22-1 Types, Effects, and Sources of Water Pollution

What Are the Major Types and Effects of Water Pollutants? Unseen Threats

Water pollution is any chemical, biological, or physical change in water quality that has a harmful effect on living organisms or that makes water unsuitable for desired uses.

The World Health Organization (WHO) estimates that 5 million people die prematurely each year from waterborne diseases. This means that during your lunch hour about 570 people died from such diseases. Each year, diarrhea alone kills about 1.5 million people—about 90% of them children under 5 in developing countries. The number of children killed by largely preventable diarrhea in the past 10 years is greater than the number of people killed in all armed conflicts since World War II.

How Do We Measure Water Quality? Biology and Chemistry in Action

To be considered safe for drinking, water should contain no colonies of coliform bacteria in a sample of 100 millilitres (about 1/2 cup). To be considered safe for swimming, it should have no more than 100 (200 in the United States) colonies per 100 millilitres. By contrast, raw sewage may contain several million coliform bacterial colonies in 100 millilitres of water. When dangerous levels of fecal coliform bacteria are detected, scientists try to determine whether the source is from humans, various types of livestock, or wild animals such as birds or raccoons. A new field of science, called bacterial source tracking (BST), uses molecular biology techniques to determine subtle differences in strains of E. coli based on their animal hosts.

Another important indicator of water quality is the level of dissolved oxygen (DO) found in water at 20°C (68°F). It can be determined using a variety of electronic probes or chemical tests, and is measured in parts per million (ppm) or milligrams of oxygen per litre (mg O/L).

Scientists also determine the biological oxygen demand (BOD) of a sample taken from a body of water. This is the amount of oxygen consumed by aquatic decomposers such as bacteria over five days at 20°C. It reflects the amount of organic waste that is polluting the water, and which, by allowing “oxygen-demanding” decomposers to flourish lowers the dissolved oxygen that is available for other aquatic life forms.

What Are Point and Nonpoint Sources of Water Pollution? Concentrated and Diffuse Sources

Point sources discharge pollutants at specific locations through drain pipes, ditches, or sewer lines into bodies of surface water (Figure 22-4). Examples include factories, sewage treatment plants (which remove some but not all pollutants), underground mines, and oil tankers. Because point sources are at specific places, they are easy to identify, monitor, and regulate. Most developed countries control point-source discharges of many harmful chemicals into aquatic systems. But there is little control of such discharges in most developing countries. Nonpoint sources are scattered and diffuse and cannot be traced to any single site of discharge (Figure 22-4). Examples include acid deposition and runoff of chemicals into surface water from croplands, live- stock feedlots, logged forests, urban streets, lawns, golf courses, and parking lots. There has been little progress in controlling water pollution from nonpoint sources because of the difficulty and expense of identifying and controlling discharges from so many diffuse sources.

What Are the Major Sources of Water Pollution? Supplying Food and Goods

Agricultural activities, such as ploughing fields to grow crops or raising large numbers of livestock, are by far the leading cause of water pollution. Sediment eroded from agricultural lands and overgrazed rangeland is the largest source. Other major agricultural pollutants include fertilizers and pesticides, bacteria from live- stock and food-processing wastes, and excess salt from soils of irrigated cropland. Industrial facilities are another large source of water pollution. Mining is a third source. Surface mining disturbs the Earth’s surface, creating a major source of eroded sediments and runoff of toxic chemicals. Acidic compounds draining from active and abandoned sub- surface and surface mines into streams can kill fish and other aquatic life

Connections: How Might Projected Climate Change Affect Water Quality? More Pollution

Global warming projections include changes in precipitation: Some areas will get much more precipitation and other areas will get less. A moisture-laden atmo- sphere generates more intense downpours, which can flush more harmful chemicals, plant nutrients, and microorganisms into waterways. Massive flooding can spread disease-carrying pathogens by contaminating water treatment facilities and wells. It can also cause lagoons that store animal wastes, as well as sewer lines that carry both sewage and storm runoff, to overflow and release raw sewage into rivers and streams.