111

94: Point and NonPoint Source Pollution

KEITH LOAGUE1 AND DENNIS L CORWIN2

1Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, US2USDA-ARS, George E. Brown, Jr. Salinity Laboratory, Riverside, CA, US

The information age has ushered in a global awareness of complex environmental problems that do notrespect political or physical boundaries: climatic change, ozone layer depletion, deforestation, desertification,and pollution from point and nonpoint sources. Among these global environmental problems, point andnonpoint source pollution represent a perfect example of a complex multidisciplinary problem that exists overmultiple scales with tremendous spatial and temporal complexity. A point source of pollution discharges to theenvironment from an identifiable location, whereas a nonpoint source of pollution enters the environment from awidespread area. The ability to accurately assess present and future point and nonpoint source pollution impactson ecosystems ranging from local to global scales provides a powerful tool for environmental stewardship andguiding future human activities.

INTRODUCTION

The objective of this chapter is to introduce the subjectof point and nonpoint source pollution, the entirety ofwhich could easily fill several volumes. The point and non-point source problems are defined and important relatedlegislation is identified in the following two subsections.Monitoring and modeling (with consideration for uncer-tainty) are discussed, relative to characterizing the impactsfrom point and nonpoint source pollution, in the sectionon assessment. Excerpts from three case studies are pre-sented in the example section; two of the case studies arefor nonpoint source pollution, the third case study is forpoint source pollution. It should be pointed out that thechapter is focused, with the exception of one of the casestudies, on the United States.

Point versus Nonpoint Source Pollution

Point source pollutants, in contrast to nonpoint sourcepollutants, are associated, as the name suggests, with apoint location such as toxic-waste spill site (see Figure 1).As such, point source pollutants are, compared to non-point source pollutants, characteristically (i) easier to con-trol, (ii) more readily identifiable and measurable, and(iii) generally more toxic. Nonpoint sources of pollution

(see Figure 1) are the consequence of agricultural activities(e.g. irrigation and drainage, applications of pesticides andfertilizers, runoff and erosion); urban and industrial runoff;erosion associated with construction; mining and forest har-vesting activities; pesticide and fertilizer applications forparks, lawns, roadways, and golf courses; road salt runoff;atmospheric deposition; livestock waste; and hydrologicmodification (e.g. dams, diversions, channelization, overpumping of groundwater, siltation). Point sources includehazardous spills, underground storage tanks, storage pilesof chemicals, mine-waste ponds, deep-well waste disposal,industrial or municipal waste outfalls, runoff, and leachatefrom municipal and hazardous waste dumpsites, and sep-tic tanks. Compared to point source pollution, nonpointsource pollution is more difficult, related to monitoring andenforcement of mitigating controls, due to the heterogeneityof soil and water systems at large scales. Characteristically,nonpoint source pollutants (i) are difficult or impossible totrace to a source, (ii) enter the environment over an exten-sive area and sporadic timeframe, (iii) are related (at leastin part) to certain uncontrollable meteorological events andexisting geographic/geomorphologic conditions, (iv) havethe potential for maintaining a relatively long active pres-ence on the global ecosystem, and (v) may result in long-term, chronic (and endocrine) effects on human health andsoil-aquatic degradation.

Encyclopedia of Hydrological Sciences. Edited by M G Anderson. 2005 John Wiley & Sons, Ltd.

1428 WATER QUALITY AND BIOGEOCHEMISTRY

Sep

tic s

yste

m(P

S)

Cro

p du

stin

g (N

PS

)

Live

stoc

kw

aste

(NP

S)

Soi

ler

osio

n(N

PS

)

Exc

essi

vefe

rtili

zer

appl

icat

ion

(NP

S) Ir

rigat

ion

(

NP

S)D

efor

esta

tion

(

NP

S)

Silt

atio

n

(N

PS

)Str

om-w

ater

runo

ff

(P

S/N

PS

)

Che

mic

al (

NP

S)

appl

icat

ion

to p

arks

and

law

nsS

ewag

e-tr

eatm

ent p

lant

(PS

)

Mar

ine

was

te (

PS

/NP

S)

Con

stru

ctio

ner

osio

n(N

PS

)

Min

e-w

aste

pon

d (P

S)

Indu

stria

lem

issi

on (

NP

S)

(NP

S)

Str

ip-m

inin

g

Und

ergr

ound

stor

age

tank

(PS

)

Und

ergr

ound

min

ing

(PS

/NP

S)

Haz

ardo

us-

was

te d

ispo

sal

(PS

/NP

S)

Mun

icip

al-

sew

age

(PS

)di

scha

rge

Land

fill (

PS

)F

resh

wat

er a

quife

r

(PS

)D

eep-

wel

lw

aste

dis

posa

l

Wat

er-b

earin

gsa

ndst

oneLim

esto

ne

Wat

er ta

ble

Sat

urat

ed z

one

Wat

er w

ell

(NP

S)

Roa

d-sa

ltru

noff

Fig

ure

1E

xam

ple

so

fpo

inta

nd

no

np

oin

tso

urc

ep

ollu

tio

n(R

epro

du

ced

fro

mC

orw

inet

al.(

1999

)by

per

mis

sio

no

fAm

eric

anG

eop

hys

ical

Un

ion

.Ad

apte

dfr

om

am

app

ub

lish

edb

yN

atio

nal

Geo

gra

ph

icS

oci

ety,

1993

).A

colo

rve

rsio

no

fth

isim

age

isav

aila

ble

ath

ttp

://w

ww

.mrw

.inte

rsci

ence

.wile

y.co

m/e

hs

POINT AND NONPOINT SOURCE POLLUTION 1429

Nonpoint source pollution was generally not recog-nized until the mid-1960s. Initially, nonpoint source pol-lution was associated entirely with pollution from stormwater and runoff. Subsequently, nonpoint source pollu-tion has expanded to encompass all forms of diffusepollutants. Nonpoint pollutants are defined as “contami-nants of [air, and] surface and subsurface soil and waterresources that are diffuse in nature and cannot be tracedto a point location” (Corwin and Wagenet, 1996). It isimportant to note the statutory definitions of point andnonpoint source pollution (see Novotny and Olem, 1994).In essence, point sources of pollution “were originallydefined as pollutants that enter the transport routes atdiscrete identifiable locations and that can usually bemeasured”, while nonpoint source pollution was “every-thing else”. Table 1 provides a list of statutory point andnonpoint sources of pollution compiled by Novotny andOlem (1994).

