CHARACTERIZING NET PERCOLATION THROUGH COAL MINE WASTE ROCK DUMPS USING HIGH-RESOLUTION PROFILES OF STABLE ISOTOPES OF WATER

Chemical constituents from unsaturated mine waste rock can be mobilized through the net percolation of snow melt or precipitation. Naturally occurring stable isotopes of water have been used to track recharge; however, there have been few cases for thick deposits (up to 100 m) of coarse-textured, unsaturated waste rock. In this study, deep coring of large coal waste rock spoils and natural soil profiles down gradient of the spoil piles was undertaken as part of a program examining the influence of surface mining on hydrological and water quality responses in the Elk Valley, British Columbia. Coring was undertaken using both dry (no water addition) and wet (water addition) sonic drilling methods. In the case of wet drilling, drill water was spiked with a deuterium tracer to identify contamination of the core samples by drilling fluids. The stable isotopes of water (d²H and d¹⁸O) in the pore-water of the core samples were measured in the laboratory using established vapour equilibration methods on core samples and on pore-water squeezed from the core to yield high-resolution depth profiles.

Substantial heating of the core occurred during both wet and dry drilling resulting in water loss, and concomitant stable isotope fractionation. A protocol for correcting the d²H and d¹⁸O profiles was developed based on equilibrium and kinetic fractionation and was verified in controlled laboratory tests. Subsequently, the corrected profiles were used to develop statistical distributions of d²H and d¹⁸O within the dump that were then compared to the isotope distributions for precipitation (snowmelt and rainfall), waste rock drain outflow, and stream flow for both natural and waste rock affected watersheds. Profiles through the natural soil profiles had a more unimodal distribution, typical of snowmelt recharge events. In contrast, the profiles from the spoils had a wider and bi-modal distribution, indicative of net percolation from both spring freshet (snow melt) and summer rainfall events. The vertical profiles within the waste rock dump were also indicative of two cycles of annual recharge with similar volumes of recharge from both snowmelt and rainfall. Isotopic profiling showed that the net percolation through un-reclaimed coal waste rock spoil is greater than through undisturbed material profiles.