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dissolved surface water mineral concentrations tend to occur in the arid West (Figure 3.16), while the highest ground water mineral concentrations tend to occur in the central and southern U.S. (Figure 3.17). This difference can occur because ground waters in the West are less diluted by rain when they enter surface streams.
Sediment is generally the most common water pollutant. Not only can it fill stream channels and harbors, but it can also degrade habitat by reducing the amount of light that reaches stream bottoms and covering spawning beds and submerged aquatic vegetation. Additionally, chemical pollutants often sorb to sediment particles and thus move through and contaminate the environment.
A broad range of factors cause marked differences in water composition and quality at various locations in the United States. First, precipitation varies in chemical composition. For instance, rain near the coasts contains a much higher concentration of sea salts than further inland. However, only in areas with no anthropogenic pollution and very slow weathering rates does this have a significant effect on stream composition. More important, acid rain created by fossil fuel use has dramatically reduced pH and acid neutralizing capacity (ANC) and increased levels of sulfate and nitrate. In areas lacking carbonate bedrock, serious acidification of surface waters can result. In the United States, acid sources are concentrated in the Ohio River Valley. Downwind from this region is a broad area underlain mainly by granitic bedrock (New York and New England) which is especially vulnerable to the atmospheric acid inputs.
Lead added to gasoline and released during combustion was the dominant source of this metal to aquatic ecosystems during most of the 20th century. The historical record of trace-metal analyses of fresh waters is so fiddled with contamination artifacts that it is impossible to know whether streams contained significant lead levels before the days of the automobile—a question further complicated by the ability of natural processes to greatly attenuate lead as it passes through soil (Wang et al., 1995). Yet substantial documentation suggests that lead from gasoline combustion significantly increased lead levels in surface waters of the world's oceans (Flegal and Patterson, 1983) and in soils throughout the United States. The elimination of lead in gasoline has caused a dramatic decline in atmospheric delivery, and surface waters now contain almost immeasurably low amounts, especially in bioavailable dissolved forms (Windom et al., 1991; Benoit, 1994).
Changes in forest ecosystems can also change water quality. For instance, gypsy moth defoliation of forested uplands in Virginia led to dramatic increases in concentrations of nitrates in stream waters. Other solute changes included increasing concentrations of calcium, magnesium, potassium, and hydrogen ions as well as decreasing concentrations of acid-neutralization capacity and SO42-. After several years, the composition of the study stream returned to predefoliation concentration levels. Short-term effects of the defoliation included an increase in the frequency and severity of episodic acidification. Long-term effects included reduction of base nutrient supplies in the catchment basin (Webb et al., 1997).