Chemical evolution of groundwater in a drainage basin of Holocene age, east-central Alberta, Canada

Abstract

Chemical evolution of groundwater in a small drainage basin of glacial origin (10,250 yr. B.P., based on radiocarbon age dating of gyttja from a closed saline lake in the basin) was studied in order to understand the generation of salts in surface-mined areas on the interior plains of Alberta. The basin was considered to be a natural analogue of a surface-disturbed area because of the large volumes of rock that had been redistributed by glaciers with the resulting change in topography and drainage. The distributions of hydraulic head, total dissolved solids (TDS), and environmental isotopes essentially reflect the superimposition of groundwater flow systems associated with the post-glacial topography upon a regional bedrock flow system of older but undertermined age. In the glacial drift aquifers and aquitards (sands and till), the groundwater composition was typically Ca-Mg-bicarbonate type at depths less than 30 m, but at depths of 30–100 m, the composition was Na-bicarbonate-sulfate type. In the deeper bedrock aquifers (> 100 m), Nabicarbonate-sulfate and Na-bicarbonate-chloride types were present. TDS was as low as 400 mg/l in the shallow drift aquifer, generally constant at ∼1000 mg/l in the deep drift and shallow bedrock aquifer, and over 1700 mg/l in the deep bedrock aquifer system. Chemical evolution of groundwater in the area appears to be dominated by two depth zones having different types of water-rock interaction. In the shallow drift zone, the dissolution of soil CO 2 in infiltrating groundwater, oxidation of organic carbon, sulfur and pyrite result in the formation of carbonic and sulfuric acids that attack carbonate and silicate minerals. On the basis of X-ray diffraction analysis, these minerals were calcite, dolomite, plagioclase feldspar, and smectite clays. However, in the deep regional bedrock aquifer, conditions are reducing (presence of methane), groundwater is alkaline (pH 8.6–10.3), and the Na-bicarbonate-chloride composition of groundwater is believed to result from the hydrolysis of volcanic glass or feldspar crystals of oligoclase-andesine composition under conditions of very slow leaching of reaction products and low partial pressure of CO 2. Under such conditions, calcite and possibly Ca-zeolite are sinks for Ca ion, but Na can accumulate in the pore water. As a result of groundwater movement induced by the postglacial hummocky topography, water from the drift aquifers mixes with water from the deep bedrock aquifers in the groundwater discharge area, yielding a range of intermediate compositions that may be explained by dilution and calcite precipitation, using the MIX2 chemical equilibrium model. Chemical diffusion was shown to be of negligible importance in comparison with mechanical dispersion to explain the mixing effect.