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Nickerson N.,Dalhousie University | Nickerson N.,Forerunner Research Inc. | Nickerson N.,St. Francis Xavier University | Egan J.,St. Francis Xavier University | Risk D.,St. Francis Xavier University
Journal of Geophysical Research: Biogeosciences | Year: 2014

Measurements of the stable isotope composition of soil flux have many uses, from separating autotrophic and heterotrophic components of respiration to teasing apart information about gas transport physics. While soil flux chambers are typically used for these measurements, subsurface approaches are becoming more accessible with the introduction of field-deployable isotope analyzers. These subsurface measurements have the unique benefit of offering depth-resolved isotopologue flux data, which can help to disentangle the many soil respiration processes that occur throughout the soil profile. These methods are likely to grow in popularity in the coming years and a solid methodological basis needs to be formed in order for data collected in these subsurface studies to be interpreted properly. Here we explore the range of possible techniques that could be used for subsurface isotopologue gas interpretation and rigorously test the assumptions and application of each approach using a combination of numerical modeling, laboratory experiments, and field studies. Our results suggest that methodological uncertainties arise due to poor assumptions and mathematical instabilities but certain methods, particularly those based on diffusion physics, are able to cope with these uncertainties well and produce excellent depth-resolved isotopologue flux data. Key Points A rigorous test of subsurface isotopologue flux methodologies is performed Methodological uncertainties arise due to poor assumptions The gradient approach was best for estimating the isotopic composition ©2014. American Geophysical Union. All Rights Reserved.

Creelman C.,St. Francis Xavier University | Creelman C.,Forerunner Research Inc. | Nickerson N.,St. Francis Xavier University | Nickerson N.,Dalhousie University | Risk D.,St. Francis Xavier University
Soil Science Society of America Journal | Year: 2013

A variety of chamber methodologies have been developed in an attempt to accurately measure the rate of soil CO2 respiration. However, the degree to which these methods perturb and misread the soil signal is poorly understood. One source of error in particular is the introduction of lateral diffusion due to the disturbance of the steady-state CO2 concentrations. The addition of soil collars to the chamber system attempts to address this perturbation, but may induce additional errors from the increased disturbance. Using a numerical three-dimensional (3D) soil-atmosphere diffusion model, we have undertaken a comprehensive and comparative study of existing static and dynamic chambers. Specifically, we are examining the 3D diffusion errors associated with each method and opportunities for correction. The impacts of collar length and diffusion parameters on lateral diffusion around the instruments are quantified to provide insight into obtaining more accurate soil respiration estimates. Results suggest that while each method can approximate the true flux in low diffusivity environments, the associated errors can be large and vary substantially in their sensitivity to both method-specific and environmental parameters. In some cases, factors such as collar length and soil diffusivity are coupled in their effects on accuracy. © Soil Science Society of America.

Risk D.,St. Francis Xavier University | McArthur G.,Forerunner Research Inc. | Nickerson N.,Forerunner Research Inc. | Phillips C.,Lawrence Livermore National Laboratory | And 3 more authors.
International Journal of Greenhouse Gas Control | Year: 2013

