Advanced diagnostic tools to support monitored natural attenuation for DNAPL plume in bedrock

Documenting biodegradation of contaminants is often necessary for acceptance of passive remedies for groundwater, such as monitored natural attenuation (MNA). For sites where contaminants can be degraded under a variety of mechanisms or where degradation products are not easily quantified, identifying degradation mechanisms can be difficult. A mature remediation site that employed an underground storage tank for methylene chloride (DCM) leaked an undetermined quantity over the span of several years and is the source of a contaminant plume in a sandstone, siltstone and shale bedrock aquifer. Because of the complex nature of the site, the minimal risk to receptors and the robust intrinsic biodegradation occurring within the fracture porosity, the site is being proposed for a technical impracticability (TI) waiver and MNA remedy. Rapid attenuation of the dissolved phase plume resulted in limited migration of DCM, although there was significant uncertainty in whether biodegradation was occurring due to a lack of degradation by-products, which jeopardized the acceptance the MNA remedy. A detailed characterisation effort, including the use of bedrock core sample analysis, contaminant passive flux meters (PFM), compound specific isotope analysis (CSIA) and meta-omics of bacterial populations, was conducted to build a robust conceptual site model (CSM) that would support remedy decision making. (author)