By: Brad Carlson, Extension educator
The focus on nitrate concentration in water, both surface and groundwater, has prompted many farmers and other rural residents to submit samples for testing. Most individuals who participate in testing opportunities have genuine curiosity regarding the results, but may not be completely sure as to what they mean. Interpretation of the numbers is not difficult, but it is important to realize that it may not be conclusive either.
Sample collection and storage
Be sure the sample was collected properly. The sample vessel should be clean prior to collection. Avoid storing the sample at ambient temperature, as this can lead to nitrate loss through denitrification depending on what else might be in the water. If you are not going to have the sample analyzed within a day or two, it is preferable to freeze the sample (making sure the container can withstand this).Interpreting results from tap water
You will receive your results in either parts per million (ppm) or milligrams per liter (mg/l). These two labels mean the same thing. The benchmark most people use for comparison is 10 ppm. This threshold was established decades ago as the point when nitrate levels are considered unsafe for drinking and is based on the sensitivity of human infants to nitrate. If the water is being used for other purposes different standards may apply. For instance, published standards for livestock consumption set the safe value at 100 ppm.The presence of nitrate in drinking water is an indication of one of several things. First, some municipalities obtain their water from surface water sources. It is possible that water obtained in this way has low levels of nitrate, but should always be within the safe range. Most groundwater sources should have little to no measurable nitrate.
Well water from some parts of the state is sensitive to nitrate contamination due to the texture and depth of soil as well as the nature of the bedrock underneath. Specifically, sandy soils near major rivers (such as the Mississippi and Minnesota) allow fast movement of water, and all that is picked up by it, to groundwater. The water table is often shallow in these areas due to proximity to the river, and means nitrate doesn’t have far to travel before it is in the groundwater. Other areas with sandy soils are also prone to have issues, but problems are less common than near rivers.
Another part of the state that experiences high nitrate levels in groundwater is southeast Minnesota, where lighter texture loess soils are frequently shallow and underlain by fractured limestone that allows for a direct conduit to groundwater. Finally, the far southwest corner of the state also has loess soils associated with the Missouri River basin. These soils are underlain by gravel deposits which allow for relatively unimpeded movement from the surface.
If you live in one of these areas and have high nitrates you should consider either installing a filtration system or establishing a new and deeper well. Finally, if you live in an area where it is uncommon to find nitrates in groundwater and find concentrations, you need to consider that there may be contamination via your well head. A well driller can inspect to ensure that the casing isn’t either cracked or experiencing flooding leading to surface water mixing with the well water.
Interpreting results from outdoor water sources
Samples taken from tile outlets can give you a glimpse of how well you are managing nitrogen (N) fertilizer. It is very difficult to determine total N loss using nitrate concentrations, as you need to know the total area contributing to the drainage system at the point of collection as well as the flow rate over time. Research data from U of M drainage plots has shown that nitrate concentrations are usually between 5 and 15 ppm. Much of this nitrate is attributable to accumulations from mineralized organic matter post growing season. Data from the Southern Research and Outreach Center in Waseca shows that nitrate concentrations have tended to be below 10 ppm for most of the past decade. If you see numbers higher than this, it is worth investigating whether a fertilizer management change is in order.Samples taken from surface water (such as drainage ditches, streams or lakes) are even more difficult to interpret. The area of a watershed is knowable but not all parts of the watershed contribute equally. Goals established for nitrate in surface water are primarily geared toward total load. As with tile water, calculating this is difficult because you need to be able to quantify flow over time. Typically, high flow will have lower nitrate concentration due to dilution. This may balance out when higher levels are found during low flows, but you can’t be sure unless it is calculated. At this time, Minnesota does not have concentration standards for surface water with the exception of places contributing to drinking water and some cold-water streams. The few places that do have a concentration standard are using 10 ppm, which is considered a goal for most surface water in the state.
Additional resources:
- MN Corn offering free nitrate screenings
- Video: Nitrogen management and water quality in drained soils
- Podcast: Watershed planning and nitrate reduction
- Six observations from six years of tile sampling for nitrates in southeast Minnesota
- Drinking water in Minnesota
- Nitrogen Smart
- U of M corn fertilizer guidelines
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Support for Minnesota Crop News nutrient management blog posts is provided in part by Minnesota's fertilizer tonnage fee through the Agricultural Fertilizer Research & Education Council (AFREC). Learn more at MNsoilfertility.com
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