Stable isotope geochemistry of sulfate in relation to hydrogeology in southern Saskatchewan, Canada

Abstract

Isotope and chemical analyses were carried out on soil and water samples collected from testholes, drilled into different stratigraphic layers at six locations in the Assiniboia area, southern Saskatchewan, to understand the origin and distribution of SO 4 and sources of groundwater recharge. The δ 34S (SO 4) andδ 18O (SO 4) values were mostly negative indicating oxidation of highly reduced sulfide in subsurface environments as the major source of SO 4 in the soils and groundwaters. However, positive or near positive δ 34S and δ 18O values in some of the samples indicated that oxidation of sulfide with positive or near positive δ 34S values or dissolution of a SO 4 mineral, such as evaporite gypsum may serve as alternate sources of S. Salt concentration and isotope compositions in the Eastend sand were nearly uniform, in comparison with till and shale layers. Total S content in the sediments varied from 0.7 to 3.2 g/kg with greater accumulations in the till layer, which were mostly 1.0 g/kg. The insoluble S fraction of the sediments was < 0.1 g/kg and SO 4-S was the major form of S. The δ 34S and δ 18O values of soluble SO 4 and δ 18O and δD values of groundwaters provide supporting evidence of groundwater flow direction determined by other techniques. The δD and δ 18O values of the groundwater samples varied between −131 and −119%, and −15.2 to −11.4%, respectively, without any correlation with stratigraphy. These data plot on a δD vs δ 18O line with a slope of 2.0. This departure from the meteoric water line (with slope of 8.0) appears to be the result of evaporation or oxygen exchange with soil minerals. The O isotope data suggest that exchange between SO 4 and H 2O is very slow, under the conditions prevailing in the subsurface.