Nitrate in Wisconsin's Public Water Systems



Created by: Grant Moser, Jennifer Dierauer, and Kevin Masarik
Center for Watershed Science and Education
Last modified: 2020-07-21 . Contact Kevin Masarik for questions

Overview

Public water systems are required to submit annual nitrate samples to the Wisconsin Department of Natural Resources. These data create an opportunity to learn about current quality in addition to exploring changes in nitrate concentrations over time in these wells. For more information on the data and methods used to determine trends click on the 'Data' Section.


Individual Well Data

Nitrate Concentration

Represents the most recent nitrate-nitrogen concentration of those public wells that have submitted a nitrate sample within the past 6 years. If multiple samples were submitted in a calendar year, the maximum value is displayed. Additional information can be obtained by clicking on the individual well location on the map or exploring the 'Individual Well' tab.

Nitrate Trend

Linear regression was used to determine nitrate trends for each individual public water supply system. Size of point represents magnitude of the rate of change. Determination of a trend does not mean that water quality will continue to increase or decrease indefinitely, ultimately changes to the surrounding land use could result in changes to nitrate concentrations in these wells. To see the annual data or to learn more about how the individual nitrate trend was determined go to 'Individual Well'.

Decreasing Wells

Individual wells with a decreasing trend.

Increasing Wells

Individual wells with an increasing trend.


County Summary

Nitrate Concentration

Mean annual nitrate-nitrogen concentration of public water systems for 2018.

Nitrate Trends

Because public water supply wells may not be statistically representative of land use and geology of the county as a whole, trend data is currently not summarized at the county level. We hope to develop ways to investigate this question at a county level soon.

Increasing Wells

Percent of public water supply wells by county with a statistically significant increasing trend.

Decreasing Wells

Percent of public water supply wells by county with a statistically significant decreasing trend.

Nitrate-nitrogen Concentrations by Wisconsin Unique Well Number

Nitrate-nitrogen concentration by year for each public water system with more than 6 years of data. If multiple samples were submitted in a calendar year, the maximum value is displayed and used in regression analysis. Use the drop down menu below to select any public water system using its Wisconsin Unique Well Number.

A linear model was used to fit a regression line to the annual maximum nitrate concentrations. The gray 'bands' around the regression line in the plot below represent the range in which the true regression line lies at a certain level of confidence (95% in the plot).

The regression line or trend is considered significant if the p-value < 0.05 and the slope is greater than 0.10 (or results in a change of greater than 1 mg/L for a 10 year period)


Trend Line Equation, R-squared, and p-value:


Median and Mean Annual Nitrate Concentration by County

Annual county-wide median and mean nitrate-nitrogen concentration for all public water system with more than 20 years of data. Wells with less than 20 years of nitrate data were excluded to reduce bias when contaminated wells are decommissioned or new wells come online.

Box plots represent the distribution of annual data. Annual mean concentration represented by the blue diamond. Number of samples listed below box plot for each year.

Use the drop down menu below to view data for any county.



Statewide Overview


Trends in Public Water Supply Systems

Trends in the 9739 public water supply systems for which more than 6 years of data exist suggest no significant trend in 89.4% of wells, an increasing trend in 6.8% of wells, and a decreasing trend in 3.9% of wells. While most wells show no significant trend over time, these data suggest of those that demonstrate a significant trend, slightly more are increasing than are decreasing.

For the purposes of figure below a slight increasing/decreasing trend is considered to be any well with a p-value less than 0.05 and a rate of change greater than 1 mg/L over a 10 year period; while a significant increasing/decreasing trend is one in which the p-value is less than 0.05 and the rate of change is greater than 2.5 mg/L over a 10 year period.

Disclaimer: A small number of systems have unknown treatment devices that may indicate decreasing trends when none exist or mask an increasing trend.



Annual Statewide Nitrate-Nitrogen Summary

Summary statistics of nitrate-nitrogen concentrations by year for all public water system with more than 20 years of data. Wells with less than 20 years of data were excluded to reduce bias when contaminated wells are decommissioned or new wells come online.

Plot A: Annual summary statistics with wells grouped by the trend classification (Significant Increase, Slight Increase, etc.). Annual mean concentration is represented by circles. Hover over circles for more information, e.g., number of samples, maximum value, etc.


Plot B: Annual summary statistics. Annual mean concentration is represented by circles; interquartile range is represented by black vertical lines. Hover over circles for more information, e.g., number of samples, maximum value, etc.

Data Overview

Samples for nitrate are generally submitted annually from public water systems and are required to be reported to the Department of Natural Resources. Data is publicly available from the Wisconsin Department of Natural Resources' Groundwater Retrieval Network and has been aggregated here to better visualize nitrate levels and long-term nitrate trends in Wisconsin's groundwater.

While these data provide a long-term dataset, it is important to consider that public water wells are often not as representative of groundwater quality in more rural parts of counties. In addition, municipal wells may be drilled deeper and often are not reflective of nitrate concentrations in the shallow groundwater.

These data provide valuable information about groundwater quality, however there are many areas that are underrepresented by this dataset. Counties may need additional strategies to better understand water quality trends countywide, particularly in the more rural areas of the county where private wells predomoninantly rely on groundwater.


Municipal Community (MC) - water systems with 15 or more service connections, or serve a community of at least 25 residents for at least 6 months of the year. MC systems are owned by a city, town, village, or other government entity.


Other-than-municipal community (OTM) - water systems have 15 or more service connections, or serve a community of at least 25 residents for at least 6 months of the year, but are not owned by municipalities. OTM systems include mobile home parks, subdivisions, apartment buildings and condominium complexes.


Transient Non-Community Wells (TN) - systems serve at least 25 people, but not necessarily the same people, for 60 days a year or more. TN systems include motels, restaurants, taverns, campgrounds, parks and gas stations.


Non-transient non-community (NN) - water systems serve at least 25 of the same people for at least 6 months of the year. NN systems include schools, day care centers, factories, or businesses with 25 or more employees.


Data Clean-up

Well Selection

The Groundwater Retrieval Network represents data from all known wells, including wells that may no longer be in use. Wells without a sample collected in the past 6 years were excluded from the analysis to reduce the amount of data from public water supply wells that may no longer be in use.


Annual Nitrate Value

Some public water supply systems are sampled more than once per year. Others may also have treated samples represented in the original dataset. To account for these issues, only the maximum nitrate value for each calendar year was selected for use in the trend analysis of individual wells and the statewide/county summaries.

Potential for Decreasing Trends to be Incorrectly Identified

In the event that well water may be treated, a decreasing trend may be indicative of treatment and not an actual decreasing trend in well water quality. This can usually be identified by a sudden drop in nitrate-nitrogen concentrations to near zero, when previous tests results where consistently elevated. This is a small number of the overall wells represented.