API PUBL 4601 : 1994

Description / Abstract:

API PUBL 4601, 1994 Edition, April 1994 - Transport and Fate of Dissolved Methanol, Methyl-Tertiary-Butyl- Ether, and Monoaromatic Hydrocarbons in a Shallow Sand Aquifer Appendix H: Laboratory Biotransformation Studies

BACKGROUND

This report presents the findings of a natural gradient tracer test and related laboratory experiments that investigate the subsurface behavior and impacts of methanol and MTBE. The primary goals of the research were to:

1) describe the transport and fate of methanol and MTBE in groundwater; and

2) determine the influence of methanol and MTBE on the transport and fate of BTEX in groundwater.

The research was designed to compare and contrast the effects of groundwater contamination by three fuel blends:

1) 100% gasoline (control)

2) 90% gasoline plus 10% MTBE

3) 15% gasoline plus 85% methanol

The study was confined to an evaluation of the dissolved constituents of plumes expected to emanate from spills of these fuels. Behavior of the nonaqueous phase and near-source dissolved phase is being addressed elsewhere (Poulsen, et al., 1991; API/CPPI in review).

Research to date has emphasized the transport and fate of the principal monoaromatic hydrocarbons in gasoline - benzene, toluene, ethylbenzene, and the xylene isomers (collectively termed BTEX) - in groundwater. These compounds represent some of gasoline's most water soluble, mobile components. Benzene, a known human carcinogen, is of particular concern (USEPA, 1984) and has a drinking water limit of 5 µg/L in many areas.

Previous studies of the behavior of BTEX in the subsurface have investigated the fate of these compounds in pure form or as derivatives of gasoline with no additives. The research has lead to a broad understanding of BTEX fate, particularly in shallow, aerobic groundwater settings. However, many retail gasoline now contains octane-enhancing additives, typically oxygen-bearing compounds (oxygenates) such as alcohols and ethers.

These additives pose special concerns with respect to groundwater quality because they have higher water solubilities than most other gasoline constituents. They can be expected to occur in extremely high concentrations in groundwater contacted by oxygenate-bearing fuels (API, 1991), and have the potential to influence the subsurface behavior of BTEX. Little is known about the effects of oxygenates on the migration and fate of BTEX in groundwater. The subsurface behavior of the oxygenates themselves has also received little scientific attention. Investigations of the environmental consequences of oxygenate use are needed for informed decision-making on alternative fuel policies.

Two oxygenates, methanol and methyl-tertiary-butyl-ether (MTBE) are the focus of this study. MTBE is used extensively in gasoline in the U.S., principally to meet Clean Air Act requirements, but also as an octane enhancer. Use of methanol as an alterative fuel for vehicles, both in pure form and in an 85% methanol and 15% gasoline blend, has been considered in the United States at both the state and federal level.

Properties of methanol, MTBE and the monoaromatics are listed in Table 1-1. Both oxygenates are highly soluble in water: MTBE has a solubility of 48,000 mg/L and methanol is completely miscible. In contrast, pure-phase BTEX solubilities range from about 200 mg/L (xylenes) to about 1,800 mg/L (benzene). Therefore, the two oxygenates can be expected to occur in high concentrations relative to BTEX at gasoline spill sites. The oxygenates will also be more mobile in the groundwater environment than hydrophobic compounds such as BTEX.

When gasoline is spilled, the nonaqueous phase moves through the soil zone and pools at the water table, leaving behind residual hydrocarbons. Soluble gasoline constituents dissolve into the groundwater and are carried in the direction of flow. The near-source concentrations of the solutes will depend on a variety of factors, in particular their relative proportions in the gasoline blend.

As they move away from the source, dissolved gasoline constituents are affected by advection, dispersion, sorption, volatilization at the capillary fringe, and biological or chemical transformation. Dispersion reduces peak concentrations, providing some natural attenuation, but it also increases the volume of the subsurface that is contaminated. Sorption slows the bulk migration rate of the solutes. Volatilization transfers the contaminant problem to the soil gas, and is probably not an important mass removal mechanism for the dissolved phase below the water table. Chemical and biological transformations are the only processes that provide permanent removal of contaminant mass from the environment.

