As with many other urban areas of the United States, the Upper Mississippi River was grossly polluted early in the twentieth century because of growing urban populations and inadequately treated municipal and industrial wastewater discharges. Municipal officials simply relied on the natural flushing of rivers to dilute the human and industrial waste products of the growing metropolitan areas. City sewers, first constructed in 1871 in the Twin Cities, collected stormwater and sewage and discharged them directly into the river. By the early 1900s, the Upper Mississippi River was unable to biologically assimilate the untreated wastewater collected from the Twin Cities (MWCC, 1988).
Before construction of a lock and dam in Minneapolis in 1917, annual peak spring flows maintained a minimally acceptable degree of water quality by the physical removal of raw sewage and other waste materials accumulated during the previous year in the Twin Cities area. Construction of the lock and dam, however, drastically altered this natural cycle by slowing the current of the river and reducing the flushing effect of the peak spring flows. By 1920, 3 million cubic yards of sewage sludge had accumulated in the pool created by the lock and dam. Water quality was severely degraded by depletion of dissolved oxygen from decomposition of the sludge bed. Bacteria levels were extremely high, sewage sludge mats floated on the surface, and the river was noxious from hydrogen sulfide gas caused by septic conditions during the warm summer months. The Upper Mississippi River was grossly polluted for a distance of 30 miles from St. Anthony's Falls in Minneapolis to the St. Croix River at Prescott, Wisconsin (MWCC, 1988).
A 1928 joint report by the Minnesota and Wisconsin State Boards of Health stated that "a zone of heavy pollution extends from Minneapolis to the mouth of the St. Croix." The state report pronounced "the river in this zone... unfit for use as a water supply . . . fish life has been exterminated." The report stated that the river was "a potential danger from a health standpoint." Beginning with a river survey in 1926, the State Board of Health documented DO levels of less than 1 mg/L over a 25-mile reach from St. Paul to Hastings, Minnesota, that could not support a healthy aquatic ecosystem, including pollution-tolerant carp (Mockavak, 1990). From 1926 to 1937, minimum DO levels of 1 to 2 mg/L indicated less than 10 percent of oxygen saturation over a 20- to 25-mile reach downstream from St. Paul (Wolman, 1971). Bacteria levels were also extremely high, with total coliform concentrations of 105 to 106 MPN/100 mL measured downstream of St. Paul (MRI, 1976). The extent of the public health risk incurred from the discharge of raw sewage by the Twin Cities was made painfully clear in 1935, when a failure of the chlorination units at the public water supply plant resulted in a serious typhoid epidemic with 213 cases and 7 deaths (USPHS, 1953).
In adopting the 1928 Board of Health recommendations, the Twin Cities became the first major city on the Mississippi River to implement primary treatment and chlorination for its municipal water pollution control plant in 1938. Water quality quickly improved dramatically as the floating mats of sludge disappeared, and DO levels increased to better than 3 mg/L from 1942 through 1955 (Mockovak, 1990; Wolman, 1971). Within 2 years, fish returned and anglers reported catching walleye and other game fish in parts of the river that had been devoid of game fish prior to 1938. Maurice Robbins, a former deputy administrator of the Metropolitan Waste Control Commission (MWCC), recalled that "The impact [of waste treatment] on the river was tremendous ... no more dead fish, no more sewage smell" (MWCC, 1988).
With increasing population (Figure 12-5), growth eventually overwhelmed the capacity of the river to assimilate the wastewater discharge from the primary Metro plant during the mid-1950s through the mid-1960s. Water quality once again deteriorated to conditions reminiscent of the 1920s and 1930s. During the summer of 1964, the Federal Water Pollution Control Administration (FWPCA) conducted a water pollution survey of the Upper Mississippi River that documented severe degradation of water quality (FWPCA, 1966). In contrast to an average of about 30,000 MPN/ 100 mL near St. Paul during the 1950s, total coliform densities ranged from 460,000 to 17,000,000 MPN/100 mL 9 miles downstream of St. Paul. Minimum DO levels of less than 1 mg/L were also recorded for 15 miles downstream of St. Paul. The biological health of the river abruptly changed, with a zone of degradation and decay ex tending 20 miles from St. Paul to Lock and Dam No. 2 at Hastings, Minnesota. The river bottom, thick with sewage sludge, was found to be devoid of the benthic organisms usually associated with clean waters (FWPCA, 1966; WRE, 1975).
In 1966, the Metro plant was upgraded to secondary treatment using the activated sludge process. Water quality once again improved, surpassing the 1928 guidelines. The rapidly growing suburban population, however, tended to generate more residual wasteload than could be removed by upgrading the plant to secondary treatment. Regardless of the Metro plant upgrades, annual high spring flows caused flooding of the plant, resulting in the discharge of raw sewage into the river. During the late 1960s, only 4 of the 33 suburban treatment plants provided adequate levels of treatment, thus contributing to the overall pollution loading of the river. Minneapolis and St. Paul further contributed to periodic pollution loading to the river through a network of combined stormwater and sewage collection sewers that discharged raw sewage during rainstorms.
In 1984, the Metro plant was upgraded once again to advanced secondary treatment with nitrification, designed to reduce effluent levels of ammonia. After implementation of secondary and advanced secondary waste treatment for the wastewater treatment plants of the Twin Cities area by the mid-1980s, water quality of the Upper Mississippi River routinely has been in compliance with water quality standards for dissolved oxygen and un-ionized ammonia. In contrast to the record of compliance for oxygen and un-ionized ammonia, turbidity levels have exceeded water quality objectives as a result of nonpoint source runoff of sediment from the Minnesota River basin (MWCC, 1994). Because the land uses of the Minnesota River basin are dominated by agricultural row crops and the fine-textured soils further contribute to sediment losses, the annual mean (1976-1996) sediment yield of 134 lb/acre-yr from the Minnesota River watershed is almost five times greater than the annual mean sediment yield of 28 lb/acre-yr estimated for the Upper Mississippi River basin upstream of Lock & Dam No. 1 (Meyer and Schellhaass, 1999). Fecal coliform levels also remained high, and often violated state water quality standards through the mid-1980s because of combined sewer overflows during rainstorms. Fecal coliform bacteria samples are in compliance with Minnesota water quality standards if the monthly geometric mean is less than 200 MPN/100 mL and any individual sample does not exceed 2,000 MPN/100 mL.
In 1984, it was estimated that 4.6 billion gallons per year of raw sewage and stormwater were discharged to the Upper Mississippi River. In response to this water quality problem and public pressure, the Twin Cities implemented an aggressive $320 million (1996 dollars) construction program from 1985 to 1995, intended to accelerate the completion of the ongoing project to separate the combined sewers (MCES, 1996). As a result of the separation of stormwater and raw sewage from the combined sewer system, fecal coliform bacteria levels have declined considerably, and compliance with state water quality standards has improved greatly at stations monitored at Lock and Dam No.1, St. Paul, Grey Cloud Island, and Pool 2 (Buttleman and Moore, 1999). Figure 12-6 shows the reduction in bacteria levels and the corresponding improvement in compliance with water quality standards. The monitoring station at St. Paul exhibits the greatest improvement, with compliance achieved at the
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