Section – 10.3 Objectives • Explain how groundwater is withdrawn from aquifers by wells. • Describe the major problems that threaten groundwater supplies.
Section 10.3 Water is not always available in the quantities and in the locations where it is needed and might be compromised by pollution. ReviewVocabulary • runoff: water flowing downslope along Earth’s surface
New Vocabulary Wells drawdown recharge Artisian wells
Section 10.3 Wells The simplest wells are those that are dug or drilled below the water table, called a water-table aquifer. Wells are holes dug or drilled into the ground to reach an aquifer. Ordinary wells
Section 10.3 Wells Overpumping from one well or multiple wells can result in a cone of depression and a general lowering of the water table. Ordinary wells
Section 10.3 Wells Groundwater Supply The difference between the original water-table level and the water level in a pumped well is called the drawdown. Water from precipitation replenishes the water content of an aquifer in the process of recharge. Ordinary wells
Section 10.3 Wells An artesian aquifer contains water under pressure. The pressurized water in a well drilled into an artesian aquifer can spurt above the land surface in the form of a fountain known as an artesian well. Artesian wells
Section 10.3 Threats to Our Water Supply Freshwater is Earth’s most precious natural resource. Changes to groundwater supplies can lead to environmental issues such as a lowered water table, subsidence, and pollution. Overuse – If groundwater is pumped out at a rate greater than the recharge rate, the groundwater supply will decrease and the water table will drop. Subsidence - Another problem caused by the excessive withdrawal of groundwater is ground subsidence—the sinking of land.
Section 10.3 Threats to Our Water Supply The most easily polluted groundwater reservoirs are water-table aquifers, which lack a confining layer above them. When the recharge areas of confined aquifers are polluted, those aquifers can also become contaminated. Sources of groundwater pollution include sewage from faulty septic tanks and farms, landfills, and other waste disposal sites. Pollution in groundwater
Section 10.3 Threats to Our Water Supply Groundwater Supply Pollutants can spread rapidly through a highly permeable aquifer. Note how the polluted well has drawn the pollution toward it as it has withdrawn water from the water table. Pollution in groundwater
Section 10.3 Threats to Our Water Supply Groundwater Supply Once chemical contaminantssuch as arsenic have entered groundwater, they cannot be easily removed. Over time, an entire aquifer can become contaminated and toxic to humans. Salt pollution is one of the major threats to groundwater supplies, especially in coastal areas, where the intrusion of salt water into groundwater is a major problem. Pollution in groundwater
Section 10.3 Threats to Our Water Supply Groundwater Supply Another source of natural groundwater pollution is radon gas, which is one of the leading causes of cancer in the United States. Radon found in ground water is one of the products of radioactive decay of uranium in rocks and sediments. USEPA advises homeowners for regular test for radon gas since it can easily get accumulated in poorly ventilated basements. Pollution in groundwater
Section 10.3 Protecting Our Water Supply Humans must be aware of how their activities impact the groundwater system so that they can protect the water supply. Major pollution sources like • Infiltration from fertilizers • Leaks from storage tanks • Drainage of acids from mines • Seepage from faulty septic tanks • Saltwater intrusion • Radon gas • need to be identified and eliminated.
Summary Water is not always available in the quantities and in the locations where it is needed and might be compromised by pollution. Wells are drilled into the zone of saturation to provide water. Overpumping of shallow wells produces cones of depression. Artisian wells tap confined aquifers in which water is under pressure. When ground water withdrawal exceeds recharge, it lowers the water table. The most common sources of ground water pollution include sewage, landfills, and other waste disposal sites.