Environmental Problems, Their Causes, and Sustainability 1

WaterQualityGoodSlightlypollutedModeratelypollutedHeavilypollutedGravelypollutedDO (ppm) at 20°CBelow 4.5Below 44.5–6.76.7–8Figure 22-3 Water quality and dissolved oxygen (DO) contentin parts per million (ppm) at 20°C (68°F). Only a few fishspecies can survive in water with less than 4 ppm of dissolvedoxygen at this temperature.8–9The level of dissolved oxygen is related to theamount of oxygen-demanding wastes, so called becausethey are broken down by oxygen-requiringbacteria, and plant nutrients in a sample of water (Figure22-3). Scientists also measure the biological oxygendemand (BOD), the amount of dissolved oxygen consumedby aquatic decomposers.They also use chemical analysis to determine thepresence and concentrations of inorganic and organicchemicals that pollute water. They measure sedimentcontent by evaporating the water in a sample andweighing the resulting sediment. Suspended sedimentclouds water. Scientists use an instrument called a colorimeterto measure the turbidity or clarity (transparency)of a water sample.Scientists can also monitor water pollution by usingliving organisms as indicator species. For example,they remove aquatic plants such as cattails andanalyze them to determine pollution in areas contaminatedwith fuels, solvents, and other organic chemicals.Bottom-dwelling species such as mussels thatfeed by filtering water through their bodies can also beanalyzed to determine water quality.Genetic engineers are working to develop bacteriaand yeasts (single-celled fungi) that fluoresce or glowin the presence of specific pollutants such as toxicheavy metals in the ocean, toxins in the air from chemicalweapons, and carcinogens in food. This developmentof biomonitors or biosensors is a rapidly growingfield that might interest you as a career choice.What Are Point and Nonpoint Sourcesof Water Pollution? Concentrated and DiffuseSourcesWater pollution can come from single sources or avariety of dispersed sources.Point sources discharge pollutants at specific locationsthrough drain pipes, ditches, or sewer lines into bodiesof surface water (Figure 22-4). Examples includefactories, sewage treatment plants (which removesome but not all pollutants), underground mines, andoil tankers.NONPOINT SOURCESRural homesUrban streetsCroplandAnimal feedlotSuburbandevelopmentPOINTSOURCESFactoryWastewatertreatmentplantFigure 22-4 Natural capital degradation:point and nonpoint sources of water pollution.It is much easier to identify and controlpoint sources than more dispersednonpoint sources.494 CHAPTER 22 Water Pollution