IN SITU EXPERIMENT TO DETERMINE ADVECTIVE-DIFFUSIVE CONTROLS ON SOLUTE TRANSPORT IN A CLAY-RICH AQUITARD

Solute transport in clay-rich aquitards is often assumed to be dominated by diffusion; however, the groundwater velocity under which this assumption is valid has not been measured. In this study, we are conducting an in situ experiment to determine at what groundwater velocity diffusion dominates the dispersion process in aquitards. In our experiment, the screen intake of a purpose-built piezometer (73 mm ID) was completed at 13 m below ground surface in a thick clay till located in Saskatchewan, Canada. At t = 0, the standing water in the well (Co = -163 ‰ V-SMOW) was spiked with deuterium (Ci = +457 ‰ V-SMOW; Hi = 563.87 m asl). Water levels were measured and water samples collected and analyzed for deuterium during the first 1054 days of this experiment (n = 48). On day 340, the well water was spiked with oxygen-18 (Co = -19.8 ‰ V-SMOW and Ci = +40.9 ‰ V-SMOW at H = 570.95 m asl). From day 340 to 1054, the collected water samples were also analyzed for oxygen-18 (n = 24). On day 1054, the water level in the well (H = 573.62 m asl) was approaching its static level. Based on the water level recovery data, the hydraulic conductivity of the aquitard was determined to be 3 x 10-10 m/s, in keeping with other laboratory and field determinations at this site. Mixing and mass balance calculations suggest that solute transport during the 1054 day test period was dominated by advection, not diffusion. During this test period, groundwater velocities decreased from about 0.9 m/year near the start of the experiment to 0.09 m/year at 1054 day. This experiment is ongoing.