Nitrate contamination in southeast Minnesota’s karst landscape: impacts of land use and precipitation

Abstract

Minnesota’s karst landscape is highly susceptible to elevated concentrations of nitrate due to land use practices and precipitation patterns. Minnesota’s karst landscape covers the southeast portion of the state that is drained by the Root and Zumbro River Watersheds and is home to Niagara Cave. Karst topography is characterized by porous limestone and sandstone which promotes the interaction between surface and groundwater through landscape features such as sinkholes, springs, and caves. However, this rapid water movement makes water resources of the region vulnerable to contamination from non-point sources like fertilizers used on dominant crops such as corn [Zea mays L.] and soybean [Glycine max (L.) Merr.]. Therefore, to understand the interaction between land use and fertilizers, I compiled data from the Minnesota Caving Club, the National Water Quality Portal, CropScape, and the Minnesota Department of Natural Resources. Nitrate concentrations were analyzed temporally using scatter plots and linear regression. I mapped the data using ArcGIS Pro to evaluate spatial patterns. There was little variation in nitrate concentration in surface water over time, with 24.8% of samples exceeding the health risk limit of 10 mg/L. Concentrations of nitrate in groundwater samples were more consistently above 10 mg/L (50.7% of samples) and exhibited larger seasonal variability. Concentrations often decreased after large rainfall events, such as the 2019 flood year, and increased during years with lower precipitation. On average, 47.8% of land use in the study area is classified as either in corn or soybean production. My results highlight karst landscape’s sensitivity to surface inputs and precipitation patterns, thus emphasizing the importance of expansion of subsurface monitoring and agricultural best management practices.