MINNESOTA'S EXTENSIVE MONITORING OF URBAN CONTAMINANTS IN GROUNDWATER

Minnesota has one of the most robust groundwater quality monitoring systems in the U.S, and the Minnesota Pollution Control Agency (MPCA) participates in this system by sampling a large network of 270 wells for an extensive list of contaminants. The network is mainly located in urban settings, such as residential and commercial/industrial areas. The network wells are sampled annually and are analyzed for routine constituents including nitrate, chloride, sulfate, and trace elements. The sampling frequency allows the MPCA to quantify changes in groundwater quality. Additionally, a large suite of organic constituents are measured in this network including volatile organic compounds, per- and polyfluoroalkyl substances (PFAS), and over 100 other contaminants of emerging concern such as prescription and non-prescription medications, detergent breakdown products, and plasticizers.

This important dataset reveals how the distribution of contaminants varies in the groundwater beneath Minnesota’s urban areas. The highest chloride concentrations and most frequent detections of the solvents tetrachloroethylene and trichloroethylene generally were in the shallow groundwater underlying commercial/industrial areas in the Twin Cities Metropolitan Area, as well as in other cities. Examination of chloride/bromide ratios indicated the most likely chloride sources in commercial/industrial areas were deicing chemicals or water softener salt. Forty percent of the wells analyzed for chloride trends had a significant upward trend in chloride concentrations; most of these were in deep bedrock wells. Chloroform was the most-frequently detected volatile organic compound; the occurrence of this chemical likely is related to the use of water supplies that likely were disinfected using chlorine-containing chemicals. Periodic PFAS sampling shows that these chemicals are detected more frequently in the groundwater underlying urban areas compared to undeveloped forested lands. Short-chain PFAS, such as perfluorobutanoic acid, are the most-commonly detected chemicals in the ambient groundwater.

This information is key to keeping the state’s water clean. Knowing the settings that contribute the most contamination along with their trends helps target efforts to reduce groundwater pollution.