Ground water is the major source of water supply in Minnesota. The quantity, quality, and availability vary greatly throughout the State. This study describes the State's groundwater resources as found in each of seven major drainage basins. Water is obtained from Quaternary glacial deposits and bedrock aquifers. Most supplies are from the drift except in the southeastern and extreme northeastern parts of the State. In the southeastern part, large quantities of water are withdrawn from sedimentary bedrock aquifers. In the northeastern part, Precambrian igneous and metamorphic rocks underlie the drift and groundwater availability is poor. Large quantities of water are available from surficial outwash, particularly in the central part of the State. Buried outwash aquifers are difficult to locate, delineate, and evaluate. Accordingly, few have been mapped. Water quality in much of the State is suitable for most uses. Ground water is typically hard and high in iron, Water from deep drift and bedrock in the western part of the State is highly mineralized. It may contain excessive amounts of some constituents, such as sodium, chloride, sulfate, boron, and iron. Most groundwater use is centered around cities, the largest amounts being withdrawn in the Minneapolis-St. Paul metropolitan area. Increasing amounts are being used for irrigation, most being obtained from shallow wells in the surficial outwash deposits. Although Minnesota is generally rich in groundwater resources, it is not without associated problems. In the western part of the State, groundwater quality is often a problem, especially in deep aquifers. Throughout the State, few buried outwash aquifers have been delineated or evaluated as to their water-yielding capabilities. Some aquifers are highly susceptible to pollution. Planned development and monitoring of water levels and water quality would be beneficial.

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The water resources were studied within an area whose natural ground-water flow is largely towards the center of the metropolitan area. This area coincides with the extent of the Hinckley Sandstone aquifer. Thus, the general geohydrology of the area bounded by the extent of the Hinckley Sandstone (about 6,000 square miles) as it relates to the hydrology of the Minneapolis-St. Paul metropolitan area is described. Greater emphasis is placed on the area underlain by the Prairie du Chien-Jordan aquifer (about 2,000 square miles), from which approximately 75 percent of the ground-water for the metropolitan area is pumped.The study indicates that the surface-water resources of the Twin Cities metropolitan area are used to such an extent that a supply adequate for domestic and industrial needs as well as power plant and sanitary effluent assimilation will not be available during severe drought.Ground-water is obtained primarily from two aquifer systems: The Prairie du Chien-Jordan and the Mount Simon-Hinckley. In 1970, these aquifers supplied about 90 percent (175 mgd) of the ground-water used in the metropolitan part of the study area. The probable level of development that can be sustained by these two aquifers in the metropolitan area is estimated to be 1,100 mgd; thus, substantial additional ground-water supplies could be developed. However, considerable management and planning would be needed to sustain this level of development.Maps in this report can be used to select general well-field locations based on consideration of 1) aquifer, 2) depth needed for completion, 3) head availability, 4) location of natural recharge and discharge boundaries, and 5) distance from areas where over-development of ground-water resources is imminent. Because of complexities in the ground-water system, yield estimates, boundary effects, and effects of aquifer interaction may best be determined in a study incorporating the use of a hydrologic model.Future detailed studies might include elaboration on some of the topics described in this report and the acquisition and interpretation of new data. Major items on which future work might focus are 1) data collection, 2) geohydrologic mapping, 3) hydraulic characteristics of subsurface geohydrologic units, 4) hydrology of lakes, and 5) hydrologic systems modeling.Potential evapotranspiration, PE, and actual evapo• transpiration, AE, highly dependent on climate, are

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