Confirming the Source of High-Sulfate Concentrations in Dead Horse Creek, Winkler, Manitoba, Canada, Using a Dual-Isotope Bayesian Probability Mixing Model

Sulfate (SO42−) is an essential nutrient for the growth of several agricultural crops. However, higher than normal SO42− runoff into rivers and streams can cause environmental impacts including acidification, heavy metal release, phosphorus remobilization, and methylmercury release in aquatic systems. The city of Winkler, Manitoba, Canada, uses groundwater for its municipal needs. Some of the wells have a high SO42− content that is stripped off and discarded into the municipal sewage lagoons which are biannually emptied into an adjacent creek (Dead Horse Creek, DHC). However, sewage lagoon effluent is not the only input source of SO42−. Culvert drainage and soil runoff from the surrounding agricultural fields also contribute SO42− to DHC. During the summers of 2011, 2012, and 2018, sewage lagoons, culverts, wells, precipitation, soil, and fertilizers were sampled from the DHC area near Winker, Manitoba. Isotopic compositions (sulfur and oxygen) for all potential SO42− inputs to DHC have been determined. In this study, a dual-isotope approach was employed in combination with Stable Isotope Analysis in R (SIAR) Bayesian probability analysis to estimate the contributions of all potential SO42− sources to DHC. The Winkler sewage lagoons were found to be the primary SO42− contributor, an outcome which is directly related to the disposal of concentrated SO42− waste from the City of Winkler’s water treatment plant. In addition, runoff of soils (through culverts) was also determined to be a significant contributor of SO42− to DHC. Based on the results of this study, it is recommended that the City of Winkler’s Waste Water Treatment Plant incorporate a process to remove the high SO42− concentrations from the lagoons prior to the release of sewage effluent into DHC.