Spatial Profiles of Sediment Denitrification at the Ground Water - Surface Water Interface in Cobb Mill Creek on the Eastern Shore of Virginia

Abstract

Intensive use of nitrogenous fertilizers on agricultural fields has contaminated the shallow, unconfined Columbia aquifer on the Eastern Shore of Virginia, which contains high NO3 levels at 10-20 mg N L, which are above USEPA drinking water limits of 10 mg N L. In a small, low-relief stream in one watershed, Cobb Mill Creek (CMC), on the Eastern Shore, which is fed almost exclusively from the surrounding sandy aquifer, the stream NO3 concentrations of 1-2 mg N L are considerably lower under base flow conditions. Synthesis of hydrological and biogeochemical ground water and stream data collected at the experimental hillslope draining to Cobb Mill Creek indicated that high concentrations of NO3 were maintained along deep, oxidized ground water flow paths approximately greater than two meters deep and discharge vertically into the stream. The NO3 was not removed until the pore water reached the ground watersurface water interface approximately 20 cm below the streambed. The objectives of this research were to determine the contribution of, and controls on denitrification in removing NO3 at the ground water/ surface water interface. Vertical and lateral profiles of NO3 levels, total organic carbon, redox potential (platinum electrode potential), and denitrification potential rates (acetylene block method) were measured in intact cores collected from the streambed. Vertical profiles showed a decrease of 8.27 mg NO3-N L or 84.24% between the open channel and 60 cm below the stream sediment surface in cores taken on July 11, August 4, August 26, and October 26, 2003. The greatest decrease in NO3 occurred in the top 20 cm of the stream sediments. In these shallow sediments, conditions of high organic matter content, low redox potential, and available NO3 promoted high ii denitrification rates. Organic matter content was significantly positively correlated to potential denitrification rates for all four sampling events. The results of this research indicate that denitrification is responsible for NO3 removal from the discharging ground water, and reducing conditions stimulated by surface-derived organic matter drive the reaction. Along lateral profiles, increased total organic matter and increased potential denitrification at depths greater than 20 cm below the stream sediment surface were found only along the right bank facing downstream in all sampling dates. Due to the curvature of the stream at the sampling reach, organic matter deposited into the stream from the surrounding riparian forest may be consolidated and buried specifically along the right bank of the stream. The average denitrification potential rates were 661, 567, and 16.8μmol N m h in the stream sediments on July 11, August 4, and October 26, 2003, respectively. The lowest denitrification rate was observed in October due to lower temperatures. The rates observed this study are similar to denitrification rates observed in other studies in the literature. While denitrification rates in Cobb Mill Creek sediments were spatially and temporally variable, denitrification was a significant NO3 sink at the ground watersurface water interface. Riparian buffers surrounding streams supply an important input of organic matter that fuels denitrification. The maintenance of riparian buffers with organic-rich soils is essential for natural remediation of NO3 concentrations in streams sediments, to prevent nutrient enrichment of downstream environments.