Info

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saturation o.ol_Ml_1_I_ £ o.ol_I_I_I_L__l o.ol_I_I_I_I_

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saturation

DISTANCE (MILES)

CALIBRATION FOR SEPT 1983 Primary Effluents

Figure 8-12 Potomac Eutrophication Model simulated water quality data for primary effluent loading scenario. Flow regime is based on September 1983 calibration. Sources: Fitzpatrick, 1991; Fitzpatrick and DiToro, 1991.

loading from the Blue Plains treatment plant, DO concentrations in the vicinity of Washington, DC are computed to be ~ 1 mg/L under the primary effluent scenario. With minimum levels less than 1 mg/L in the vicinity of the Blue Plains discharge (RM 106), the simulated results for the primary effluent scenario are remarkably similar to the historical data recorded for 1960-1964 and 1966 during the drought conditions of the 1960s (see Figure 8-8). The spatial extent of the depression for dissolved oxygen is relatively limited because of the oxygen production by photosynthesis of the algal bloom. Compared to current data and model results, the magnitude of the bloom would have intensified under conditions of primary effluent loads. Consistent with the known occurrence of dense algal blooms during the 1960s (Jaworski, 1990), peak chlorophyll-« concentrations greater than 250 ^g/L were simulated under the primary effluent scenario in contrast to the present range of ~ 160 ^g/L under the loading conditions of 1983. The increase in primary productivity and algal biomass resulted from the additional nutrient (TN, TP) loading delivered to the river.

Under the secondary effluent assumption (Figure 8-13), the reduction in CBOD loading significantly improved dissolved oxygen near Washington, DC. In comparison to the primary scenario, minimum monthly-averaged oxygen levels increased to almost 3.5 mg/L from approximately 0.2 mg/L under the secondary effluent scenario. When compared to the model results for the existing 1983 conditions, the results of the secondary effluent simulation show somewhat poorer water quality conditions for dissolved oxygen and algal biomass. The reason for the failure to achieve compliance with the 5 mg/L water quality standard for dissolved oxygen over only a few miles (RM 104-106) is that, under the existing loading scenario for 1983, the Blue Plains facility (the largest wastewater discharger to the Potomac River) has instituted advanced secondary treatment with greater removal of CBOD, ammonia, and phosphorus than is represented in the secondary effluent scenario (Fitzpatrick, 1991). The magnitude of the algal bloom has also been slightly attenuated as a result of reduced effluent nitrogen levels used to represent secondary effluent discharges to the Potomac estuary.

As shown with both observed data and model simulations, the implementation of secondary and better treatment has resulted in significant improvements in the DO status of the estuary. As demonstrated with the model (and actually attained), better-than-secondary treatment is required to achieve compliance with the water quality standard of 5 mg/L for DO at the critical location downstream of Blue Plains (see Figure 8-10). In contrast to the 1950s and 1960s, the occurrence of low oxygen conditions has been virtually eliminated in the Upper Potomac estuary (see Figure 8-8). Additional improvements in Potomac water quality, in terms of reduced algal biomass, increased water clarity, and still greater improvements in dissolved oxygen levels, have been achieved as a result of implementation of advanced secondary and tertiary levels of municipal wastewater treatment for the Upper Potomac estuary.

Recreational and Living Resources Trends

In addition to public water supply withdrawals (from the free-flowing river) and wastewater disposal from a number of municipalities, the uses of the Upper Potomac estuary include recreational and commercial fishing, boating and navigation, bird-watching, and secondary contact water-based recreation (e.g., wind-surfing). Although

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Saturation

Saturation

DISTANCE (MILES)

CALIBRATION FOR SEPT 1983 Secondary Effluents

Figure 8-13 Potomac Eutrophication Model simulated water quality data for secondary effluent loading scenario. Flow regime is based on September 1983 calibration. Sources: Fitzpatrick, 1991; Fitzpatrick and DiToro, 1991.

recreational opportunities were severely limited during the 1940s, 1950s, and 1960s because of water pollution, the improvements in water quality during the 1980s have resulted in a significant increase in a variety of recreational uses of the river by the urban population of Washington, DC. Boating, canoeing, kayaking, windsurfing, walking, running, and bicycling on trails along the riverbanks, and recreational fishing are now extremely popular activities in the tidal river in the vicinity of Washington, DC.

Designated Uses and Bacterial Trends Unlike the uses of many other major urban waterways, swimming, because of limited access from the shoreline and a lack of public bathing beaches, is not considered a major use of the Upper Potomac estuary. Most of the Potomac River from the upper freshwater reaches near Point of Rocks, Maryland, to the estuarine waters near Point Lookout, Maryland, is designated for primary contact recreational uses (swimming). In the vicinity of Washington, DC, however, the waters of the tidal Potomac are designated for secondary contact recreational uses such as boating or windsurfing. The estuarine portions of the Potomac downstream of Smith Point, Maryland, have been designated for shellfish harvest and must comply with more stringent bacteria level standards than those set for primary or secondary contact recreational uses. To protect public health from risks resulting from direct contact with the waters of the Potomac or ingestion of shellfish from the estuary, water quality standards have been established by the state of Maryland, the District of Columbia, and the state of Virginia for the maximum log mean fecal coliform bacteria levels [as most probable number (MPN) per 100 mL] as follows:

• Shellfish harvest < 14 MPN/100 mL

Based on long-term historical water quality data from measurements taken downstream of the Blue Plains discharge, it is apparent that the introduction of effluent chlorination in 1968 resulted in dramatic improvements in bacterial contamination of the tidal Potomac (Figure 8-14). Prior to chlorination of wastewater effluent, summer coliform levels, typically on the order of 105 to 106 MPN/100 mL from 1940 to the mid-1960s, consistently were in violation of the secondary contact standard of 1,000 MPN/100 mL. Even with the dramatic reductions, summer bacteria levels still exceeded water quality standards during the 1970s. As bacteria loadings from the Washington area municipal wastewater plants continued to decrease during the 1980s, summer bacteria densities began to be in compliance with the water quality standard for both primary and secondary contact. Since the 1980s, periodic violations of bacteria level standards in the tidal Potomac have usually been related to storm event discharges from combined sewer overflows in the District of Columbia and Alexandria, Virginia (MWCOG, 1989).

Since the passage of the 1965 Water Quality Act, well-planned and coordinated water pollution control programs in the Washington metropolitan region have succeeded in achieving substantial reductions in pollutant discharges to the Potomac estuary. Despite the remarkable improvements in the bacteria levels of the tidal Po-

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