McGregor, R. | Remediation 29(4): 107-116(2019)

A sequential treatment approach of in situ chemical oxidation (ISCO) and enhanced bioremediation (EBR) was evaluated in a 700-day pilot test to remediate a petroleum hydrocarbon (PHC) plume containing BTEX (≤4,584 µg/L); MTBE (55,182 µg/L); and tert-butyl alcohol (TBA, 1,880 µg/L) in a saline unconfined aquifer. About 13,826 L of unactivated persulfate solution (19.4 wet weight% Na₂S₂O₈) was injected into a series of wells installed within the PHC plume. Results indicated that the BTEX, MTBE, and TBA within the PHC plume were treated over time by ISCO and sulfate reduction. The mean total concentration from the three monitoring wells within the pilot-test area decreased by as much as 91% for BTEX, by 39% for MTBE and 58% for TBA over the first 50 days post-injection. Concentrations rebounded at day 61 when no persulfate was detected in groundwater. Subsequent monitoring revealed a continued decrease of BTEX, suggesting that EBR was occurring within the plume. Between Days 51 and 487, concentrations decreased an additional 84% for BTEX and 33% for MTBE compared to Day 61. The TBA concentration decreased initially but increased as the sulfate concentration decreased from MTBE degradation. Isotope analyses supported the conclusions that ISCO and EBR processes were occurring at different stages and locations within the plume over time.