Groundwater and soil at a one-third acre urban location (Site) is impacted with tetrachloroethene (PCE) and there are multiple lines of evidence indicating the likely presence of dense non-aqueous phase liquid (DNAPL). The Site is surrounded with commercial businesses and residences and therefore safety was a significant concern while implementing remediation. The primary health threat posed by the Site has been the detection of elevated levels of PCE in the indoor air of adjacent commercial buildings caused by the vapor intrusion pathway. The remedial objectives for the Site are to reduce PCE concentrations in soil and groundwater and eliminate the threat posed by vapor intrusion. The remedy selected was in situ chemical oxidation (ISCO) treatment using sodium permanganate (NaMnO4) and a surfactant in the source areas, and NaMnO4 only for the dissolved plume.
As part of the pre-design investigation; 1) the permanganate native oxidant demand (PNOD) was calculated, 2) a treatability study was conducted to confirm NaMnO4 effectiveness on PCE and to screen and select an ideal surfactant, and 3) a clean water pilot test was performed to determine the aquifer’s capacity to accept injection water. Full-scale design consists of three rounds of injections with the first injections delivering approximately 51,000 pounds (lbs.) of NaMnO4 (40% strength) and 2,000 lbs. of surfactant (35% strength) into the soils and groundwater in the summer of 2017. Approximately 44,000 gallons of 5% to 20% NaMnO4 and surfactant were delivered into the groundwater through 108 Geoprobe®-driven injection points. The injections were carried out safely with no ISCO breakouts to the ground surface, utilities or basements of adjacent buildings. The distribution of NaMnO4 in groundwater as monitored by a colorimeter or visual field kit revealed adequate distribution and that oxidant had dispersed (as planned) under several impacted buildings via migration with groundwater.
The first round of oxidant injections revealed promising results. Before the injections, 12 of the 17 monitoring wells sampled exceeded the site cleanup standard of 50 μg/L for PCE. Following 7 months of injections, PCE exceedance was limited to two of these 17 wells. At 10 months post-injection, a slight rebound was observed and the number of wells with PCE exceedance increased to four. The overall decrease in PCE concentrations in groundwater across the Site has coincided with a significant decrease in the vapor concentrations (monitored as part of sub-slab depressurization system), thus helping to meet the key objective of mitigating the vapor intrusion concerns. A second injection event was conducted in the summer of 2018 and these results are being reviewed for presentation. It is anticipated that a total of three rounds of injections will be required to remediate the Site.