Rate of Natural Attenuation of Tert-Butyl Alcohol at a Chemical Plant

Tert-butyl alcohol (TBA) may be present in groundwater as an original component of leaked gasoline, or as a degradation product of methyl tert-butyl ether (MTBE). Evidence for natural attenuation of TBA in groundwater is presented from a chemical plant in Pasadena, Texas. Shallow groundwater in several areas of the plant has been affected by historic leaks and spills of TBA. A decade of regular groundwater monitoring of one groundwater plume, consisting primarily of TBA, shows generally declining concentrations and a plume area that is shrinking. Natural attenuation mechanisms are limiting the advective transport of TBA. The principal mechanism of attenuation in this case is probably biodegradation as the other physical components of natural attenuation (dilution, dispersion, diffusion, adsorption, chemical reactions, and volatilization) cannot explain the behavior of the plume over time. Biodegradation was also indicated by the enrichment of stable carbon isotope composition (¹³C/¹²C) of TBA along the flow path. Preliminary dissolved gas and electron acceptor analyses indicate the groundwater is at least under sulfate reducing condition in the core of the plume and the process responsible for biodegradation of TBA may include fermentation under aerobic (plume fringes) and possible anaerobic conditions. This case history demonstrates that natural attenuation of TBA is important, and can be used as a groundwater management tool at this site.