Real-Time Assessment of VOCs in Fractured Bedrock Using Innovative Core Discrete Fracture Network (DFN) Techniques

Following an initial program of site investigation using a variety of High Resolution Site Characterization (HRSC) techniques in accordance with the U.S. EPA Triad framework, chlorinated solvent impact was identified beneath an active manufacturing site within unconsolidated alluvial deposits at the base of the alluvial deposits and upper margins of underlying shale bedrock.

In order to determine vertical contaminant distribution and extent, a second stage of works was implemented comprising detailed assessment of the shale bedrock via the Core DFN approach. This included collection of cores for geological logging, field screening (with a photoionization detector) and on-site pore water extraction using Microwave Assisted Extraction (MAE) to rapidly obtain circa 450 crushed rock samples for on-site VOC analysis using a GC/MS.

The use of the MAE equipment enabled the investigation to be completed using a dynamic Triad style approach and provided near real-time on-site analyses of bedrock matrix contamination data that was used to progressively refine the investigation scope in a sustainable manner.

The results of the fractured bedrock assessment provided a refined and detailed Conceptual Site Model showing that the contaminant plume migrated laterally through the weathered shale profile and the interface with deeper, fresher fractured shale bedrock provided a “barrier” to significant vertical contaminant migration.

The results of the chemical analysis identified two trichloroethene source zones and indicated that significantly greater contaminant concentrations were present within pore water samples (up to 1620 mg/L), than the dissolved phase concentrations detected in samples collected from monitoring wells had initially shown (up to 40 mg/L total VOCs), reflecting a typical mass distribution for fractured rock with the greatest concentrations present within the rock matrix.

The refined conceptual site model was instrumental in the development of a technically appropriate and cost effective sustainability-led remediation approach, which was subsequently implemented at the site.