Formatted Title
Pilot-Scale Remediation of Residual TCE Contamination with Passive In Situ Chemical Oxidation, Naval Air Station Pensacola, Florida
Background/Objectives
Chlorinated solvents such as trichloroethylene (TCE) can be quickly remediated in sandy aquifers, but TCE often persists in adjacent (and often deeper) less permeable material. This scenario exists at a site at Naval Air Station (NAS) Pensacola, Florida, where a TCE plume in a shallow, sandy water-table aquifer had been successfully remediated. However, TCE concentrations in groundwater still ranged from 100 to 1,200 micrograms per liter (µg/L) near the bottom the sandy aquifer where a confining unit was encountered.
Approach/Activities
In 2016, the United States Geological Survey (USGS) collected core material from the base of the aquifer/confining unit interface at 40- to 50-ft below land surface at sites adjacent to three previously existing monitoring wells. Because the residual TCE was providing a long-term source of TCE to the aquifer, a pilot-scale study of passive in situ chemical oxidation (P-ISCO) was implemented. Prior to P-ISCO implementation, the total oxygen demand (TOD) of the sand and clay material was determined and borehole geophysical surveys, such as vertical profiling of gamma and electromagnetic conductivity (EM), were run. P-ISCO, utilizing SOCORE wax cylinders that contained potassium permanganate, was initiated during July 2021. Soon thereafter, additional borehole geophysical data were collected in August 2021 to provide initial evidence of permanganate dissolution and transport from the wax cylinders into the clay and overlying aquifer formation material. In 2023, groundwater samples were collected.
Results/Lessons Learned
The core material indicated that higher concentrations of TCE and other chlorinated solvents were associated with wet, silty-clay zones compared to either dry-silty clay or the overlying sand. Concentrations of TCE in groundwater from two of the three wells have decreased by 67 to 97%; cis-1,2-DCE by more than 95%, and vinyl chloride by 96%. Moreover, borehole geophysical data collected in 2022 and 2023 indicate that EM have returned to levels observed prior to P-ISCO.