Where is it found?
Ground water is, appropriately, found underground. Aquifers carry most of the ground water. Some is accessible above this level however in the natural water table. Ground water is usually very pure, clear and free of most sediment and pollution. The many layers of earth and rock that the water must pass through to reach the aquifer effectively filter most of the pollutants out. There may be a few pathogens present however and a few dissolved solids that leached from the rocks into the water such as calcium carbonate and iron. groundwater discharging into streams is what keeps streams moving steadily all year long. Except during a rainfall or snow melts when the flow is supplemented all of the water in the stream is from ground water seeping through the stream banks and beds. This is called base flow.
For ground water to reach an aquifer, it must first percolate down through layers of soil and rock (the zone of unsaturation) to reach the zone of saturation. In this zone, all of the spaces between the bits of rock are filled with water. The top of this zone is the water table. The rock and soil layers that can easily store and move this water are known as aquifers. Aquifers may be as large as several states or only a few acres. They could be only a few feet deep to hundreds of feet deep. The are some layers of rock that are impermeable to water, these are known as aquitards. An aquifer contained above and below by aquitards is a confined or artesian aquifer. One with no aquitard is unconfined.
Because of Pennsylvania's complex geological history, there are many types of aquifers, classified by the type of rock that surrounds the water. The four principal types are sand and gravel, sandstone and shale, carbonate rock, and crystalline rock. Pennsylvania is predominantly sandstone and shale. An aquifer with mostly sandstone contains softer water while the shale has a tendency to have harder water. The hard or softness of water refers to the amount of calcium carbonate in the water. The harder the water, the more soap is needed to produce a lather.
Water Use Issues
Many people in rural areas have wells. Almost half of the groundwater use in Pennsylvania is for the domestic water supply. Depending on the depth of the well it may draw on the water table or take directly from an aquifer. Artesian wells draw from an aquifer and are usually extremely deep. An advantage of this well is that is rarely runs dry, even during droughts. Surface wells, those that draw from the water table, may be very shallow, only a hundred feet deep. It is much easier for these wells to run dry, especially during drought conditions. Over pumping of aquifers can reduce stream quality by decreasing the amount and quality of groundwater available for base flow in the streams.
Most cities depend upon surface water for their needs, there are a few though, that draw from ground water sources. Unfortunately ground water is not unlimited and these cities often have to supplement their supply with surface water. Many of these cities are considering switching to surface water resources.
As the population of the country increases, the demands on our water sources increase as well. We must learn to conserve and preserve our water resources to prevent the overuse of them and an eventual water shortage.
Land Use Issues
The effects of land use on our ground water supply wasn't truly considered until the 1970's human-made organic chemicals were found in the groundwater. Four groups of activities can have adverse affects on ground water: waste disposal, agriculture, resource extraction, and urbanization.
Coal mining is especially harmful to groundwater. Mines often intersect aquifers and the water will flow into the mine, draining the aquifer. This drainage will seep through the mine and intersect another aquifer, carrying the contamination gained from contact with the mine, such as iron, manganese, and sulfate. Many of the aquifers in Pennsylvania are no longer suitable for drinking because of mine contamination.
Oil and natural gas drilling may release brine, salt water, into fresh water aquifers. Waste disposal facilities and septic systems may leach contaminants into the groundwater. The worst problem from agriculture is the seepage of nitrogen from fertilizers into the soil and groundwater.
Urbanization affects the recharging or groundwater. The construction of impervious surfaces such as roads, roofs, or parking lots and even compacted dirt from machinery prevents water from percolating into the ground. The water that would have entered the aquifer system instead flows into nearby streams.
For more general, easy to under stand explanations of ground water and other aquatic topics see Aquatic Resources of Pennsylvania from KARE (Keystone Aquatic Resource Education Program) and the PA Fish and Boat Commission.
Understanding Groundwater by University of Nebraska Cooperative Extension