Characterizing groundwater flow in the Twin Cities metropolitan area, Minnesota a chemical and hydrostratigraphic approach.

Abstract

Historic chemical and isotopic data for groundwater within the 11 county Twin Cities Metropolitan Area, extended (TCMAx) were used to distinguish three regional groundwater types based on similar chemical and isotopic composition: 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 based on carbon-14 dating and low chloride to bromide ratios where sufficient data exists. The three-dimensional distribution of these hydrochemical facies were compared to permeability of unconsolidated sediments, the distribution of macropores within sedimentary (Paleozoic) bedrock, and the regional distribution of vertical hydraulic head gradient. Results of this investigation demonstrate 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 regional changes in hydraulic gradient due to high-capacity pumping. Chloride content and chloride to bromide ratios, in particular, can be used to identify the presence of recently recharged groundwater in bedrock aquifers and further characterize the movement of these recent waters through bedrock macropores. Urban groundwater systems present unique challenges for resource management and scientific investigations due in large part to the transient nature of hydraulic head gradients and changing landuse. For urban planners charged with groundwater resource management, results in this thesis demonstrate the utility of having groundwater hydrochemical types fully integrated with a hydrogeologic framework model in a three-dimensional geographic information system (GIS) environment, where age and chemical quality of groundwaters can be compared with other, more familiar factors, such as locations and pumping levels of high capacity wells. For groundwater modelers of urban aquifers, these same results can guide conceptual models of recharge to bedrock aquifers and constrain model calibration to produce flux estimates in agreement with flowpaths indicated by the distribution of recent waters.