Theme: 10. Characterization, Fate and Transport

Topic Area: 10a. Advanced Investigation Tools and Techniques

Formatted Title:

GWETS Optimization for an EDC- and Chloride-Impacted Aquifer: Accounting for Variable Density-Dependency

Background/Objectives:

Groundwater extraction and treatment systems (GWETS) designed and constructed without accurate conceptual site models (CSM) often operate ineffectively or inefficiently. GWETS optimization facilitated by improved understanding of site hydrostratigraphy, plume extent, and/or contaminant fate and transport mechanisms may be desirable to ensure regulatory compliance or to realize long-term cost savings through reduced operational costs or remedial timeframes. GWETS optimization is being undertaken at a chemical manufacturing facility with groundwater impacted by multiple volatile organic compounds (VOCs) and a high-salinity brine. Site contaminants have a significant variance with respect to their density and partitioning coefficients, with implications for their fate and transport properties, current distribution within the aquifer system, and the optimal strategy for extraction and treatment. The project objectives were to update the CSM through advanced site characterization approaches to facilitate optimization of the existing GWETS design.

Approach/Activities:

To achieve project objectives new data were collected and existing data were re-evaluated using approaches not previously employed at the site. Data collection included installation of new monitoring wells, high resolution vertical profiling of electrical conductivity within new and existing wells, groundwater sampling at multiple discrete vertical intervals within new and existing wells, and down-well camera survey of existing wells to confirm reported screen intervals. Re-evaluation of existing data included an equivalent freshwater elevation density correction using historic synoptic groundwater gauging and sampling data. Geological and analytical data were incorporated into a three-dimensional visualization model (3DVM) developed in Leapfrog Works to compare the spatial distribution of site contaminants to the existing extraction well network and hydrostratigraphic framework.

Results/Lessons Learned:

The results of the supplemental site characterization activities and data evaluation resulted in a significantly revised CSM and enabled GWETS optimization planning and implementation. High resolution vertical conductivity profiling within screen intervals of new and existing wells established a consistent trend of increasing brine concentration in groundwater with depth. VOC concentration gradients identified during the vertical aquifer sampling did not consistently follow this trend. Because historic groundwater sampling had been conducted from well screen midpoints (some site wells have screen lengths up to 100 feet), previous results did not represent maximum concentrations of site contaminants at those locations. Therefore, plume maps for both EDC and brine that formed the basis for historic GWETS layout and operating parameters were found to be misleading. Historic groundwater elevations were not previously corrected for density. Equivalent freshwater elevation density correction for a historical dataset provided significantly revised inferred GW flow directions. The updated CSM and 3DVM helped determine the optimal location for a new center-of mass extraction well. Additionally, these efforts facilitated the identification of a subset of existing extraction wells to be further evaluated to determine feasibility of rehabilitation and reactivation as part of GWETS optimization. The spatial relationships between contaminant concentrations, well screened interval, specific capacity, and aquifer framework were evaluated for the selected wells. If these screening criteria were favorable, the selected wells were included in iterative capture zone analyses that incorporated the revised plume extents and groundwater flow directions to propose an optimized GWETS design.

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Jesse Drummond

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