Environmental Problems, Their Causes, and Sustainability 1

What Is Groundwater? Water DownBelowSome precipitation infiltrates the ground and isstored in spaces in soil and rock.Some precipitation infiltrates the ground and percolatesdownward through voids (pores, fractures,crevices, and other spaces) in soil and rock (Figure 15-3).The water in these spaces is called groundwater—oneof our most important sources of fresh water. Groundwaterfound within 1 kilometer (0.6 miles) of the earth’ssurface contains more than 100 times all the waterfound in the world’s rivers, streams, freshwater lakes,and reservoirs combined.Close to the surface in the zone of aeration, thepores of the soil contain a mixture of air and somewater. Lower layers of soil where the spaces are completelyfilled with water make up the zone of saturation.We drill shallow wells to tap into groundwater inthis zone. The water table is located at the top of thezone of saturation. It falls in dry weather or when weremove groundwater faster than it is replenished, andrises in wet weather.Deeper down are geological layers called aquifers:porous, water-saturated layers of sand, gravel, orbedrock through which groundwater flows. They arelike large elongated sponges through which groundwaterseeps. Fairly watertight layers of rock or clay belowan aquifer keep the water from seeping out. Aboutone of every three people on the earth depends on waterpumped out of aquifers for drinking and other uses.Most aquifers are replenished naturally by precipitationthat percolates downward through soil androck in what is called natural recharge. But some arerecharged from the side by lateral recharge from nearbystreams. Most aquifers recharge extremely slowly.PrecipitationUnconfined Aquifer Recharge AreaEvaporation and transpirationEvaporationConfinedRechargeAreaRunoffFlowingartesian wellInfiltrationWatertableRechargeUnconfinedAquiferInfiltrationStreamWellrequiringa pumpLakeUnconfined aquiferLesspermeable materialsuch as clayConfined aquiferConfining impermeable rock layerFigure 15-3 Natural capital: groundwater system. An unconfined aquifer is an aquifer with a water table. Aconfined aquifer is bounded above and below by less permeable beds of rock. Groundwater in this type ofaquifer is confined under pressure. Some aquifers are replenished by precipitation and others are not.308 CHAPTER 15 Water Resources