Planet earth groundwater notes

Prof. C.ValentiPlanet Earth Groundwater Notes 1

IntroductionAlthough fresh water on the surface of the earth is most visible in streams and lakes, the greatest amount of fresh water is actually found beneath the surface of the ground as groundwater. Groundwater is water that exists in the pore spaces and fractures in rock and sediment beneath the Earth's surface. It originates as rainfall or snow, and then moves through the soil into the groundwater system, where it eventually makes its way back to surface streams, lakes, or oceans.

Groundwater makes up about 1% of the water on Earth, but groundwater makes up about 35 times the amount of water in lakes and streams.

Most of the earth's fresh water is stored below the ground (besides ice). ~15% of precipitation ends up as groundwater and groundwater is brought up to the surface by wells, springs, geysers and streams.

Groundwater occurs everywhere beneath the Earth's surface, but is usually restricted to depths less that about 750 meters. Groundwater includes water in cracks, crevices, pore spaces and caves underground.

Used as a resource (pump) for drinking and irrigation. Initiates karst topography (dissolution of carbonates). Groundwater flows, doesn't always remain stagnant. The surface below which all rocks are saturated with groundwater is the

water table.

Planet Earth Groundwater Notes


The Water TableRain that falls on the surface seeps down through the soil and into a zone called the zone of aeration or unsaturated zone where most of the pore spaces are filled with air. This is also known as the vadose zone where water is moving downward towards the water table. As it penetrates deeper it eventually enters a zone where all pore spaces and fractures are filled with water. This zone is called the saturated zone. Between the two levels, and marking the upper limit of the zone of saturation is an undulating surface called the water table. The phreatic zone is below the water table where water flows in response to pressure gradients and will find the path of least resistance to flow. The water table occurs everywhere beneath the Earth's surface. In desert regions it is always present, but rarely intersects the surface.

The water table does not remain fixed in position but (1) Fluctuates with the quantity

of recent precipitation. The depth to the water table may change as the amount of water flowing into and out of the saturated zone changes. During dry seasons, the depth to the water table increases. During wet seasons, the depth to the water table decreases.

(2) Mimics topography. In more humid regions the water table is often so high that it intersects the ground surface producing streams and lakes, and generally tends to follow surface topography.

(3) Flows. Water in the ground flows in the phreatic zone, where rate of flow is dependent on slope and hydraulic conductivity of the rock.



Movement of GroundwaterGroundwater is in constant motion, although the rate at which it moves is generally slower than it would move in a stream because it must pass through the intricate passageways between free space in the rock. The groundwater moves downward due to the pull of gravity. The groundwater moves upward because it will flow from higher pressure

areas to lower pressure areas, as can be seen by a simple experiment illustrated below.

Imagine that we have a "U"-shaped tube filled with water. If we put pressure on one side of the tube, the water level on the other side rises, thus the water moves from high pressure zones to low pressure zones.

The same thing happens beneath the surface of the Earth, where pressure is higher beneath the hills and lower beneath the valleys

The amount of groundwater in an area depends on a variety of factors.(1) Amount of precipitation. Large amounts of precipitation results in

elevated water table.(2) Rate of evaporation. Humid climates have less evaporation due to

high humidity in the atmosphere.(3) Vegetation. Although dense vegetation transpires, it prevents rapid

runoff of rainfall, encourages percolation of water into the ground and prevents rapid evaporation by providing shade. The effect of forests increases groundwater supply/recharge.

(4) Porosity and permeability of the aquifer. An aquifer is a rock or sediment layer that can hold large quantities of water and transmits water well. The amount of groundwater in an area as well as the rate of groundwater flow are controlled by two properties of the rock: porosity and permeability.

Porosity is the percentage of the volume of the rock that is open space (pore space). It is therefore the amount or proportion of space between the particles that make up the soil or rock. This determines the amount of water that a rock can contain. In sediments or sedimentary rocks the porosity depends on



1. grain size, 2. the shapes of the grains, 3. the degree of sorting, and 4. the degree of cementation.

