Formatted Title
Adaptive Management in Groundwater Remediation via Source Zone Depletion at the Nation’s Most Impacted Carbon Tetrachloride Site
Background/Objectives
Approximately 200,000 drums of pesticide manufacturing waste were disposed of in shallow trenches in three disposal areas totalling 24 acres from 1964 to 1973, which created the country’s most impacted carbon tetrachloride groundwater contamination site. The disposal area stratigraphy includes a 90-foot sandy vadose zone with an underlying sandy unconfined aquifer. Carbon tetrachloride concentrations in groundwater have ranged from three to four orders of magnitude above its respective MCL standard of 5 micrograms per liter (µg/L). The first-generation site remedy consisted of a RCRA multi-layer cap with traditional groundwater pump and treat. The groundwater pump and treat system operated for seven years from 1996 to 2003 and removed 50,000 Kg of VOCs. Five years following shutdown of the groundwater pump and treat system, the extraction well carbon tetrachloride concentrations rebounded by factor of 2 to 2.5 from ~15,000 µg/L in 2003 to ~ 38,000 µg/L in 2008.
Approach/Activities
EPA opted to defer further groundwater treatment until vadose zone source area treatment could be implemented. An SVE pilot test was completed at the site’s southern 1-acre drum disposal area from 2009 to 2013. Nested SVE extraction wells were installed through the drum disposal trenches in Level B and SVE off-gas vapors were treated with vapor phase carbon. The SVE treatment system operated for 22,000 hours, extracted ~ 1,500 pore volumes from the targeted treatment zone, removed ~ 17,000 Kg of VOCs, and reduced vadose zone VOC soil gas concentrations by two orders of magnitude. The baseline (2009) VOC soil gas concentration of ~ 3,300 (parts per million by volume) ppmv was reduced to ~36 ppmv at shutdown (2013) and rebounded to 115 ppm after a three-year post-remediation period (2016).
Results/Lessons Learned
Three groundwater monitoring wells are located ~ 150, 300, and 500 meters downgradient from the SVE pilot test remediation area. Following vadose zone treatment during the pilot test, carbon tetrachloride groundwater concentrations in these three wells decreased by one to two orders of magnitude since 2012. The baseline (pre-SVE) carbon tetrachloride groundwater concentrations at these three wells of 3,350, 3,750, and 2,050 µg/l have decreased to 15, 280, and 43 µg/l, respectively, by 2023.
Based on 45 years of historical site groundwater data, the empirical carbon tetrachloride groundwater transport velocity is approximately 150 m/yr. Accordingly, the three monitoring wells downgradient from the SVE pilot test remediation area have experienced approximately 3 to 10 aquifer volume flushes since 2012.
SVE treatment of a drum disposal landfill with drums left in place has been effective to reduce the vadose zone source area soil gas concentrations by two orders of magnitude. Once the ongoing source of vadose zone contamination was effectively depleted, VOC contamination levels in the site’s highly permeable sandy aquifer reduced by one to two orders of magnitude in ten years’ time through natural flushing.
SVE treatment of the site’s 3-acre middle landfill was completed in 2021 and construction of the site’s 20-acre northern disposal area SVE system is underway. EPA’s decision in 2003 to cease groundwater pump and treat operations and to forgo further other groundwater remediation actions until source area remediation was fully underway has proven to be wise and effective.