Formatted Title
Integration of Non-Invasive Surface Geophysics, High-Resolution Site Characterization, Borehole Geophysics, and Vertical Rock Core Profiling to Delineate CVOC and 1,4-Dioxane Source Areas in Weathered and Fractured Bedrock
Background/Objectives
Restoration of weathered and fractured bedrock impacted with recalcitrant compounds, such as chlorinated volatile organic compounds (CVOCs) and 1,4-dixoane, continues to be one of the biggest challenges in society. Remediation of chlorinated volatile organic compounds (CVOCs) in weathered and fractured bedrock without proper source area characterization and delineation is an unfortunate and frequent practice in the environmental industry. This is because source area characterization and delineation are challenging to implement effectively, not expeditious, and expensive. Incomplete source area characterization can lead to selection of an ineffective remedial strategy. Incomplete source area delineation often results in untreated plumes associated with unidentified sources or source areas that have larger volumetric extent than originally suspected. Most importantly, proper characterization of source areas to assess the presence of dense non-aqueous phase liquid (DNAPL) is critical to assess feasibility of remedial technologies to meet cleanup goals. In this talk, we will discuss our approach for characterization of CVOC and 1,4-dixoane source areas at different scales (i.e., fracture scale and plume scale) to understand the physical processes (e.g., advection, dispersion, matrix diffusion, sorption). We will discuss our phased approach for deployment of appropriate tools (e.g., surface geophysics, high-resolution site characterization [HRSC], borehole geophysics, and vertical rock core profiling) to collect geological and hydrogeological data sets necessary to evaluate these processes and their control on observed contaminant distribution.
Approach/Activities
Using a phased approach, CVOC and 1,4-dioxane source areas were characterized and delineated at a site located in the Piedmont physiographic province in North Carolina. The geology underling the site consists of crystalline metamorphic protolith overlain by a variably thick regolith. Pump and treat has been the remedy for CVOCs in groundwater for over 20 years. Historical data (e.g., a GORE-SORBER® passive soil gas survey, unsaturated soil analytical data, and 20 years of groundwater analytical data) facilitated the design of the source areas characterization and delineation program. Non-invasive surface geophysical surveys including shallow and deep two-dimensional (2D) electrical resistivity imaging (ERI), multi-channel analysis of surface waves (MASW), seismic reflection (both P and S waves), and unmanned aerial vehicle (UAV) magnetics were completed to provide valuable information about subsurface conditions affecting groundwater flow and contaminant transport. Data from these surveys were used to design a multi-phase source area characterization and delineation program. During the first phase, HRSC tools including Membrane Interface Probe Hydraulic Profiling Tool (MIHPT), Waterloo Advanced Profiling System™ (Waterloo APS), and Cone Penetration Testing (CPT) were deployed in known or suspected source areas. During the second phase, a sonic corehole pilot study followed by a full scale sonic corehole profiling program was completed at the plume scale. Following completion of the multi-phase source area program, nuclear magnetic resonance (NMR) profiling was completed in select boreholes and PVC monitoring wells to provide insights on the hydrogeologic conditions within the source areas and plume cores. Mineralogical and petrographic analyses were also completed to better understand geological controls on contaminant transport and fate.
Results/Lessons Learned
We will show key results from individual tools deployed at the site and how we integrated results from those tools to build a robust conceptual model for the CVOC and 1,4-dixoane source areas and resultant plume cores. We will discuss the observed physical processes at the fracture scale and plume scale. We will discuss geological and hydrogeological features (e.g., a narrow curvilinear corridor of fractured and weathered bedrock that may be faulted) controlling distribution and transport of CVOCs and 1,4-dioxane. We will discuss how the revised source area CSM was used for design of alternative remedial strategies. We will discuss how the current pump and treat system was optimized for improved capture and contaminant mass removal. Lastly, we will provide insights on lessons learned.