New MPCA Report Describes Minnesota’s Groundwater Quality

Man-made chemicals, like the polyfluorinated alkyl substances (PFAS) and volatile organic chemicals (VOCs), have also polluted the groundwater in many parts of the state, such as the 100-square mile PFAS contamination plume in the in the eastern part of the Twin Cities Metropolitan Area (TCMA)². The current status and trends in these and other chemicals in the state’s groundwater recently was described in a report “The Condition of Minnesota’s Groundwater Quality, 2013-2017” released by the MPCA. This article shares some of the highlights from the investigation.

This study focused on water quality conditions and trends in aquifers that are vulnerable to human-caused contamination. Familiar groundwater pollutants like nitrate, chloride, and VOCs were discussed as well as newly recognized ones like PFAS and contaminants of emerging concern (CECs).

Several information sources were used to provide a comprehensive assessment of the state’s water quality. This approach was necessary because groundwater quality monitoring in Minnesota is a coordinated effort among several state agencies. One excellent example of this cooperation between the state agencies is the ambient groundwater monitoring. Minnesota state law splits these responsibilities between the MPCA and MDA—the MPCA monitors for non-agricultural chemicals in the groundwater and the MDA conducts similar monitoring for pesticides and fertilizers. In this report, data from the MPCA’s Ambient Groundwater Monitoring Network and the MDA’s ambient groundwater monitoring were used to describe water-quality conditions and recent trends near the water table. Water-quality conditions in the aquifers used to obtain drinking water were described using information collected by the MPCA as well as published reports or data from the MDA’s private well monitoring networks³, MDA’s Township Testing Program⁴, and the DNR’s County Atlas Program⁵.

The installation of new monitoring wells for the MPCA’s network enhanced the understanding of groundwater contamination in urban areas in Minnesota. The MPCA’s network mainly is comprised of shallow monitoring wells which intersect the water table but also includes some deep wells. The shallow wells act as an “early warning system” which allows the agency to understand what chemicals can readily be transported to the groundwater as well as discern the effect land use has on groundwater quality and quickly identify any emerging trends. From about 2011-2015, the MPCA added approximately 150 new wells to its network to fill identified monitoring gaps. This included installing wells which represent ambient conditions in commercial/industrial areas and improving the network’s coverage in residential areas that rely on subsurface sewage treatment systems (SSTS) for wastewater disposal and treatment.

These monitoring network improvements refined our knowledge on the distribution of nitrate and chloride in the shallow groundwater underlying urban areas throughout the state. Nitrate concentrations rarely exceeded state standards in urban parts of the state– less than five percent of the sampled wells that intersected the water table had concentrations exceeding the state class 1 domestic consumption use standard of 10 mg/L as nitrogen (Minn. R. ch. 7050, 7060)⁶. The median nitrate concentrations in the shallow groundwater underlying the monitored urban land use settings ranged from 1.1 to 1.8 mg/L as nitrogen, with the highest median concentration in the shallow groundwater underlying residential areas served by centralized wastewater treatment systems. In contrast, most wells with high chloride concentrations were located in urban areas, especially the 11-county TCMA. Chloride concentrations also had very distinct variations with urban land use. The highest median concentration, 81.9 mg/L, was in the shallow groundwater underlying commercial/industrial areas. Chloride concentrations in these areas were almost twice as high as those in residential areas where the median concentrations ranged from 44.6 mg/L in areas using centralized wastewater treatment to 16.1 mg/L in areas using SSTS.

Deicing chemicals or water softener salt likely was the source of the majority of the high chloride concentrations in commercial/industrial areas and residential areas. Chloride/bromide (Cl/Br) ratios were computed to determine the potential chloride sources in the state’s groundwater. Various chloride sources can be distinguished using Cl/Br ratios because chloride is typically 40-8000 times more abundant in groundwater. As a result, small differences in bromide concentrations yield vastly different ratios. Almost three-quarters of the wells used to sample the shallow groundwater underlying commercial/industrial areas of Minnesota had a Cl/Br ratio greater than 1,000 which indicted the chloride source was halite (also known as rock salt). Salt in this form usually is applied as a deicing chemical or to regenerate the resins in softeners that remove calcium and magnesium from the water. Halite was also an important chloride source to the shallow groundwater underlying the state’s residential areas, with 51 to 62% percent of the wells having Cl/Br ratios consistent with a halite source.

Increasing chloride concentrations in the state’s bedrock aquifers indicated that the high concentrations found near in the unconsolidated, water table aquifers were slowly seeping into the deep aquifers used to obtain drinking water. Chloride trends in the bedrock aquifers were largely untested until now because insufficient data had been collected from most of the deep wells to perform the analyses. Overall, 40 percent of the tested wells had increasing chloride concentrations from 2005-2017. A greater percentage of the wells with increasing trends were installed in bedrock aquifers as opposed to near the water table. This suggested that chloride inputs to the land surface have stabilized but a pulse of high chloride concentrations from past years was slowly making its way down to the deep bedrock aquifers. Almost 70 percent of the bedrock aquifer wells tested had increasing chloride concentrations. The percentage of unconsolidated water table wells with increasing trends was much smaller—22 percent. Most of the bedrock aquifer wells with increasing trends were installed in the Prairie du Chien aquifer, and the remainder were installed in the Galena and St. Peter aquifers.