There can be a fine line between point and nonpointsource pollution. The distinction depends entirely upon thescale of interest. One person’s point source pollution caneasily be another person’s nonpoint source pollution. Forexample, a single quarter-section (65 ha), in a watershed ofhundreds of thousands of hectares, might be considered apoint source of nitrogen fertilizer, and, yet, within the areaof interest, the fertilizer could be viewed as a nonpointsource because of the broadcasting of millions of granulesof the active ingredient. The scale of reference ultimatelydetermines whether a pollutant is viewed as coming froma point or nonpoint source.

Historically, point source pollutants have received thegreatest attention, both publicly and scientifically, becauseof the conspicuous severity of their impacts at a localizedpoint (e.g. Love Canal (Mercer et al., 1983) and Woburn(Harr, 1995)). However, over recent years, public, polit-ical, and scientific attention has shifted more and moretoward pollutants that are widespread. This shift reflectsan awareness of the scope and potential impact of the non-point source pollution problem (see Corwin and Loague,1996; Corwin et al., 1999).

A Bit of the Legal History, for the United States

Up until the 1800s, the rural environment remained largelypristine in contrast to the filth of the urban areas asexemplified by historic centers such as ancient Rome,and, in the Middle Ages, London and Paris. By themid-1800s, the sewage and runoff of the urban areasthat polluted the surface waters became associated withwaterborne disease. The concern for public health led to thefirst awareness of the repercussions of the environmentaldegradation and the close association of humankind’swell being to environmental quality, thereby heraldingthe first environmental activist period. By the twentieth

Table 1 Statutory point and nonpoint sources of pollution(after Novotny and Olem, 1994)

Statutory Point Sources• Municipal and industrial wastewater effluents• Runoff and leachate from solid waste disposal sites• Runoff and infiltrated water from concentrated animal

feeding operations• Runoff from industrial sites not connected to storm

sewers• Storm sewer outfalls in urban centers with

populations of more than 100 000• Combined sewer overflows• Leachate from solid waste disposal sites• Runoff and drainage water from active mines, both

surface and underground, and from oil fields• Other sources, such as discharges from vessels,

damaged storage tanks, and storage piles of chemicals• Runoff from construction sites that are larger than 2 haStatutory Nonpoint Sources• Return flow from irrigated agriculture• Other agricultural and silvicultural runoff and

infiltration from sources other than confinedconcentrated animal operations

• Unconfined pastures of animals and runoff from rangeland

• Urban runoff from sewered communities with apopulation of less than 100 000 not causing asignificant water quality problem

• Urban runoff from unsewered areas• Runoff from small and/or scattered (less than 2 ha)

construction sites• Septic tank surfacing in areas of failing septic tank

systems and leaching of septic tanks effluents• Wet and dry atmospheric deposition over a water

surface (including acid rainfall)• Flow from abandoned mines (surface and

underground), including inactive roads, tailing, andspoil piles

• Activities on land than generate wastes andcontaminants, such as:

• Deforestation and logging• Wetland drainage and conversion• Channeling of streams, building of levees, dams

causeways, and flow-diversion facilities on navigablewaters

• Construction and development of land• Interurban transportation• Military training, maneuvers, and exercises• Mass outdoor recreation

century, sewage systems and sewage treatment plants cameinto being.

Public interest in the environment through the first halfof the twentieth century was almost negligible primarilybecause the epidemics associated with the Middle Ageshad been controlled or eliminated, and because the useof chemical insecticides and fertilizers did not occur toan appreciable extent until the late 1950s. Once human-made chemicals, such as DDT (1,1,1-trichloro-2,2-bis-(4-chloropheny)-ethane)), were introduced in the mid-1950s


and the dumping of outflows from post-World War IIfactories continued unabated, pollution of soil and waterresources rapidly increased. The book Silent Spring byRachel Carson in 1962, which revealed the spread andpotential danger of toxic human-made chemicals to theunwitting public, is heralded as initiating the second envi-ronmental activist period.

Freeze (2000) astutely points out that environmentalpolicy and perspective over the last half century havebeen driven by the prevailing socio-politico-economic cli-mate. The environmental pendulum, as Freeze refers to it,has swung from underkill to overkill and from concernthat breeds action to disillusionment that breeds reaction.Table 2 provides a timetable of some of the significantenvironmental and statutory events that have shaped thelast six decades and an overview of the pendulum swingfrom the environmental perspective to a retrospective fromthe New Right.

The positive result of the environmental activism of the1960s and the 1970s was the formation of the Environmen-tal Protection Agency (EPA) in 1970 and the Clean WaterAct (CWA) of 1972 (Public Law 92–500). It should bepointed out, however, that the CWA called for “zero dis-charge” into surface waters, which, in actuality, shifted theonus of pollution from surface water to soil and groundwa-ter. Prior to the CWA, the Refuse Act of 1899 and the WaterPollution Control Act of 1948 (Public Law 80–845) wereused to control water pollution from industrial sources, butthese archaic statutes were generally ineffective in control-ling pollution.

