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 (CaCO₃ treatment). Baseline O₂-consumption rates, the effects of reacidification, and the effects of CaCO₃ and CaCl₂ additions were determined. Oxygen consumption rates in pond sediments were higher in the CaCO₃-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 NO₃^- and SO₄^2-, basic cation (C_B) and Fe²⁺ release, and the production of acid-neutralizing capacity (ANC), while the release of Ca²⁺ from cation-exchange sites dominated ANC in the treated wetland. Reacidification of CaCO₃-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, SO₄^2- and NO₃^-) from atmospheric deposition, and as sinks and transformation zones for Al associated with acidic upland drainage.