Genesis and evolution of HCO 3 -rich and SO 4 -rich groundwaters of Quaternary sediments, Pinawa, Canada

Details Genesis and evolution of HCO 3 -rich and SO 4 -rich groundwaters of Quaternary sediments, Pinawa, Canada Genesis and evolution of HCO 3 -rich and SO 4 -rich groundwaters of Quaternary sediments, Pinawa, Canada

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.4933n&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 57, Issue 20, pp.4933-4946

Computer Science

2

Scientific paper

Groundwaters collected from sediments at AECL Research (Whiteshell Laboratories) include Ca-Mg-Na-HCO 3 , Na-Mg-Ca-HCO 3 , and Ca-Mg-Na-SO 4 -HCO 3 water types. The type produced is determined by the reactive phases encountered during migration through the enclosing sediments. These include calcite, gypsum, and a clay substrate on which cation exchange occurs. CO 2 -charged soil waters undergo cation exchange reactions with solids of the enclosing sediments and dissolve calcite to saturation, yielding Ca-Mg-HCO 3 groundwaters containing up to 700 mg·-L -1 total dissolved solids (TDS). More saline sulphate-rich groundwaters (1100-3000 mg·L -1 TDS) are produced through combined reaction with gypsum, calcite, and a Mg-rich exchange substrate. The type of groundwater produced is critically dependent upon the proportions of exchangeable cations on the bulk exchange substrate. Dissolution of gypsum increases SO 4 of the solutions but much of the released Ca is taken onto the exchange substrate, displacing Na + K first then Mg. Continued gypsum dissolution causes precipitation of calcite and a decrease in solution pH. The resulting saline waters are enriched in Mg and Na (relative to Ca) compared with most groundwaters, and are enriched in SO 4 relative to HCO 3 . Many sulphate-rich groundwaters common to the northern North American Prairies may have similar origins. Three solid phases (exchange substrate, calcite, and gypsum) are needed to model successfully these waters but interactions among them are complex. Calcite, for example, dissolves initially but is subsequently precipitated during evolution of sulphate-rich waters; consequently, changes to major dissolved constituents and carbon isotopic ratios are difficult to predict without a comprehensive understanding of the geochemical evolution of these waters.

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