Well-rounded coarse-grained sediments usually have higher porosity than fine-grained sediments, because the grains do not fit together well.

Well sorted sediments and rocks hold greater amounts of water because there is a constant space between grains even when closest packed. Poorly sorted sediments usually have lower porosity because the fine-grained fragments tend to fill in the open space.

Since cements tend to fill in the pore space, highly cemented sedimentary rocks have lower porosity.

In igneous and metamorphic rocks porosity is usually low because the minerals tend to be intergrown, leaving little free space. Highly fractured igneous and metamorphic rocks, however, could have high porosity

Permeability is a measure of the degree to which the pore spaces are interconnected, and the size of the interconnections. It therefore refers to the ability to allow water passage through an aquifer.



Porosity and permeability are not synonymous terms. A rock can have a high porosity but a low permeability. It is possible to have a highly porous rock with little or no interconnections between pores. E.G. A good example of a rock with high porosity and low permeability is

a vesicular volcanic rock, where the bubbles that once contained gas give the rock a high porosity, but since these holes are not connected to one another the rock has low permeability.

Grain size is a function of permeability. A thin layer of water will always be attracted to mineral grains due to the unsatisfied ionic charge on the surface. This is called the force of molecular attraction. If the size of interconnections is not as large as the zone of molecular attraction, the water can't move. Thus, coarse-grained rocks are usually more permeable than fine-grained rocks, and sands are more permeable than clays. Coarser grains have greater interconnections between the pore spaces so water can move. Clay size particles have electrical charges packing them tightly and retarding or ceasing water flow.

Fractures and joints can increase permeability drastically because water doesn’t percolate through the rock, it flows quickly along bedding, fractures and joints. Aquifer - a body of rock or sediment layer that conducts groundwater in usable amounts; a highly porous and permeable layer of rocks or sediment.

Aquiclude – a body of rock or sediment layer that does not conduct groundwater in usable amounts; an impermeable rock or sediment layer, also called an aquitard.

Movement in the Zone of AerationRainwater soaks into the soil where some of it is evaporated, some of it adheres to grains in the soil by molecular attraction, some is absorbed by plant roots, and some seeps down into the saturated zone. During long periods without rain the zone of aeration may remain dry.

Movement in the Saturated Zone



In the saturated zone (below the water table) water percolates through the interconnected pore spaces, moving downward by the force of gravity, and upward toward zones of lower pressure. Where the water table intersects the surface, such as at a surface stream, lake, or swamp, the groundwater returns to the surface.

Recharge Areas and Discharge Areas The Earth's surface can be divided into areas where some of the water falling on the surface seeps into the saturated zone and other areas where water flows out of the saturated zone onto the surface.

Areas where water enters the saturated zone are called recharge areas, because the saturated zone is recharged with groundwater beneath these areas.

Areas where groundwater reaches the surface (lakes, streams, swamps, & springs) are called discharge areas, because the water is discharged from the saturated zone. Generally, recharge areas are greater than discharge areas.

Confinement. Aquifers can be ‘confined’ that is, a highly permeable and porous aquifer can be trapped in between two aquicludes. When an aquifer is confined between aquicludes, it can only be recharged in certain areas (upslope exposures) where the aquifer is exposed to the surface of the earth. Because water is trapped, it is under a great hydrostatic pressure.

MOVEMENT OF GROUNDWATERGroundwater velocity. Most groundwater moves slowly relative to stream flow. Water velocity varies greatly for a variety of reasons. Water flow underground flows in response to differences in water pressure and elevation. 1. Elevation. Water within the upper part of the saturated zone (phreatic

zone) moves down hill from high head to low head. The greater the change in elevation, the faster it will flow.

2. Water table slope. The slope of the water table strongly influences ground water velocity where the steeper the slope the faster the water flows. Slope is controlled by topography (mimics topography). Groundwater will not move if the water table is flat.



