Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

High-Resolution Site Characterization of Sequence Stratigraphy and Contaminants in Soil for Effective Remedial Design

Background/Objectives:

An unspecified site in western Kansas that is under the jurisdiction of USACE was identified as a contributing source to local chlorinated solvent plumes that extend 1,300 meters downgradient and impact the former local water supply. Despite time critical removal actions (TCRAs) completed at the source area, including excavation of impacted soil in 2013 and in situ chemical oxidation (ISCO) injections in 2015, COC concentrations in soil and groundwater continued to exceed soil-to-groundwater migration screening levels and EPA maximum contaminant levels (MCLs), respectively. A remedial investigation (RI) was completed between 2016 and 2020 to determine nature, extent, fate, and transport of COCs and complete a risk assessment; however, data gaps in the extent of COCs and lithology in the source area remained. To address the identified data gaps from the RI and to support the design of a remedy for soil, high-resolution site characterization (HRSC) was completed combining continuous lithologic sampling, field screening, quick-turn volatile organic compound (VOC) analysis by on-site mobile laboratory, and 3D modelling.

Approach/Activities:

To delineate the area in soil with COC concentrations greater than the soil-to-groundwater migration screening levels remaining after the 2012 soil excavation and 2015 ISCO treatment, a combination of data collected by high-density soil sampling, continuous lithologic logging, and rapid-turn VOC analysis provided by an onsite laboratory were incorporated into a Leapfrog Works 3D model daily. The real-time models of site geology and VOC distributions allowed the project team to optimize step-out locations and more accurately target contaminated lithology. Pump testing and groundwater sampling were completed in the source area to confirm hydrogeologic conditions for the design basis of an in-situ thermal resistivity (ITSR) remedial alternative and to evaluate the effectiveness of the 2015 ISCO treatment. Soil samples were collected to analyze for physical soil properties including static and dynamic resistivity for the ITSR alternative. Pump test data and physical soil properties data were used to determine the applicability of potential remedy alternatives in the context of the revised CSM.

Results/Lessons Learned:

The high-resolution lithologic and COC 3D model identified a thin (1 to 5 ft) zone of carbonate nodules in a sandy clay matrix just below a 20 ft thick unit of low permeability clay. The carbonate zone exhibited greater permeability than the overlying slit and clay unit and contained higher VOC impacts in soil that were more laterally extensive than the clay unit above or the sand layer below. The HRSC completed at the site refined the CSM and provided a more complete assessment of contaminant migration and subsequent extent of COCs greater than the soil-to-groundwater screening levels. Combined with the hydrogeologic data and physical soil properties data, this application of HRSC refined the CSM and established a well-defined and well-characterized target treatment zone (TTZ) necessary for ITSR and other potential alternatives. The new TTZ allows for precise remedial designs to treat any heterogeneities in COC distribution identified within TTZ efficiently and effectively while reducing costs of materials and resources.

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Allan Ddamulira