GEO-113: ENVIRONMENTAL GEOLOGY
WATER RESOURCES LECTURE NOTES
I. The Hydrosphere: All H2O at or near earthıs surface.
1) 97.5% is in the oceans.
2) 1.8% is in ice or snow.
3) 0.7% is freshwater (98% of that is groundwater, the largest source of fresh, liquid water).
4) A very tiny fraction is in the atmosphere as water vapor and in surface streams or lakes.
All water moves between these reservoirs 10^17 gallons per year 100 million billion gallons. May take 10,000 million years to move from one reservoir to another. A slow rate of exchange on a human time scale. Therefore, water is considered a non-renewable resource in the short term.
Scarcity of water often leads to political conflicts. Much of the conflict between the Israelis and the Palestinians is over water rights. May be easier to settle the land issues than the water issues, since much of the water supply in the area comes from underground and can't be seen directly.
II. Ground water: Water in the zone of saturation. Lies beneath the zone of aeration (vadose zone or soil moisture).
The water table is the top surface of the zone of saturation. It fluctuates with topography (may be higher than elevation of land such as at a lake) and with rate of recharge (infiltration from surface plus migration in from side) versus rate of discharge (loss due to pumping and migration out). Varies with changes in seasonal or daily precipitation.
Ground water is the most important source of fresh water today, not surface waters (lakes and rivers). Supplies 34 out of 100 largest U.S. cities. Why?
1) Precipitation varies dramatically, particularly in arid areas where little or no surface water exists.
2) Surface water often polluted.
3) Often naturally filtered
4) Largest available source of fresh, liquid water.
Large volumes of ground water require an aquifer that holds and transmits the water. Material has a high porosity and permeability. Usually a well-sorted sediment rock or sedimentary rock). Flow rates of 1-100 meters per day for sand and sandstone and 100-500 per day for gravel or conglomerate are typical.
Aquiclude: impermeable to water flow. Acts as a barrier (i.e. shale).
Aquitard: intermediate condition
III. Aquifer Types
1) Unconfined: no confining aquiclude on top of aquifer. Water is not under any pressure. Aquifer is open to surface waters (and pollution) throughout its entire area. Usually is regional in extent (100s of square miles or more).
2) Perched water table: localized (10s of square miles or less) unconfined aquifer usually at a shallower depth than the regional aquifer. Caused by underlying aquiclude of limited extent. Cheaper to exploit, but can be quickly depleted and is more sensitive to local precipitation and pollution.
3) Confined of artesian: overlain by an aquitard or aquiclude. Water movement restricted to the sandwiched aquifer. Water usually under pressure due to its own weight (hydrostatic head). If drilled, water will rise to the potentiametric surface. This surface decreases in altitude with distance from the recharge area because of loss of ability to due work due to friction. Most municipal water supply systems are synthetic artesian systems. Due to overlying confining layer, confined aquifers are less susceptible to pollution over most of their area. However, in the restricted recharge area, the aquifer can be greatly impacted by local pollution and the reduction of water infiltration due to urbanization.
Urbanization cuts recharge rates by covering the recharge area with impermeable material (asphalt, concrete), particularly for confined aquifers were recharge area is small.
Problem can be corrected by building retention basins to hold the water in place for a time and allow it to percolate into the ground. This also helps in upstream flood control.
IV. Consequences of Excessive Ground Water Withdrawal
If rate of water extraction is greater than rate of recharge a cone of depression develops in the water table around the extraction well. Depth of the cone is greatest at the well. The difference between the elevation of the regional water table outside the cone of depression and the water level in the well is known as the drawdown.
Bigger drawdowns occur in aquifers of lower permeability. Nearby shallower wells may dry up unless they are drilled deeper. Overlapping cones of depression can cause a regional lowering of the water table causing many wells to dry up. May require 10-100ıs of years after the water withdrawal stops before recharge raises the water table and allows the wells to be used again.
Cones of depression may locally reverse the direction of ground water flow. May impact the contamination of local water supply.
1) If a lowering of the water table empties an underground cavern (usually in limestone), roof of cavern is left unsupported and it may collapse. Forms a surface depression or sinkhole. These may be 1000ıs of feet across and 10ıs of feet deep and cause drastic property damage.
2) Large areas of land may subside as water is removed from pore spaces between grains. Large problem in Houston, New Orleans, coastal New Jersey, California, and Venice.
3) As water is removed compaction of the aquifer occurs. Porosity and permeability may be permanently reduced. Aquifer may now become an aquitard.
4) In coastal areas freshwater withdrawal may be replaced by saltwater recharge. Causes saltwater intrusion into the aquifer. Makes aquifer and any wells tapping it useless. This is a large problem on Long Island, coastal New Jersey, Gulf Coast, and California.
5) May cause a stream to switch from being effluent (gaining ground water) to influent (losing water). Polluted surface streams may now pollute ground water.
V. U.S. Water Supply
1) 4.2 X 1012 gallons per day fall as precipitation. 2.8 X 1012 gallons per day lost back to the atmosphere because of evaporation and transpiration. Therefore, 1.4 X 1012 gallons per day is potentially available for use. However, much of this is lost because of pollution streams and surface runoff (lost to surface streams and enters ocean). Also much of it falls were it is not needed.
2) 4.5 X 1011 gallons per day is used. This represents approximately 1800 gallons per person per day in the U.S. Most of this is used for industry or agriculture. Each of us is personally responsible for about 150 gallons per day (home and work usage). Only about 1 gallon per person per day is needed to sustain human life.
3) Only 1 X 1011 gallons per day is consumed. The rest is returned to the hydrosphere, although often in a polluted (chemically, physically, or thermally) form.
VI. U.S. Water Uses
1) Hydroelectric generation
2) Public (municipal) water supplies
3) Rural (individual) water supplies
4) Irrigation - biggest consumer of water. Causes the biggest depletion of ground water supplies and lowering of water tables. An example is the Ogallala Aquifer. Water table has decreased 100+ feet in some areas. Lowering 0.5 to 3 inches per year now. May have 40-50 years of usable water left in some areas. There are no cheap alternative sources of water. Will make crops grown from expensive water more expensive themselves. Problem will only get worse due to global warming.
5) Industrial - biggest user of water. Most water is returned to the hydrosphere.
VII. Extending Water Supplies
1) Conservation: Use less water for many activities. Use high efficiency appliances. Cut down on irrigation or make it less susceptible to evaporation by using drip irrigation techniques. Biggest saving would come in the agricultural area.
2) Interbasin Water Transfer: Move water from where it is plentiful, but not used to areas where it is used, but not plentiful. Often draw on very distance sources, such as the municipal water supplies for Los Angeles and New York City. They get their water from 100s of miles away. May have to do this in the Midwest where water could be piped in from Canada. It will cost billions of dollars to build such an extensive water distribution system. Transfer of large volumes of water may have a very negative environmental impact on the source. For example, there is now more demand for Colorado River water than the river can supply. As a result the river dries up entirely it enters Gulf of California. Often there are legal questions as to whose water it is.
3) Desalination: Turn salty ocean water in to fresh water. Two methods to do this:
1) Filtration - relatively cheap process, but can only process limited amounts of water. Good process for individual homes or factories and small towns.
2) Distillation - Evaporate large quantities of salt water and then precipitate the water vapor as fresh water. Can be powered by burning fuels or solar energy. Used extensively in Saudi Arabia and other desert environments. Very expensive technology with the derived water costing 5-15 times more than naturally fresh water. Food grown will cost 2-5 times as much.
We are reaching the limit of our water supplies in some locations in this country and around the world. Where are we going to find more and at what cost, both economically and environmentally?