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data about the quality of groundwater near homes or businesses or get an estimate of groundwater availability.
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GCC Report to the Legislature Groundwater Quality

Nearly three-quarters of us in Wisconsin rely on groundwater for our primary drinking water supply. Many schools, food processing and other businesses, hotels, taverns and restaurants are public water systems that need good groundwater quality. What are the trends in groundwater quality? Non–community public water systems are required to regularly test water for nitrate and report results to the DNR, which provides a unique opportunity to evaluate changes in groundwater nitrate concentrations over time.

Nitrate is Wisconsin's most widespread groundwater contaminant and studies show its correlation with other contaminants*, so trends in nitrate concentrations can be considered a proxy for the state of groundwater quality.

To create the map below, all non–community public drinking water wells were evaluated for statistically significant trends** in nitrate concentration over the past 10 years. The map depicts the percent of wells with an increasing trend minus the percent of wells with a decreasing trend (% difference = % increasing - % decreasing). Counties where a higher percentage of wells have increasing nitrate concentrations are shown in red, those with a higher percentage of wells with decreasing nitrate concentrations are shown in blue, and those with similar numbers of increasing and decreasing wells are shown in yellow. Statewide, the trend is slightly increasing, but is less than 2%. About 11% of non-community wells had nitrate above 5 mg/L in the last reported sample.


map legend

* Chowdry, S.H., A.E. Kehew, R.N. Passero. 2003. Correlation between nitrate contamination and ground water pollution potential. Ground Water, 41(6):735-45.

** Analysis of trends in groundwater nitrate concentrations — this analysis was based on work by Kevin Masarik and others in 2013 [PDF], but has been updated and revised slightly. Nitrate data from all Transient and Non–transient Non–community water systems in Wisconsin was compiled from DNR internal databases. Wells are eliminated from further analysis if there are no nitrate samples (DNR storet parameters 618, 620, 625, 630, or 631) or if the most recent sample was collected more than three years ago. For remaining wells, linear regression analysis is performed on all reported nitrate data through the last full year. If the rate of change is not statistically significant at the 95% confidence level or if it is less than 1 mg/L over the ten year period, the system is classified as "no trend/not significant" (approximately 75% of wells). Otherwise, it is classified as "increasing" or "decreasing", as appropriate.

Last revised: Wednesday January 04 2017