3. Permeability. Groundwater flows faster in aquifers with high permeability.

4. Pressure. Water under pressure will move faster than water not under pressure.



Discharge and VelocityThe rate at which groundwater moves through the saturated zone depends on the permeability of the rock and the hydraulic gradient. The hydraulic gradient is defined as the difference in elevation (h1-h2) divided by the distance between two points on the water table (L).

Velocity, V, is then: V = K(h2 - h1)/L

where K is the coefficient of permeability (Hydraulic conductivity); depends on the type of aquifer; it’s a constant for a particular aquifer type.

If we multiply this expression by the area, A, through which the water is moving, then we get the discharge, Q.

Q = AK(h2 - h1)/L, which is Darcy's Law.

GROUNDWATER MOVEMENT/DARCY'S LAW Water flows from high head to low head due to gravity. Unless under

pressure. Slope of the water table. The steeper the slope, the faster the water will

flow downhill. Permeability of the aquifer. Groundwater flows faster in highly permeable

aquifers. Pressure. If the aquifer is under pressure, water flows faster.



Wells. Groundwater is an extremely vital resource to many areas of the United States and most of the world. 50% of the American population derives its drinking water from the ground. In Florida 90% of the population utilizes groundwater resources. 2/3 of groundwater is used for irrigation.

Wells. A well is an artificial opening in the land surface that is dug or below the water table. Wells are usually used as a source for groundwater. When a well is dug beneath the water table, water will fill the open space to the level of the water table, and can be drawn out by a bucket or by pumping.

Shallow wells supply of water is related to the fluctuations in the water table. Deeper wells that penetrate into lower aquifers provide more reliable sources of water and are less affected by seasonal periods of drought.

When water is pumped from a new well, the rate of withdrawal initially exceeds the rate of local groundwater flow. This imbalance in flow rates creates a cone of depression in the water table immediately surrounding the well. This local lowering of the water table is called drawdown. The locally steepened slope of the water table increases the flow of water to the well. Once the rate of inflow balances the rate of withdrawal, the slope of the water table stabilizes. Pumping of groundwater changes the local hydrology, that is, the more

water pumped, the bigger the cone of depression. Wells cannot be constructed too close together, if the cones merge, wells will run dry.

Because drilling wells can be expensive, pump tests are preformed. To determine the nature of the aquifer, one would drill a cheap test drill hole. Water is pumped from the well at a known rate, and then it is monitored to see how quickly water levels come back to normal. The rate at which they return is influenced by the properties of porosity and permeability of the aquifer.



AquifersAn aquifer is a large body of permeable material where groundwater is present in the saturated zone. Good aquifers are those with high porosity and high permeability such as poorly cemented sands, gravels, and sandstones or highly fractured rock. Large aquifers can be excellent sources of water for human usage. Aquifers can be of two types:

Unconfined Aquifers - the most common type of aquifer, where the water table is exposed to the Earth's atmosphere through the zone of aeration (at lakes and streams, where the water table intersects the land surface).

Confined Aquifers - these are less common, but occur when an aquifer is confined between layers of impermeable strata.

Artesian Wells Artesian wells are constructed in confined aquifers Because the water is under pressure, the water rises to the

potentiometric surface (potential surface that the water table would be at if it were not under pressure). If the potentiometric surface is above the level of the well, the water will spout above the land’s surface.

Aquifer must be exposed at the surface as some place to be recharged.



Problems with Pumping GroundwaterWhen discharge of groundwater exceeds recharge of the system, several adverse effects can occur. Most common is lowering of the water table, resulting in springs

drying up and wells having to be dug to deeper levels.

Subsidence; sinking of the ground when water is pumped faster than recharge. Occurs in layers of sediments; pore spaces with water has pore water pressure keeping the grains pushed apart. Remove water, the grains pack together, take up less space and the weight of the overlying sediments settle.

