THE ROLE OF EPHEMERAL PONDS IN GROUNDWATER RECHARGE IN THE CANADIAN PRAIRIES

The Canadian prairies are characterized by a mosaic of uplands and wetlands on the surface, and in the subsurface a sequence of surficial unconfined transmission aquifers, overlying clay-rich glacial till aquitards which are up to 100s of meters thick, with sporadic coarse grained inter-till confined aquifers There is consensus that the relatively shallow (< 50 m) confined aquifers in this environment are recharged with modern water, but the mechanism of recharge is not well understood. Published isotope data are ambiguous, and most researchers have suggested these aquifers are recharged by “depression-focused recharge”, which is to say snow and snowmelt runoff collect in wetland depressions, infiltrate after the soil thaws, and produce groundwater recharge. At our field site, St Denis, Saskatchewan, there is a confined aquifer approximately 30 m below the ground surface. In this study we used physical measurements and stable isotopes to explore the recharge mechanisms to the confined aquifer at St Denis. We propose a refinement to the conceptual depression-focused recharge model of this system, whereby ephemeral ponds are the dominant supply of groundwater recharge. This idea is consistent with all of the physical and chemical observations at our site.