CHARACTERIZING GROUNDWATER FLOW IN THE TWIN CITIES METROPOLITAN AREA, MINNESOTA; A CHEMICAL AND HYDROSTRATIGRAPHIC APPROACH

Historic chemical and isotopic data for groundwater within the extended 11 county Twin Cities Metropolitan Area (TCMAx) were used to distinguish three regional hydrochemical types: 1) recent waters, characterized by detectable tritium, elevated chloride, and/or the presence of anthropogenic compounds; 2) waters with elevated strontium to calcium plus magnesium ratios; and 3) naturally elevated chloride distinct from recent waters. The three-dimensional distribution of these hydrochemical facies were compared to permeability of unconsolidated sediment, bedrock hydrostratigraphy, and the regional distribution of vertical hydraulic head gradient. This evaluation demonstrated that groundwaters within the TCMAx can be broadly categorized by chemical composition, and that their distribution is controlled both by regional differences in subsurface permeability and natural hydraulic head gradients, and by changes in vertical hydraulic gradient due to high-capacity pumping. Results are currently being used by the Metropolitan Council to guide conceptual models of recharge to bedrock aquifers in their regional groundwater model.