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 SO₄^2? 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 SO₄^2? 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 SO₄^2? export, we measured concentration and isotopic composition (δ³⁴S and δ¹⁸O) of SO₄^2? in stream water, and concentration and δ³⁴S 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 SO₄^2? concentrations <100 µeq L^?1 and isotopic values consistent with those of atmospheric SO₄^2? that had been cycled through the organic soil pool. Catchments with S‐rich bedrock had stream‐water SO₄^2? concentrations ranging from 56 to 229 µeq L^?1. Isotopic values deviated from those of SO₄^2? in atmospheric deposition, clearly indicating a mineral weathering source in some cases, whereas in others spatial variability of mineral δ³⁴S 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.