OFR14-05, A HYDROCHEMICAL SURVEY OF GROUNDWATER FLOW IN THE ROCHESTER METROPOLITAN AREA, MINNESOTA

Abstract

Historical chemical and isotopic data from Olmsted County, Minnesota were used to distinguish groundwater types based on similar chemical and isotopic composition. The extent of recent waters, identified by detectible tritium, chloride, nitrate or sulfate concentrations above background levels, along with groundwaters having elevated calcium to magnesium molar ratios were mapped in three dimensions. The distribution of these waters can be explained, in part, by the permeability of unconsolidated sediments overlying bedrock, bedrock hydrostratigraphy, and vertical hydraulic gradients within the Rochester Central Metropolitan Area (RCMA) dueto high capacity pumping. The spatial distribution of groundwater chemical types is also a function of changes in vertical hydraulic gradients with time. Within the last 20 years, the extent of recent waters within the RCMA has expanded both horizontally and vertically. Groundwater calcium to magnesium ratios in the Prairie du Chien Group (Shakopee aquifer) and the Jordan Sandstone (Jordan aquifer) within the RCMA have also increased and show greater variability through time, indicating a greater percentage of recharge to these aquifers moving vertically within the RCMA and from the Decorah edge than before high-capacity pumping began.