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. Additional information can be obtained by clicking on the individual well location on the map.
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. 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
10008
public water supply systems for which more than 6 years of data exist suggest
no significant trend in
90.7%
of wells, an increasing trend in
3.8%
of wells, and a decreasing trend
in
5.4%
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.
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.
What is Nitrate
This test measures the amount of nitrate-nitrogen in your well. Nitrate is a form of
nitrogen, commonly found in agricultural and lawn fertilizer,
that easily dissolves in water. It is also formed when waste materials such as manure or
septic effluent decompose. The natural level of nitrate in Wisconsin's groundwater is
less than 1 mg/L. Levels greater than this suggest groundwater has been impacted by
various land-use practices.
Why Test for Nitrate
Nitrate is an important test for determining the safety of well water for drinking. Nitrate is a test that
allows us to understand the influence of human activities on well water quality. Because it moves
can come from a variety of sources and moves easily through soil, it serves as a useful indicator of certain land-use activities.
An annual nitrate test is useful for better understanding whether water quality is getting better, worse, or staying
the same with respect to certain land-uses (see Sources).
Interpreting Nitrate-Nitrogen Concentrations
Health Effects of Nitrate in Drinking Water
Ways to reduce nitrate in your drinking water
One way to reduce nitrate is to install a water treatment device approved for removal of nitrate;
testing is the only way to make sure these devices are properly functioning
Point-of-use devices treat enough water for drinking and cooking needs
- Reverse Osmosis
- Distillation
Point-of-entry systems treat all water distributed throughout the house
Additionally, you may want to investigate the potential that drilling a new well or well reconstruction
may provide water with safe nitrate levels
Sources of Nitrate
- Agricultural Fertilizers
- Manure and other biosolids
- Septic Systems
- Lawn Fertilizers
Strategies to reduce nitrate in groundwater
- Applying fertilizer at the right rate, time, source, place will maximize
profitability and minimize excessive losses of nitrogen to groundwater; additional
practices may be needed to improve water quality in areas with susceptible soils and geology
- You may not need as much nitrogen fertilizer as you think, conduct your own on-farm rate trials
to develop customized fertilizer response curves for your farm
- Utilize conservation incentive programs to take marginal land or underperforming
parts of fields out of production
- Diversify cropping systems to include less nitrogen intensive crops in the rotation
- Explore and experiment with the use of cover crops, perennial cropping systems, or
managed grazing to reduce nitrate losses to groundwater
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.
Trasient 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.
