Toxicological and Performance Aspects of Oxygenated Motor Vehicle Fuels (1996)

This chapter reviews the interagency report's discussion of fuel oxygenates and water quality. The purpose of that discussion was to address water-quality issues arising from the use of fuel oxygenates (primarily methyl tertiary-butyl ether (MTBE)) and their movement in the hydrologic cycle. In general, the interagency report provides a clear and factual presentation of our current understanding of MTBE fate and transport in the environment. However, the water-quality section of that report does not adequately emphasize that there is only a small amount of data available on MTBE—a very widely used, potentially hazardous industrial chemical. The large majority of states do not have any current programs or requirements to monitor concentrations of MTBE or other fuel oxygenates in water. Yet the available data suggest that MTBE is sometimes present in precipitation, storm-water runoff, groundwater, and drinking water. The interagency report should clearly recommend that state and federal agencies immediately begin monitoring for MTBE and related oxygenates immediately. Until these data are collected, we will not be able to

evaluate nationwide the range of aquatic-resource contamination or the exposure of humans to fuel oxygenates.

The limited data available suggest that currently the waterborne exposure pathway of greatest concern is consumption of groundwater contaminated by point-source releases from leaking underground storage tanks (USTs). It is common practice in those states that currently monitor MTBE to close or treat water supplies contaminated with higher concentrations of this compound, and thus exposures via this pathway are expected to be minimized. In the majority of states that do not monitor for MTBE, it is not known to what degree individuals are exposed to higher levels of MTBE via drinking water.

Monitoring data show that releases of MTBE through nonpoint sources (e.g., precipitation and small surface spills) can enter storm-water-runoff and shallow groundwater and potentially enter drinking water supplies. At present, the relative risk of exposure to point and nonpoint sources of MTBE are not adequately characterized. MTBE concentrations in water associated with nonpoint-source releases need to be carefully monitored to determine whether these concentrations are increasing with continued use of MTBE. In addition, the effect of landuse, storm-water-management practices, and hydrogeologic factors on MTBE concentrations should be assessed.

The interagency report provides a summary of major sources of fuel oxygenates in the hydrologic cycle; monitoring data on MTBE concentrations in surface water, storm-water runoff, and groundwater, including drinking water supplies; and a review of the major processes expected to influence MTBE transport and fate in the

environment. Most of the MTBE monitoring data presented in the interagency report were obtained directly from data collected by regulatory agencies or by the U.S. Geological Service.

Oxygenates added to gasoline have the potential to degrade water quality due to environmental releases during gasoline production, handling, and use. MTBE is the most commonly used oxygenate, followed by ethanol (EtOH). Much lower amounts of ethyl tertiary-butyl ether (ETBE), tertiary-amyl methyl ether (TAME), diisopropyl ether (DIPE), and methanol (MeOH) are also used. Monitoring of groundwater, storm-water, and drinking water have shown the presence of MTBE and have raised questions about the extent to which MTBE and possibly other oxygenates might move through the hydrologic cycle.

Historically, the primary concern over oxygenate contamination in water has been associated with substantial surface spills and leakage from USTs. Because of the oxygenates' high solubility in water, they can partition readily from gasoline into water, resulting in high aqueous concentrations. In shallow groundwater downgradient of USTs, MTBE concentrations up to 200,000 µg/L have been observed (Davidson, 1995). EtOH is more soluble in water than MTBE and may occur in higher concentrations in groundwater and surface water. However, compared to MTBE, EtOH is biodegraded more rapidly under a wide range of environmental conditions. MTBE is much more difficult to biodegrade and likely to persist when introduced into groundwater or surface water.

Recent results from monitoring of storm-water and shallow groundwater have raised concerns about the potential for diffuse or nonpoint sources of MTBE. In studies of 16 U.S. cities with populations greater than 100,000, the interagency report indicates that MTBE was detected in storm-water runoff in eight cities. Of these eight cities, only three had an oxygenated-fuels program in place at the time of sampling. In the other five cities where MTBE was detected, it was likely used as an octane enhancer in gasoline.

When data from all 16 cities were examined, MTBE was detected in 6.9% of the 592 samples analyzed. When detected, the concentration ranged from 0.2 to 8.7 µg/L. (The limit of detection of MTBE in water reported in these studies was 0.2 µg/L.)

