Controls on the long-term downward transport of δ2H of water in a regionally extensive, two-layered aquitard system

Abstract

Seven high-resolution (0.3-0.6 m depth intervals), 1-D vertical profiles of the δ2H of pore water were collected across a 300 km 2 study area in southern Saskatchewan, Canada, to define the vertical controls on solute transport in a >120 m thick, two-layered aquitard system. The 1-D profiles were augmented with an existing δ2H profile collected from a previous study. The surficial aquitard in the area consists of Quaternary deposits (either glacial till or lacustrine deposits; 13 to 128 m thick) underlain by an upper Cretaceous claystone aquitard (80-110 m thick). The shape of the individual δ2H profiles and associated 1-D transport modeling suggest diffusion is the regionally dominant vertical transport mechanism across the aquitards. The profile shape is controlled by the thickness of the Quaternary deposit and the δ2H value at the upper boundary, which coincides with the depth of the water table. The upper boundary δ2H value varies considerably across the area (-149‰ to -101‰), perhaps due to differences in local hydrological conditions (e.g., slope, aspect, infiltration) across the landscape. Modeling of all profiles shows the timing for till deposition and the timing of climate change during the Holocene are consistent across the area (∼30 ka and 7-10 ka before the present, respectively), corroborating other studies. This study provides insights into the hydrogeologic controls on solute transport in an aquitard system and associated geologic and climatic changes for a prairie region over the past 30 ka, and improves our understanding of initial and time-dependent transport boundary conditions for the study of aquitards. Copyright 2011 by the American Geophysical Union.