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BOD5 Load (metric tons/day)

Figure 7-7 Point and nonpoint source loads of BOD5 (ca. 1995) for the Lower Delaware River-Delaware Bay case study catalog units. Source: Bondelid et al., 2000.

105.4 mt/day accounts for 89 percent of the total estimated BOD5 load of 117.1 mt/day from point and nonpoint sources. Municipal facilities contribute 57.3 mt/day (49 percent), while industrial dischargers account for 47.5 mt/day (40 percent) of the total point and nonpoint source BOD5 load (Figure 7-7). Nonpoint sources of BOD5 account for 12.6 mt/day; rural runoff contributes approximately 8 percent, and urban land uses account for approximately 5 percent of the total point and nonpoint load of 117.1 mt/day (Figure 7-7).

One of the major trends indicative of water quality improvement in the estuary has been that for DO. A comparison of mean summer DO levels between 1968-1972, 1975-1979, 1981-1985, and 1988-1994 (Figure 7-8) clearly shows the water quality improvements achieved as a result of the point source loading reductions of ultimate CBOD (Figure 7-6). Mean summer DO concentrations have increased by approximately 1 mg/L between River Mile 110 and River Mile 55 (DRBC Zones 3, 4, and 5) between 1957-1961 and 1981-1985 (Brezina, 1988). DO concentrations have increased from less than 2 mg/L to 5 mg/L at the critical DO sag point at the mouth of the Schuylkill River in Philadelphia (RM 92) during the period from 1968-1972 to 1988-1994. DO concentrations increased steadily farther downstream of Philadelphia (RM 83), reaching a level of approximately 5.5 mg/L during 1988-1994.

The historical summer DO spatial transects data (Figure 7-8) show that waste-water discharges from the Philadelphia area result in minimum DO conditions between the Ben Franklin Bridge (RM 100), the Philadelphia Navy Yard (RM 93), and

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