The CWA was intended to be a comprehensive waterquality program. For example, under the CWA, each Statewas to establish water quality standards based upon a totalmaximum daily load (TMDL), which is the amount of apollutant that a body of water can handle from all sources.Once a TMDL is established, it is used as a basis forputting limits on the amount of that pollutant that can bedischarged into the system. In reality, most of the empha-sis of the CWA was placed on controlling point sourcerather than nonpoint source pollution (Novotny and Olem,1994). The CWA established three crucial tasks (Novotnyand Olem, 1994): (i) the regulation of point source dis-charge, (ii) the regulation of oil spills and other hazardoussubstances, and (iii) financial assistance for wastewatertreatment plant construction. The most significant con-tribution of the CWA was the formulation of a meansof enforcement through the National Pollution DischargeElimination System (NPDES). This system has served asthe basic mechanism for enforcing the implementation ofpollution abatement of point sources and some nonpointsources legally classified as point sources (Novotny andOlem, 1994).

The Water Quality Act (Clean Water Act) of 1987 (Pub-lic Law 100–24) provided three sections (Section 319,402, and 404) that are significant to the control of non-point source pollution. Of these, Section 319 most directlyapplies to nonpoint pollution, requiring each State to pre-pare a State Nonpoint Source Assessment Report, and pro-vides the incentive of matching Federal funds to encourageStates to develop and implement management programs.

Table 2 Timetable of environmental and statutory events (after Freeze, 2000)

DecadeEnvironmental

perspectiveEnvironmental

eventsUS Federalresponse

Prevalentenvironmental

conceptsInfluential

booksNew right

retrospective

1940s ChemicalRevolution

A Sand CountyAlmanacLeopold (1949)

ChemicalRevolution

1950s Age ofCarelessness

Throwawaysociety

Age of EconomicProsperity

1960s Age ofAwakening

EPA(established,1970)

Conservation,wildernessprotection

Silent SpringCarson (1962)

Age of SocialUpheaval

1970s Age ofAwarenessand Action

Earth Day LoveCanal

CWA, SDWA,TSCA, RCRA,PP list, NOR

Limits to growth The ClosingCircleCommoner(1971)

Age ofOverreaction

1980s Age ofDisillusion

Three mi Island CERCLA, HSWA,SARA

Soft energy paths Age ofVindication

1990s Age of Reaction Earth Summit PPA in Rio Sustainable yield Earth in theBalance Gore(1992)

Age of Reason

CERCLA = Comprehensive Environmental Response, Compensation, and Liability Act; CWA = Clean Water Act; EPA = EnvironmentalProtection Agency; HSWA = Hazardous and Solid Waste Amendments (to RCRA); NOR = National Organics Reconnaissance;PPA = Pollution Prevention Act; PP list = Priority Pollutant List; RCRA = Resource Conservation and Recovery Act; SARA = SuperfundAmendments Reauthorization Act; SDWA = Safe Drinking Water Act; TSCA = Toxic Substances Control Act.


Section 319 establishes a regulatory link between non-point source pollution and groundwater quality; specifi-cally requiring States to develop best management practices(BMPs) that consider the impact of management on bothsurface water and groundwater quality. Examples of BMPsinclude grassy waterways, buffer strips, reduced applicationrates for pesticides and fertilizers, and the proper storageand disposal of animal wastes.

Besides the CWA, there are a plethora of Federal statu-tory laws concerning the environment, floodplain man-agement, the US Department of Agriculture, and miningthat affect point and nonpoint source pollution and waterquality (e.g. National Environmental Policy Act; FederalEnvironmental Pesticide Control Act; Rare and EndangeredSpecies Act; Safe Drinking Water Act; Federal Insecticide,Fungicide, and Rodenticide Act; Toxic Substance Con-trol Act; The Wild and Scenic River Act; Coastal ZoneManagement Act; Flood Control Act (and amendments);National Flood Insurance Programs; Flood Disaster Pro-tection Act; Rural Development Act; The Food SecurityAct; Surface Mining Control and Reclamation Act; Fed-eral Land Policy and Management Act). Novotny andOlem (1994) provide an excellent discussion of the laws,regulations, and policies related to point and nonpointsources of pollution. Table 3 shows the relative impor-tance of pollutants with respect to their source. Domenico

and Schwartz (1990) provide a list of the EPA prior-ity pollutants.

ASSESSMENT OF POINT AND NONPOINTSOURCE POLLUTION

Assessment involves the determination of change ofsome constituent over time and space. This changecan be measured in either real time or simulated witha model. Real-time measurements reflect the activitiesof the past, whereas simulations can provide usefulglimpses into the future. Both means of assessmentare valuable. Related to real-time measurement andmonitoring, the distribution and trends of pesticides inthe atmosphere (Majewski and Capel, 1995), groundwater(Barbash and Resek, 1996), surface water (Larson et al.,1997), and fluvial sediments and aquatic biota (Nowellet al., 1999) have been carefully assessed. For morethan 10 years, the US Geological Survey’s (USGS)National Water Quality Assessment (NAWQA) program(see http://water.usgs.gov/nawqa) has played a keyrole in the assessment of nonpoint source contaminationfrom various pollutants to both surface water andgroundwater, with studies in more than 50 major riverbasins and aquifers (Gilliom, 2001). An example ofthe importance of real-time measurement is the field

Table 3 Relative importance of pollutant concentration in soil-water systems (after Peirce et al., 1998)

Nonpoint source

Suspendedsolids andsediment BOD Nutrients

Toxicmetals

Traceelements Pesticides Pathogens

Salinity/TDS

Urban storm runoff M L-M L H M L H MConstruction H N L N-L N-L N N NHighway de-icing N N N N N N N HInstream hydrologic

modificationH N N N-H N-H N N N-H

Noncoal mining H N N M-H M-H N N M-HAgriculture• Nonirrigated crop

productionH M H N-L N-L H N-L N

• Irrigated cropproduction

L L-M H N-L H M-H N H

• Pasture and range L-M L-M H N H-L N N-L N-L• Animal production M H M N-L N-L N-L L-H N-LForestry• Growing N N L N N L N-L N• Harvesting M-H L-M L-M N N L N NResiduals

managementN-L L-H L-M L-H N-H N L-H N-H

On-site sewagedisposal

L M H L-M L-M L H N

Instream sludgeaccumulation

H H M-H L-H L-H M-H L N

Direct precipitation N N N-M L L L N-L NAir pollution fallout M L L-M L-H N-M L-M N-L NNatural background L-M L-H M N-M N-M N N-L N-H

N = Negligible; L = Low; M = Moderate; H = High; TDS = Total Dissolved Solids.


characterization of the April 1977 accidental spill ofapproximately 1900 L of the pesticide EDB (ethylenedibromide) within approximately 20 m of a well thatprovided drinking water to the village of Kunia on theHawaiian island of Oahu. The point source area aroundthis spill would eventually become a Superfund site.

