Researchers from Nagoya University in Japan reported that when it rains, the accumulation of nitrate in the soil near the river plays an important role in the increase in nitrate content in river water. Their results, published in the journal Biogeosciences, may help reduce nitrogen pollution and improve water quality in downstream water bodies such as lakes and nearshore waters.
nitrate is an essential nutrient for plants and phytoplankton , but high concentrations of nitrate in rivers can damage water quality, causing eutrophication (over-enrichment of nutrients in water) and pose a threat to the health of animals and humans. It is well known that nitrate content in streams rises when it rains, but the reason is not clear.
There are two main theories about how nitrates increase when it rains. According to the first theory, nitrates in the atmosphere dissolve in rainwater and fall directly into streams. The second theory is that when it rains, soil nitrates along the river (called riparian zone) flow into the river water.
To further investigate the source of nitrates, a research team led by Professor Urumu Tsunogai of the University of Environmental Research College, conducted a study in collaboration with the Asian Air Pollution Research Center to analyze the composition changes of nitrogen isotopes and oxygen in nitrates and the increase in nitrate concentrations in rivers during heavy rains.
Previous research reports said that nitrate concentrations increased significantly during the storm in rivers in the upper reaches of the Kaji River in Niigata Prefecture in northwestern Japan. Researchers collected water samples from the Kajikawa River basin, including streams upstream of the river. During the three storms, they used an automatic water sampler to collect samples from the collecting stream every hour for 24 hours.
The team measured the nitrate concentration and isotope composition in the stream water, and then compared the results with the concentration and isotope composition of the soil nitrate in the stream bank. As a result, they found that most of the nitrates came from the soil in the area, not the rainwater.
"We concluded that the surge of river nitrates into rivers due to rising rivers and groundwater levels is the main reason for the increase in river nitrates during the storm," said Dr. Ding Weitian, the corresponding author of the study.
Research team also analyzed the effect of atmospheric nitrate on increased nitrate flow during storms. Despite increased precipitation, the atmospheric nitrates in the stream remained unchanged, indicating that there was little effect on the source of atmospheric nitrates.
Researchers also found that soil nitrates in the riparian zone are produced by soil microorganisms. “It is believed that microbial-derived nitrates accumulate in the riparian soil only in the summer and autumn of Japan,” Professor Tsunogai explained. “From this perspective, we can predict that river nitrate increases due to rainfall will only occur in these seasons.” Understanding the seasonality of nitrate increases may be an important finding that helps ensure safe freshwater.
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