Combining high resolution spring monitoring, dye tracing, watershed analysis, and outcrop and borehole observations to characterize the Galena Karst, Southeast Minnesota, USA

A combination of spring monitoring and springshed delineation using dye tracing and hydrograph separation methods has elucidated aquifer characteristics of the Ordovician Galena Group in southeast Minnesota. The Galena Group is primarily composed of carbonate rock with subordinate beds of fine siliciclastics, mainly shale. It underlies eleven counties in southeastern Minnesota and is used as a source of water for domestic and agricultural use. In two counties, Fillmore and Olmsted, it is a thinly covered well-developed karst that supplies substantial baseflow to streams. Continuous spring monitoring is being conducted to measure aquifer properties and recharge response following precipitation and snowmelt events. Data are also being used to characterize sources, fate, and transport of nitrate-nitrogen and other pollutants within springsheds. Dye tracing and hydrograph analysis are utilized to determine groundwater flow direction and time of travel and to assist in determining the size and areal extent of the springsheds that supply perennial discharge to springs. Discharge, water temperature, and nitrate concentration are the emphasis of monitoring at several of the springs where data are paired with additional small watershed-scale hydrologic studies conducted at edge of field and small watershed in-stream locations. This allows an estimate of annual nitrate loading and assessment of agricultural pollutant sources and agricultural best management practice effectiveness. Base flow temperatures of the monitored Galena springs are variable, ranging from annual sinusoidal temperature cycles that are out-of-phase with average air temperature to nearly constant temperature interrupted by event-scale variations caused by snowmelt and precipitation events. Recharge events at each of the springs show characteristic multi-fold increases in discharge with rapid initial dilution of nitrate-nitrogen consistent with surface water runoff into sinkholes rapidly emerging via conduits at the springs. Following these dilution events, mobilization of nitrate-nitrogen in the soil and in aquifer storage in the Bear Spring Springshed frequently increases nitrate-nitrogen concentrations to above pre-event levels.