Modeling

A distinct advantage of simulation is that it can be used toalter the occurrence or future impact of detrimental con-ditions. Obviously, simulation cannot replace real data.It should also be pointed out that field observations andsimulation are not mutually exclusive entities. Simulationrequires considerable information to establish (i) the prob-lem of interest (e.g. climatic data, land-cover characteristics,spatial distributions of near-surface soil-hydraulic proper-ties) and (ii) test (in some cases) the model’s performance(e.g. spatial and temporal distribution of chemical concen-trations). The use of mathematical models for assessingpoint and nonpoint sources of pollution, especially afterground truth comparisons, fills in information gaps, identi-fies critical areas and chemicals for future monitoring, andprovides the what if capability needed for both regulationand remediation.

By definition, a mathematical model integrates existingknowledge into a framework of rules, equations, and rela-tionships for the purpose of quantifying how a systembehaves. As long as a model is applied over the rangeof conditions from which it was developed, it serves asa useful tool for prognostication. When sufficient informa-tion exists to characterize point and nonpoint pollution at agiven time, a model can be calibrated (e.g. adjusting modelparameter values within a reasonable range so that simu-lated concentrations closely match observed concentrations)and subsequently employed to make predictions (Loagueand Green, 1991). When sufficient information does notexist, a model can still be used in a what if mode to addressquestions related to potential impacts (Abrams and Loague,2000). Models can range in complexity from the simplestempirical equation to complex sets of partial-differentialequations that are only solvable with numerical approxima-tion techniques.

Addiscott and Wagenet (1985) present a categorizationfor models on the basis of a conceptual approach, distin-guishing between deterministic and stochastic, and mech-anistic and functional. The key distinction between deter-ministic and stochastic models is, according to Addiscottand Wagenet (1985), that deterministic models “presumethat a system or process operates such that the occurrenceof a given set of events leads to a uniquely definable out-come” while stochastic models “presuppose the outcometo be uncertain”. Stochastic models consider the statisticalcredibility of both input conditions and model predictions,whereas deterministic models ignore any uncertainties in

their formulation. The second level of model distinction isbetween mechanistic and functional models. As describedby Addiscott and Wagenet (1985), “mechanistic is taken[here] to imply that the model incorporates the most fun-damental mechanisms of the process, as presently under-stood”, whereas the term functional is used for “modelsthat incorporate simplified treatments of solute and waterflow and make no claim to fundamentality, but do, thereby,require less input data and computer expertise for use”.

Three categories of, for example, deterministic modelsare: (i) regression models, (ii) overlay and/or index models,and (iii) transient-state solute transport models. Regressionmodels generally use multiple linear regression techniquesto relate various causative factors. Overlay and/or indexmodels, which can be property or process based, computean index of pollutant mobility. Transient-state, process-based models are capable of simulating the movementof a pollutant in a dynamic flow system. Transient-state,process-based models describe some or all of the processesinvolved in, for example, solute transport (e.g. advection,dispersion, diffusion, and retardation). Corwin et al. (1997)summarize several geographical information system (GIS)based nonpoint source pollution modeling efforts.

Uncertainty

There can be considerable uncertainty in modeling pointand nonpoint source pollution. So, why model? In general,there are two idealized uses for simulation. The firstis the prediction (or forecasting) of future events basedupon a calibrated and validated model. The second useis the development of concepts for the design of futureexperiments to improve the understanding of processes.There are three sources of error inherent to modeling:(i) model error, (ii) input error, and (iii) parameter error.Model error results in the inability of a model to simulatethe given process, even with the correct input and parameterestimates. Input error is the result of errors in the sourceterms (e.g. soil-water recharge, chemical application rates).Input error can arise from measurement, juxtaposition,and/or synchronization errors. Parameter error has twopossible connotations. For models requiring calibration,parameter error usually is the result of model parametersthat are highly interdependent and nonunique. For modelswith physically based parameters, parameter error resultsfrom an inability to represent aerial distributions on thebasis of a limited number of point measurements. Theaggregation of model error, input error, and parametererror is the total (or simulation) error. For multiple-process and comprehensive model simulation, error iscomplicated further by the propagation of error betweenmodel components.

In general, the methods for characterizing uncertaintycan be grouped into three categories (Loague and Corwin,1996): (i) first-order analysis, (ii) sensitivity analysis, and


(iii) Monte Carlo analysis. First-order analysis is a sim-ple technique for quantifying the propagation of uncertaintyfrom input parameter to the model output. The first-orderapproximation of functionally related variables is obtainedby truncating a Taylor-series expansion (about the mean) forthe function after the first two terms. Sensitivity analysis isused to measure the impact that changing one factor hason another. The sensitivity of a model’s output to a giveninput parameter is the partial derivative of the dependentvariable with respect to the parameter. Monte Carlo analy-sis is a stochastic technique of characterizing the uncertaintyin complex hydrologic response model simulations. TheMonte Carlo method considers each model input parameterto be a random variable with a probability density function(PDF). Monte Carlo simulations are based upon a largenumber of realizations, from every input parameter distri-bution, created through sampling the different PDFs with arandom number generator. A separate hydrologic responsesimulation is made for each parameter realization. The num-ber of possible simulations, based upon all the combinationsof parameter realizations, is infinite; therefore, a finite num-ber of cases (e.g. several hundred) are usually investigated.Estimates of the average simulated hydrologic response,and the associated uncertainty, are made from the com-bined outputs of the simulations (i.e. the total ensemble ofthe different realizations). Loague and Corwin (1996) pro-vide examples of first-order uncertainty analysis, sensitivityanalysis, and Monte Carlo simulation.