MTBE present in storm-water runoff is hypothesized to originate from two sources: (1) contact of surface runoff with minor surface spills associated with petroleum refueling; and (2) partitioning of MTBE out of the ambient air into precipitation. The half-life of MTBE in the atmosphere is longer during winter months than in warmer months, and MTBE is expected to build up to somewhat higher concentrations in the atmosphere. At lower temperatures, there is an increased tendency of MTBE to partition from the air into water (Squillace et al., 1995a). Both these factors likely combine to cause higher concentrations of MTBE in winter precipitation and winter storm-water runoff. While no direct measurements of MTBE in precipitation are available, according to the interagency report the frequency of MTBE detection in the 16-city storm-water-monitoring study was substantially higher in the winter than in the summer.

In cases where precipitation or storm-water runoff contains MTBE or other oxygenates, there is a potential for contamination of shallow groundwater supplies to result in low concentrations of these compounds. MTBE is not routinely monitored as part of assessments of groundwater when no contamination is expected. However, in the few studies that did look for MTBE, the interagency report indicates that MTBE was detected at low levels (0.2-10 µg/L) in 13% of the 540 wells sampled. Because of the relatively low MTBE levels detected and the physical location of the wells, the report indicates that these detections were not due to releases from concentrated spills or UST leakage. The measurement of low concentrations of MTBE at scattered locations was used as an indication of diffuse contamination of shallow groundwater by contaminated precipitation or storm-water runoff.

At present, the potential health and environmental impacts of oxygenate contamination in storm-water runoff and shallow groundwater are not adequately characterized. EPA had published a draft lifetime health advisory with a lower limit concentration, 20 µg/L, of MTBE in drinking water. (The interagency report mentions that the health advisory for MTBE is expected to be revised in 1996.) For the small number of water supplies sampled, high levels (exceeding the 20-µg/L health advisory) were rarely observed. However, the interagency report indicates that MTBE concentrations between 0.2 and 10 µg/L were occasionally observed in drinking water supplies. If EPA's recommended health-advisory concentrations were reduced considerably, substantial concerns would arise about the potential hazards of MTBE in drinking water.

The impacts of storm-water and groundwater containing oxygenates on aquatic life are very poorly understood. Although acute-toxicity assays have been performed for several important species, chronic-toxicity data are lacking. Without chronic-toxicity data, it is not possible to determine whether important aquatic biota might be impaired by exposure to MTBE or other oxygenates.

The interagency report indicates that the concentration of MTBE in surface water will normally be reduced by volatilization, which occurs more rapidly when the water is shallow and the overlying atmosphere contains little or no MTBE. In the subsurface, transport of MTBE will be influenced by exchange with the gas phase and possibly by biodegradation. Sorption to sediment or aquifer material is not believed to be an important attenuation mechanism, because of the high solubility and low octanol-water partition coefficient of MTBE and other oxygenates.

At this time, the influence of biodegradation on the transport of MTBE through the subsurface is not well understood. While a few studies have shown limited MTBE biodegradation, it has occurred only under restricted environmental conditions (Daniel, 1995;

Thomas et al., 1988; Mormile et al., 1994; Yeh and Novak, 1995). Also, in most biodegradation studies, MTBE biodegraded in only a small fraction of the total samples examined, suggesting that MTBE biodegradation may not be common in the environment. The presence of MTBE or other oxygenates does not appear to affect the biodegradation of other fuel contaminants adversely in most cases (Hubbard et al., 1994). However, because of MTBE's recalcitrance to biodegradation, the presence of MTBE in the fuel mixture might reduce the applicability of intrinsic bioremediation or natural attenuation for the management of UST releases. The importance of abiotic degradation processes or other nonbiological processes (e.g., chemisorption) are not addressed at all in the interagency report.

The interagency report should be revised to state clearly the actual sources of the monitoring data used in the analyses. It is not clear in the report whether the data presented were obtained directly from the raw data files or from summary reports. For example, it is not clear whether the groundwater-monitoring data were obtained directly from the National Water Quality Assessment (NAWQA) files or from summaries prepared by Squillace et al. (1995).

Nearly half the reported detections of MTBE in groundwater were from the Denver-area NAWQA study. These data might not be representative of nationwide trends, for two reasons. First, a number of the wells sampled in this study were up-gradient wells monitored during UST investigations where the MTBE detection could have been related to a UST release, not nonpoint-source contamination. Second, dry wells and other infiltration devices are often used in the Denver area for storm-water management. These

devices should rapidly introduce contaminants into the subsurface and may greatly increase the likelihood of groundwater contamination by storm-water runoff. While Denver is not the only area of the country that uses these devices, in a great many areas lower-permeability soils restrict the use of infiltration measures for storm-water management.

