Sulfur and boron isotope variations to track air pollutant deposition in the Castle River of southern Alberta, Canada

Sulfur and boron isotopes were applied as natural tracers to determine origins of contamination in the watershed of the Castle River in southern Alberta. Combining sulfur and boron isotopes may be a sensitive detection technique since the isotopic composition of local industrial emissions is characteristic and distinct from background contributions to sulfate and boron in the region. Boron in rocks could be isotopically heavier with respect to the boron in groundwater. However, an earlier study found industrial boron emission was isotopically lighter in comparison. Sulfur from glacial tills in Alberta typically have low delta34S values around -16 ‰, while sulfur from industrial source in the region have an isotope composition of +20 ‰ or higher.;With distance downstream from headwaters, the watershed area is increased, and the air pollutant deposition area becomes larger. In addition, more tributaries enter the river which brings natural sulfate into the river and results in the accumulation of sulfate and other major ions downstream. Fluxes of the major ions increased downstream with the exception of boron. The delta 34S values for sulfate ranged from +14.2 +/- 0.8 ‰ to +23.6 +/- 0.4 ‰, and declined downstream. A tentative interpretation is that natural sulfate from glacial tills contributes a large proportion of sulfate downstream. delta11B values ranged from -1.7 +/- 3.5 ‰ to +7.5 +/- 9.5‰‰. They are consistent with each other within the range of uncertainties along the river, which could be the combination of the results of (1) limited boron emissions, (2) dilution caused by the water infiltration, and (3) boron adsorption and desorption.