Characterizing the hydrogeology of a complex clay-rich aquitard system using detailed vertical profiles of the stable isotopes of water

High-resolution 1D profiles of the stable isotopes of porewater (delta(2)H and delta(18)O) obtained by direct porewater-equilibration of core samples were used to identify hydrogeologic zones in a thick, complex, till aquitard system in Saskatchewan, Canada. Detailed ID isotope profiles revealed the presence of three distinct hydrogeological zones: an upper (5- 10 m), an intermediate (10-30 m), and a lower (30-62.5 m) till zone. The upper hydrogeological zone was controlled by a sand layer having a high hydraulic conductivity (K). The lack of vertical variability in the stable isotope data in the intermediate zone suggested groundwater flow and solute transport was controlled by a system of interconnected vertical fractures and lateral sand layers. The curved shape of the isotope profile in the lower till zone suggested that solute transport in this zone was controlled mainly by molecular diffusion. We show that hi.-h-resolution vertical stable isotope profiles of porewater obtained from core samples and direct equilibration yield cost-effective and invaluable hydrogeologic information on groundwater flow in aquitard systems that cannot be realistically obtained by construction of costly conventional piezometer bundles (i.e. delta(2)H and delta(18)O analyses of water samples and K determinations). Further. the direct equilibration approach overcomes the problem of piezometer leakage in low K aquitards. (C) 2004 Elsevier B.V. All rights reserved.