Formatted Title
Correlating MIHPT Results to Soil and Groundwater Analytical Results and Lithology When Characterizing TCE Contaminant Mass Distribution and Clay Lenses in an Aquifer
Background/Objectives
As part of the pre-design and investigation phase of a Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) site, a membrane interface hydraulic profiling tool (MIHPT) was used to assist with the characterization of site hydrogeology and trichloroethylene (TCE) contaminant mass distribution in an approximately 360-acre commingled volatile organic compound (VOC) groundwater plume (Project Area). Air sparge (AS) / soil vapor extraction (SVE) is the selected interim remedy to treat areas where TCE concentrations in the groundwater exceed 500 micrograms per liter (µg/L). Clay zones are present throughout the Project Area and can limit contact with the sparged air and the plume by displacing the air along the submerged impermeable zones and potentially preventing contact with the plume located over the clay zones and directing sparged contaminated air outside the influence of the SVE wells. The MIHPT profiling tool was used to help understand the general lithology, vertical distribution of TCE contaminant mass in the aquifer, and the TCE mass loading to the aquifer matrix within the target treatment areas to optimize the location and depth of the proposed horizontal AS and SVE wells. Upon completion of the MIHPT profiling, soil borings were advanced in the target treatment areas with a sonic drilling rig. Soil lithology and soil and groundwater samples were collected to help confirm the results of the MIHPT probing, to fill remaining data gaps, and to update the conceptual site model (CSM) from the Remedial Investigation.
Approach/Activities
Vertical profiling using a MIHPT profiling tool was advanced along the proposed the treatment areas to a maximum target depth of 80 feet (ft) below ground surface (bgs) (or probe refusal) using a direct push technology drilling rig. The MIHPT provided a continuous log regarding the degree of VOC impacts and information relating to soil types and hydraulic conductivity with depth. The output from the MIHPT investigation in combination with sampling data from existing monitoring wells were inputted into a 3-Dimensional Data Visualization and Analysis (3DVA) model to identify areas with TCE concentrations greater than or equal to 500 µg/L and potential clay lenses. Multiple conversion factors for 500 µg/L of TCE correlating to the electron capture detector (ECD) data were developed and the most reasonable factor was selected to generate the TCE greater than 500 µg/L plumes in the 3DVA model. The MIHPT responses with elevated VOC concentrations and/or low permeability layers were identified and used to inform the locations and depths for the sonic drilling investigation. Sonic borings were advanced to 100 ft bgs to truth the results of the MIHPT profiling and 3DVA model output, to fill in remaining data gaps, and to update the CSM. Soil and groundwater samples for VOCs were collected from each boring based on observations in the field and select intervals identified on the corresponding MIHPT profiling. Additionally, soil samples were collected for sieve, permeability and dry weight analyses.
Results/Lessons Learned
Preliminary results from the sonic investigation have shown that the MIHPT results were beneficial in identifying areas with elevated TCE concentrations and identifying the clay lenses throughout the Project Area. However, the ECD results typically maxed out at a corresponding TCE concentration considerably less than 500 µg/L in corresponding groundwater samples indicating that the selected converstion factor was more conservative. The evaluation of the MIHPT and sonic investigation is scheduled to be completed in December 2023 and will be presented.