Strontium isotopic evidence on the chemical evolution of pore waters in the Milk River Aquifer, Alberta, Canada

Abstract

Strontium isotope ratios were measured on 13 rock, 18 leachate and 28 pore-water samples from the Milk River aquifer, the confining argillaceous formations, and the glacial till mantling the recharge area. Strontium isotope ratios ( 87Sr/ 86Sr) of pore waters from the aquifer, confining units, and the glacial till ranged from 0.7069 to 0.7082. The 87Sr/ 86Sr ratios in aquifer pore waters decrease with increasing distance from the aquifer recharge area, and this is interpreted to be the result of mixing and water–rock interaction within the aquifer. The solute composition of the recharging groundwater is modified by the local lithology, causing distinct geochemical patterns along different flow paths within the aquifer. Whole-rock 87Sr/ 86Sr ratios indicate that the shales and till are generally more radiogenic than the aquifer sandstone. The authigenic carbonate cements and rock-forming minerals comprising the major lithologic units had little apparent influence on the pore-water Sr chemistry. Carbonate cement leachates from the till and the aquifer sandstone are more radiogenic than those from the confining shale formations. Feldspar separates from the aquifer sandstone have relatively radiogenic Sr isotope ratios, whereas bentonites from the Milk River and Colorado Shale Formations have whole-rock and leachate Sr isotope ratios that are relatively unradiogenic. Ratios of most Milk River aquifer pore waters are lower than those of any leachates or whole rocks analyzed, except the bentonites. The 87Sr/ 86Sr ratios of exchangeable Sr in the bentonites are similar to ratios found in the more evolved pore waters. Simple rock–water interaction models calculated for the whole-rock, leachate, and exchangeable-ion/pore-water pairs indicate that ion exchange with bentonite clays within the Milk River and Colorado Shale Formations appears to influence the isotopic evolution of the pore-water Sr in each of these units.