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Figure 12-14 Long-term trends of mean, tenth percentile, and ninetieth percentile summer BOD5 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.

provement" or "degradation" in DO (Figure 12-13) are caused primarily by year-to-year variability of summer streamflow (see Figure 12-4). An accurate evaluation of the long-term trend in improvement of DO is possible only by filtering the time series of oxygen records to extract only those summers that are characterized by dry streamflow conditions.

Figure 12-15 shows long-term trends in DO conditions for "dry" summers for a 10-mile subreach of the RF1 reach (07010206001) for the critical oxygen sag location from Newport, Minnesota (UM milepoint 820), to Grey Cloud, Minnesota (UM milepoint 830). The time series record of DO data in Figure 12-15 is extracted to highlight the trend in improvement for summers of comparable "dry" streamflow conditions when the flow at the St. Paul USGS gage was less than 75 percent of the long-term (1951-1980) summer mean. During the 1960s, low-flow summer mean DO levels violated water quality standards with concentrations as low as less than 1 mg/L in 1961 to approximately 4 mg/L in 1964. After the upgrade of the Metro plant to advanced secondary with nitrification in the late 1980s, mean summer DO levels in the critical subreach had improved to levels as high as approximately 6 to 7 mg/L even during the extreme drought conditions of 1987-1988. Using before and after data in a post-audit model applied to the low-flow summers of 1976 and 1988, Lung (1996a) has clearly demonstrated that the improvements in DO can be directly related to upgrades of the Metro plant.

As shown by the historical records for fecal coliform bacteria (Figure 12-6), DO (Figure 12-15), and levels of sediment mercury in Lake Pepin (Figure 12-7), investments in water pollution control programs of the 1970s and 1980s have succeeded in

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