『Abstract
　Throughout the last few decades, sulfate concentrations in streamwater have received considerable attention due to their dominant role in anthropogenic acidification of surface waters. The objectives of this study conducted in the Oldman River Basin in Alberta (Canada) were to determine the influence of geology, land use and anthropogenic activities on sources, concentrations and fluxes of riverine sulfate on a watershed scale. This was achieved by combining hydrological, chemical and isotopic techniques. Surface water samples were collected from the main stem and tributaries of the Oldman River on a monthly basis between December 2000 and March 2003 and analyzed for chemical and isotopic compositions. At a given sampling site, sulfate sources were primarily dependent on geology and did not vary with time or flow condition. With increasing flow distance a gradual shift from δ³⁴S values ＞10‰ and δ¹⁸O values ＞0‰ of riverine sulfate indicating evaporite dissolution and soil-derived sulfate in the predominantly forested headwaters, to negative δ³⁴S and δ¹⁸O values suggested that sulfide oxidation was the predominant sulfate source in the agriculturally used downstream part of the watershed. Significant increases in sulfate concentrations and fluxes with downstream distance were observed, and were attributed to anthropogenically enhanced sulfide oxidation due to the presence of an extensive irrigation drainage network with seasonally varying water levels. Sulfate-S exports in an artificially drained subbasin (64 kg S/ha/yr) were found to exceed those in a naturally drained subbasin (4 kg S/ha/yr) by an order of magnitude. Our dataset suggests that the naturally occurring process of sulfide oxidation has been enhanced in the Oldman River Basin by the presence of an extensive network of drainage and irrigation canals.

Keywords: Watershed; Sulfate; Stable isotopes; Sulfide oxidation; Anthropogenic activities』

1. Introduction
2. Study area
　2.1. Description
　2.2. Geology and soils
3. Materials and methods
　3.1. Sampling
　3.2. Concentration and isotope measurements
4. Results
　4.1. Concentration, flux, and isotopic composition of sulfate in tributaries
　4.2. Concentration, flux, and isotopic composition of sulfate in the Oldman River
5. Discussion
　5.1. Sulfate sources at respective sampling sites
　5.2. Influence of geology and land use on sulfate sources and fluxes
　5.3. Anthropogenic control on riverine sulfate loading
6. Conclusions
Acknowledgments
References