Controls on regional sulphate distribution in shallow groundwater in the western Canadian Interior Plains.

Sulphate (SO), predominantly derived from sulphur (S)-bearing glacial sediments distributed widely across the Canadian Interior Plains, contributes to high groundwater salinity and can be detrimental to riparian and dry-land ecosystems, agricultural production, and water use. While previous researchers investigated SO distribution and dynamics in shallow groundwater at local scales (<1500 km), we examine SO occurrence in groundwater at larger scales, and to depths of ∼150 m, considering variations in geology, glacial history, climate, and geochemical and hydrogeological settings in the Canadian province of Alberta. Sulphate concentrations in groundwater vary considerably, with 15 % of 139,130 samples above the 500 mg/L Canadian drinking water aesthetic objective. Analysis of [Formula: see text] and [Formula: see text] from 179 wells throughout Alberta shows SO is dominantly derived from oxidation of S-bearing material. At this large scale, marine shale bedrock subcrops, glacial ice flow directions, and climate control SO distribution. Groundwater contains less SO (<200 mg/L) in western Alberta compared to the east due to a lack of S-bearing bedrock, higher groundwater recharge rates, thinner glacial sediments, and increased groundwater circulation. Focusing on a region in south-central Alberta (Red Deer area), hydrogeological characteristics relating to SO formation, recharge, permeability, hydraulic gradient, and travel time are included in a principal component analysis to identify six groundwater groups at varying stages of evolution and SO concentrations depending on their hydrogeological setting. Low SO concentrations are associated with areas of high recharge, where SO has been leached from the sediments, or with more evolved bedrock groundwater where SO has been reduced or was recharged in pre-glacial times. High SO concentrations are found in areas of lower recharge and permeability, where flushing occurs more slowly. Combining the Red Deer area and province-wide analyses, we identify seven regions across Alberta with characteristic distribution and controls on SO occurrence in shallow groundwater.