Monitoring conducted as part of the MDA’s Township Testing Program showed the townships with the largest percentage of drinking water wells with nitrate concentrations exceeding the state class 1 standard tend to be located in the southeastern Minnesota. Result maps produced by this program classified townships most impacted by nitrate contamination as having at least 10% of the tested wells with concentrations equal to or exceeding the state class 1 standard of 10 mg/L as nitrogen. The majority of these townships were located in southeastern Minnesota.

VOCs were not detected very frequently in the ambient groundwater. From 2013-2017, the MPCA tested 275 ambient network wells for these chemicals. The percentage of the sampled wells with detectable VOC concentrations ranged from 5% in 2015 to 8% in 2013 and 2014. When VOCs were detected, they usually were found in monitoring wells screened near the water table and the measured concentrations were typically low (less than 1 ug/L).

The most frequently detected VOCs were the disinfection byproduct, chloroform, and the solvents tetrachloroethylene (PERC) and trichloroethylene (TCE). Chloroform detections generally were sporadic. In the majority of the wells with detections, chloroform was only detected once from 2013-2017. The use of disinfected public water and its eventual recharge into the groundwater was the likely source of the chloroform found in the ambient groundwater. The one common feature among all of the wells with any chloroform detections from 2013-2017 was that they were located in areas served by municipal water-supply systems that disinfect their water using chlorine or chloramines TCE, a solvent whose major use is to degrease metal parts, was detected in five wells from 2013-2017. TCE mostly was detected in shallow monitoring wells that were located near or within commercial/industrial areas. The highest TCE concentrations were measured in the two monitoring wells in St. Paul that were located a few hundred feet apart. In these wells, concentrations as high as 46 ug/L were reported. These two wells also had PERC detected in them.

PFAS were another set of chemicals that predominantly were found in the groundwater underlying urban areas. A 2013 MPCA study, which targeted wells in areas naturally vulnerable to contamination, found at least one of these chemicals in almost 70% of the tested wells. Perfluorooctanic acid (PFOA) was detected in about 30% of the tested wells and perfluorosulfonic acid (PFOS) was detected in about 12% of them. Concentrations in about five percent or less of the wells exceeded the PFOA HBV of 35 ng/L set by MDH in 2017 and the PFOS HBV set by MDH in 2019. Some of the wells with exceedances of the PFOA and PFOS HBVs were located in the vicinity of the known industrial contamination from the 3M Company in Washington County, and the others were shallow wells located in other parts of the TCMA and cities in outstate Minnesota.

in almost 70% of the tested wells. Perfluorooctanic acid (PFOA) was detected in about 30% of the tested wells and perfluorosulfonic acid (PFOS) was detected in about 12% of them. Concentrations in about five percent or less of the wells exceeded the PFOA HBV of 35 ng/L set by MDH in 2017 and the PFOS HBV set by DH in 2019. Some of the wells with exceedances of the PFOA and PFOS HBVs were located in the vicinity of the known industrial contamination from the 3M Company in Washington County, and the others were shallow wells located in other parts of the TCMA and cities in outstate Minnesota.

A very limited resampling of some of the wells with PFAS detections was conducted in 2017 and indicated that concentrations of the measured PFAS declined in most of the wells sampled outside of Washington County. Figure 2 illustrates the extreme variability in PFAS detections and concentrations measured in some shallow monitoring wells outside of the eastern TCMA. The PFAS decreases measured in 2017 may have been due to changes in the types used in products over the last 10 years. For example, the production of PFOA and PFOS was reduced in the U.S. and other countries as part of a global stewardship program. New PFAS were developed to replace these chemicals in products, and one caveat of this study is that the replacement PFAS chemicals were not measured in either the 2013 and 2017 ambient groundwater studies. 

The full report is available on the MPCA’s website at: https://www.pca.state.mn.us/water/groundwater-data

References

1. Steil, M. Minn. moves to reduce nitrates in groundwater; environmental groups wonder, wait. (accessed 3/7/2018).
2. Marcotty, J., 3M Settles groundwater lawsuit for $850 million. StarTribune 2/20/2018.
3. (a) Minnesota Department of Agriculture Characterizing nitrate in private drinking water wells, (accessed 8/7/2018); (b) Minnesota Department of Agriculture Private Well Pesticide Sampling Project. (accessed 8/7/2019).
4. Minnesota Department of Agriculture Township Testing Program.(accessed 8/7/2019).
5. Minnesota Department of Natural Resources County Groundwater Atlas Program. (accessed 8/7/2019).
6. Minnesota Legislature Minnesota Administrative Rules. (accessed 8/7/2019)