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1976-85 1986-95 1996-98

Figure 12-6 Pre- and post-CSO separation project trends in fecal coliform bacteria for the Upper Mississippi River at St. Paul. State standard for fecal coliform bacteria is 200 MPN/100 mL based on monthly geometric mean from May to October. Source: Buttleman and Moore, 1999.

71 percent level for samples collected from 1996 to 1998. High bacteria levels, however, do occasionally occur in the heavily urbanized area upstream of Lock and Dam No. 1; the high levels apparently are associated with urban stormwater runoff (Buttleman and Moore, 1999).

To remedy the periodic flooding of the Metro plant that resulted in the discharge of raw sewage to the river, flood protection projects were completed in 1975 and effluent pumps were installed in 1977. The pumps allowed the Metro plant to treat wastewater during the annual spring floods. The success of the flood control efforts at the Metro plant was dramatically demonstrated during the flood events of 1993 and 1997 when the plant recorded 100 percent compliance with NPDES permit limits during these two extreme events. Many other water pollution control plants in the region were forced to bypass waste treatment as a result of these extraordinary floods (Larson, 1999). In addition to their use for flood control, the effluent pumps are used during low-flow conditions when DO levels are depressed to aerate the effluent to increase ambient oxygen levels in the river.

Responding to federal industrial pretreatment requirements promulgated in 1979, the Twin Cities initiated a program to reduce discharges of heavy metals to the Upper Mississippi River. A comprehensive strategy was adopted in 1981 to reduce the discharge of heavy metals from municipal water pollution control plants contributed by sanitary sewer discharges from industrial sources. By 1992, a decade after beginning the program, the loading of heavy metals to the river had been reduced by an average of 82 percent, with declines in ambient levels of heavy metals. Using sediment cores collected in Lake Pepin, Balogh et al. (1999) have reconstructed historical loading rates of mercury from ca. 1800 to 1996 from the Upper Mississippi River watershed to Lake Pepin (Figure 12-7). Averaging the sediment core data by 10-year intervals, Balogh et al. estimated a loading rate of 3 kg/yr to characterize naturally occurring deposition of mercury under pristine conditions before European settlement began ca. 1830. Mercury deposition progressively increased during the nineteenth and twentieth centuries, with about one-half of the total mercury load deposited from 1940 to 1970 and the peak accumulation rate of 357 kg/yr identified during the 1960s. As a result of decreasing the discharges of mercury from municipal and industrial wastewater plants, the deposition rate in Lake Pepin has declined by almost 70 percent from the maximum loading during the 1960s to 110 kg/yr during 1990-1996. Although the investment in water pollution control has been very successful in reducing mercury in the Upper Mississippi River, ambient levels of mercury are still 30 times greater than the pristine conditions of the early 1800s (MCES, 2000). As of the late 1990s, the MCES is actively working to monitor and reduce even further the remaining sources of heavy metals, including mercury discharges to the river by wastewater treatment plants (MCES, 2000).

During the 1950s and 1960s, the depletion of dissolved oxygen in Pool 2 of the Upper Mississippi River near St. Paul adversely affected pollution-intolerant fish and other aquatic organisms. Studies during the early 1960s, for example, documented that burrowing mayflies (Hexagenia), an aquatic organism that is very sensitive to low DO conditions, were very scarce or absent from Pools 2 and 3 and Lake Pepin (Pool 4) of the river (Fremling, 1964). With the restoration of healthy levels of dissolved oxygen beginning in the mid-1980s, an abundance of mayflies once again col

2000. 19801 19601 1940: 19201 19001 1880: 18601 18401 1820] 18001

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Mercury Accumulation (kg/yr)

Figure 12-7 Historical mercury loading rates in the Upper Mississippi River reconstructed from sediments of Lake Pepin. Sediment core data averaged at 10-year intervals from 18001810 through 1980-1989 and 1990-1996. Source: Reprinted with permission from S. J. Balogh, D. E. Engstrom, J. E. Almendinger, M. L. Meyer, and D. K. Johnson, Environ. Sci. Technol. 33(19): 3297-3302, Copyright © 1999, American Chemical Society.

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Mercury Accumulation (kg/yr)

Figure 12-7 Historical mercury loading rates in the Upper Mississippi River reconstructed from sediments of Lake Pepin. Sediment core data averaged at 10-year intervals from 18001810 through 1980-1989 and 1990-1996. Source: Reprinted with permission from S. J. Balogh, D. E. Engstrom, J. E. Almendinger, M. L. Meyer, and D. K. Johnson, Environ. Sci. Technol. 33(19): 3297-3302, Copyright © 1999, American Chemical Society.

onized suitable habitats in the Upper Mississippi River from St. Paul to Lake Pepin after a 30-year absence from the river (Fremling and Johnson, 1990; MDNR, 1988). The resurgence of mayflies, significant improvements in ambient levels of DO and fecal coliform bacteria in Pool 2, and the reduction of mercury loading to the sediments of Lake Pepin demonstrate the successes of the water pollution control efforts implemented beginning in the 1980s. The Metro plant was upgraded to advanced secondary treatment with nitrification in 1984; the industrial pretreatment program was begun in 1982; and the accelerated CSO separation project, initiated in 1985 to jumpstart an ongoing sewer separation project, was completed in 1995.

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