MODELING TRAVEL TIME DISTRIBUTIONS WITHIN KARST AND BEDROCK AQUIFERS SYSTEMS OF SOUTHEASTERN MINNESOTA

Thousands of samples taken from wells, streams and springs located in southeastern Minnesota show nitrate concentrations at levels that are concerning. The Minnesota Department of Agriculture has a groundwater protection plan that establishes a rule for managing nitrogen fertilizer applications to reduce the impact on groundwater resources and surface waters fed by groundwater. As a result of the established rule there are efforts to implement best management practices within agricultural production systems to reduce nitrate contamination of groundwater. A number of BMPs have been implemented during the past decade, and there is interest to evaluate whether those practices are having a measureable impact on nitrate concentrations in groundwater and groundwater fed surface waters. Nitrate data for samples taken from groundwater, surface streams and springs show a confounding picture as to whether the established BMPs are effective at reducing nitrate concentrations. It is thought that a main issue related to the lack of clear trends in the nitrate data is the residence time of water in the aquifer systems. Changing precipitation patterns might also have a large influence on nitrate concentrations. We hypothesize that mapping out the age of groundwater in the aquifers, and the time of travel of water through the aquifers and into streams or springs, will help with the interpretation of monitoring data. To test this hypothesis we are taking on a campaign to age-date groundwater, and model groundwater flows and travel times within impacted aquifers. The location of this work is within the MDA-sponsored Field to Stream Partnership program located within the South Branch of the Root River, where there is an intense effort to implement nitrogen BMPs and assess their benefits. The age-dating work is just beginning in spring of 2020 so results will not be available for the GSA conference. However, groundwater modeling has been initiated and this presentation will show the analysis of travel time calculations for one of the monitored areas. Through scenario analysis, we will demonstrate the potential impact of changing surface loadings of nitrogen on nitrate concentrations in groundwater and receiving streams/springs. We hope also to demonstrate the possible impact of changes in precipitation pattern on nitrate trends.