Tyler K. Birkham

Microbial respiration and diffusive transport of O2, 16O2, and 18O16O in unsaturated soils: a mesocosm experiment

Abstract Although the flux of molecular O 2 between the atmosphere and the subsurface is intrinsically linked to the net soil production of greenhouse gasses, few studies have focused on the controls affecting the isotopic composition of O 2 in the subsurface. Here, we developed and tested a stable oxygen isotope tracer technique and gas transport modeling approach to evaluate O 2 cycling and fluxes from the subsurface that used an environmentally controlled soil mescosm. We measured the O 2 and δ 18 O 2 profiles in a model unsaturated soil zone and quantified the O 2 consumption rates and the O 2 isotope fractionation factors resulting from the combined processes of subsurface microbial (including bacteria, fungi, and protozoa) consumption of O 2 and diffusive influx of O 2 from the atmosphere. We found that at high respiration rates in the mesocosm, there appeared to be very little isotope fractionation of O 2 by soil microorganisms. Although the mesocosm respiration rates are not typical of natural soils in northern temperate climes, they may be more representative of soils in warm and moist tropical environments. Our findings caution against the indiscriminate application of laboratory-determined oxygen isotope fractionation factors to field settings. The oxygen isotope tracer and modeling approach demonstrated here may be applied to gain a better understanding of biogenic gas production and O 2 cycling in subsurface systems and soils.

Assessing groundwater discharge to streams with distributed temperature sensing technology

Characterization of the interactions of groundwater with surface stream water is fundamental to understanding and managing stream water quality in natural and altered watersheds. The spatial and temporal variability in the exchange of water and solutes across the streambed affects stream water quality evolution along the channel length, especially in coarse-grained alluvial sediments. Identifying and delineating areas of groundwater discharge has the potential to inform water quality management strategies including the location and design of intake structures, groundwater cut-off systems, or permeable reactive barriers. Distributed temperature sensing (DTS) is an emerging technology that has found wide application in stream hydrology. A DTS system integrates a fibre-optic (FO) cable to measure continuous temperature profiles (manufacture reported accuracy of ±1 °C and resolution of ±0.01 °C) with spatial resolutions of 1 m or better over distances of up to several kilometres. In September 2013, a DTS cable was installed in a predominantly coarse-textured stream channel at a coal mining operation in the Elk Valley of British Columbia. The objective was to evaluate the utility of a FO DTS system in a location where identification of groundwater discharges to a stream was being studied as part of Teck Resources Limited (Teck) applied research and development program focused on managing water quality in mine watersheds. This study was intended as a verification of methods for in-stream DTS installation and measurements with recommendations made for improved calibration procedures. Temperature was measured along a 160-m stretch of stream and streambed at 20 minute sampling intervals using a 5 minute signal integration time over six days. In-stream DTS measurements were successful in delineating a localized area of lower temperature identified as a groundwater discharge zone. The measurement of cooler groundwater discharge in the streambed relative to the stream was corroborated by the presence of well-established riparian vegetation and a streambank seep.

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.

Near-surface water balances of waste rock dumps

The near-surface water balance of mine impacted landscapes is a key control on re-vegetation performance, and on the hydrologic and water quality impact at the watershed scale. As part of Teck Resources Limited’s applied research and development program focused on managing water quality in mine-affected watersheds, 12 sites in western Canada (southeastern British Columbia and western Alberta) representing a range of waste rock dump reclamation surface management options (i.e. soil cover, surficial mounding) were instrumented in 2012 to measure meteorological and soil water response and to quantify the near-surface water balances with a focal objective to improve estimates of ranges of net percolation into waste rock dumps under a range of scenarios. Subsurface water and meteorological conditions varied substantially, as expected for the range of elevation, slope aspects, vegetation, soil covers, geographic location and surface preparation of the selected sites. Patterns in water balance trends emerged in the first year of analysis with net percolation (NP) into underlying waste rock typically decreasing for increased vegetation and soil cover, as well as for decreases in rainfall or snowmelt. Increased vegetation cover resulted in a greater volume of water removed from near-surface through evapotranspiration. The lowest NP (as % of water input) was estimated for a mature, reclaimed conifer forest site and a dense agronomic grass/alfalfa covered site. Net percolation estimated for a soil covered waste rock slope was approximately 15% (of water inputs) less than an adjacent bare waste rock slope. Decreased NP was partly attributed to greater water storage in the finer-textured soil cover. Net percolation through the soil cover is expected to further decrease with time as vegetation establishes relative to the bare waste rock slope. Net percolation for a mounded, bare waste rock slope was less than estimated for an adjacent smooth slope. Net percolation below a trough was similar to the smooth slope, but decreased at the crest and mounded mid-slope positions due to thinner snowpack (less snowmelt) from wind scouring. Additional monitoring and analysis of site-specific water balances will help define the shift in the relative proportions of water entering the deposits as vegetation matures.