Chemical fluxes from sediments in two Adirondack wetlands : Effects of an acid-neutralization experiment

As a strategy of acid deposition mitigation, the application of neutralizing agents to hydrologic source areas has received substantial attention for the past decade. To compare mass balance-determined fluxes with field measurements at the sediment-water interface, we used benthic enclosures to determine chemical fluxes from the sediments of a reference beaver pond (no chemical treatment) and a beaver pond within the watershed of an acid-neutralization experiment (CaCO 3 treatment). Baseline O 2 -consumption rates, the effects of reacidification, and the effects of CaCO 3 and CaCl 2 additions were determined. Oxygen consumption rates in pond sediments were higher in the CaCO 3 -treated wetland, indicating stimulation of microbial activity and the subsequent enhancement of organic-matter decomposition. In the reference wetland, anoxia was followed by the sequential consumption of NO 3 - and SO 4 2- , basic cation (C 8 ) and Fe 2+ release, and the production of acid-neutralizing capacity (ANC), while the release of Ca 2+ from cation-exchange sites dominated ANC in the treated wetland. Reacidification of CaCO 3 -treated sediments caused an immediate increase in Al concentration in the water column, initially in the inorganic monomeric form (Al IM ), followed by increasing concentrations of the organic monomeric form (Al OM ). Hydrolysis of Al inputs from upland drainage, complexation of Al with dissolved organic carbon (DOC), and the formation of less toxic Al OM were all observed. Our evidence reveals that these sediments may act as sinks for inputs of strong acid anions (e.g., SO 4 2- and NO 3 - ) from atmospheric deposition, and as sinks and transformation zones for Al associated with acidic upland drainage.