Salt water intrusion. Freshwater meets seawater near coastal areas. Urbanization of coastal areas increases groundwater usage; In coastal areas, the water table is close to sea level. Excessive pumping causes freshwater to be sucked up, cone of depression forms, and then if withdrawal exceeds recharge, salt water contaminates the well. Deep wells and wells close to the coastline are more affected.

Contamination; Sewers and septic tanks Waste dumps (both industrial and

residential); leaching from landfills. Gasoline Tanks (like occur beneath all

service stations) Biological waste products Agricultural pollutants such as fertilizers and pesticides.

Groundwater Lab



Water Quality and Groundwater ContaminationWater quality refers to such things as the temperature of the water, the amount of dissolved solids, and lack of toxic and biological pollutants. The elements and compounds dissolved in groundwater consist mainly of chlorides, sulfates, and bicarbonates of calcium, magnesium, sodium, potassium, and iron from the common minerals in the rocks from which they were weathered. But the exact composition and characteristics of groundwater varies from place to place according to the kind of rock and the type of environment in which it occurs.

Water that contains a high amount of dissolved material (hard water) through the action of chemical weathering can have a bitter taste , makes bathing difficult as soap does not lather easily, and can cause the deposition of scaly crusts in pipes restricting water flow.

Water from sulfur rich rocks may contain dissolved hydrogen sulfide giving it the odor of rotten eggs.

Groundwater flowing through rocks with high arsenic and lead content may dissolve these elements making it dangerous to drink.

Hot water can occur if water comes from a deep source or encounters a cooling magma body on its traverse through the groundwater system. Such hot water may desirable for bath houses or geothermal energy, but is not usually desirable for human consumption or agricultural purposes.

Most pollution of groundwater is the result of biological activity, much of it human.



Geologic Activity of GroundwaterDissolution - water is the main agent of chemical weathering. Groundwater is an active weathering agent and can leach ions from rock, and, in the case of carbonate rocks like limestone, can completely dissolve the rock.

Chemical Cementation and Replacement - Water is also the main agent acting during diagenesis. It carries in dissolved ions which can precipitate to form chemical cements that hold sedimentary rocks together. Groundwater can also replace other molecules in matter on a molecule by molecule basis, often preserving the original structure such as in fossilization or petrified wood.

Caves and Caverns - If large areas of limestone underground are dissolved by the action of groundwater these cavities can become caves or caverns (caves with many interconnected chambers) once the water table is lowered. Acidic water follows joints and planes of weakness. As water flows, it dissolves and creates bigger and bigger holes gradually forming caves/caverns. A drop in water table (or uplift of caverns tectonically above water table) causes caverns to fill with air (and tourists).Once a cave forms, it is open to the atmosphere and water percolating in can precipitate new material such as the common cave decorations like stalagtites (hang from the ceiling), stalagmites (grow from the floor upward), and dripstones, and flowstones. Caverns



Sinkholes - If the roof of a cave or cavern collapses, this results in a sinkhole. Sinkholes, likes caves, are common in areas underlain by limestones. For example, in Florida, which is underlain by limestones, a new sinkhole forms about once each year, gobbling up cars and houses in process.If sinkhole is lined with an impermeable layer; forms a sinkhole lake.

Karst Topography - In an area where the main type of weathering is dissolution (like in limestone terrains), the formation of caves and sinkholes, and their collapse and coalescence may result in a highly irregular topography called karst topography. Show overhead.

Disappearing Streams Streams appear to disappear off the surface into the ground. Occurs

where limestone is overlain by an impermeable layer which ends.Karst Towers As sinkholes and caverns grow larger and overburden collapses,

remaining columns/pillars remain.Springs and Geysers Groundwater heats up, some steam forms and pressure builds; pressure

is released, cool fresh groundwater enters caverns and process repeats. Eruptions are intermittent; takes time to heat the water up and form

steam.


Planet earth groundwater notes

Technology

Transcript of Planet earth groundwater notes