THREE EXAMPLES

There are thousands of documented cases of point and non-point source pollution, far too many to list here. Threeexamples (from the first author’s work) that combine fieldobservations and modeling are briefly discussed in thissection. The second example (II) is a local-scale legacyassessment of point source pollution from three Manufac-tured Gas Plants. The first (I) and third (III) examples areboth for nonpoint source pollution. Example I is a regional-scale legacy assessment associated with agrochemical use inCalifornia’s Central Valley. Example III is a regional-scaleassessment focused on the future management of forestedareas on the Canary Island of Tenerife. Assessments of thetype shown in Examples I, II, and III can be useful forregulatory decisions and remediation efforts. It is worthpointing out that both examples I and II were undertakenrelated to major civil actions.

Example I: Nonpoint Source Pollution, the Fresno

Case Study

Between the late 1950s and the time of its statewidecancellation in August of 1977, there was widespread use ofDBCP (1,2-dibromo-3-chloropropane) throughout the San

Joaquin Valley in California. More than two decades afterits cancellation, DBCP-contaminated groundwater persistedas a problem in the San Joaquin Valley. The objectiveof the Fresno Case study (Loague et al., 1998a,b) was toaddress, from a simulation perspective supported by fieldobservations, if label recommended NPS applications werelikely to be the principal source of the DBCP groundwatercontamination in Fresno County.

The San Joaquin Valley, at the southern end of Califor-nia’s Central Valley, extends in a southeasterly direction forapproximately 400 km from just south of Sacramento. East-central Fresno County, situated between the San JoaquinRiver to the north and the Kings River to the south, is thelargest agricultural county in the valley. The spatial distri-bution of DBCP use in the study area, between 1960 and1977 (see Figure 2a), was estimated using land-cover mapsfor different years.

The numerical model used for the 1D simulations(Loague et al., 1998a) of dissolved phase DBCP concentra-tion profiles in the unsaturated zone was PRZM-2 (Mullinset al., 1993). The potential fate and transport of DBCPbetween the surface and the water table for multiple NPSapplications, related to different and changing land usebetween 1960 and 1977, was quantitatively estimated for1172 elements for a 35-year period. The aggregate of theDBCP concentrations loaded to the water table for eachgrid element make up the annual loading files for the3D saturated transient transport simulations. The numericalmodels used for the 3D simulations (Loague et al., 1998b)of saturated subsurface fluid flow and DBCP transport areMODFLOW (McDonald and Harbaugh, 1988) and MT3D(Zheng, 1992), respectively. Recharge to the water tablefor the saturated simulations was estimated as the residual(precipitation plus irrigation minus evapotranspiration) inthe PRZM-2 water-balance simulations. The area focusedupon for the saturated simulations was represented by a 3Dfinite-difference grid made up of 76 440 elements (i.e. 21841 km2 surface elements with 35 layers).

The 13-step approach used by Loague et al. (1998a,b) tosimulate the impact of multiple DBCP applications underchanging land use and groundwater pumping/rechargein the Fresno study area are summarized in Table 4.Figure 2(b) shows a 1999 snapshot of the simulated DBCPloading water table from the Fresno Case study. The simula-tion results from the Fresno Case study lead to the followinggeneral comments (Loague et al., 1998a,b):

• The areas most likely to facilitate DBCP leachingthrough the entire unsaturated soil profile were targeted.

• The first appearance of DBCP above the detectable limitat the water table was simulated as most likely occurringbetween 1961 and 1965.

• The estimated DBCP concentrations reaching the sat-urated subsurface exceed the maximum contaminantlevel (MCL) at several locations at different times. The


37°00'

119°57'30" 119°52'30" 119°50'119°55' 119°35' 119°32'30" 119°30' 119°27'30" 119°25'119°42'30" 119°37'30"119°40'119°45'119°47'30"

119°57'30" 119°55' 119°50' 119°40' 119°37'30" 119°35' 119°32'30" 119° 30" 119°27'30" 119°25'119°47'30" 119°42'30"119°45'119°52'30"

36°57'30"

36°55'

36°52'30"

36°50'

36°47'30"

36°45'

36°42'30"

36°40'

(a)

(b)

37°00'

36°55'

36°57'30"

36°52'30"

36°50'

36°47'30"

36°45'

36°42'30"

36°40'

Total DBCP Applications 1960–1977 (kg ha−1

)

0–30 71–90 131–150 Latitude/longitude Rivers/canalsHighways/roads1Km grid151–17091–110

111–13031–5051–70

DBCP at the Water table 1980 (µg L−1)

>0.241 0.121–0.150 0.001–0.030 Latitude/longitude Rivers/canals

Highways/roads1 Km grid0.091–0.1200.061–0.0900.031–0.060

0.211–0.2400.181–0.2100.151–0.180

Figure 2 Components of the Fresno Case study example (i.e. Example I) of nonpoint source pollution (After Loagueet al., (1998a) 1998, with permission from Elsevier). (a) Aggregate of the estimated DBCP applications. (b) Simulated1990 DBCP loading at the water table (Note, this snapshot is taken from a continuous simulation animation). A colorversion of this image is available at http://www.mrw.interscience.wiley.com/ehs


Table 4 Steps used in the Fresno Case study (after Loagueet al., 1998a,b)

1. Approximate the climatic history (1950–1994).2. Approximate the distribution of soils.3. Approximate the land-cover history (1958–1994).4. Approximate the average irrigation history

(1960–1994).5. Approximate the water table depth history

(1960–1994).6. Simulate (with PRZM-2) transient unsaturated fluid

flow and DBCP transport (1960–1994).7. Abstract the DBCP concentration at the water table

for each element (1960–1994).8. Approximate the geology.9. Approximate the distribution of saturated hydraulic

conductivity.10. Approximate the recharge history (1960–1994).11. Approximate the pumping history (1960–1994).12. Simulate (with MODFLOW) groundwater flow

(1960–1994).13. Simulate (with MT3D) saturated transient DBCP

transport (1960–1994).

first appearance above the MCL was between 1965and 1970 (note, by 1990, the concentrations are belowthe MCL).

