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Abstract. Detailed vertical profiles of dissolved C1-and its isotopes (36C1 and1537C1of C1 relative to C1 because of diffusion in this aquitard system. IntroductionClay-rich deposits are common throughout the world. Because of their low hydraulic conductivity and widespread occurrence, these deposits are important as sites for waste disposal and as protective covers on regional aquifers. To assess the viability of such deposits to contain contaminants, an understanding of the processes controlling the fate and transport of dissolved constituents in these clay-rich deposits is critical. Despite their importance, most of our knowledge on the processes and mechanisms controlling dissolved constituents in aquitards has been inferred from studies on adjacent aquifers Wassenaar, 1999a]. Prior to further investigation of the geochemistry of a wide range of reactive solutes (e.g., major ions, metals, and organics) in these aquitards, it was deemed important to determine if additional information on the origin and migration of solutes could be obtained from studying the distribution and origins of an additional conservative tracer, chloride, and its stable and radioactive isotopes.Chloride is widely believed to be among the most conservative of dissolved constituents in groundwater and has been used to evaluate solute origins and transport processes [Fabryka-Martin et at., 1987]. Dissolved chloride (C1-) is commonly used as a conservative tracer in groundwater studies because it exhibits a lack of adsorption in its anionic form, exists at low concentrations in most rock-forming minerals, has high solubility in natural waters, and does not exhibit volatility in the subsurface (excluding hydrothermal systems) [Feth, 1981]. Although C1-has been used as a conservative tracer in aquifer studies, there is little knowledge on the behavior of C1-in aquitards. Desautniers et at. [1981] showed that dissolved C1-present in a clay-rich till in southern Ontario was derived from the underlying bedrock by upward diffusion. However, with the exception ofDesautniers et at. [1981], no comprehensive studies have been published on the systematics of dissolved C1-geochemistry in thick, clay-rich aquitards.

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