Distribution of sulfate and organic carbon in a prairie till setting: Natural versus industrial sources

Abstract

Large lateral variations in the concentrations of sulphate and dissolved organic carbon (DOC) have been observed near a sour gas (natural gas containing hydrogen sulfide) processing facility in southern Alberta, Canada. High concentrations of sulfate are correlated with high concentrations of dissolved organic carbon. The spatial distribution of the high concentrations indicates that the pattern is not related to facility activities. Drilling, chemical analyses, and electrical conductivity measurements produced results consistent with the depression‐focused recharge model that has been proposed for the Dalmeny site in Saskatchewan. Unleached soils contain gypsum and soluble sulfur, and the electrical conductivity is relatively high. In the leached settings, groundwaters have lower total dissolved solids, lower sulfate concentrations, and lower concentrations of DOC. Leached soils contain little soluble sulfur, and the electrical conductivity is relatively low. In contrast to the Dalmeny site, the soils at this study site showed little spatial variation in total organic carbon, because the till at the site contains numerous coal fragments. Spatial variability of sulphate concentrations may be due to natural or industrial sources. Stable isotopes are used to distinguish between the two sources. The highest concentrations of sulfate found in groundwater samples are mainly derived from weathering gypsum and pyrite. The highest concentrations of DOC are interpreted to be predominantly fulvic acid. The isotopic signature of industrial sulfur is found in groundwater near suspected contamination sources, in shallow soils, and possibly in plant material. This case study demonstrates the importance of integrated analysis when studying the effects of industrial contamination in complex prairie till environments and the effectiveness of using stable sulfur isotopes to distinguish between industrial impact and natural site conditions.