Environmental Problems, Their Causes, and Sustainability 1

Groundwater normally moves from points of highelevation and pressure to points of lower elevationand pressure. This movement is quite slow, typicallyonly a meter or so (about 3 feet) per year and rarelymore than 0.3 meter (1 foot) per day.There is a hydrological connection between groundwaterand surface water because eventually mostgroundwater flows into rivers, lakes, estuaries, andwetlands. Thus if we disrupt the hydrological cycleby removing groundwater faster than it is replenished,some nearby streams, lakes, and wetlands candry up.Some aquifers get very little, if any, recharge andon a human time scale are nonrenewable resources.Typically these nonreplenishable aquifers—also calledfossil aquifers—are found fairly deep underground andwere formed tens of thousands of years ago. Withdrawalsfrom them amount to water mining that, if keptup, will deplete these ancient deposits.How Much of the World’s Reliable WaterSupply Are We Withdrawing? Taking HalfNow and More LaterWe are using more than half of the world’s reliablerunoff of surface water and could be using 70–90%by 2025.Withdrawal is the total amount of water we removefrom a river, lake, or aquifer for any purpose. Some ofthis water may be returned to its source. For example,most water withdrawn from a river or lake to help coolpower plants may be returned to its source. However,this input of heated water can disrupt aquatic life, aphenomenon known as thermal pollution.Some of the water withdrawn from a source maybe returned to that source for reuse. Consumptive wateruse occurs when water withdrawn is not available forreuse in the basin from which it was removed—mostlybecause of losses such as evaporation, seepage into theground, transport to another area, or contamination.During the last century, the human populationtripled, global water withdrawal increased sevenfold,and per capita withdrawal quadrupled. As a result, wenow withdraw about 34% of the world’s reliablerunoff. We leave another 20% in streams to transportgoods by boats, dilute pollution, and sustain fisheriesand wildlife. Thus we directly or indirectly use about 54%of the world’s reliable runoff of surface water.Because of increased population growth alone,global withdrawal rates of surface water could reachmore than 70% of the reliable surface runoff by 2025—90% if per capita withdrawal of water continues risingat the current rate. This is a global average, with withdrawalrates already exceeding the reliable runoff in agrowing number of areas.How Do We Use the World’s Fresh Water?Watering Crops Is Number OneIrrigation is the biggest user of water (70%)followed by industries (20%), and cities andresidences (10%).Worldwide, we use about 70% of the water we withdraweach year from surface waters and aquifers to irrigateone-fifth of the world’s cropland. This producesabout 40% of the world’s food, including two-thirds ofthe rice and wheat. About 85% of the water withdrawnfor irrigation is consumed and not returned to its waterbasin, mostly because of evaporation and seepageinto the ground. Some of this water is also contaminatedwith salts and pesticides.Industry uses about 20% of the water withdrawneach year, and cities and residences use the remaining10%. Uses of withdrawn water vary from one regionor country to another (Figure 15-4)..Just about anything you do uses water. Readinga newspaper, driving a car, eating a hamburger or abowl of rice, wearing cotton clothing, or drinking abeverage out of an aluminum can involve processes orproducts that require large amounts of water (Figure15-5, p. 310).There is also a difference in water priorities betweendeveloped and developing nations. Accordingto the United Nations, for example, the daily minimumamount of water needed to support three fourthsof the world’s people is equal to the amount of waterused each day to irrigate the world’s golf courses.Case Study: Freshwater Resources in theUnited States—Unequal DistributionThe United States has plenty of fresh water, butsupplies vary in different areas depending onclimate.The United States has more than enough renewablefresh water. But much of it is in the wrong place at thePower plantcooling38%United StatesAgriculture41%ChinaAgriculture 87%Industry 11% Public 10% Public 6% Industry 7%Figure 15-4 Use of water withdrawn in the United States andChina. In the United States, about two-thirds of irrigation watercomes from surface sources and the rest comes from underground.(Worldwatch Institute and World Resources Institute)http://biology.brookscole.com/miller14309