The impacts of a watershed CaCO3 treatment on stream and wetland biogeochemistry in the Adirondack Mountains

Temporal and longitudinal variations in the chemistry of two tributary streams of Woods Lake in the Adirondack Mountains of New York were monitored before and after a watershed CaCO3 application. One subcatchment of the lake had a large beaver pond and wetland at its headwaters, while the second was free-flowing. Treatment of both subcatchments with CaCO3 resulted in an immediate increase in acid neutralizing capacity (ANC) associated with Ca2+ release. The extent and duration of the response to the treatment were greater in the wetland-impacted stream. Aluminum was retained and complexed with organic solutes generated within the beaver-pond. In the free-flowing stream, NO3− concentration increased significantly after the manipulation; this pattern was not evident in the wetland-impacted stream. Net retention of SOkinf4/sup2− was evident in the beaver pond prior to and following treatment, and this response was enhanced after the watershed liming. Comparisons of beaver pond inlet/outlet concentrations, mass balance calculations, and in-pond profiles of chemical parameters revealed patterns of retention of SO42−, NO3− and Al, and release of Fe2+, dissolved organic carbon (DOC) and NH4+ in the wetland during the summer before CaCO3 treatment. Post-treatment releases of Ca2+ from the near-sediment zone in the beaver pond corresponded to anoxic periods in mid- to late-summer and under ice in winter. These findings demonstrate the importance of increased microbial processing of organic matter, along with high partial pressure Of CO2 (Pco2) in facilitating the dissolution of the applied CaCO3. Dissolved silica (H4SiO4) was retained in the wetland during the summer prior to treatment but was released after the manipulation. This phenomenon may reflect the dissolution of diatom frustules or silicate minerals in the wetland at higher pH and DOC concentrations. Within two years of the CaCO3 treatment 60% of the CaCO3 applied to the beaver pond and surrounding wetland was dissolved and transported from the pond, in contrast to only 2.2% of the CaCO3 applied to the upland subcatchment draining into the wetland. These results, coupled with high quantities of exchangeable Ca2+ found in sediments and onSphagnums mosses in the pond, demonstrate the importance of hydrologic source areas and wetlands in facilitating the dissolution of added CaCO3 and in regulating the production of chemical species important in ANC generation.