Selenate bioreduction in a large in situ field trial.

Removing selenium (Se) from mine effluent is a common challenge. A long-term, in situ experiment was conducted to bioremediate large volumes (up to 7500 mc d-1) of Se(VI)-contaminated water (mean 87 µg L-1) by injecting the water into a saturated waste rock fill (SRF) at a coal mining operation in Elk Valley, British Columbia, Canada. To stimulate/maintain biofilm growth in the SRF, labile organic carbon (methanol) and nutrients were added to the water prior to its injection. A conservative tracer (Br-) was also added to track the migration of injected water across the SRF, identify wells with minimal dilution and used to quantify the extent of bioreduction. The evolution of the Se species through the SRF was monitored in time and space for 201 d. Selenium concentrations of <3.8 µg L-1 were attained in monitoring wells located 38 m from the injection wells after 114 to 141 d of operation. Concentrations of Se species in water samples from complementary long-term (351-498 d) column experiments using influent Se(VI) concentrations of 1.0 mg L-1 were consistent with the results of the in situ experiment. Solid samples collected at the completion of the column experiments confirmed the presence of indigenous Se-reducing bacteria and that the sequestered Se was present as insoluble Se(0), likely in Se-S ring compounds. Based on the success of this ongoing bioremediation experiment, this technology is being applied at other mine sites.

Quantifying denitrification in a field-scale bioremediation experiment.

Nitrate (NO3-) in mine waste rock derived from undetonated NH4NO3 can contaminate receiving waters. An in-situ bioremediation experiment was conducted at a coal mining operation in Elk Valley, British Columbia, Canada to remediate NO3- from large volumes of mine water. Over the test period (201 d), 5000 to 7500 m3 d-1 of NO3--rich (mean concentration 22 mg N L-1) mine water was injected into saturated waste rock along with methanol, nutrients, and a conservative tracer (Br-). Complete denitrification (<0.5 mg N L-1) was recorded in monitoring wells located 38 m from the injection wells after 114 to 141 d of operation. Plots of δ15N- and δ18O-NO3- versus NO3--N concentrations for monitoring wells yielded isotopic enrichment factors (ε) for δ15N- and δ18O-NO3- of -25.7 and -13.2 ‰ for high C/C0 NO3- concentrations (>10.5 mg N L-1) and -5.5 and -3.6 ‰ for lower C/C0 values. The fraction of NO3- denitrified (Dp) calculated using bi-linear ε values for δ15N- and δ18O reproduced the Dp determined independently using a conservative tracer indicating that stable isotope tracers of the NO3- reducing processes in bioremediation are invaluable to determine Dp. Based on the success of this ongoing bioremediation experiment, the technology is being applied at other sites.

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.

Fate and Transport of Shale-derived, Biogenic Methane.

Natural gas extraction from unconventional shale gas reservoirs is the subject of considerable public debate, with a key concern being the impact of leaking fugitive natural gases on shallow potable groundwater resources. Baseline data regarding the distribution, fate, and transport of these gases and their isotopes through natural formations prior to development are lacking. Here, we define the migration and fate of CH4 and δ13C-CH4 from an early-generation bacterial gas play in the Cretaceous of the Williston Basin, Canada to the water table. Our results show the CH4 is generated at depth and diffuses as a conservative species through the overlying shale. We also show that the diffusive fractionation of δ13C-CH4 (following glaciation) can complicate fugitive gas interpretations. The sensitivity of the δ13C-CH4 profile to glacial timing suggests it may be a valuable tracer for characterizing the timing of geologic changes that control transport of CH4 (and other solutes) and distinguishing between CH4 that rapidly migrates upward through a well annulus or other conduit and CH4 that diffuses upwards naturally. Results of this study were used to provide recommendations for designing baseline investigations.

Measuring Concentrations of Dissolved Methane and Ethane and the 13 C of Methane in Shale and Till.

Baseline characterization of concentrations and isotopic values of dissolved natural gases is needed to identify contamination caused by the leakage of fugitive gases from oil and gas activities. Methods to collect and analyze baseline concentration-depth profiles of dissolved CH4 and C2 H6 and δ13 C-CH4 in shales and Quaternary clayey tills were assessed at two sites in the Williston Basin, Canada. Core and cuttings samples were stored in Isojars® in a low O2 headspace prior to analysis. Measurements and multiphase diffusion modeling show that the gas concentrations in core samples yield well-defined and reproducible depth profiles after 31-d equilibration. No measurable oxidative loss or production during core sample storage was observed. Concentrations from cuttings and mud gas logging (including IsoTubes® ) were much lower than from cores, but correlated well. Simulations suggest the lower concentrations from cuttings can be attributed to drilling time, and therefore their use to define gas concentration profiles may have inherent limitations. Calculations based on mud gas logging show the method can provide estimates of core concentrations if operational parameters for the mud gas capture cylinder are quantified. The δ13 C-CH4 measured from mud gas, IsoTubes® , cuttings, and core samples are consistent, exhibiting slight variations that should not alter the implications of the results in identifying the sources of the gases. This study shows core and mud gas techniques and, to a lesser extent, cuttings, can generate high-resolution depth profiles of dissolved hydrocarbon gas concentrations and their isotopes.

Defining near-surface groundwater flow regimes in the semi-arid glaciated plains of North America.

The dominant transport mechanisms controlling the migration of contaminants in geologic media are advection and molecular diffusion. To date, defining which transport mechanism dominates in saturated, non-lithified sediments has been difficult. Here, we illustrate the value of using detailed profiles of the conservative stable isotope values of water (δ(2)H and δ(18)O) to identify the dominant processes of contaminant transport (i.e. diffusion- or advection-dominated transport) in near-surface, non-lithified, saturated sediments of the Interior Plains of North America (IPNA). The approach presented uses detailed δ(18)O analyses of glacial till, glaciolacustrine clay, and fluvial sand core samples taken to depths of 11-50 m below ground at 22 sites across the IPNA to show whether transport in the fractured and oxidized sediments is dominated by advection or diffusion. Diffusion is by far the dominant transport mechanism in fine-textured lacustrine and glacial till sediments, but lateral advection dominates transport in sand-rich sediments and some oxidized, fine-textured lacustrine and glacial till sediments. The approach presented has a number of applications, including identifying dominant transport mechanisms in geomedia and potential protective barriers for underlying aquifers or surface waters, constraining groundwater transport models, and selecting optimum locations for monitoring wells. These findings should be applicable to most glaciated regions of the world that are composed of similar hydrogeologic units (i.e. low K clay till layers overlain by higher K coarse-textured aquifers or weathered clay till layers) and may also be applicable to non-glaciated regions exhibiting similar hydrogeologic characteristics.