LABORATORY EXPERIMENTS FOR EVALUATION OF SOLUTE TRANSPORT WITH CONSTRUCTION OF BACKFILL AQUIFERS IN RESTORED COAL MINE PITS, POWDER RIVER BASIN

Mined waste rock is commonly stored for post-mine land reclamation where the waste rock may become part of local aquifers. The generation and disposal of waste rock may increase contaminants in local aquifers due to enhanced weathering of newly exposed mineral surfaces and the production of transportable particles. The potential release and weathering of transportable particles (e.g., colloids, nanoparticles) is difficult to predict but may be a substantial transport pathway for contaminants during early-stage weathering of waste rock. Such issues are present in the Powder River Basin where waste rock is used to restore coal mine pits, which has resulted in groundwater contamination from previously bound elements, such as cadmium and selenium. To evaluate the influence of new mineral surfaces and particle transport in early-stage weathering of siliciclastic waste rock from a coal mine in the Powder River Basin, laboratory column experiments were conducted over 20 weeks under warm (20 °C) and cold (5 °C) conditions. The kinetic columns were exposed to a cycle of 72-hr saturation, 2-hr leachate collection, and a 6-hr unsaturated condition that was repeated throughout the experiment. Anion and cation species were analyzed for unfiltered and 0.45-μm and 0.2-μm filtered leachate. Results from particle size analysis of unfiltered samples indicated a quick decrease in the first two weeks followed by a median particle size range of 100 to 200 nm for the warm condition and 300 to 1,000 nm for the cold condition from week 3 to 16. Cadmium and selenium were released into solution over the first 3 weeks of the experiment after which the elements were no longer detected in the leachate, which is suggestive of only particle transport of these elements. Typical aluminosilicate-associated elements, such as potassium and manganese, were released in substantial concentrations during the first 3 weeks followed by a steady decline in concentration from week 4 to week 8 and a slow decline in concentrations from week 9 to week 20, suggestive of early particle transport followed by high mineral weathering and a final lower mineral weathering state. These weathered leachate elements also exhibited reduced concentrations in leachate at colder temperatures, indicating activation energy control on early-stage weathering solute release.