REACTIVE AND INERT GASES IN GROUNDWATER CONTAMINATION AND REMEDIATION STUDIES

The role of gases has often been overlooked in groundwater contamination and remediation studies, despite the fact that processes involving gases can play a significant role in these environments. For example, groundwater contamination involving light non-aqueous phase liquids (LNAPLS) typically straddles the water table in shallow unconfined aquifers, a highly dynamic environment that is affected by gas entrapment and release due to water table fluctuations. In addition, biodegradation under natural attenuation conditions tends to consume O₂, and based on redox conditions generates N₂, CO₂ and CH₄, often exceeding the solubility of gases and leading to gas exsolution and ebullition from the saturated groundwater zone. Groundwater remediation is similarly affected by gas generation and migration. For example, gases including CH₄ and H₂ are produced and entrapped in permeable reactive barriers affecting the pore connectivity and permeability of the treatment media, CO₂ is generated and exsolved during treatment of chlorinated solvents with permanganate, and biostimulation may lead to excessive gas generation possibly increasing the mobility and spreading of volatile contaminants that are target of the treatment. This presentation provides a conceptual overview on the role of reactive and inert gases at contaminated sites and discusses the fate of gases based on selected case studies including: analysis of noble gas concentrations and ratios to investigate ebullition and vadose zone gas transport at a hydrocarbon contaminated site, the use of Ar and N₂ to investigate gas exsolution in a permeable reactive barrier, monitoring of O₂ and CO₂ to assess seasonal gas migration dynamics in a waste rock pile, and surficial gas efflux measurements at hydrocarbon and denatured ethanol fuel spill sites to locate and quantify hotspots of subsurface biodegradation. Methods developed in these studies are also applicable for investigating gas migration near oil and gas production wells (with an emphasis on assessing wellbore leakage at shale gas and fracking sites) and in oil sands tailings.