Delaware Estuary 196894

River Miles from Cape May-Cape Herilopen

Figure 7-8 Long-term trends of the spatial distribution of summer DO in the Delaware estuary. Sources: Scally, 1997; Patrick, Ruth, Faith Douglass, and Drew Palavage, Surface water quality: Have the laws been successful? Copyright © 1992 by Princeton University Press, Reprinted by permission of Princeton University Press.

River Miles from Cape May-Cape Herilopen

Figure 7-8 Long-term trends of the spatial distribution of summer DO in the Delaware estuary. Sources: Scally, 1997; Patrick, Ruth, Faith Douglass, and Drew Palavage, Surface water quality: Have the laws been successful? Copyright © 1992 by Princeton University Press, Reprinted by permission of Princeton University Press.

Marcus Hook (RM 78). Figure 7-9 shows long-term summer (July-September) trends in DO measured at stations within the RF1 reach from the Ben Franklin Bridge to the Philadelphia Navy Yard. The long-term trend documents improvements in oxygen during the 1980s and 1990s from the water pollution control efforts initiated during the 1970s. Most dramatic, however, is the progressive improvement in the minimum oxygen levels during the 1980s and early 1990s. Summer minimum values increased from approximately 1 mg/L or less in the 1960s and 1970s to approximately 4-5 mg/L during 1990-1995. Although oxygen conditions improved tremendously between the 1960s and the early 1990s, a continued trend of further improvements through the mid-1990s has not been recorded. Minimum oxygen concentrations still can approach 4 mg/L near Chester, Pennsylvania (River Mile 84) and can drop lower than 4 mg/L in the 10-mile oxygen sag reach between River Mile 95 and River Mile 85 (HydroQual, 1998).

Spatial water quality trends recorded during the late 1960s, 1970s, 1980s, and 1990s include documentation of temporal declines in BOD5 (Figure 7-10), ammonia-N (Figure 7-11), total nitrogen (Figure 7-12), and total phosphorus (Figure 7-13). Effluent reductions of oxygen-demanding loads from industrial and municipal sources have resulted in significant declines in ambient levels of BOD5 and ammonia-N. An interannual temporal trend for ambient ammonia-N (Figure 7-14) at a station near Marcus Hook (RM 78) shows a considerable improvement in water quality, with a steep decline from approximately 1.4 mg N/L during the late 1960s to approximately 0.5 mg N/L by the late 1970s, followed by relatively unchanging ambient concentrations (approximately 0.15 mg N/L) recorded during the mid-1980s through the mid-1990s (Santoro, 1998). The decline in ambient ammonia-N during this 30-year

Figure 7-9 Long-term trends of summer (July-September) DO in the Delaware estuary near Philadelphia, PA (RF1-02040202030, River Mile 100-80). Source: USEPA STORET.
Figure 7-10 Long-term trends of the spatial distribution of BOD5 in the Delaware estuary (mean of data from 1968-1970, 1978-1980, and 1988-1990). Source: Marino et al., 1991.

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