I. Groundwater

I. Groundwater

A. Introduction

of all the world's water, only about six-tenths of one percent is found underground

nevertheless, the amount of water stored in rocks and sediment beneath Earth's surface is vast

if just fresh water is considered, the largest volume -- by far -- is glacial ice

second in rank is groundwater, with slightly more than 14% of the total fresh water

however, when ice is excluded and just liquid water is considered, more than 94% is groundwater

groundwater provides the water needs for great numbers of people, crops, livestock, and industry

in the United States, groundwater provides drinking water for more than 50% of the population, as well as 40% of the water for irrigation, and 26% of industry's needs

in some areas, however, overuse of this basic resource has resulted in streamflow depletion, land subsidence, saltwater intrusion, and increased pumping costs

additionally, groundwater contamination due to anthropogenic activities is a real and growing threat!

geologically, groundwater is an important erosional agent

the dissolving action of groundwater is responsible for producing the surface depressions known as sinkholes, as well as creating subterranean caverns

groundwater also plays another important role as an equalizer of streamflow

much of the water that flows in streams is not transmitted directly to the channel after falling as rain, but instead infiltrates through the unsaturated zone to the saturated zone where it moves slowly underground to stream channels

groundwater thus is a form of storage that sustains streams during periods when rain does not fall

B. Distribution of Underground

Water

recall that water beneath Earth's surface is distributed within two zones: the unsaturated zone and the saturated zone

water (soil water) within the unsaturated zone is held between soil grains by capillary action

water (groundwater) within the saturated zone completely fills the pore spaces of soil particles and moves under the influence of gravity

C. The Water Table

the upper limit of the saturated zone is referred to as the "water table"

the water table is a very significant feature of the groundwater system

it is important in predicting the productivity of wells, explaining the changes in the flow of streams and springs, and accounting for fluctuations in the levels of lakes

while the water table cannot be observed directly, its position can be mapped and studied in detail in areas where wells are numerous because the water levels in wells coincides with the upper boundary of the saturated zone

such maps reveal that the water table is rarely level as we might expect a table to be

instead, its shape is usually a subdued replica of the surface topography, reaching its highest elevations beneath hills and then descending toward valleys

where a swamp is encountered, the water table is right at the surface, while lakes and streams generally occupy areas where the water table is above the land surface

a number of factors contribute to the irregular surface of the water table, such as variations in rainfall and permeability, which can lead to uneven infiltration and thus differences in the water table

most importantly, however, is simply the fact that groundwater moves very slowly and at varying rates under different conditions

because of this, water tends to "pile-up" beneath high areas between stream valleys

if rainfall were to cease completely, these water table "hills" would slowly subside and gradually approach the level of the valleys

however, new supplies of rainwater are usually added frequently enough to prevent this

nevertheless, in times of extended drought, the water table may drop enough to reduce stream flow and dry-up shallow wells

in humid regions, even during dry periods, the movement of groundwater into the stream channel maintains a flow in the stream - such streams are said to be "effluent"

an "effluent stream" - is a stream whose channel intersects the water table -- consequently, groundwater feeds into the stream

by contrast, in arid regions, where the water table is far below the surface, groundwater does not contribute to stream flow

therefore, the only permanent streams in such areas are those that originate in wet regions and then happen to traverse the desert

under these conditions the zone of saturation below the valley floor is supplied by the downward seepage from the stream channel, which in turn, produces and upward bulge in the water table

such streams that provide water to the water table are called "influent streams"

D. Posrosity and Permeability

depending on the nature of the subsurface material, the flow of groundwater and the amount of water that can be stored are highly variable

recall that water soaks into the ground because bedrock, sediment, and soil contain voids or openings -- "pore spaces"

the quantity of groundwater that can be stored depends on the "porosity" of the material; that is "the percentage of the total volume of rock or sediment that consists of pore spaces"

sediment is commonly quite porous, and open pore spaces may occupy from 10-50% of the sediment's total volume

the amount of pore space depends on the size and shape of the grains, as well as the packing, degree of sorting, and in the case of sedimentary rocks, the amount of cementing material

for example, clay may have a porosity as high as 50%, whereas in some gravels voids make up only 20% of the material's volume

where sediments of various sizes are mixed, the porosity is reduced because the finer particles tend to fill the openings between the larger grains

most igneous and metamorphic rocks, as well as some sedimentary rocks, are composed of tightly interlocking crystals -- thus, the amount of pore space between the grains may be negligible

thus, if these rocks are to have a greater porosity, fractures must provide a significant proportion of the open space

porosity alone is not a satisfactory measure of a material's ability to yield groundwater

rock or sediment may be very porous and still not allow water to move through it!

the ability of a material to transmit a fluid -- "permeability" -- is also very important

groundwater moves by twisting and turning through small openings

the smaller the pore spaces, the slower the water moves

if the spaces between particles are very small, the films of water clinging to the grains will come in contact or overlap

as a result, the force of molecular attraction binding the water to the particles extends across the opening and the water is held firmly in place

this idea is illustrated by examining the water-yielding potential of different materials

we can divide groundwater into 2 categories: (1) that portion which will drain under the influence of gravity -- called "specific yield"; and (2) that part which is retained as a film on particle and rock surfaces and in tiny openings --- called "specific retention"

specific yield represents how much water is actually available for use, whereas specific retention indicates how much water remains bound to the material

e.g., although clays' ability to store water is high, its pore spaces are so small that water is unable to move

that is: its porosity is high, but because its permeability is poor, clay has a very low specific yield

impermeable layers composed of materials such as clay that hinder or prevent water movement are termed "aquicludes"

on the other hand, larger particles, such as sand or gravel, have larger pore spaces, and therefore the water in the center of the pore spaces is not bound to the particles by molecular attraction and can move with relative ease

thus sand and gravels have low specific retentions and high specific yields

permeable rock strata or sediment that transmit groundwater freely, are called "aquifers"

In Summary:

(1) porosity is not always a reliable guide to the amount of groundwater that can be produced

(2) the property of permeability is a significant factor in determining the rate of groundwater movement and the quantity of water that might be pumped form a well