Water resources of the Minneapolis-St. Paul area, Minnesota

Abstract

The water supply of the Minneapolis-St. Paul area is adequate to satisfy present requirements and requirements for many years to come if the area continues to develop at about the present rate. The flow of -the Mississippi River at the Twin Cities is more than sufficient to meet the demands of the water-supply systems of Minneapolis and St. Paul. The lowest momentary flow during the period 1931-51 was more than twice the present combined maximum demand of Minneapolis and St. Paul. The lake storage of the St. Paul system combined with possible regulations by the Mississippi River headwater reservoir system, in case of an emergency, provides a reserve supply ample to meet a greatly expanded demand. The lowest average daily flow of the Mississippi River at the intakes of the Minneapolis and St. Paul water supply was 389 mgd (602 cfs), The flow at the water supply intakes has been less than 452 mgd (700 cfs) for not more than 6 consecutive days. Except for the Mississippi River, the streams in the Twin Cities area have not been extensively developed for water supply. The only known use of them for water supply is for the steam-electric. generating plant on the Minnesota River at Savage. Thus, the St. Croix River, within 12 miles on the east, the Minnesota River entering the Twin Cities from the southwest, the Vermilion within 12 miles on the south, and the Crow River within 25 miles on the west offer untapped supplies for industrial and municipal uses. Many water-bearing formations occur in the area. A blanket of glacial deposits, as much as 400 feet thick, covers the area. Small domestic ground-water supplies can be developed practically everywhere in the glacial deposits, and larger industrial supplies can be obtained by exploring and testing. Below the glacial materials is a thick series of rock formations including several prolific sandstone aquifers. The formations dip toward the center of the area forming an artesian basin. The estimated average daily withdrawal of ground water from all aquifers in the area is about 90 mgd. Practically all the communities that are not supplied by the Minneapolis or St. Paul water-supply systems obtain their water from wells. Where many large-capacity wells have been concentrated in relatively small areas, there has been a great lowering of artesian pressures. However, there are large areas, distant from the centers of concentrated pumping, which are favorable for the development of additional ground water. With an adequate program of exploration and testing to determine precisely the geologic and hydrologic characteristics of the waterbearing formations, it is likely that large additional supplies of ground water can be developed for municipal and industrial uses. Both Minneapolis and St. Paul obtain their municipal water supplies from the Mississippi River above the TwinCities and are thus assured of a large supply that is not subject to contamination by industrial wastes and sewage effluents, Treatment at municipal plants for both cities provides water for diversified industrial use and for domestic use that meets U. S. Public Health Service drinking water standards., The treated water is remarkably uniform in chemical composition throughout the year and is virtually free of all color, iron, manganese, and turbidity. Currently, (1952). the two supplies are softened to about 75 ppm (as CaC03), which is an average reduction of about 55 percent in hardness of river water. The dissolvedsolids content of the treated water for St. Paul currently (1952) averages about 100 ppm; the dissolved-solids content of the Minneapolis water is slightly higher. As a matter of further interest to industrial consumers, temperatures of the untreated river water, which is only slightly altered at the Minneapolis treatment plant, averages less than 60 F for about 8 months of the year and is less than 40 F for 4 winter months. The Mississippi River as it enters the Twin Cities is moderately mineralized, averaging 241 ppm dissolved solids and 179 ppm hardness during the period 1940-49, Average turbidity is very low and silica is moderately low, but the quantities of iron and color in solution are relatively high. Color increases markedly during the period March to July in response to an increase in streamflow. The average chemical composition of the water has remained virtually unchanged except for seasonal variations since 1907. Data collected by the Minneapolis-St. Paul Sanitary District have shown improved sanitary conditions of the river at the Twin Cities lock and dam since the sewage plant went into operation in 1939. The Minnesota River is more than twice as mineralized and hard as the Mississippi River, and it exerts a noticeable effect on the chemical and sanitary quality of the Mississippi River at St. Paul. Other principal tributary streams to the Mississippi River, including Crow River, Vermilion River, and Bassett Creek, were sampled during the 1952 flood season, at which time they were of the calcium-bicarbonate type, more dilute, and of lower hardness than the Minnesota River. Lake waters in the Twin Cities area generally are less mineralized than those of the streams. Waters from the drift deposits and bedrock formations overlying the Hinckley sandstone are hard and calcareous and generally contain troublesome quantities of iron. Regular treatment is required of some public-supply wells for removal of iron encrustations. Water fr.om these sources generally exceeds 300 ppm hardness, but in some places the St. Peter sandstone and St. Lawrence formation yield water of better quality. The Hinckley sandstone yields the best quality ground-water because of its comparatively lower hardness and uniform temperature (about 52 F). However, the average hardness of the treated municipal supplies of St. Paul and Minneapolis is considerably less than water from the Hinckley.