Surface Water as a Natural Resource
Fresh surface water is a basic natural resource essential to human agricultural and industrial activities. Runoff held in reservoirs behind dams provides water supplies for great urban centers, such as New York City and Los Angeles, as well as irrigation water for agriculture. We can also generate hydroelectric power from surface water where the gradient of a river is steep. If the gradient is gentle, we can travel along surface water. But our heavily industrialized society requires enormous supplies of fresh water to sustain it, and demand is increasing. Urban dwellers consume 150 to 400 l (50 to 100 gal) of water per person per day in their homes. We use large quantities of water in air conditioning units and power plants, and much of this water is obtained from surface water.
Unlike ground water, which represents a large water storage body, fresh surface water in the liquid state is stored only in small quantities. (The Great Lakes system is an exception.) About 20 times as much ground water is available globally than water held in freshwater lakes. And water in streams is about one one-hundredth of that in lakes. We build dams to help store runoff that would otherwise escape to the sea. But even if the reservoir is full, we can still only use as much water as is naturally supplied by inflowing rivers in the long run.
POLLUTION OF SURFACE WATER
Streams, lakes, bogs, and marshes provide specialized habitats for plants and animals. These habitats are particularly sensitive to changes in the water balance and in water chemistry. Our industrial society not only makes radical changes to the flow of water by constructing dams, irrigation systems, and canals, but it also pollutes and contaminates our surface waters with a large variety of wastes.
There are many different sources of water pollutants. Some industrial plants dispose of toxic metals and organic compounds directly into streams and lakes. Many communities still discharge untreated or partly treated sewage wastes into surface waters. In urban and suburban areas, deicing salt and lawn conditioners (lime and fertilizers) enter and pollute streams and lakes, and also contaminate ground water. In agricultural regions, fertilizers and livestock wastes are important pollutants. Mining and processing of mineral deposits also pollute water. Surface water can even be contaminated by radioactive substances released from nuclear power and processing plants.
Many chemical compounds dissolve in water by forming ions—charged forms of molecules or atoms. Among the common chemical pollutants of both surface water and ground water are sulfate, chloride, sodium, nitrate, phosphate, and calcium ions. Sulfate ions enter runoff from both polluted urban air and sewage. Chloride and sodium ions come from polluted air and from deicing salts used on highways. Fertilizers and sewage also contribute nitrate and phosphate ions. Nitrates can be highly toxic in large concentrations and are difficult and expensive to remove.
Phosphate and nitrate are plant nutrients and can encourage algae and other aquatic plants to grow to excessive amounts in streams and lakes. In lakes, this process is known as eutrophication and is often described as the “aging” of a lake. Nutrients stimulate plant growth, producing a large supply of dead organic matter in the lake. Microorganisms break down this organic matter but require oxygen in the process. But oxygen is normally present only in low concentrations because it dissolves only slightly in water. The microorganisms use up oxygen to the point where other organisms, including desirable types of fish, cannot survive. After a few years of nutrient pollution, the lake takes on the characteristics of a shallow pond that has been slowly filled with sediment and organic matter over thousands of years.
Acid mine drainage is a particularly important form of chemical pollution of surface water in parts of Appalachia, where abandoned coal mines and strip-mine workings are concentrated. Ground water emerges from abandoned mines as soil water percolates through strip-mine waste banks. This water contains sulfuric acid and various salts of metals, particularly of iron. In sufficient concentrations, the acid from these sources is lethal to certain species of fish and has at times caused massive fish kills.
Plants and animals have also been killed by toxic metals, including mercury, pesticides, and a host of other industrial chemicals introduced into streams and lakes. Sewage introduces live bacteria and viruses—classed as biological pollutants—that can harm humans and animals alike. Another form of pollution is thermal pollution, which is created when heat, generated from fuel combustion and from the conversion of nuclear fuel to electricity, is discharged into the environment. Heated water put into streams, estuaries, and lakes can have drastic effects on local aquatic life. The impact can be quite large in a small area.