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

Sulphate (SO₄), 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 SO₄ distribution and dynamics in shallow groundwater at local scales (<1500 km²), we examine SO₄ 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 SO₄ is dominantly derived from oxidation of S-bearing material. At this large scale, marine shale bedrock subcrops, glacial ice flow directions, and climate control SO₄ distribution. Groundwater contains less SO₄ (<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 SO₄ 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 SO₄ concentrations depending on their hydrogeological setting. Low SO₄ concentrations are associated with areas of high recharge, where SO₄ has been leached from the sediments, or with more evolved bedrock groundwater where SO₄ has been reduced or was recharged in pre-glacial times. High SO₄ 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 SO₄ occurrence in shallow groundwater.