Interface Treatment of Petroleum Hydrocarbon‐impacted Lower Permeability Layers by Activated Sodium Persulfate to Reduce Emissions to Groundwater

Abstract

AbstractNonaqueous phase liquid (NAPL)‐impacted lower permeability layers in heterogeneous media are difficult to fully remediate and can act as persistent sources of groundwater contamination through diffusive emissions to transmissive aquifer zones. This work investigated the benefits of partial remediation involving treatment focused near the high‐low permeability interface, with the performance metric being emissions reduction. A sequential base‐activated persulfate (S2O8 2−) delivery treatment strategy was studied in this work, involving 13–14 d deliveries of 10% w/w sodium persulfate (Na2S2O8) and 14–28 d deliveries of 19 g/L sodium hydroxide (NaOH). Treatment and control experiments were conducted in 1.2‐m wide × 1.2‐m tall × 5‐cm thick physical model tanks containing two soil layers differing in hydraulic conductivity by three orders of magnitude. The top 10 cm of the lower permeability layers contained 7400–7800 mg‐NAPL/kg‐soil; the NAPL was comprised of benzene, toluene, ethylbenzene, p‐xylene, o‐xylene, n‐propylbenzene, and 1,3,5‐trimethylbenzene (TMB) mixed in octane. Approximately 0.1 g‐Na2S2O8 was delivered per cm2‐interface area over 13–14 d. The S2O8 2− and SO4 2− concentration profiles suggest higher oxidant reaction rates when NaOH is delivered prior to, rather than after Na2S2O8. After 264 d and two treatments, hydrocarbon emissions from the NAPL source were reduced by 60% to 73% compared to a no‐treatment control tank. The incremental benefit of the second treatment was only about 10% of the effect of the first treatment.