• Relative to the size of the study area, the extentand duration of the estimated DBCP contaminationwas small.

• DBCP concentrations are a function of the spatialand temporal variations in (i) the application rates,(ii) the application frequency, (iii) the unsaturatedprofile thickness, (iv) the soil-hydraulic properties, and(v) the near-surface sorption.

• The DBCP plume evolves (grows and retracts) withtime owing to the loading rates at the water table.

Example II: Point Source Pollution, Three

Manufactured Gas Plants

From the early 1800s to about 1950, manufactured gasplants (MGPs) were operated in the United States to pro-duce gas from coal or oil. By the time they began toclose down, when the distribution of natural gas becamemore economical, approximately 2000 MGPs had been con-structed throughout the USA. The contamination legacyresulting from the production of manufactured gas poses asignificant and ongoing groundwater quality problem. Thedisposal of by-products from the manufacturing processat MGPs contributed to the contamination of MGP sites.Although coal tar was perhaps the most abundant manufac-turing by-product, it was not always considered waste asit was sold for multiple uses (e.g. dyes, explosives, pesti-cides, pharmaceutical preparations, pipe coatings, plastics,road tar, roofing pitch, and wood preservatives). Becauseof its commercial value, coal tar was usually stored inholding tanks. After a MGP was decommissioned, the coal

Table 5 Steps used in the simulations of three formermanufacture gas plants (after Abrams and Loague, 2000)

1. Approximate the climatic history (MGP inception* –2000).

2. Approximate the soil properties for each site.3. Approximate the operational history of each MGP.4. Simulate (with LEACHM) unsaturated fluid flow and

dissolved naphthalene transport for subsurfacesources (MGP inception – 2000).

5. Abstract the dissolved naphthalene concentration atthe water table for each source (MGP inception –2000).

6. Approximate the geology.7. Approximate the distribution of saturated hydraulic

conductivity of each site.8. Simulate (with MODFLOW) groundwater flow for

each site (MGP inception – 2000).9. Simulate (with MT3D) saturated dissolved

naphthalene transport for each site (MGPinception – 2000).

tar was often left in the holding tanks. Fifty years later,many of these holding tanks remained as point sources ofcontamination.

The purpose of the study reported by Abrams and Loague(2000) was to address, in a what if (forensic) mode, usingall available data, if contamination emanating from theholding tanks at three former MGP sites (located in Indi-ana), could cause undesirable impacts to groundwater qual-ity. The nine-step approach used by Abrams and Loague(2000) to simulate the subsurface transport of dissolvednaphthalene, under variable precipitation for ∼150 years,is summarized in Table 5. The numerical model used forthe 1D unsaturated naphthalene transport simulations inthis study was a slightly modified version of LEACHM(Hutson and Wagenet, 1992). The characterization, throughsimulation, of the temporally variable naphthalene loadinghistories at the water table was used for the input to subse-quent 3D simulations of saturated subsurface fluid flow withMODFLOW (McDonald and Harbaugh, 1988) and solutetransport with MT3D (Zheng, 1992).

The simulation results reported by Abrams and Loague(2000) indicate that accidental releases of dissolved naph-thalene, from the holding tanks at the three formerMGP sites, most likely resulted in severe, negativegroundwater quality impacts. Figure 3 illustrates, for oneof the three sites, components of effort reported byAbrams and Loague (2000). The assumptions made byAbrams and Loague (2000) in conducting these sim-ulations had little impact on the overall conclusionthat leaks from the holding tanks can reasonably beexpected to lead to groundwater contamination. Thesensitivity analyses performed by Abrams and Loague(2000) support this conclusion. It is interesting to notethat the approaches used in Examples I (see Table 4)


0 100

m

hr2 = 226.46 m

h3 = 227.09 m

hr1 = 226.37mh5 = 226.16m

h = hr

h = h2,3

River

y

xz

1

2

3

228

227

226

B

A

05

1015

Fill (0.03 m d−1)Alluvium (0.4 m d−1)Till (0.03 m d−1)

Dep

th (

m)

A B

Hydrostratigraphic unit and saturated hydraulic conductivity

Depth ofhorizontal slice

(b)

Naphthaleneconcentration(µg L−1)

≥ 10010

10.1

≤ 0.01

Advection

(c)

Advection and dispersion

h = h3,4h4 = 228.62 m

h = h4,5

h = h1,5

(a)

Figure 3 Components of the manufactured gas plant example (i.e. Example II) of point source pollution (Reproduced fromAbrams and Loague (2000), by permission of Springer). (a) Site map showing finite-difference grid and boundary-valueproblem. (b) Site map and vertical cross section showing the spatial distribution of hydrostratigraphic units andsaturated hydraulic conductivity values. (c) Snapshots from two saturated-zone solute transport simulations (Note,these snapshots are taken from continuous simulation animations). A color version of this image is available athttp://www.mrw.interscience.wiley.com/ehs

and II (see Table 5) are very similar. The major dif-ferences between the nonpoint and point source pollu-tion examples are the scale of the two problems andthe characterization of the sources (i.e. dispersed andwell defined).

Example III: Nonpoint Source Pollution, the

Forested Areas of Tenerife

It is widely known that the use of pesticides in mod-ern agriculture can result in groundwater contamination.


