The Minnesota Pollution Control Agency (MPCA) has released results of its study, “Nitrogen in Surface Waters,” which shows elevated nitrate levels, particularly in southern parts of the state.
Concern about nitrate has grown in recent years because studies show that nitrate in surface water is toxic to fish and the aquatic life food chain and nitrate in drinking water is potentially harmful to humans.
The comprehensive study was conducted to better
understand the effect nitrates are having in Minnesota’s surface waters and to identify the nitrate sources and potential reduction strategies. The study shows that primary source of the nitrates (70 percent) is cropland agriculture.
Several Minnesota streams exceed standards established to protect potential drinking water sources. Minnesota also contributes to the oxygen-depleted zone in the Gulf of Mexico. That “dead zone” is currently the size of Massachusetts.
The MPCA looked at the extent of nitrate pollution using monitoring results from more than 50,000 stream samples from across Minnesota. In the north, nitrate levels are relatively low; whereas in the southern part of the state, especially south-central Minnesota, nitrate levels are either high or very high.
The University of Minnesota worked with the MPCA to examine how the nitrates are getting into our surface waters. Results showed that more than 70 percent is coming from cropland, and the remaining 30 percent is from other regulated and unregulated sources, such as wastewater treatment plants, forests, the atmosphere, septic systems, and urban runoff.
In the Minnesota River, Missouri River, Cedar River, and Lower Mississippi River basins, cropland accounts for an estimated 89 to 95 percent of the nitrate load. The amount reaching surface waters from cropland varies widely, depending on the crop, tile drainage practices, cropland management, soils, climate, geology, and other factors. Scientists also believe crops that do not have deeper rooting systems to actively remove nitrate during the spring and fall months allow nitrate to leach through the soil and into groundwater. Another contributor in much of south-central Minnesota and increasingly in other areas of the state are agricultural drainage lines that are installed just a few feet below ground, allowing nitrate-rich water to be intercepted and conveyed into ditches and streams.
To make progress in reducing nitrate from agricultural lands, farmers are encouraged to better optimize their use of fertilizers. Nitrogen fertilizer efficiency has improved during the past two decades. Further refinements in fertilizer rates and application timing can be expected to reduce nitrogen loads by roughly 13 percent statewide, according to the study. Additional and potentially more costly practices are also needed to achieve the overall statewide nitrate-reduction goal of 30 percent or more to meet downstream needs.
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Several ongoing efforts are already working to reduce nitrates in surface waters, including the State-level Nutrient Reduction Strategy, the Minnesota Agriculture Water Quality Certification Program, and research for perennial biomass energy crops.
Nitrogen is essential for all living plants and animals and it is one of the most widely distributed elements in nature. Nitrate, a form of nitrogen, is commonly found in ground and surface waters throughout the country. Human activities can increase nitrate levels in lakes, streams and groundwater. Typically, nitrate levels are quite low in undisturbed landscapes.