Nitrate pollution comes from the nitrate used in crop fertilizers that goes to rivers and streams through drainage systems and agricultural ditches. High concentrations of nitrate in rivers and streams can be toxic to ecosystems and human health, as they may contaminate drinking water. As a result, a team of researchers from the University of Minnesota looked at strategies on how to reduce nitrate in rivers and streams more effectively.
In their study, which was published in the journal Nature Geoscience, they found that wetlands mitigate nitrate pollution in rivers and streams five times more effectively than land-based mitigation approaches.
In conducting the study, the research team used water samples gathered for more than four years from more than 200 waterways within intensively managed, 17,000-square-mile Minnesota River basin. In addition, they used geo-spatial data about land use in the watershed. The team managed to isolate the effect of wetlands on stream and river nitrate concentrations within large watersheds.
Results of the study revealed that wetlands are five times more effective per unit area at nitrate reduction compared to the best land-based conservation practices, such as cover crops and land retirement, when stream flows are high. Although other common conservation strategies are efficient at lower flow conditions, they are overwhelmed with higher stream flows.
The location of wetlands within a watershed was also found to be an essential predictor of the magnitude of nitrate removal. Wetlands are three times more efficient at nitrate reduction if they intercept 100 percent of the drainage area in comparison to that of 50 percent of the drainage area.
"The location of the wetlands are important predictors of how effective they are at reducing nitrate," Amy Hansen, one of the lead authors of the study, explained. “Water quality programs that target these locations would be more cost effective as well."
The researchers also found that the reduction of nitrate as a result of temporary wetlands, such as riparian floodplains and wetlands that are not connected to the river network by surface water, was measurable and was greatest in high stream flows, when such characteristics are hydrologically connected to surface water.
"Our work shows that wetland restoration could be one of the most effective methods for comprehensive improvement of water quality in the face of climate change and growing global demand for food," said study co-author Jacques Finlay, of the University of Minnesota.
The findings of the study suggested that maintaining or restoring wetlands in intensively managed agricultural watersheds would reduce nitrate in rivers and improve water quality more efficiently. (Related: Manmade wetlands can help reduce agricultural fertilizer pollution by up to 50%.)
The effect of nitrates on water quality
Nitrates are highly soluble, which means they are easily dissolved in water. Moreover, they are colorless and odorless, thus their presence cannot be known without the use of special testing equipment. The levels of nitrates are observed in municipal water supplies and food in order to prevent exposing people to the potential dangerous harmful effects of high levels of nitrates. Nitrates can be harmful to humans because they can disrupt the ability of red blood cells to carry oxygen.
Moreover, excess levels of nitrates in water can create conditions that reduce the chance of survival of aquatic insects and fish. Aquatic plants and algae need nitrates in order to grow. However, too much nitrates can also cause them to grow excessively, which in turn creates an unstable amount of dissolved oxygen. There will be high levels of dissolved oxygen during the day, while the levels of oxygen can decrease greatly at night, creating stressful conditions for aquatic species. This will adversely affect the normal behavior and reproductive ability of fish and other aquatic animals.
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