REMEDIATION OF A TCE GROUNDWATER PLUME USING HYDRAULIC FRACTURING TO EMPLACE ZVI/CARBON AMENDMENT

Hydraulic fracturing was used to emplace granular reactive material into a moderate permeability aquifer to enhance remediation of contaminated groundwater. A large-scale pilot test of hydraulic fracturing to inject zero valent iron (ZVI)/complex carbon amendment resulted in 94% trichloroethene (TCE) mass reduction within 21 months in the source area of a contaminated perched sandstone aquifer at a former Atlas Missile Site. TCE concentrations declined 94%, 95% and 99% in wells with the highest initial concentrations of 3,600 ug/L, 1,700 ug/L, and 1,500 ug/L, respectively. Given the success of the pilot test and the ZVI lifetime (5+ years), the full-scale remedy will be designed to target the remaining untreated portions of the plume using the following design components.

Correlation of amendment loading rates/distribution with TCE reduction. The predicted fracture propagation radius was 30 feet; however, tiltmeter mapping during the pilot test showed that the actual radius was larger, averaging 79 ft (vertical) and 65 ft (horizontal). A fracture radius of 60 to 80 feet is assumed for full-scale design. Amendment loading rates will be selected based on correlation of pilot test loading rates (corrected for volume due to larger fracture radius) with corresponding TCE reductions.

Effectiveness of TCE degradation pathways. The two amendment components facilitate abiotic (ZVI) and biotic (carbon) degradation. The abiotic pathway dominated during the pilot test, as indicated by simultaneous declines in TCE and cis-DCE. The same ZVI to carbon ratio will be used during the full-scale remedy.

Plume configuration is not always a good indication of contaminant transport in groundwater. Previous investigations and conceptual site model (CSM) development were constrained by limited information to assess groundwater flow. The initial CSM was based on transport through an isotropic aquifer, despite initial characterization data indicating a low permeability aquifer. Pilot test data allowed for reinterpretation of the CSM and semi-quantitative determination of low groundwater flow rates required a significant component of overland flow in a surficial drainage system as the source of contaminants in the distal portion of the groundwater plume.