To help evaluate surface monitoring tools for Weyburn, it is important to establish ranges of natural variation, and signal to noise ratio (SNR) of MMV tools in their intended setting. This study took place at three sites, two of which were in the injection field. For six months, we measured parameters at various temporal scales from half-hourly (CO2 surface flux and meteorology), to monthly (soil gas CO2 and δ13CO2), to bi-monthly (soil gas CO142), to compare SNRs of promising MMV techniques for Weyburn. Our summary of findings is as follows:1.All observed data fell within the range of values considered normal for Weyburn and for proximal control sites such as the Minard farm.2.High temporal variation in CO2 surface fluxes were observed. Lower atmospheric CO2 concentrations were also highly variant, and coupled with abiotic factors. A modelling strategy was able to reduce observed variability by 80-95%. When used together, soil CO2 surface flux + modelling methods can produce high SNRs for leak detection.3.Temporal variability in soil profile CO2 concentration was controlled by soil gas diffusivity (soil wetting/drying) and not biological production. Despite various sources of noise, we conclude that soil gas bulk CO2 investigations can still be useful for MMV.4.There were many possible influences on δ13CO2, including biological variation, normal steady and non-steady state physical transport (several ‰), spatial differences (0-3‰), and temporal fluctuations (0-3‰). The effects of these influences are cumulative. Relative to this background variation, the Cenovus-source δ13CO2 is not highly differentiated, and δ13CO2 is not a robust tracer.5.High precision radiocarbon soil profile data indicates that CO2 produced within the soil profile is modern and its average age is less than decades old. This age is consistent with other studies, and recent Kerr investigations (Trium, 2011). There was a tendency towards older CO142 production with increasing depth. There is a marked differentiation in CO142 signature from deep gases, and low variation. Radiocarbon is a very promising tracer for Weyburn with high SNR. © 2013 Elsevier Ltd.

Nickerson N.,Forerunner Research Inc. | Nickerson N.,Dalhousie University | Nickerson N.,St. Francis Xavier University | Risk D.,Forerunner Research Inc. | Risk D.,St. Francis Xavier University
International Journal of Greenhouse Gas Control | Year: 2013

In order to fulfill a role in demonstrating containment, surface monitoring for Carbon Capture and Geologic Storage (CCS) sites must be able to clearly discriminate between natural, and reservoir-source CO2. The CCS community lacks a clear metric for quantifying the degree of discrimination, for successful inter-comparison of monitoring approaches. This study illustrates the utility of signal-to-noise ratio (SNR) to compare the relative performance of three commonly used soil gas monitoring approaches, including bulk CO2, δ13CO2, and Δ14CO2. For inter-comparisons, we used a simulated northern temperate landscape similar to that of Weyburn, Saskatchewan (home of the IEAGHG Weyburn-Midale CO2 Monitoring and Storage Project), in which realistic spatial and temporal CO2 and isotopic variation is simulated over multiple annual cycles, so that the techniques may also be inter-compared seasonally. Results show that methods with strong difference between biological and seepage source CO2, such as Δ14C signatures in this study, have the best overall SNR values. However, our analysis also shows each monitoring approach could be useful, depending on the desired seepage certainty level and characteristics including site spatial variability and injection gas attributes. This study emphasizes both the importance of developing clear metrics for monitoring performance, the need to evaluate SNR and MMV approaches on a site specific basis, and the benefit of modeling for decision support in CCS monitoring design. © 2013 Elsevier Ltd.

Risk D.,St. Francis Xavier University | Lavoie M.,St. Francis Xavier University | Nickerson N.,Forerunner Research Inc.
International Journal of Greenhouse Gas Control | Year: 2015

Monitoring is cited as a mechanism by which operators can drive public acceptance in CO2 geosequestration and energy projects. In this synthesis study we inter-compared a broad suite of geochemical monitoring indicators, to identify which could contribute best to an affordable, yet effective, monitoring system. We examined two very comprehensive datasets, collected during a leak investigation, from a CO2 enhanced oil recovery (EOR) project in Weyburn, Canada, and applied an existing signal to noise ratio (SNR) approach to build a quantitative benchmark for each indicator. Despite the fact that a leak would be composed mostly of CO2, the analysis shows that a leak would be difficult to identify at this site based on CO2 or 13C-CO2, owing to the high variability of natural sources. Affordable alternatives such as N2, Ar, He, and CO2/O2 ratio would be less impacted by background variability. The most definitive indicators were 4He/20Ne (a proxy), and radiocarbon-CO2 (a direct indicator), though cost limits their everyday applicability. We conclude that a narrowed suite of indicators could achieve effectiveness at reduced cost, but that definitive indicators should be favoured in cases where leakage is suspected. © 2015 Elsevier Ltd.

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