Configuration of water table and distribution of downward leakage to the Prairie du Chien-Jordan Aquifer in the Minneapolis-Saint Paul metropolitan area, Minnesota

Abstract

The configuration of the water table as plotted at a contour interval of 20 feet (6 metres) on quadrangle maps (scale 1:2,500) of the Minneapolis-Saint Paul metropolitan area. Control points used for mapping were water levels in wells, lakes and sloughs, and places where topographic contours cross perennial streams. A computer program, using a variation of Darcy's law, was developed to determine distribution of 1) downward leakage to the Prairie du Chien-Jordan aquifer under steady-state conditions, using estimated vertical-hydraulic conductivity values for overlying materials; 2) calculated vertical hydraulic conductivity values, assuming uniform leakage to the aquifer; and 3) additional leakage to the aquifer resulting from increased pumpage during the summer. For data determination and data input to the computer program, the area was gridded into units of 1-minute longitude by 1-minute latitude, about 600 acres (243 hectares) per unit. Previous work estimated the increased summer pumpage (1971) of ground water to be 127 million gallons (481x106 litres) per day. Calculations, made Within the limits of governing assumptions, indicate that 10 to 20 percent of increased summer pumpage is derived from increased leakage. Most of the remainder is probably from captured natural discharge and induced recharge from major streams within the influence of summer cones of depression. Based on available data and estimates of vertical hydraulic conductivity for geologic units, major leakage to the Prairie du Chien-Jordan aquifer is indicated to occur in formation subcrop areas, especially where these areas are. overlain by the most permeable glacial drift.