The quality of water in aquifers and streams of the Potomac River Basin likely will continue to be stressed by population growth and associated pressures well into the 21st century. Basin population will increase by an estimated 19 percent to 6.2 million between the years 2000 and 2020 (Carlton Haywood, Interstate Commission on the Potomac River Basin, oral commun., 1998). Analysis of data collected by the NAWQA Program suggests that, although water quality in the basin is improving in some respects, water managers will continue to wrestle with several long-term water-quality problems while addressing the pressures and consequences of population growth and associated land-use changes.
Despite an estimated 44 percent increase in population in the Potomac River Basin from 1970 to 1990, total phosphorus concentrations in the Potomac River at Washington, D.C., have decreased since 1979, and nitrogen concentrations have apparently stabilized (fig. 28). Large-scale water-quality-management practices such as improving municipal wastewater-treatment facilities and widespread implementation of phosphate-detergent bans and agricultural best-management practices (BMPs) are apparently working effectively to curb nutrient concentrations in the Potomac River.
Different forms of nitrogen show conflicting patterns in long-term trends in the Potomac River at Washington, D.C., which complicates the forecasting of trends in nutrient concentrations (fig. 28). Improved treatment of municipal wastewater is likely to decrease discharges of both ammonia and organic nitrogen significantly but increase nitrate discharge to streams. Agricultural BMPs that minimize field runoff and increase infiltration to ground water may cause similar nitrate increases in surface water by diverting nitrogen through the ground-water system prior to its discharging to streams. These practices may delay the movement of nitrogen to streams by years or even decades (Focazio and others, in press) as well as increase contamination of ground water by nitrogen and other agricultural chemicals. Much of the ground water underlying agricultural lands in the Potomac River Basin already contains elevated concentrations of nitrate and detectable concentrations of herbicides; and small streams at base flow contain concentrations of nitrate and pesticides similar to those found in ground water. At this point, it is impossible to predict the long-term effect(s) that BMPs may have on ground-water quality or resulting effects on the Potomac River and its tributaries.
Figure 28. Flow-adjusted nutrient concentrations in the Potomac River at Washington, D.C., 1979-96. Total phosphorus concentrations have decreased over this period. Recent trends in nitrogen concentrations are more complicated; ammonia plus organic nitrogen concentrations have decreased, whereas nitrate concentrations have increased, resulting in apparently stable total nitrogen concentrations since about 1985 (modified from Darrell and others, in press).
Contamination by pesticides and industrial compounds will likely persist in the Potomac River Basin for years to come. Chlordane and DDT, two long-banned insecticides, were detected in tissues and sediment and will likely remain present at low concentrations in urban and suburban areas for the foreseeable future. Mercury and PCBs will likely continue to be carried downstream in sediments of the Shenandoah River from their industrial sources until the reservoir of the contaminants is depleted. Many newly developed pesticides and industrial compounds are being released into the environment. The occurrence of these compounds can be tracked only with continued monitoring initiatives.