Controls on the Distribution of Denitrification in Streambeds on the Eastern Shore of Virginia

To examine controls on nitrate removal in groundwater discharging to streams on Virginia’s Eastern Shore, organic matter, and nitrate (NO3-) concentrations, were examined in four different streams. Depth profiles of organic matter, chloride, and NO3- concentration were developed from intact sediment cores from the streams. Denitrification was then estimated as NO3- removal (NO3- ) from an additional set of cores in the lab, where in situ flow conditions were simulated. An artificial groundwater solution containing a known concentration of NO3- was pumped upward through the core, recovered at the top, and analyzed for NO3- with an ion chromatograph, and NO3- removal was then determined as the influx minus the efflux. Multilinear regression combined with principal component analysis examined the relative effect of each measured factor on NO3- i.e., denitrification. Pumped columns were shown to remove up to 100% of added NO3-, though some columns did not show NO3- removal. In 3 of the 4 streams, denitrification in situ was primarily limited by NO3-. Regression analyses for NO3- concentration in porewater showed that NO3- is primarily influenced by Cl- and depth below streambed. Organic matter (OM) was found to not be limiting in these streams, although it was the main driver for denitrification. Additionally, canopy cover had a significant effect on streambed organic matter content with forest-canopied streams having about twice as much OM as streams with open canopies. While this study shows the ability of certain streams to remove large amounts of NO3-, the large variability among streams does not support allowing more NO3- inputs to ESVA streams.