The impact of pesticide use in forestry has receivedconsiderably less attention. Pesticide use in forest man-agement falls into two main categories. The first cate-gory consists of applying herbicides, soil insecticides, andfumigants for site preparation before reforestation, usingherbicides to control undesired vegetation during initialtree growth, and applying insecticides and fungicides toprevent and control sporadic outbreaks of pests. The sec-ond category of pesticide use in forest management isthe application of herbicides to clear firebreaks and roadedges. Pesticide applications in forest management beganduring the late 1950s and early 1960s, with chlorinatedinsecticides, such as DDT. In many cases, organochlo-rines are still manufactured today for use in less devel-oped countries.

The focus of the study reported by Diaz–Diaz andLoague (2001) was to estimate the potential for groundwa-ter contamination on the Canary Island of Tenerife resultingfrom the use of pesticides for forest management purposes.In Tenerife, there are currently no guidelines for the useof pesticides in forested areas despite the ongoing use ofsome chemicals. Diaz–Diaz and Loague (2001) used anindex-based model to rank the leaching potential of 50pesticides that are or could be used for forest manage-ment in Tenerife forests. Once the pesticides having thegreatest leaching potential were identified, regional-scalegroundwater vulnerability assessments with considerationfor uncertainty were generated using soil, climatic, andchemical information in a GIS framework for all of thepine forest areas.

On the basis of the leaching potential ranking, Diaz–Diazand Loague (2001) suggest that, for the pine forestareas of Tenerife (see Figure 4a), the potential leachersare picloram (potassium salt of 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), tebuthiuron (N -(5-(1,1-dimethyl)-1,3,4-thiadiazol-2-yl)-N ,N ′-dimethylurea), carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl-n-methylcarbamate),triclopyr (triethylamine salt of 3,5,6-trichloro-2-pyridinyl-oxyacetic acid), and hexazinone (3-cyclohexyl-6-(dimethyl-amino)-1-methyl-1,3,5-triazine-2,4-(1H ,3H )-dione). Fol-lowing the identification of the five potential leachers,Diaz–Diaz and Loague (2001) prepared regional-scalegroundwater vulnerability assessments in a GIS-driven for-mat, for the pine forest areas of Tenerife. Two hot spotswere identified on the northern side of the study area, theirlocations corresponding to soils classified as Inceptisolsand high recharge rates. The uncertainties in the leach-ing assessments, related to the uncertainties in the soil,recharge, and pesticide data, were shown to be significant.Figure 4(b) illustrates the distribution of soils and rechargeacross the pine forest areas of Tenerife. Figure 4(c) showsthe regional-scale groundwater vulnerability assessments,and their uncertainty, for picloram and carbofuran.

Li Li + SLi

Picloram

Carbofuran

Li < 0.20.2 < Li < 0.50.5 < Li

N

0km

10

Pine forest1995 Fires1998 Fire

(a)

0–250250–500500–750

mm/year

InceptisolsEntisols

Soil orders Recharge

(b)

(c)

Figure 4 Components of the Tenerife example (i.e.Example III) of nonpoint source pollution (Reproducedfrom Diaz–Diaz and Loague (2001), by permission ofAlliance Communications). (a) Distribution of pine for-est areas, showing the locations of fires in 1995 and1998. (b) Distribution of soil orders (left) and averagerecharge rates (right) for the pine forest areas in (a).(c) Distribution of groundwater vulnerability estimates(left) and groundwater vulnerability estimates with con-sideration of uncertainty (right) for two pesticides [notes:the leaching index (Li) values range between zero andone, the larger the value, the more likely it is that thepesticide will leach; the uncertainty is represented by onestandard deviation (S) in Li]. A color version of this imageis available at http://www.mrw.interscience.wiley.com/ehs

EPILOGUE AND FOOD FOR THOUGHT

This article only provides an introduction to the hugesubject of point and nonpoint source pollution. The valueof real-time measurements for assessing point and nonpointsources of pollution and the heroic contributions of theNAWQA program are stressed. The use (and types) of


mathematical models as well as the characterization ofuncertainties associated with modeling point and nonpointsource problems are covered, illustrated by three examples.

There has been no effort in the allotted space here toaddress the exposure/human-health outcome question as itrelates to point and nonpoint source pollution. Unques-tionably, point and nonpoint sources of pollution pose apotentially serious long-term health threat. The EPA, CDC(Center for Disease Control and Prevention), NIH (NationalInstitutes for Health), and the USGS (and their counterpartsin Canada and Europe) have each focused considerableenergy on developing the linkages between environmentalindicators (contaminants), exposure routes, and human-health outcomes. Nevertheless, there is a feeling amongstsome scientists and policymakers that the perceived severityof the threats resulting from point and nonpoint source pol-lution, relative to human health, must be better documented.This view is clearly expressed in Al Freeze’s (2000) bookThe Environmental Pendulum. Anyone interested in thiscontroversy should investigate further.

The reader interested in point and nonpoint source pollu-tion may also want to read the following chapters: Chap-ter 3, Hydrologic Concepts of Variability and Scale,Volume 1; Chapter 10, Concepts of Hydrologic Model-ing, Volume 1; Chapter 66, Soil Water Flow at DifferentSpatial Scales, Volume 2; Chapter 69, Solute Trans-port in Soil at the Core and Field Scale, Volume 2;Chapter 78, Models of Water Flow and Solute Trans-port in the Unsaturated Zone, Volume 2; Chapter 79,Assessing Uncertainty Propagation Through Physicallybased Models of Soil Water Flow and Solute Trans-port, Volume 2; Chapter 91, Water Quality, Volume 3;Chapter 100, Water Quality Modeling, Volume 3; Chap-ter 122, Rainfall-runoff Modeling: Introduction, Vol-ume 3; Chapter 131, Model Calibration and Uncer-tainty Estimation, Volume 3; Chapter 152, ModelingSolute Transport Phenomena, Volume 4; Chapter 153,Groundwater Pollution and Remediation, Volume 4 andChapter 150, Unsaturated Zone Flow Processes, Vol-ume 4.

