"Legacy nitrogen" contributing to upward nitrate trends in streams

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Despite the tremendous effort invested in reducing the use of nitrogen, widespread decreases in nitrate loads in US rivers and streams remain elusive—what gives? A new study from the USGS provides more evidence that the culprit is the slow release to streams of nitrogen that has accumulated in groundwater and other storage areas, known as legacy nitrogen.

The study used a dynamic modeling technique to examine daily loads and concentrations of nitrate for 2002–2012 in 320 streams across the contiguous U.S. The study focused on winter low-flow conditions (January through March), when much of streamflow is contributed by upwelling groundwater or by point sources such as wastewater treatment plant effluent. Elevated nitrate concentrations (> 2 mg/L as nitrogen) typically were in areas with intensive use of nitrogen-containing fertilizer or in high-density metropolitan areas with wastewater treatment plant discharges.

The good news is that downward trends in nitrate concentration were more common than upward trends. However, in streams where groundwater contributed more than 40% of the total winter nitrate load, upward trends outnumbered downward trends 2 to 1, reflecting the influence of legacy nitrate in groundwater discharging to the streams.

Scientists found that the relation between stream discharge and nitrate concentration was a good predictor of whether legacy nitrate was affecting stream water quality. Determination of this relation is straightforward and requires a modest amount of data.  This information could be used to provide insights into the areas of the Nation where legacy nitrate has the greatest potential to affect stream water quality.

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