Assessing the fate of explosives derived nitrate in mine waste rock dumps using the stable isotopes of oxygen and nitrogen.

Ammonium nitrate (NH4NO3) mixed with fuel oil is a common blasting agent used to fragment rock into workable size fractions at mines throughout the world. The decomposition and oxidation of undetonated explosives can result in high NO3- concentrations in waters emanating from waste rock dumps. We used the stable isotopic composition of NO3- (δ15N- and δ18O-NO3-) to define and quantify the controls on NO3- composition in waste rock dumps by studying water-unsaturated and saturated conditions at nine coal waste rock dumps located in the Elk Valley, British Columbia, Canada. Estimates of the extent of nitrification of NH4NO3 in oxic zones in the dumps, initial NO3- concentrations prior to denitrification, and the extent of NO3- removal by denitrification in sub-oxic to anoxic zones are provided. δ15N data from unsaturated waste rock dumps confirm NO3- is derived from blasting. δ15N- and δ18O-NO3- data show extensive denitrification can occur in saturated waste rock and in localized zones of elevated water saturation and low oxygen concentrations in unsaturated waste rock. At the mine dump scale, the extent of denitrification in the unsaturated waste rock was inferred from water samples collected from underlying rock drains.

Microbial respiration and diffusive transport of O2, 16O2, and 18O15O in unsaturated soils and geologic sediments.

Molecular oxygen (O2) in unsaturated geologic sediments plays an important role in soil respiration, biodegradation of organic contaminants, metal oxidation, and global oxygen and carbon cycling, yet little is known about oxygen isotope fractionation during the consumption and transport of O2 in unsaturated zones. We used a laboratory kinetic cell technique to quantify isotope fractionation due to respiration and a numerical model to quantify both consumptive and diffusive fractionation of O2 isotopes at a field site comprised of unsaturated lacustrine sandy materials. The combined use of laboratory-based kinetic cell experiments and field-based isotope transport modeling provided an effective tool to characterize microbial respiration in unsaturated media. Based on results from the closed-system kinetic cells, O2 consumption and isotope fractionation were attributed to the alternative cyanide-resistant respiration pathway. At the field site, the modeled depth profiles for O2 and delta18O matched the measured in situ data and confirmed that the consumption of O2 was via the alternative respiration pathway. If the cyanide-resistant respiration pathway is indeed widespread in soils, its high oxygen isotope enrichment factor could help to explain the discrepancy between the predicted present-day Dole effect (+20.8/1000) and the observed Dole effect (+23.5/1000). Thus, further soil O2 isotope studies are needed to better characterize and model the fractionation of oxygen isotopes during subsurface respiration and the potential impact on the isotopic content of atmospheric O2.

Characterizing geochemical reactions in unsaturated mine waste-rock piles using gaseous O2, CO2, 12CO2, and 13CO2.

In situ determinations of geochemical reaction rates in mine waste-rock piles remain a challenge. Depth-profiles of field O2 and CO2 pore-gas concentrations, delta13C(CO2) values, and moisture contents were used to characterize and quantify geochemical reaction rates in two waste-rock piles at the Key Lake Uranium Mine in northern Saskatchewan, Canada. Traditionally, the presence of O2 concentrations less than atmospheric in waste-rock piles has been attributed to mineral oxidation. This study showed that the interpretation of O2 and CO2 concentration profiles alone could not be used to identify the depths of dominant geochemical reactions in the piles and could lead to erroneous estimates of reaction rates. Modeling of the delta13C(CO2) depth profiles clearly showed that the gas concentration profiles present in the piles were the result of the oxidation of organic matter present below the piles, a mechanism not previously reported in the literature. Based on these findings, the rates of reactions in the organic zone were determined. The oxidation of organic matter at the base of waste-rock piles should be considered in future mine-waste pore-gas studies, in addition to sulfide oxidation and carbonate buffering.