THE APPLICATION OF PUSH-PULL TESTING TO DEFINE BIOGEOCHEMICAL CONTROLS ON SELENIUM AND NITRATE ATTENUATION IN SATURATED COAL WASTE ROCK
Surface mining of steelmaking coal in the Elk Valley, British Columbia, Canada, has resulted in the release of constituents of interest, such as selenium (Se) and nitrate (NO3-). Oxidation of sulfide minerals in the unsaturated coal waste rock generates water-soluble forms of Se (selenite (Se4+) and selenate (Se6+)). Nitrate, introduced to waste rock through the blasting process, is also water soluble and mobile in the aqueous phase. Limited data suggest attenuation of Se and NO3- via reduction can occur in saturated waste rock, and therefore the placement of waste rock in topographic low areas, such as backfilled pits, could create conditions in which Se and NO3- attenuation would be enhanced. Key factors in the attenuation of these species are the reduction reaction rates and residence time of water in the saturated zone. Water level measurements, slug testing, and groundwater age dating were conducted at the Henretta saturated backfill study area at the Fording River Operation to develop an understanding of the hydraulics of the saturated backfill. Geochemical data collected over approximately 2.5 years were used to examine changes in porewater chemistry over time with respect to individual species, and compare the relationship between species over time (i.e., Se/SO42- and SO42-/NO3-). A method to examine in situ reaction rates (push-pull testing) was developed and tested. Tests were able to identify the consumption of dissolved oxygen (DO) and interpret DO consumption using a first-order reaction equation. Push-pull tests were then conducted at two wells in the study area to determine if Se6+ and/or NO3- can be attenuated over a 3-day test period. Additional information on the reduction potential of the study area was gathered from Se speciation analysis, NO3- isotopes, and dissolved organic carbon concentrations. Slug testing demonstrated that the waste rock is highly permeable (hydraulic conductivity on the order of 10-4 m/s), and tritium-helium age dating showed that water flow within the saturated fill is rapid. Although changes occurred in porewater geochemistry over time, the relative concentrations of SO42- and NO3- remained stable. On the other hand, the relationship between Se and SO42- varied over time, suggesting differences in the availability or attenuation of Se relative to SO42-. Results of push-pull testing, Se speciation, and NO3- isotope analyses indicate no observable Se or NO3- reduction occurred at the site over the period of testing (i.e., 3 d).
geochemistry, hydrogeology, waste rock, environmental geochemsitry
Master of Science (M.Sc.)