The interagency report should discuss the current understanding of the extent to which abiotic degradation mechanisms or other nonbiological mechanisms (e.g., chemisorption) reduce groundwater concentrations of MTBE and other alkyl ether oxygenates.

The rate of degradation of MTBE and other oxygenates in the atmosphere can have a substantial influence on ambient concentrations in air and precipitation. On page 28 of the water-quality chapter of the interagency report, the half-life of MTBE is reported to be 4 days at 25°C. However, in the appendix of the air-quality chapter, the winter-time half-life of MTBE is shown be vary from 20 to 300,000 days depending on latitude. This apparent discrepancy needs to be resolved.

The rate of volatilization from surface water will be controlled by the concentrations in the aqueous and gas phases and the mass-transfer efficiency. The interagency report should note that when the overlying atmosphere contains significant concentrations of alkyl ether oxygenates (AEOs) and aqueous concentrations are low (such as might occur during overland flow), the extent of volatilization might be negligible.

The discussion of volatilization effects on AEO transport to groundwater needs to be revised substantially with emphasis on the uncertainties in this area, as well as on documentation of the modeling approach, and assumptions, boundary and initial conditions, and major model parameters. The text should also clearly distinguish between aqueous and gas-phase diffusion processes. This section of the report assumes a relatively constant level of AEOs in the atmosphere, which would result in a gradual transfer of contaminants

to groundwater. A plausible alternative scenario is one in which atmospheric AEO concentrations increase in the winter and then decrease in the summer. Under this scenario, soil-water AEO concentrations might increase in the wintertime due to high atmospheric concentrations and then decrease during the summer due to diffusive losses back to the surface, resulting in a very low net flux of AEOs to the water table.

The interagency report recommends that an ad hoc panel representing public and private sectors be formed to develop a comprehensive research and assessment plan to determine the impact of alkyl ether fuel oxygenates on drinking water quality and aquatic life. Development of this plan is certainly worthwhile, but should not be a reason to delay implementation of the other recommendations in the interagency report. The interagency report has already made a substantial contribution toward defining what is known about sources and transport of alkyl ethers in the hydrologic cycle. Government agencies and industry should immediately begin work to fill the major information gaps identified in the interagency report.

The interagency report recommends that the AEOs e.g., MTBE, ETBE, TAME, and DIPE—be added to existing VOC analytical schedules and as routine target analytes for VOCs in drinking water, wastewater, surface water, groundwater, and remediation studies.

The committee supports this recommendation and suggests that these compounds also be added to the list of target analytes in EPA methods for VOCs in air. Addition of AEOs to the air and water analyte lists could be accomplished at little additional expense and

would greatly improve our understanding of the sources and fate of these compounds in the environment.

The interagency report recommends that a national database be developed to catalog analytical determinations of AEOs in air and water. This database could then be used in exposure assessments for AEOs in drinking water and for aquatic life in surface water.

Over the short term, it is unlikely that sufficient additional information will be developed that would allow accurate exposure assessments of AEOs. Over the longer term, these data can be more efficiently collected and compiled through existing monitoring programs. Consequently, there appears to be little benefit in developing a separate database on concentrations of AEOs in different media at this time. However, it would be very useful to identify a lead agency or organization that would continue to compile information as it becomes available on the occurrence, transport, and fate of AEOs in different media. As analytical methods for measuring these compounds come into more widespread use, the pool of monitoring data for AEO concentrations should rapidly increase. At that time, it may be useful to revisit this issue and develop a computerized database for use in risk assessment.

The interagency report recommends that a database be developed on AEO use in different cities and regions to identify correlations between seasonal use and water quality.

At this time, it is not clear what would be the benefit of demonstrating that AEO concentrations did or did not correlate with use. In the absence of a clear benefit, development of the administrative infrastructure required to collect and organize these data on a national scale does not appear to be warranted.

The interagency report recommends that annual releases of AEOs to the environment from all sources (e.g., industrial releases, refueling losses, auto emissions, and storage-tank releases) be determined to identify the primary sources of AEOs in the environment and aid in setting priorities for further reduction.

