GENESIS AND EVOLUTION OF HCO3-RICH AND SO4-RICH GROUNDWATERS OF QUATERNARY SEDIMENTS, PINAWA, CANADA

Groundwaters collected from sediments at AECL Research (Whiteshell Lab oratories) include Ca-Mg-Na-HCO3, Na-Mg-Ca-HCO3, and Ca-Mg-Na-SO4-HCO3 water types. The type produced is determined by the reactive phases e ncountered during migration through the enclosing sediments. These inc lude calcite, gypsum, and a clay substrate on which cation exchange oc curs. CO2-charged soil waters undergo cation exchange reactions with s olids of the enclosing sediments and dissolve calcite to saturation, y ielding Ca-Mg-HCO3 groundwaters containing up to 700 mg.L(-1) total di ssolved solids (TDS). More saline sulphate-rich groundwaters(1100-3000 mg.L(-1) TDS) are produced through combined reaction with gypsum, cal cite, and a Mg-rich exchange substrate. The type of groundwater produc ed is critically dependent upon the proportions of exchangeable cation s on the bulk exchange substrate. Dissolution of gypsum increases SO4 of the solutions but much of the released Ca is taken onto the exchang e substrate, displacing Na + K first then Mg. Continued gypsum dissolu tion causes precipitation of calcite and a decrease in solution pH. Th e resulting saline waters are enriched in Mg and Na (relative to Ca) c ompared with most groundwaters, and are enriched in SO4 relative to HC O3. Many sulphate-rich groundwaters common to the northern North Ameri can Prairies may have similar origins. Three solid phases (exchange su bstrate, calcite, and gypsum) are needed to model successfully these w aters but interactions among them are complex. Calcite, for example, d issolves initially but is subsequently precipitated during evolution o f sulphate-rich waters; consequently, changes to major dissolved const ituents and carbon isotopic ratios are difficult to predict without a comprehensive understanding of the geochemical evolution of these wate rs.