Impact of freshwater river reconnection on porewater salinity and ammonium availability in coastal brackish marsh soils.

Restoring freshwater flows to wetland ecosystems is an increasingly common tool for reversing saltwater intrusion/chronic salinization. Hydrologic restoration projects can deliver large volumes of sediment and fresh water to coastal basins, episodically exposing brackish and salt marsh vegetated soils to low surface water salinities. Yet little is known about the impacts of river reconnection/diversions to porewater salinity of the active root zone (0-30 cm) and salinity dependent soil biogeochemical processes like sorption. Intact soil cores from a brackish marsh site in mid-Barataria Basin, LA were subjected to a simulated river diversion opening to examine how porewater salinity and ammonium (NH4 + ) availability change with depth and time. Quadruplicate soil cores were inundated with continuously flowing fresh (0 salinity) water for 0, 7, or 28 d then measured for porewater salinity and NH4 + partition coefficient (exchangeable NH4 + :porewater NH4 + ) every 2 cm for the top 10 cm of soil. Porewater salinity decreased in the 0-4 cm interval between 0 and 7 d of the simulated river diversion and increased in the 8-10 cm interval between 7 and 28 d. Overall, depth-averaged porewater salinity of the top 10 cm did not significantly change between 0 and 28 d of the simulated river diversion. Ammonium partition coefficients increased only in the 0-2 cm interval between 0 and 7 d of the simulated river diversion, likely due to freshening-induced NH4 + adsorption. These results indicate that the physicochemical environment of brackish marsh soils is relatively resistant to a single surface water freshening over one month. Models utilized by the state of Louisiana are likely overpredicting freshening of the marsh soil porewater in Mid-Barataria Basin in response to the episodic operation of the Mid-Barataria Sediment Diversion. This study demonstrates the importance of measuring diffusive-adsorptive flux of major cations and anions when modeling vertical salt transfer in brackish marsh soils.