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Figure 12-12 Spatial trends of August DO in the Upper Mississippi River from 1926 to 1988-1996 from St. Paul, Minnesota (UM milepoint 840), to Lock & Dam No. 3 at Redwing, Minnesota (UM milepoint 797). Sources: Larson, 1999; Mockovak, 1990; MWCC, 1989; D. K. Johnson and P. W. Aasen, J. Minn. Acad. Sci. 55(1), 1989.

water treatment implemented in 1938, 1966, and the early 1970s at the Metro plant. Because of the hydraulic characteristics of the Upper Mississippi River, minimum DO levels have been consistently observed in a zone 5 to 15 miles downstream of the Metro plant discharge, within the oxygen sag region from Newport, Minnesota (UM milepoint 820) to Grey Cloud, Minnesota (UM milepoint 830).

Using historical data available from USEPA's STORET water quality database, the long-term trend of summer DO and BOD5 (1940-1995) has been compiled from monitoring station records extracted for RF1 reach 07010206001 from the Minnesota River (UM milepoint 844.7) to the St. Croix River (UM milepoint 811). Although DO is characterized by a high degree of interannual variability because of temporal variability in streamflow and the spatial gradient over this 34-mile-long reach, there has been a definite improvement in this long reach between the 1960s, when summer mean oxygen levels ranged from approximately 4 to 7 mg/L, to the period from the mid-1970s through the mid-1990s, when summer mean oxygen levels consistently ranged from approximately 7 to 8 mg/L even during the drought conditions of 19871988 (Figure 12-13). The trend of improvement in DO during the 1980s and 1990s is consistent with the long-term trend of improvement in ambient BOD5 extracted for the same reach (Figure 12-14). During the 1960s and 1970s, summer mean BOD5 ranged from approximately 4 to 8 mg/L. During the 1980s, mean BOD5 ranged from approximately 2.5 to 4.5 mg/L. In the period 1990-1995, mean ambient BOD5 declined even further to levels ranging from approximately 2 to 3.5 mg/L as a result of upgrading the Metro plant to advanced secondary with nitrification in the late 1980s.

In interpreting the year-to-year variability of the long-term DO data from 1940 through 1995, it is important to understand the influence of streamflow on summer

Figure 12-13 Long-term trends of mean, tenth percentile, and ninetieth percentile summer DO in the Upper Mississippi River for RF1 reach 07010206001 from the Minnesota River (UM milepoint 844.7) to the St. Croix River (UM milepoint 811). Source: USEPA STORET.

Figure 12-13 Long-term trends of mean, tenth percentile, and ninetieth percentile summer DO in the Upper Mississippi River for RF1 reach 07010206001 from the Minnesota River (UM milepoint 844.7) to the St. Croix River (UM milepoint 811). Source: USEPA STORET.

oxygen levels under the peak effluent loading conditions of the 1960s and early 1970s compared to the greatly reduced effluent loading conditions that have characterized the Twin Cities area since the mid-1970s. Under conditions of similar effluent loading rates, DO decreases during low-flow conditions in contrast to a relative increase during higher summer flow conditions. Over years of comparable effluent BOD5 loading, the interannual cycles that appear to show fluctuating trends of either "im-

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