Impacts of Ethanol on Anaerobic Production of Tert-Butyl Alcohol (TBA) from Methyl Tert-Butyl Ether (MTBE) in Groundwater

Methyl tert-butyl ether (MTBE) is a contaminant of concern to groundwater resources due to its persistence in subsurface environments. MTBE appears to be degraded readily in the presence of oxygen but is recalcitrant under the anaerobic conditions prevalent in the subsurface, and can be converted into the more toxic compound tert-butanol (TBA). As ethanol is being promoted as a renewable fuel and a replacement for MTBE in gasoline formulations, its potential impact on the biodegradation of preexisting contaminants and on other components of petroleum must be examined. The purpose of this study is to investigate the effect of ethanol release on existing MTBE plumes and the fate of TBA under sulfate-reducing conditions. Our results suggest that TBA, MTBE and ethanol-induced methane concentrations are strong determinants of the composition of the indigenous microbial community that develops during MTBE transformation. Some of the changes in microbial communities induced by ethanol may be long lasting, thus potentially altering the natural attenuation capacity of the impacted aquifer. Introduction and Problem Statement Five years after being banned in California, methyl tert-butyl ether (MTBE) continues to threaten groundwater resources due to its persistence in the subsurface. MTBE was introduced into gasoline reformulations as an oxygenate and octane enhancer, and enters the environment primarily through leaking underground storage tanks (LUSTs). Ethanol, both as an oxygenate replacing MTBE and as a new fuel alternative, is expected to interact with existing subsurface MTBE plumes when it leaks from LUSTs previously used for MTBEcontaining fuels. Previous studies have shown that ethanol may reduce biodegradation of benzene, toluene, ethyl-benzene and xylene (BTEX) and other gasoline contaminants. Moreover, ethanol may stimulate the partial degradation of MTBE to tert-butanol (TBA). TBA is a more problematic groundwater pollutant than MTBE due to its higher toxicity and mobility and because it is difficult to treat using conventional remediation methods. In this study, we focus on the role of microbial processes in the conversion of MTBE and production of TBA, and how these processes are affected by the presence of ethanol. TBA is a serious emerging ground water threat, especially as ethanol usage in reformulated gasoline is rapidly increasing. Little is known about mechanisms responsible for the transformation of MTBE to TBA in the field and the microbial processes involved. Field data on the degradation potential of TBA under anaerobic conditions is inconclusive, and no microorganisms capable of degrading TBA without oxygen have been isolated in pure culture. In our study, we are investigating changes in the composition of microbial communities to better understand the impacts of MTBE and its degradation products on native microbial communities. Objectives Our goal is to understand anaerobic MTBE transformation at an environmentally relevant scale, both chemically and at the microbial community level, by means of field experiments and controlled microcosms. Our established research site at the Vandenberg Air Force Base (VAFB) is dedicated to investigating the natural attenuation of gasoline and fuel additives in anaerobic aquifers. In the current study, controlled releases were carried out, of either MTBE combined with ethanol or of TBA alone. Groundwater samples were collected for enumeration of both bacterial and archaeal populations using quantitative polymerase chain reaction (qPCR), and analysis of community diversity using a DNA fingerprinting method, terminal restriction fragment length polymorphism (TRFLP). TRFLP profiles were analyzed using multivariate statistical methods, including canonical correspondence analysis (CCA), to determine how microbial communities are affected by the contaminants and environmental variables. A numerical model describing the VAFB field site is being assembled, coupling hydrogeological, chemical, and biological data, to investigate the role of biological degradation in MTBE transformation. Procedure The field site is located at the Vandenberg Air Force Base at a site where a leaking underground storage tank released MTBE, BTEX and other compounds into the shallow aquifer (Fig. 1). The original contaminated source area was excavated and back-filled in the mid 1990s. Following monitoring of the original plume, controlled release experiments were carried out by injecting contaminants of interest at known concentrations into the aquifer. Concentrations downstream of injection sites are monitored by sampling a dense network of monitoring wells (eight major transects 10 20 ft apart, each transect consisting of between 20 and 30 monitoring wells at ~3 ft apart; see Fig. 1). Surface boundary of excavation Aerobic in situ biobarrier Background wells B: G W + Br -