Reduction of diffusive contaminant emissions from a dissolved source in a lower permeability layer by sodium persulfate treatment

Residual contamination contained in lower permeability zones is difficult to remediate and can, through diffusive emissions to adjacent higher permeability zones, result in long-term impacts to groundwater. This work investigated the effectiveness of oxidant delivery for reducing diffusive emissions from lower permeability zones. The experiment was conducted in a 1.2 m tall × 1.2 m wide × 6 cm thick tank containing two soil layers having 3 orders of magnitude contrast in hydraulic conductivity. The lower permeability layer initially contained dissolved methyl tert-butyl ether (MTBE) and benzene, toluene, ethylbenzene, and p-xylenes (BTEX). The treatment involved delivery of 10% w/w nonactivated sodium persulfate (Na₂S₂O₈) solution to the high permeability layer for 14 days. The subsequent diffusion into the lower permeability layer and contaminant emission response were monitored for about 240 days. The S₂O₈^2- diffused about 14 cm at 1% w/w into the lower permeability layer during the 14 day delivery and continued diffusing deeper into the layer as well as back toward the higher-lower permeability interface after delivery ceased. Over 209 days, the S₂O₈^2- diffused 60 cm into the lower permeability layer, the BTEX mass and emission rate were reduced by 95-99%, and the MTBE emission rate was reduced by 63%. The overall treatment efficiency was about 60-110 g-S₂O₈^2-delivered/g-hydrocarbon oxidized, with a significant fraction of the oxidant delivered likely lost by back-diffusion and not involved in hydrocarbon destruction.