USING STABLE ISOTOPES TO UNDERSTAND THE MOVEMENT OF WATER IN WATERSHEDS INFLUENCED BY SURFACE MINING, ELK VALLEY, BRITISH COLUMBIA, CANADA

Surface mining is a common method of accessing coal. In high-elevation environments, vegetation and soils are typically removed prior to blasting of overburden rock to access coal and removed overburden is deposited in adjacent valleys as waste rock spoils. While there has been much research on the geochemistry of waste rock and the waters that emerge from waste rock spoils, there has been less emphasis on the impact of waste rock spoils on the hydrological response, particularly at larger watershed scales. Furthermore, there has been limited application of stable isotopes of water (d²H and d¹⁸O) to understand sources and residence times of water in mine-influenced systems. As part of a R&D program that is examining the influence of surface mining on watershed hydrological and water quality responses in the Elk Valley, British Columbia, a comparative study of hydrological, geochemical and stable isotope responses was done to determine the influence of waste rock on the sources and pathways of water in montane watersheds.

Stable isotopes of water in precipitation were sampled throughout the Elk Valley to establish a local meteoric water line and seasonal variability. Stream water was sampled at four reference and four mine-influenced watersheds, with two sites selected for high-frequency sampling. Flows were attenuated in mine-influenced watersheds, with the spring freshet delayed and a more muted response to precipitation events observed. Dissolved ions were an order of magnitude greater in mine-influenced streams, with more dilution-based responses to flows compared with chemostatic behavior in reference streams. Stable isotope signatures in stream water suggested that for both mine-influenced and reference watersheds, the stream water was derived from well mixed groundwater as annual variability of stream isotope signatures was dampened compared with precipitation signatures. However, stream isotope signatures are more responsive to enrichment and depletion events in reference watersheds. As a group, mine-influenced watersheds had a heavier isotope signature than reference watersheds, suggesting an enhanced influence of rainfall on recharge. Transit time distributions using the convolution approach indicate waste rock increases the average time water takes to move through the catchment.