Excess legacy sediments deposited in former impounded streams can overlay historic wetlands, decrease in-situ nitrogen removal, and increase nitrogen transport downstream, particularly where deep legacy sediments limit sediment-water interactions. This has prompted efforts in the mid-Atlantic region to remove legacy sediments and restore floodplain wetlands. By removing legacy sediments greater surface and groundwater interaction can occur along with the formation of saturated surface sediments that may accumulate the organic carbon needed to drive denitrification activity. In this study we measured nutrients (nitrate and organic carbon) in surface and groundwater sampled bi-monthly, sediment denitrification potential with acetylene block measured annually, and observed changes in stoichiometric ratios to evaluate nitrogen retention before and four years following legacy sediment removal at small stream floodplain restoration site, Big Spring Run, in Lancaster, PA. These measures were used to test the hypothesis that legacy sediment removal and restoration can enhance nitrogen retention, increase organic carbon to nitrogen ratios, and decrease nitrate concentrations.
Results/Conclusions
Our preliminary results show that surface water nitrate-N concentrations have decreased significantly from a mean of 9.6 mg L-1 to 8.7 mg L-1 (p<0.01) and dissolved organic carbon has increased significantly from 0.95 to 1.1 mg L-1 (p<0.05) in pooled pre vs. post comparisons. Groundwater showed a similar pattern with similar differences. Sediment denitrification potential measurements show a transition from carbon limited immediately after legacy sediment removal to nitrate – carbon co-limited in subsequent years. These findings suggest that local organic carbon is accumulating in a legacy sediment restoration site and functional nitrate retention can develop over time as the stoichiometric balance between carbon and nitrate shifts from nitrogen to organic carbon dominance. The stoichiometric shift to higher carbon to nitrogen ratios may be used as an indicator to show functional biogeochemical enhancement through restoration and possibly provide an indicator metric to determine biogeochemical function when nitrate retention is a goal of floodplain restoration activities. These results from a novel ecosystem in the Anthropocene may aid in future restoration activities in the region and beyond.
This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.