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

✍ Scribed by S. W. Bailey; B. Mayer; M. J. Mitchell

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 155 KB
• Volume: 18
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.1410

✦ Synopsis

Mass balance studies in forested catchments in the northeastern USA show that S losses via streamwater SO 4 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 SO 4 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 SO 4 2 export, we measured concentration and isotopic composition (υ 34 S and υ 18 O) of SO 4 2 in stream water, and concentration and υ 34 S 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 SO 4 2 concentrations <100 µeq L 1 and isotopic values consistent with those of atmospheric SO 4 2 that had been cycled through the organic soil pool. Catchments with S-rich bedrock had stream-water SO 4 2 concentrations ranging from 56 to 229 µeq L 1 . Isotopic values deviated from those of SO 4 2 in atmospheric deposition, clearly indicating a mineral weathering source in some cases, whereas in others spatial variability of mineral υ 34 S 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.