Sulfate Land Application Enhances Biodegradation in a Petroleum Hydrocarbon Smear Zone

Abstract

Delivery of sulfate to petroleum hydrocarbons (PHCs) source zones and groundwater plumes is desirable to enhance biodegradation rates when treatment has become limited due to depletion of sulfate. Sulfate land application involves spreading sulfate salts on ground surface and allowing their dissolution and infiltration of sulfate into subsurface. The objectives of this pilot-scale investigation were to capture the vertical transport of sulfate beneath an application area, confirm that sulfate reduction was occurring, and explore how the added sulfate affected biodegradation of benzene and toluene. Approximately 4,000 kg of gypsum was spread over a 30 m × 30 m study area above a smear zone located ∿2 m below ground surface. Precipitation was augmented by two irrigation events. Groundwater samples, collected over 1058 days from multilevel wells and a conventional long-screened monitoring well, were analyzed for benzene, toluene, ethylbenzene and xylenes (BTEX), sulfate, bromide, dissolved inorganic carbon (DIC) and methane. Compound-specific isotope analyses (CSIA) for benzene and toluene, and isotope analyses of 13 C-DIC and 34 S-SO4 2were performed. Following application, an increase in sulfate concentration was noted in the smear zone. 34 S-SO4 2enrichment and 13 C-DIC depletion indicated that sulfate reduction and mineralization of PHCs were enhanced. CSIA results provided unequivocal evidence of anaerobic biodegradation of benzene and toluene. After 1058 days when sulfate was depleted, methane concentrations were about three times greater than baseline conditions suggesting syntrophic benefit of the delivered sulfate. Observations from this investigation support the viability of sulfate land application to enhance biodegradation rates in shallow PHC smear zones.