PASSIVE GAS DIFFUSION SAMPLING AT MONITORED NATURAL ATTENUATION SITES

Monitored natural attenuation (MNA) is an increasingly popular remediation strategy at oil and gas impacted sites. Accurate data that demonstrate that biodegradation is occurring is critical to MNA assessment. Regulators look for both a decrease in contaminant concentrations, and evidence that these contaminants are degrading, ideally to CO2 and CH4 degradation end-products. Measurement of these gases is also valuable for interpreting sub-surface biogeochemical processes and robust mass balance calculations.

Dissolved gas concentrations from passive gas diffusion samplers can provide strong supportive evidence of MNA. Passive diffusion sampling in combination with measurement of total dissolved gas pressure (TDGP) is an attractive alternative to conventional dissolved gas sampling techniques (e.g. headspace extraction of a groundwater sample). Atmospheric contact is minimized, and gas samples are collected under in situ TDGP. Whereas traditional gas sampling provides relative dissolved gas concentrations, passive gas sampling in combination with TDGP can provide absolute concentrations of dissolved gases in groundwater.

Diffusion samplers were deployed in groundwater monitoring wells with varying degrees of hydrocarbon contamination at several MNA research sites in Alberta. Dissolved gas concentrations acquired using traditional headspace extraction techniques were compared to those collected with passive diffusion samplers combined with TDGP measurement. Samples collected using conventional methodology often underestimated the consumption (O2) and production (CO2 and CH4) of dissolved gases during biodegradation reactions. Elevated CO2 and CH4 concentrations provided strong evidence that MNA was occurring at these sites and insight into the spatial extent of biodegradation. Seasonal temperature fluctuations had notable effects on the production of dissolved gases in shallow groundwater.

Conventional groundwater gas sampling techniques may not provide sufficient accuracy for MNA sampling programs. Passive gas diffusion samplers combined with TDGP measurement can provide the more robust evidence of biodegradation that regulators require.