THE SENTINEL LAKES PAIRED GROUNDWATER/SURFACE WATER NETWORK'S INVESTIGATION OF THE MOVEMENT OF PHOSPHORUS AND CHLORIDE

Since 2013, the Minnesota Pollution Control Agency has investigated groundwater-surface water interactions using a network of shallow monitoring wells installed on the shores of lakes enrolled in a long-term study. Paired monthly groundwater and surface water samples were collected and analyzed from eight wells and four lakes, for the last seven years. Downhole instruments were placed in the wells to collect continuous records of ambient air temperature and barometric pressure, groundwater elevation and water temperature, and conductivity. This has created a dataset of groundwater and surface water information that is unparalleled for its breadth and length of sampling in the state, while providing insights into the movement of compounds of interest between groundwater and surface water in counties dominated by agriculture.

Groundwater and surface water interactions that were uncovered included the strong control groundwater exerted over surface water chloride levels at Lake Shaokatan. Chloride concentrations in groundwater and the lake were highly correlated (R²= 0.94). Phosphorus concentrations in groundwater were correlated with lake concentrations until 2018 when lake concentrations rose, likely related to the observed dramatic decrease in aquatic vegetation density, and an increase in lake-wide algal blooms in 2019. At Madison Lake, the flow gradient was downward through the lake bottom, moving through groundwater to discharge into the Minnesota River valley. Phosphorus levels in the lake appeared to be driving concentrations in both the shallow and deep groundwater wells, suggesting this lake was a source of phosphorus to the Minnesota River.

Phosphorus and chloride have become growing threats to the quality of the state’s water resources. Tracking their transport from land use into groundwater and surface water resources will become critical in understanding the source of the risk, and the environmental conditions that allow its spread.