With currently available information, it will probably not be possible to develop precise estimates of AEO releases. Over the short term, efforts should be focused on developing order-of-magnitude estimates of releases from likely AEO sources. These estimates will help to rank sources for potential reduction and to identify major data gaps that will need to be eliminated before more precise emission estimates can be developed. Over the long term, development of more accurate estimates of releases will be needed if we are to improve our understanding of the fate and transport of AEOs throughout the hydrologic cycle.

The interagency report recommends that the available data be reviewed annually to assess the effects of AEO-use, land-use, and hydrogeologic variables on temporal and spatial variations in water quality.

While a continual assessment of the effects of oxygenate use on water quality is very important, it is not clear whether this assessment should be conducted as a separate program or should be integrated into existing water-quality assessment and management systems. Each state is now required to perform a biannual assessment of surface water, groundwater, and drinking water quality and the major factors influencing water quality (305b Reports). This information is incorporated into a national water-quality assessment report prepared by EPA.

Once AEO measurements are incorporated into the major analytical protocols, the states will begin to accumulate a wealth of information on sources and transport of these compounds in various compartments of the hydrologic cycle. In the short term, these data should be compiled to update the interagency report. Over the longer term, the states should be encouraged to address the effect of AEOs on water quality and associated beneficial uses in the 305b reports. The federal government's primary role might best be to integrate this information and to support additional targeted studies that address regional and multimedia issues.

The interagency report recommends that large-scale monitoring studies be performed in selected areas to characterize the concentrations and mass fluxes of AEOs between the different compartments of the hydrologic cycle (atmosphere, surface water, and groundwater); that laboratory and field studies be conducted to identify factors controlling the degradation of AEOs in the environment, the rate of degradation, and potential degradation products; and that theoretical modeling be performed to evaluate factors influencing the transport of AEOs from the atmosphere to shallow groundwater.

The committee supports the need for additional research in all these areas. A portion of this work would probably be done most effectively by individual investigators. However, for this topic, coordinated multi-investigator projects could yield substantial benefits. Ideally, several cities or regions with differing climatic, land-use, storm-water management, and hydrogeologic conditions would be selected for multimedia studies of AEO transport and fate in the environment. Through proper coordination with industry, it should be possible to estimate seasonal use of AEOs and subsequent air emissions in these target areas. These emissions may then be compared with AEO concentrations in ambient air, precipitation, storm-water runoff, surface water, and groundwater. By conducting coordinated large-scale monitoring projects, small-scale field studies, and controlled laboratory experiments, it should be possible to evaluate current theories and mathematical models of AEO fate and transport.

The interagency report recommends that coordinated chronic-and acute-toxicity studies of AEOs be performed with a broad range of aquatic animals and plants to assess the threat to aquatic life and form a basis for federal water-quality criteria.

The committee supports this recommendation. Intrinsic bioremediation is rapidly becoming accepted as a cost-effective, environmentally acceptable strategy for managing releases from USTs.

Through this approach, naturally occurring microorganisms are allowed to degrade petroleum constituents, such as benzene, toluene, ethylbenzene, and xylenes. However, MTBE and other AEOs are believed to be more resistant to biodegradation and are assumed to be resistant to abiotic degradation processes. Thus those compounds might be transported to nearby streams. States and other regulatory bodies now need aquatic-toxicity data to determine whether discharge of AEOs to surface waters will adversely affect aquatic resources.

The large majority of states do not have any requirements in place to monitor MTBE or other fuel oxygenates in storm-water runoff, groundwater, or drinking water. The absence of these monitoring data prevents an accurate assessment of exposure of humans or aquatic biota to MTBE and implementation of control measures to prevent adverse impacts.

On the basis of the small amount of monitoring data, MTBE has been detected in less than 5% of the groundwater samples analyzed, suggesting that drinking water is not currently a major MTBE exposure pathway for much of the population.

Storm-water runoff and shallow groundwater can be contaminated with low levels of MTBE (< 20 µg/L) via precipitation or contact with small surface spills. These contamination sources should be carefully monitored to evaluate changes over time and the effect of land use, storm-water management practices, and hydrogeologic factors on MTBE concentrations in environmental

media. If EPA considerably lowers the level of its recommended health-advisory concentration for MTBE, substantial concerns would arise about the potential for nonpoint sources of MTBE to affect water supplies adversely.

More needs to be known about the biodegradation of MTBE and other AEOs in surface water, soil, and groundwater. Biodegradation processes have the potential to substantially reduce the effects of pointand nonpoint-source releases of MTBE and other oxygenates. Current information should be assessed to determine whether a better understanding of abiotic degradation is an important research need.