Contamination of ground water from point and nonpoint sources (such as landfills, feedlots, agricultural chemicals applied to fields, and septic systems) is a recognized problem in the karst area of southeastern Minnesota. The migration of contaminants to and through the aquifers in the karst area is difficult to predict because of the complicated nature of the groundwater flow system. Estimation of localized groundwater-flow directions on the basis of water-level measurements made in wells may be inaccurate because of the influence of bedrock fractures. The orientation and density of bedrock fractures and the relation of these features to localized groundwater flow is poorly understood in southeastern Minnesota.

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This report documents regional ground-waterflow models constructed by the U.S. Geological Survey in cooperation with the Minnesota Department of Health (MDH) to satisfy the requirements of their Source Water Protection Plan (SWPP). Steady-state single-layer groundwater flow models were constructed with the computer program MODFLOW to simulate flow in surficial sand and gravel aquifers along the Mississippi River between Brainerd and St. Cloud in central Minnesota. The hydrogeologic data that were used to construct the models were compiled from available sources. Calibrated values of horizontal hydraulic conductivity and areal recharge for the aquifer in a northern model area were 70 m/d and 3.0x10-4 m/d, respectively. This model was sensitive to net areal recharge, vertical hydraulic conductivity of perennial streambed sediments, and horizontal hydraulic conductivity. The major source of net inflow to the model was from edge boundary cells. The major source of net outflow was groundwater discharge to perennial and ephemeral streams. Calibrated values of horizontal hydraulic conductivity and areal recharge for the aquifer in a southern model area were 70 m/d and 6.0x10-4 m/d, respectively. This model was sensitive mostly to horizontal hydraulic conductivity. Net areal recharge and groundwater discharge to perennial streams were the major sources of net inflow and outflow, respectively.

Recharge to unconfined aquifers in the seven-county Twin Cities metropolitan area of Minnesota was estimated by five methods. Mean recharge estimated as a percentage (27 percent) of precipitation ranged from 7.7 to 8.3 in./yr across the study area. The median recharge estimated from automated analyses of streamflow-recession displacements for seven basins ranged from 1.2 to 12.2 in./yr. Median recharge estimated from graphical analyses of groundwater level fluctuations for 11 wells ranged from 4.5 to 13.6 in./yr. Recharge estimated by age dating of shallow ground water at a nested well site was 8.8 in./yr. Minimal groundwater recharge estimated by statistical analyses of watershed characteristics (geologic data and streamflow records) ranged from less than 0.1 to 5.6 in./yr. Leakage, considered recharge to confined aquifers, in the seven-county Twin Cities metropolitan area was estimated by two methods. Leakage estimated by analyses of groundwater level fluctuations for 11 wells ranged from 3.2x10-3 to 1.1x10-2 in./yr. Leakage estimated by analyses of vertical-hydraulic gradients based on application of the Darcy flow equation for seven nested well sites ranged from 4.6x10-5 to 1.1x10-1 in./yr. Recharge estimates, which generally varied within 10 in./yr for each of the methods, generally were largest based on the precipitation, groundwater level fluctuation, and age dating of shallow ground water methods, slightly smaller based on the streamflow-recession displacement method, and smallest based on the watershed characteristics method. Leakage, which was less than 1 in./yr, varied within 1 order of magnitude based on the groundwater level fluctuation method and as much as 4 orders of magnitude based on analyses of vertical-hydraulic gradients.

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