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pumped to the Vadnais chain of lakes, north of St. Paul for storage prior to treatment) Under most conditions, Mississippi River supplies far exceed the water needed by communities that rely on the river. In times of drought or contamination, however, use of river water may be limited. The Minneapolis Water Works currently has no alternative water sources to the Mississippi River. This leaves its system vulnerable to events that may limit availability of supplies from the river. St. Paul Regional Water Services maintains a reserve to supply approximately 60 days of its `practical' water demand. It stores its reserve in the Rice Creek Chain of Lakes (Centerville Lake) and tributaries to Vadnais Lake, where the water can be withdrawn for use if Mississippi River supplies are limited. The St. Paul Regional Water Services is currently evaluating groundwater as a back-up supply (Metropolitan Council, 2007a). 3.2.1 Water Treatment and Contamination Surface water must be treated before distribution to remove water-borne contaminants, such as bacteria, viruses, parasites, and a variety of natural and man-made constituents. Formerly, both the Minneapolis Water Works and the St. Paul Regional Water Services used a lime softening process to treat raw water. Minneapolis has invested considerably to change over to nano-filtration technologies that a designed, in part, to remove newly emerging contaminants such as pharmaceuticals and hormones. Both the Minneapolis Water Works and the St. Paul Regional Water Services are believed to have considerable excess capacity to treat water, or can bring such capacity on line. Surface-water sources on the Mississippi River are vulnerable to release of upstream contaminants that may overwhelm the treatment system (e.g., an oil spill) and to low-flow conditions in the river that may make it impossible for the water works' intakes to keep up with minimum demands. Surface-water supplies are susceptible to both chronic contamination and accidental or intentional releases of contaminants from a variety of sources. Water-treatment plants may shut intakes to allow responders to mitigate an upstream release, with the duration of shut downs related to a number of factors, including the types of contaminants as well as the volumes and locations of the spills (Metropolitan Council, 2007a). The federal government requires all surface-water suppliers in the United States to include filtration and disinfection in their treatment processes as final barriers of defense against contaminants. 3.2.2 Low-Flows and Drought In response to the 1988 drought, the Metropolitan Council prepared ashort-term water-supply plan which establishes a critical flow of 554 cubic feet per second (cfs) in the Mississippi River at Anoka to supply municipal water systems, generate power, and allow navigation. These minimum flows P:\Mpls\23 MN\7l\2371105 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 26