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Modelling the influence of well pads on toluene biogenesis in peatlands.

- General

The appearance of toluene in organic soils, which is not consistent with an anthropogenic source, has recently garnered attention within the oil and gas industry. Previous research has shown toluene is a biogenic intermediary product of microbial fermentation in strongly anaerobic and cold conditions similar to the conditions in northern Alberta peatlands. This suggests there is a separate, perhaps biogenic, source of toluene that is not reflected in current soil and groundwater guidelines.

Higher concentrations of toluene are often found beneath the clay pad of abandoned wellsites built on peatlands and commonly peak at the interface between pad and peat materials with concentrations up to 30 parts per million. In a recent project, we suspected that elevated toluene was due to an interruption in gas diffusion from the clay pad, and higher microbial activity related to availability of an anaerobic electron acceptor (sulphate) in clay pads. These observations suggested the clay pad was producing an indirect effect which was increasing toluene levels under well pads.

Elevated toluene levels are of concern because toluene is toxic to receptors in all trophic levels of fresh water aquatic ecosystems in parts per BILLION. However, the relevancy of current species-specific benchmarks for use in peatland ecosystems are unclear. Reference species used in ecotoxicity studies, e.g. water fleas, leopard frogs, and rainbow trout, are uncharacteristic of these ecosystems. Further investigation of this indirect impact is required to develop best management practices.

The consistency of the on-site/background toluene pattern at our field site has provided a unique opportunity to gain understanding of this emerging issue. Statistical analysis of lab results may be able to identify a background toluene guideline, which can be used as a benchmark for reclamation.

Multi-response permutation procedure (MRPP) analysis showed similarities in toluene levels between two wellsite groups. This suggests toluene biogenesis is fairly consistent and a background-based toluene guideline for bog ecosystems could be applied regionally.

Preliminary analysis supports our hypothesis and shows a separation of on-site and background samples in bog ecosystems. Sulphate and toluene levels under well pads were generally higher than background levels. Statistical modeling indicates that on-site toluene levels are higher the background levels, with a confidence level of 80%. We believe this confidence level, coupled with the toxicity of toluene and the elevated toluene levels, supports the indirect impact caused by clay pads and represents a credible risk to the environment.

Unfortunately, preliminary analyses of our existing data was insufficient to determine if there was a significant difference between on-site and background levels in fen eco-sites. This suggests further data is required to reduce the error in this analysis.

Our client has agreed to share soil chemistry data from Phase 2 Environmental Site Assessments (ESA) within a broadened study area in northwestern Alberta for inclusion in further analysis. We are hopeful the increase in data will allow us to better understand the spatial distribution of biogenic toluene in fen ecosystems and increase our confidence levels for predictions for bog ecosystems. This could produce sufficient support for model predictions that could be used as background-based toluene guidelines for bogs and fens.

A background-based guideline could be used as a benchmark for comparison to occurrences of biogenic toluene. This will create a standard to determine if intrusive remediation is required and will reduce remediation-related disturbance in this sensitive ecosystem. Further, this would reduce the management costs to industry members, regulators, and land owners.

Solstice Canada Corp. has received funding from the Petroleum Technology Alliance of Canada (PTAC) via the Alberta Upstream Petroleum Research Fund (AUPRF) to perform a meta-analyses using ESA data. Technical advice and support is being provided by various partners including Dee Patriquin (Adjunct Professor) and Anne McIntosh (Assistant Professor), of the University of Alberta, Augustana Campus.

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