Potomac Estuary Case Study

The Mid-Atlantic Basin (Hydrologic Region 2), covering a drainage area of 111,417 square miles, includes some of the major rivers in the continental United States. Figure 8-1 highlights the location of the basin and the Potomac estuary, the case study watershed profiled in this chapter.

With a length of 340 miles and a drainage area of 14,670 square miles, the Potomac River ranks forty-eighth among the 135 U.S. rivers that are more than 100 miles in length (Iseri and Langbein, 1974). Figure 8-2 highlights the location of the Potomac estuary case study catalog units identified by major urban-industrial areas affected by severe water pollution problems during the 1950s and 1960s (see Table 4-2). This chapter presents long-term trends in population, municipal wastewater infrastructure and effluent loading of pollutants, ambient water quality, environmental resources, and uses of the Potomac estuary. Data sources include USEPA's national water quality database (STORET), published technical literature, and unpublished technical reports ("grey" literature) obtained from local agency sources.

With a combined drainage area of 14,670 square miles, the freshwater and estuar-ine Potomac River basin is the second largest watershed in the Middle Atlantic region. The freshwater Upper Potomac River flows more than 220 miles from the headwaters of the North Branch in the eastern Appalachian Mountains to the fall line at Little Falls, Virginia, near Washington, DC. Tidal influences in the Potomac extend 117 miles from the fall line at Little Falls to the confluence with Chesapeake Bay at Point Lookout, Maryland (Figure 8-2).

In this 117-mile reach, the Potomac River is classified into three distinct hydro-graphic regions—tidal river, transition zone, and estuary. The tidal river, extending 38 miles from the fall line to Quantico, Virginia, is characterized as freshwater (salinity < 0.5 ppt) with net seaward flow from surface to bottom. This section of the Potomac River receives the effluent discharge from the major municipal wastewater treatment facilities in the Washington, DC, metropolitan area. The transition zone, extending 29 miles from Quantico, Virginia, to the Route 301 bridge in Maryland, is characterized by variable salinity (0.5 to 10 ppt) and significant mixing of freshwater and saltwater from Chesapeake Bay. In the mesohaline estuary region, extending 50 miles from the Route 301 bridge to Chesapeake Bay at Point Lookout, Maryland, salinity varies from 5 to 18 ppt, with estuarine circulation described as partially mixed (Haramis and Carter, 1983).

During much of the past century, the Potomac estuary has been characterized by severe water pollution problems—bacterial contamination, oxygen depletion, and nui-

Figure 8-1 Hydrologie Region 2 and the Potomac estuary watershed.

sance algal blooms—resulting from population growth in the Washington, DC, area and inadequate levels of waste treatment. Historical DO data provide an excellent indicator to characterize long-term trends in the ecological status of the Potomac estuary. The water quality benefits attributed to implementation of secondary and advanced waste treatment by Washington, DC, area municipal wastewater dischargers to the Potomac estuary represent a major national environmental success story.

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