Environmental Problems, Their Causes, and Sustainability 1

ties of acids, pesticides, and other toxic chemicals,often blown in from hundreds or thousands of kilometersaway.By the 1960s, many areas of the Great Lakes weresuffering from severe cultural eutrophication, hugefish kills, and contamination from bacteria and a varietyof toxic industrial wastes. The impact on Lake Eriewas particularly intense because it is the shallowest ofthe Great Lakes and has the highest concentrations ofpeople and industrial activity along its shores. Manybathing beaches had to be closed, and by 1970 the lakehad lost most of its native fish.Since 1972, Canada and the United States havejoined forces and spent more than $20 billion on aGreat Lakes pollution control program. This programhas decreased algal blooms, increased dissolved oxygenlevels and sport and commercial fishing catches inLake Erie, and allowed most swimming beaches toreopen.These improvements occurred mainly because ofnew or upgraded sewage treatment plants, bettertreatment of industrial wastes, and bans on use of detergents,household cleaners, and water conditionersthat contained phosphates.Despite this important progress many problemsremain. Each August a large zone severely depleted ofdissolved oxygen is likely to stretch across the centerof Lake Erie. The oxygen-poor water in this zone killsfish and microorganisms that support the lake’s foodweb. During the last 10 years, the time that the zonelasts has increased from two weeks to a month and scientistsdo not know why. Possible causes include oxygendepletion by zebra mussels (Case Study, p. 267),undetected inputs of phosphates from fertilizersthrough storm runoff sewers, an unknown naturallyoccurring cycle, or climate change.More bad news. According to a 2000 survey by theEPA, more than three-fourths of the shoreline of theGreat Lakes is not clean enough for swimming or forsupplying drinking water. The EPA and EnvironmentCanada have identified 43 highly polluted shorelineareas. Nonpoint land runoff of pesticides and fertilizersfrom urban sprawl now surpasses industrial pollutionas the greatest threat to the lakes. Sediments in 26toxic hot spots (Figure 22-8) remain heavily polluted.About half of the toxic compounds entering thelakes come from atmospheric deposition of pesticides,mercury from coal-burning plants, and other toxicchemicals from as far away as Mexico and Russia.Toxic chemicals such as PCBs have built up in foodchains and webs (Figure 22-6), contaminating manytypes of sport fish and depleting populations of birds,river otters, and other animals feeding on contaminatedfish. A recent survey by Wisconsin biologistsfound that one fish in four taken from the Great Lakesis unsafe for human consumption. Another problemhas been an 80% drop in EPA funding for cleanup ofthe Great Lakes since 1992.Some environmentalists call for banning the use oftoxic chlorine compounds such as bleach in the pulpand paper industry around the Great Lakes. Theywould also ban new incinerators (which can releasetoxic chemicals into the atmosphere) in the area, andthey would stop the discharge into the lakes of 70 toxicchemicals that threaten human health and wildlife. Officialsin the industries involved have successfully opposedsuch bans.22-4 POLLUTION OF GROUNDWATERWhy Is Groundwater Pollution Such a SeriousProblem? Not Easily CleanedGroundwater can become contaminatedwith a variety of chemicals because it cannoteffectively cleanse itself and dilute and dispersepollutants.According to many scientists, a serious threat to humanhealth is the out-of-sight pollution of groundwater,a prime source of water for drinking and irrigation.Studies show that groundwater pollution comes fromnumerous sources (Figure 22-9, p. 502). People whodump or spill gasoline, oil, and paint thinners andother organic solvents onto the ground also contaminategroundwater.Although experts rate groundwater pollution as alow-risk ecological problem, they consider pollutantsin drinking water (much of it from groundwater) ahigh-risk health problem. Once a pollutant from aleaking underground tank or other source contaminatesgroundwater it permeates the nearby porouslayers of sand, gravel, or bedrock in the aquifer likewater saturating a sponge. This makes removal of thecontaminant difficult and costly.Then the contaminated water slowly flows throughthe aquifer and creates a widening plume of contaminatedwater. If this plume reaches a well used to extractgroundwater, the polluted water can get into drinkingwater and into water used to irrigate crops.When groundwater becomes contaminated, itcannot cleanse itself of degradable wastes as flowing surfacewater does (Figure 22-5). One reason is thatgroundwater flows so slowly—usually less than 0.3meter or 1 foot per day—that contaminants are not dilutedand dispersed effectively. Another problem isthat groundwater usually has much lower concentrationsof dissolved oxygen (which helps decomposemany contaminants) and smaller populations of decomposingbacteria. Also, the usually cold temperaturesof groundwater slow down chemical reactionsthat decompose wastes.http://biology.brookscole.com/miller14501