Quantifying Sediment Nitrogen Releases Associated with Estuarine Dredging

Abstract

Experimental studies of sediment pore water NH4+ chemistry, adsorbed NH4+ concentrations, sediment–water NH4+ exchange and N2–N flux were carried out to quantify the mass of labile N that can be released during large-scale dredging activities. Pore water NH4+ concentrations below 0.5-m sediment depth averaged 5 ± 2 mmol L−1 with average adsorbed NH4+ concentrations of 11 μmol g−1. Elevated NH4+ concentrations found in rapidly accreting dredge channels are partly a result of the rapid advective burial of both reactive organic matter and pore water. Elutriate tests, a dilution of sediment with site water, yielded adsorbed NH4+ concentrations very similar to those using the more typical KCl extraction. Intact deep sediment sections exposed to overlying water, used to simulate postdredging conditions, showed high initial fluxes of ammonium and no development of coupled nitrification–denitrification under the cold incubation conditions. Despite high concentrations and effluxes of NH4+ during dredging, the amount of NH4+ release during dredging was <0.5% of northern Chesapeake Bay sediment fluxes. The likelihood of large environmental effects of nitrogen release during the dredging of navigational channels in the Chesapeake Bay is low.