"Legacy nitrogen" contributing to upward nitrate trends in streams
Why is nitrate increasing in some U.S. streams, despite best efforts?
"Legacy" nitrogen in groundwater and wastewater treatment plant effluent may be the culprits
Learn MoreDespite 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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Nitrate in streams during winter low‐flow conditions as an indicator of legacy nitrate
Winter low‐flow (LF) conditions in streams provide a potential opportunity to evaluate the importance of legacy nitrate in catchments due to the dominance of slow‐flow transport pathways and lowered biotic activity. In this study, the concentration, flux, and trend of nitrate in streams during winter low‐flow conditions were analyzed at 320 sites...
Johnson, Henry M.; Stets, Edward G.