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Why Water Quality Varies
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Text and illustrations from An Introduction to Michigan's Water Resources, 1987 Michigan State University Institute of Water Research.
Just as the distribution and availability of water varies across the state, so does its quality. Water quality is affected by the many substances water contacts during its movement through the hydrologic cycle. Water dissolves a wide variety of minerals, nutrients, and other substances from soils, rocks, and the atmosphere, and carries them in solution. Lakes, streams, and groundwater accumulate these dissolved substances and reflect the distinctive characteristics of their watershed's soils, geology, and land use.
Human activities also can change the composition of surface runoff and groundwater. Water is vulnerable to contamination at all points in the hydrologic cycle, and all pathways that transport water can also carry pollutants. Land use activities and wastewater discharges can degrade water quality. Many practices of the past, especially for waste disposal, which were not known at that time to have serious adverse water quality impacts have left behind long lasting contamination problems. An increasing awareness of and concern for the problems caused by these activities has developed in recent years, and Michigan has made a commitment to cease environmental degradation and clean up the problem areas.
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Water Discharges
Agriculture & Forestry
Urban Runoff
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Water Discharges
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Approximately 1,000 industrial and 400 publicly owned wastewater treatment facilities discharge effluent, or wastewater, to the surface waters of Michigan. Heavy manufacturing has discharged acids, solvents, oils, hazardous wastes, and heavy metals, such as mercury and lead, into surface water. Paper and pulp mills have discharged organic chemicals such as polychlorinated biphenyls (PCBs) and waste wood fibers. Municipal sewage treatment plants have discharged inadequately treated wastewater into rivers, streams, and lakes. Major efforts have been undertaken to eliminate these pollution sources, and all discharges now require a permit and are monitored. New sewage treatment plants have been built, and a statewide ban on phosphorus containing detergents has resulted in reduced nutrient loading.
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Agriculture & Forestry
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Some agricultural and forestry practices have had significant impacts on water quality. Soils denuded of protective vegetation have the potential for rapid erosion. Sediment entering lakes and streams from surface runoff reduces water clarity, thus shading aquatic plants, smothering eggs, or interfering with an animal's ability to catch prey or avoid predators. Nutrients, primarily nitrogen and phosphorus, enter water through surface runoff and stimulate plant growth which may become a nuisance and indirectly contribute to periodic reductions in dissolved oxygen, causing fish kills. Many pesticides and herbicides are extremely toxic to aquatic species and may enter surface waters because of over application, wind drift, or surface runoff. Even when these substances are present in nonlethal quantities, they can interfere with fish reproduction and growth or kill aquatic insects, which are a major food source of many game fish.
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Urban Runoff
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Urban areas also contribute pollution through surface runoff and storm sewers. When the land surface is sealed by concrete and asphalt, very little infiltration occurs. During a storm, rainwater washes over the surface, picking up oils, grease, soil, salt, and anything else in its path. The contamination can enter surface waters through direct surface runoff, or through storm sewers. When storm sewers are separate from sanitary or human waste sewers, the stormwater, often untreated, is released directly into surface waters. When storm and sanitary sewers are combined, as is the case in many of Michigan's older urban areas, street runoff and sewage are treated by the wastewater treatment plant prior to discharge. During a storm, the combined sewer often overflows the capacity of the plant, and untreated sewage is released into the river with the contaminated stormwater.
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This page last updated on 9/3/2003.
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