Library Index » Science Encyclopedia » Surface Water: Rivers and Lakes - Characteristics Of Rivers And Lakes, The Need For Pollution Control, Assessing Water Quality, Water Quality Of The Nation's Riversand Streams

Surface Water: Rivers and Lakes - Water Quality Of The Nation's Lakes

acres impaired nutrients figure

In the 2000 National Water Quality Inventory report, which assessed 43% of the nation's 40.6 million acres of lakes, ponds, and reservoirs, 47% were fully supporting their designated uses. However, about 8% of the lake acres were threatened. Of the lakes assessed, 45% could only partially support their designated uses.

Leading Pollutants in Lakes, Ponds, and Reservoirs

A lake's water quality reflects the condition and management of its watershed, that is, the land area that drains to the lake. Elevated levels of nutrients were identified as the most common pollutants, contributing to 50% of the impaired water quality in lakes. Figure 3.7 shows the top pollutants and the percentage of impaired lake acres affected by each type.

NUTRIENTS.

The leading pollutant of lakes, ponds, and reservoirs in 2000 was an excess of nutrients, which was reported in 22% of the assessed lake acres and 50% of the impaired lake acres. Nutrients in small quantities are found in healthy lake ecosystems. The presence of excess nutrients disrupts the balance of a lake's ecosystem by creating an environment in which algae and aquatic weeds become too abundant. As these plants die they sink to the bottom of the lake, pond, or reservoir and decompose. In the process of decomposing, dissolved oxygen is used, leaving oxygen levels lower than they would be in a more balanced environment. Two things that typically occur when a lake environment is low on dissolved oxygen is that fish die off and the lake emits foul odors.

METALS.

Metals were the second most prevalent pollutant, affecting 42% of the impaired lake acres. This finding was caused mostly by the widespread detection of mercury in fish tissue. Because it is difficult to measure mercury in water, and because mercury readily accumulates in tissue (bioaccumulates), most states measure mercury contamination using fish tissue samples. Evaluating the extent of the mercury problem is complex because it involves atmospheric transport from power-generating facilities, waste incinerators, and other sources.

SILTATION.

The third most common pollutant of lakes reported in the EPA's 2000 inventory was siltation or sedimentation. Nine percent of the lakes assessed in the report were shown to have been impaired by siltation, FIGURE 3.7
Leading pollutants and sources impairing assessed lakes,
reservoirs, and ponds, 2000

SOURCE: Adapted from "Figure 3-4. Leading Pollutants in Impaired Lakes" and "Figure 3-5. Leading Sources of Lake Impairment," in 2000 National Water Quality Inventory, U.S. Environmental Protection Agency, August 2002
making this pollutant responsible for 21% of the lake acres designated as impaired. (See Figure 3.7.)

Sources of Pollutants in Lakes

AGRICULTURE.

As in the case of rivers and streams, agricultural runoff was the most extensive source of pollution for lakes, affecting 41% of impaired lake acres. (See Figure 3.7.) Pasture grazing and both irrigated and nonirrigated crop production were the leading sources of agricultural impairments to lake water quality.

HYDROLOGIC MODIFICATIONS.

The second most commonly found cause of lake impairment was hydrologic modifications. These modifications, resulting from flow regulation, dredging, and construction of dams, degraded 8% of the assessed lake, pond, and reservoir acres and 18% of the impaired acres.

URBAN RUNOFF AND STORM SEWERS.

A nearly equal percentage of lake acres were degraded by urban runoff and storm sewers as were degraded by hydrologic modifications.

Trophic Status of U.S. Lakes

Lakes naturally change over the years, filling with silt and organic material that alter many of the basic characteristics, such as lake depth, biological life, oxygen levels, and the inherent clearness of the water. This natural aging process is called eutrophication. Human activities often speed up eutrophication by increasing nutrient levels. Figure 3.5 compares a healthy lake system with a system impaired by excessive nutrients.

Naturally occurring eutrophication progression includes several stages. Oligotrophic lakes are clear waters with little organic matter or silt; mesotrophic waters contain more organic material, and the oxygen level is being depleted; eutrophic waters are extremely high in nutrients, and the water is murky and shallow with lots of algae and a depleted oxygen level. Under natural conditions, eutrophication of a large lake can take thousands of years. Human activity can speed up this process, reducing the time to a few decades.

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