HYDROGEOLOGICAL CHARACTERIZATION OF A GROUNDWATER SYSTEM DOWNGRADIENT OF A COAL WASTE ROCK SPOIL

Chemical solutes can be mobilized from waste rock spoils and move downgradient to ground water and surface water systems. The geochemical conditions within the groundwater system can also help to attenuate some of the solutes in mine-affected water. Drainage from a coal waste rock spoil in the Elk Valley, British Columbia is known to contain selenium (Se), sulphate (SO₄), derived from the oxidation of pyrite, and nitrate (NO₃) from blasting; however, the impact of the waste rock spoil on groundwater quality has not been studied in detail. This study was conducted as part of a R&D program identifying controls on waste rock chemistry, hydrology, and water quality in the region and specifically focused on the hydrogeology and geochemistry of a natural groundwater system within a small mine-affected watershed (the West Line Creek watershed). The geology of the study site consisted of a complex mix of glacial and alluvial deposits underlain by shale bedrock. An unconfined aquifer, formed at the alluvial/fractured bedrock interface, was identified as the primary conduit for the migration of solutes toward Line Creek. Total elemental analyses were conducted on core samples from the geologic formations (n=182) and water chemistry analyses were conducted on samples collected from pore waters squeezed from core samples (n= 267), water leached from cores (n= 53), piezometers (n= 120), springs (n= 192), a rock drain (n= 133), and surface waters (n= 111) between May 2013 and September 2014. The distribution of NO₃, Se and SO₄ across the study site varied markedly. The Se/SO₄ molar ratios of rock drain samples were uniform (at 10^-4) and consistent with pyrite oxidation in the waste rock. The Se/SO₄ molar ratios of water samples collected from piezometers screened in shallow bedrock and pore waters squeezed from fine grained sediments (10^-5) suggests Se attenuation in the groundwater system.