Near an oxygenated gasoline source, high concentrations of methanol have the potential to increase the levels of BTEX in groundwater by cosolvency effects, but such increases are not expected for MTBE fuels (API, 1991). Elevated BTEX solubilities could increase the mobility of these contaminants, since sorption and solubility are generally inversely related for nonionic organic compounds (Chiou et al., 1979). Oxygenates could also affect the long term fate of BTEX in groundwater systems by inhibiting biotransformation of these compounds.

Research has shown that the limiting factor in BTEX persistence in groundwater is biodegradation (Barker et al., 1989). The monoaromatics are biotransformed under a wide range of environmental conditions (Gibson and Subramanian, 1984). In aerobic groundwater, if oxygen does not become limiting, the biodegradation rate is rapid and no toxic breakdown products are found (Barker et al., 1987). Conversely, anaerobic biodegradation of BTEX has been shown to occur at a significantly slower rate. In natural gradient injection experiments in a shallow, aerobic sand aquifer, BTEX has been found to persist only in oxygen-depleted zones (Patrick et al., 1986).

Methanol is considered to be a readily biodegradable compound (White, 1986), and could potentially act as a preferred substrate in the groundwater environment. The oxygen demand exerted by high concentrations of methanol within a gasoline plume could cause rapid oxygen depletion, limiting oxygen availability and thereby increasing BTEX persistence. Little is known about MTBE biodegradation, but ethers are generally considered to be recalcitrant (Harada and Nagashima, 1975; Ludzack and Ettinger, 1960), so MTBE is unlikely to represent a preferred substrate.

Other inhibitory effects are also possible. For example, high concentrations of either of the oxygenates could harm microbial communities that degrade BTEX. In their review of the effects of alcohols on microorganisms, Ingram and Buttke (1984) report that alcohols can inhibit microbial cell function by partitioning into the cell membrane, causing increased leakage of ions and metabolites and decreased growth rates. Ethers, since they are also polar compounds, may behave similarly.

Regardless of mechanism, the inhibition of BTEX biodegradation by methanol or MTBE could lengthen the interval of time that BTEX remains in groundwater before natural remediation occurs. Enhanced mobility of BTEX in groundwater could lead to longer transport distances over a given time period. Either occurrence would be undesirable because of the greater likelihood that BTEX from gasoline spills would reach drinking water supplies or environmentally sensitive discharge sites.

STUDY APPROACH

The natural gradient tracer test was conducted in a shallow, aerobic sand aquifer at Canadian Forces Base (CFB) Borden, Ontario. Pulses of groundwater contacted by each of the three fuels were simultaneously released below the water table. Chloride served as a conservative tracer. BTEX, oxygenate, and chloride concentrations were monitored for sixteen months using a dense network of multilevel samplers. Solute concentration data were reduced to plume-scale estimates of the location, mass, and degree of spreading of the solute plumes using spatial moment analysis. The subsurface mobility of BTEX and the oxygenates was defined by comparison with the conservative tracer. Effects of the oxygenates on BTEX migration and fate were distinguished by comparison to the control case, which contained no additives. Dissolved oxygen measurements were taken to investigate the role of oxygen availability in the fate of the organic solutes.

Laboratory microcosm experiments were performed to assess the ability of the microbial population at the CFB Borden test site to degrade BTEX and the oxygenates. The biotransformation process was isolated through the use of sterile controls. The microcosm experiments also explored the role of oxygen availability and oxygenate presence as constraints on BTEX metabolism. The laboratory work provided an assessment of biotransformation potential under static conditions, while the field experiment afforded an opportunity to examine biotransformation in a dynamic system in which solute transport phenomena in large part control the persistence of the compounds.