The DNAPL Remediation Challenge: Is There a Case for Source Depletion?

Permanganate remediation was conducted in southern Florida at a small industrial site with TCE and 1,1,1-TCAcontamination down to 70 ft bgs in a sand aquifer. The permanganate solution was injected into the DNAPL source zone(30 ft diameter in plan view) where a small mass of residual TCE DNAPL caused this zone to have TCE concentrationsexceeding 10,000 μg/L, with small zones approaching TCE saturation. This remediation project comprised three injectionepisodes, each occurring over several days during which small volumes of KMnO4 solution were injected at depthsbetween 5 and 65 ft bgs in each of 6-8 direct push holes. During the 2-3 months allowed between injection episodes,the large initial density contrast caused the permanganate to spread out laterally and move downward while fingeringand diffusion occurred. The combination of these migration and mixing processes caused complete KMnO4 coverageof the source zone, even though the coverage immediately after each episode was only 1-8% of the total pore volumeof this zone. By keeping the initial injection volumes small relative to the total aquifer pore space within the targetedtreatment zone, displacement of TCE mass outward from the source zone was avoided. The post-treatment monitoringshowed that the combined effect of the three permanganate injection episodes was a 90% reduction in the aquifervolume where TCE in groundwater was above 100 μg/L and an even larger reduction in volume above 10,000 μg/L.The injections caused the maximum TCE concentration in groundwater to drop from 635,000 μg/L to 56,000 μg/L, withthis latter value occurring only in a very small zone. These results indicate that all, or nearly all, of the DNAPL hasbeen destroyed. These encouraging results are based on groundwater sampling of an exceptionally detailed networkof depth-discrete multilevel monitoring systems. Based on the results of this initial phase, we expect that completeremediation of the source zone will be achieved with one additional injection episode focused on the two small zoneswhere moderate TCE concentrations exist.