REFERENCES

Abrams R.H. and Loague K. (2000) Legacies from threemanufactured gas plants: groundwater quality impacts.Hydrogeology Journal, 8, 594–607.

Addiscott T.M. and Wagenet R.J. (1985) Concepts of soluteleaching in soils: a review of modelling approaches. Journalof Soil Science, 36, 411–424.

Barbash J.E. and Resek E.A. (1996) Pesticides in Ground Water:Distribution, Trends, and Governing Factors, Ann Arbor Press:Chelsea.

Carson R. (1962) Silent Spring, Houghton Mifflin: Boston.Commoner B. (1971) The Closing Circle, Knoph: New York.

Corwin D.L. and Loague K. (Eds.) (1996) Applications of GISto the Modeling of Non-Point Source Pollutants in the VadoseZone, Special Publication 48, Soil Science Society of America:Madison.

Corwin D.L., Loague K., and Ellsworth T.R. (Eds.) (1999)Assessment of Non-Point Source Pollution in the Vadose Zone,Geophysical Monograph 108, AGU Press: Washington.

Corwin D.L., Vaughan P.J. and Loague K. (1997) Modelingnonpoint source pollutants in the vadose zone with GIS.Environmental Science and Technology, 31, 2157–2175.

Corwin D.L. and Wagenet R.J. (1996) Applications of GIS to themodeling of non-point source pollutants in the vadose zone:a conference overview. Journal of Environmental Quality, 25,403–411.

Diaz–Diaz R. and Loague K. (2001) Assessing the potentialfor pesticide leaching for the pine forest areas of Tenerife.Environmental Toxicology and Chemistry, 20, 1958–1967.

Domenico P.A. and Schwartz F.W. (1990) Physical and ChemicalHydrogeology, John Wiley & Sons: New York.

Freeze R.A. (2000) The Environmental Pendulum: A Quest for theTruth About Toxic Chemical, Human Health and EnvironmentalProtection, University of California Press: Berkeley.

Gilliom R.J. (2001) Pesticides in the hydrologic system – whatdo we know and what’s next? Hydrological Processes, 15,3197–3201.

Gore A. (1992) Earth in the Balance: Ecology and the HumanSpirit, Houghton Mifflin: New York.

Harr J. (1995) A Civil Action, Vintage Books: New York.Hutson J.L. and Wagenet R.J. (1992) LEACHM: A Process-

Based Model of Water and Solute Movement, Transformations,Plant Uptake, and Chemical Reactions in the UnsaturatedZone (Version 3), Research Series 92–3 , Department of Soil,Crop, and Atmospheric Sciences, New York State College ofAgriculture and Life Sciences, Cornell University: Ithaca.

Larson S.J., Capel P.D. and Majewski S.S. (1997) Pesticides inSurface Waters: Distribution, Trends, and Governing Factors,Ann Arbor Press: Chelsea.

Leopold A. (1949) A Sand County Almanac, Oxford UniversityPress: New York.

Loague K., Abrams R.H., Davis S.N., Nguyen A. and StewartI.T. (1998b) A case study simulation of DBCP groundwatercontamination in Fresno County, California: 2. Transport in thesaturated subsurface. Journal of Contaminant Hydrology, 29,137–163.

Loague K. and Corwin D.L. (1996) Uncertainty in regional-scaleassessments of non-point source pollutants. In Applications ofGIS to the Modeling of Non-Point Source Pollutants in theVadose Zone, Corwin D.L. and Loague K. (Eds.), SpecialPublication 48, Soil Science Society of America: Madison, pp.131–152.

Loague K. and Green R.E. (1991) Statistical and graphicalmethods for evaluating solute transport models: overview andapplication. Journal of Contaminant Hydrology, 7, 51–73.

Loague K., Lloyd D., Nguyen A., Davis S.N. and AbramsR.H. (1998a) A case study simulation of DBCP groundwatercontamination in Fresno County, California: 1. Leachingthrough the unsaturated subsurface. Journal of ContaminantHydrology, 29, 109–136.


Majewski S.S. and Capel P.D. (1995) Pesticides in theAtmosphere: Distribution, Trends, and Governing Factors, AnnArbor Press: Chelsea.

McDonald M.G. and Harbaugh A.W. (1988) A Modular Three-Dimensional Finite-Difference Groundwater Flow Model,Scientific Software Group: Washington.

Mercer J.W., Silka L.R. and Faust C.R. (1983) Modeling ground-water flow at Love Canal, New York. Journal of EnvironmentalEngineering, 109, 924–942.

Mullins J.A., Carsel R.F., Scarbough J.E. and Ivery A.M. (1993)PRZM-2, A Model for Predicting Pesticide Fate in the CropRoot and Unsaturated Soil Zones: Users Manual for Release2, EPA-600/R-93/046, USEPA Environmental Research Labo-ratory: Athens.

Novotny V. and Olem H. (1994) Water Quality – Prevention,Identification, and Management of Diffuse Pollution, VanNostrand Reinhold: New York.

Nowell L.H., Capel P.D. and Dileanis P.D. (1999) Pesticides inStream Sediments and Aquatic Biota: Distributions, Trends, andGoverning Factors, CRC Press: Boca Raton.

Peirce J.J., Weiner R.F. and Vesilind P.A. (1998) Environ-mental Pollution and Control, Fourth Edition , Butterworth-Heinemann: Boston.

Zheng C. (1992) MT3D: A Modular Three-Dimensional Model forSimulation of Advection, Dispersion, and Chemical Reactionsof Contaminants in Groundwater Systems (Version 1.5), S.S.Papadopulos and Associates: Bethesda.

111

Documents

Transcript of 111