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January 4, 2021

A new USGS study has identified factors that contribute to high nitrate in base flow in streams flowing into Chesapeake Bay. Land use, organic carbon in soils, and geology were among the most important factors affecting nitrate in stream base flow, based on data collected at 156 stream sites.

The ecosystem of the Chesapeake Bay, a national jewel, has been impaired by nitrate. The source of much of that nitrate is groundwater, which provides base flow to the many streams that flow into Chesapeake Bay. Nitrate can be removed from groundwater by denitrification, a biochemical process that transforms nitrate into harmless nitrogen gas. Organic-rich sediments, often found in riparian zones, enhance denitrification, but dissolved oxygen in groundwater prevents it.

Armed with this information, the study predicted nitrate concentrations during base flow for unmonitored reaches of small streams throughout the Chesapeake Bay watershed. The highest predicted concentrations tended to be near agricultural areas, a potential source of nitrate from fertilizer, and in areas of carbonate rocks, which have geologic characteristics that can facilitate rapid groundwater transport and thereby limit denitrification.

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