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Municipal Effluent Scenario

Figure 4-4 (continued) (c) James estuary; and (d) Upper Mississippi River. Sources: Lung, 1991a; Fitzpatrick and Di Toro, 1991; Fitzpatrick, 1991; HydroQual, 1986; Lung, 1998; Lung, 1991b.

Municipal Effluent Scenario

Figure 4-4 (continued) (c) James estuary; and (d) Upper Mississippi River. Sources: Lung, 1991a; Fitzpatrick and Di Toro, 1991; Fitzpatrick, 1991; HydroQual, 1986; Lung, 1998; Lung, 1991b.

tremely poor conditions with DO levels of less than 1 mg/L for the Potomac, James, and Upper Mississippi cases, and 2 mg/L for the Delaware.

The model results for the primary scenario of severe oxygen depletion are, in fact, consistent with historical oxygen data recorded for these rivers during the 1960s. Simulating an upgrade to secondary treatment, as mandated by the 1972 CWA for municipal facilities, DO conditions are improved but are still less than the benchmark concentration of 5 mg/L often used to describe compliance with water quality standards. As demonstrated with the models, and actually achieved, better than secondary levels of municipal treatment are needed to exceed a benchmark of 5 mg/L for DO. In contrast to the poor water quality conditions common in these rivers during the 1960s, the occurrence of low DO levels has been effectively eliminated, even under severe drought conditions, as a result of upgrades beyond primary treatment to better than secondary levels of wastewater treatment.

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