Stable isotope profiling for estimation of net percolation through a reclaimed overburden dump

Abstract

Characterising the hydrology and hydrogeology of reclaimed overburden fills at oil sands mines is required to evaluate contaminant loadings to surface water receptors. Conventional monitoring of water balance does not provide an accurate estimate of one of the most critical parameters, net percolation (groundwater recharge). Here we describe a novel method to estimate recharge rates in a saline-sodic overburden fill at the Syncrude Canada Ltd. Mildred Lake Mine in the Athabasca Oil Sands region of Alberta, Canada. This site has been used to study cover performance and monitor the surface water balance for different configurations of reclamation cover over the past decade. Core samples of the cover soil and overburden fill were collected from nine locations across the site. The core samples were analysed in the laboratory for the stable isotopes, δ2H and δ18O, in the pore water. A local meteoric water line (LMWL) was developed from precipitation samples collected at the site in 2009 and was used to assess whether evaporation had altered the isotopic signatures of the pore water. Stable isotope profiles were generated from the soil sample analysis results at each sampling location. The one dimensional δ2H profiles were mathematically simulated to estimate the long-term net percolation rates at different topographic positions. The estimated net percolation rates ranged from 32–50 mm/yr for the plateau and mid-slope bench to 0–12 mm/yr for locations on the slope.