwaterVulnerability to Agricultural Land Uses

Two key impediments exist to GIS developmentfor non-technical decisionmakers concerned withwater quality protection at all levels of government.

These are: 1) lack of needed data; and 2) difficultyintegrating information from many sources at scalessuitable for local, regional, and national assess-ments.

Congress could mandate development of inter-agency GISs for management of groundwaterprotection. 6 A first focus could be on completing anddigitizing soil survey maps developed by the SoilConservation Service. This should be extended to alldata sets identified as important for water qualityassessment and protection. To assist with this, datasets developed outside Federal agencies might beencouraged to meet specifications and standardsestablished by agencies with lead responsibility forcollecting and interpreting the data. Such an effortcould be coordinated through current OMB/FICCDC efforts to ensure orderly GIS developmentwithin Federal agencies, or could be assigned to aconcomitantly expanded Council on EnvironmentalQuality. Development of such a system also could behandled through a centralized Office of Environ-mental Data Acquisition, Integration, and Manage-ment mentioned earlier.

A comprehensive and carefully developed ap-proach to provide an “open architecture” GIS—allowing users to combine databases with new dataand add models as well as interpret interrelationships-could eventually lead to integration of national-leveldatabases into geographic information of specifiedaccuracy and scientifically supportable applications(43). By being “open,” such a “core” GIS couldallow incorporation of decision support, and expertsystems to provide a powerful and accessibleinformation management system. Such a systemcould also be developed to allow regional and locallevels of detail to be added together with regionallyappropriate factors including local climate, croppingsystems, chemical use, location of livestock confine-ment facilities, watershed characteristics, and otherregionally specific factors. A core GIS might pro-vide a model for local systems and could assist inintegrating national-level databases, Geographic In-formation Systems, and expert systems into power-ful information management and decision-aid sys-tems. These, in turn, could foster development andintegration of voluntary, incentives-based, and regu-latory systems to protect groundwater (54).

sDi@t~ed &tabas~ also can support development of computerized f~n decisio~“ g aids, such as ‘‘expert systems’ (see ch. 5).

Chapter 3---Contamination of the Hydrogeological System: A Primer ● 75

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