Evidence for influence of mineral weathering on stream water sulphate in Vermont and New Hampshire (USA)

Mass balance studies in forested catchments in the northeastern USA show that S losses via streamwater SO4 exceed measured atmospheric S inputs. Possible sources of the excess S loss include underestimated dry deposition, mineralization of organic S in soils, desorption of soil sulphate, oxidation of recently formed sulphides and mineral weathering. Evaluating the relative contribution of these sources and processes to SO4 export is important to our understanding of S cycling as well as to policy makers in their evaluation of the efficacy of S emission controls. In order to evaluate the potential for mineral weathering contributions to SO4 export, we measured concentration and isotopic composition (υ34S and υ18O) of SO4 in stream water, and concentration and υ34S values of four S fractions in bedrock and soil parent material in catchments of varying geological composition. Geological substrates with low S concentrations were represented by catchments underlain by quartzite and granite, whereas geological substrates with high S concentrations were represented by catchments underlain by sulphidic slate, schist and metavolcanic rocks. Catchments with S-poor bedrock had stream-water SO4 concentrations <100 μeq L 1 and isotopic values consistent with those of atmospheric SO4 that had been cycled through the organic soil pool. Catchments with S-rich bedrock had stream-water SO4 concentrations ranging from 56 to 229 μeq L 1. Isotopic values deviated from those of SO4 in atmospheric deposition, clearly indicating a mineral weathering source in some cases, whereas in others spatial variability of mineral υ34S values precluded the isotopic detection of a weathering contribution. These results, along with evidence suggesting formation of secondary sulphate minerals in bedrock weathering rinds, indicate that mineral weathering may be an important source of S in the surface waters of some forested catchments in the northeastern USA. Copyright  2004 John Wiley & Sons, Ltd.