Environmental Problems, Their Causes, and Sustainability 1

Because point sources are at specific places, theyare easy to identify, monitor, and regulate. Most developedcountries control point-source discharges ofmany harmful chemicals into aquatic systems. Butthere is little control of such discharges in most developingcountries.Nonpoint sources are scattered and diffuse andcannot be traced to any single site of discharge (Figure22-4). Examples include acid deposition and runoffof chemicals into surface water from croplands, livestockfeedlots, logged forests, urban streets, lawns, golfcourses, and parking lots. There has been little progressin controlling water pollution from nonpoint sourcesbecause of the difficulty and expense of identifyingand controlling discharges from so many diffusesources.What Are the Major Sources of WaterPollution? Supplying Food and GoodsThe leading sources of water pollution areagriculture, industries, and mining.Agricultural activities are by far the leading cause ofwater pollution. Sediment eroded from agriculturallands and overgrazed rangeland is the largest source.Other major agricultural pollutants include fertilizersand pesticides, bacteria from livestock and food processingwastes, and excess salt from soils of irrigatedcropland.Industrial facilities are another large source of waterpollution. Mining is a third source. Surface miningdisturbs the earth’s surface, creating a major source oferoded sediments and runoff of toxic chemicals. Acidiccompounds draining from active and abandoned subsurfaceand surface mines into streams can kill fishand other aquatic life (Figure 16-14, p. 344).Is the Water Safe to Drink? For Mostbut Not AllOne of every five people in the world lack accessto safe drinking water.Good news. About 95% of the people in developedcountries and 74% of those in developing countrieshave access to clean drinking water.Bad news. According to the WHO, about 1.4 billionpeople in developing countries do not have access toclean drinking water. As a result, each day about 9,300people die prematurely from infectious diseasesspread by contaminated water or the lack of water foradequate hygiene.The United Nations estimates it would cost about$23 billion a year over 8–10 years to bring low-cost safewater and sanitation to the people in the world whodo not have it. If developed countries paid half of thatcost, it would amount to an average of $19 a year foreach person in such countries.Connections: How Might Projected ClimateChange Affect Water Quality? MorePollutionIn a warmer world, too much rain and too little raincan increase water pollution.Global warming projections include changes in precipitation:some areas will get much more precipitationand other areas will get less. A moisture-laden atmospheregenerates more intense downpours, which canflush more harmful chemicals, plant nutrients, and microorganismsinto waterways. Massive flooding canspread disease-carrying pathogens by contaminatingwater treatment facilities and wells. It can also causelagoons that store animal wastes, as well sewer linesthat carry both sewage and storm runoff, to overflowand release raw sewage into rivers and streams.Prolonged drought can reduce river flows that dilutewastes. It can also spread infectious diseases morerapidly among people who lack enough water to stayclean. Warmer water temperatures can threatenaquatic life by reducing dissolved oxygen levels, andcan increase the growth rates of populations of harmfulbacteria.22-2 POLLUTION OF FRESHWATERSTREAMSWhat Are the Water Pollution Problemsof Streams? Pollution Overload andLow FlowFlowing streams can recover from a moderate levelof degradable water pollutants if their flows are notreduced.Rivers and other flowing streams can recover rapidlyfrom moderate levels of degradable, oxygen-demandingwastes and excess heat through a combination ofdilution and biodegradation of such wastes by bacteria.But this natural recovery process does not workwhen streams are overloaded with pollutants or whendrought, damming, or water diversion for agricultureand industry reduce their flows. Also, these natural dilutionand biodegradation processes do not eliminateslowly degradable and nondegradable pollutants.In a flowing stream, the breakdown of degradablewastes by bacteria depletes dissolved oxygen and createsan oxygen sag curve (Figure 22-5, p. 496). This reducesor eliminates populations of organisms withhigh oxygen requirements until the stream is cleansedof wastes.http://biology.brookscole.com/miller14495