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MTBE Contamination in Groundwater: Identifying and Addressing the Problem.
Subcommittee on Environment and Hazardous Materials
Dr. Pamela R. D. Williams
Thank you Mr. Chairman and
Members of the Subcommittee for the invitation to appear here today.
I appreciate the opportunity to discuss the impact of MTBE on drinking
water supplies, particularly in California, and the potential threat to public
health from exposure to MTBE in drinking water.
Over the past few years, I have conducted extensive research on MTBE and
other volatile organic compounds (VOCs). I
have presented my findings in approximately 7 published manuscripts and 12
presentations at various conferences nationwide.
During this period, I have found that much misinformation has been
circulated, and many misperceptions exist, about MTBE.
I hope to help clarify some of these issues for you in my testimony
today. MTBE in Drinking Water
A common perception is that
there is widespread contamination of drinking water supplies in the U.S. due to
MTBE, particularly in California, and that the degree of contamination is
increasing over time. It is also
believed that ground water sources are at greater risk of MTBE contamination
than surface water sources. Analysis
of the available drinking water data, however, does not support these claims.
In fact, MTBE was detected in 1% or less of all sampled drinking water
sources in California from 1996 to 2001, and MTBE was detected approximately 5
to 10 times more often in surface water sources than ground water sources during
this period. Furthermore,
detections of MTBE in California surface water sources decreased by about 50%
from 1998 to 2001, most likely due to a ban on the use of two-stroke engines in
selected surface water bodies. Contrary
to prior claims and projections, detections of MTBE in California drinking water
have not increased over time, and the annual rate of new MTBE detections has
actually decreased in recent years for both ground water and surface water
sources (likely due to the implementation of a new underground tank program in
California and the ban on two-stroke engines). Even when MTBE is detected in drinking water, the concentrations are typically very low. For example, approximately 87% of detected MTBE concentrations were below California's primary (health-based) standard of 13 parts per billion (ppb), and about 72% were below the State secondary (aesthetic-based) standard of 5 ppb from 1995 to 2001.[1] The average concentration of MTBE detected in California drinking water sources was less than 8 ppb from 1997 to 2001. The average concentration of MTBE in water sources where it was detected was higher before 1997; the average was made higher due to the sampling of the Arcadia and Charnock wells in Santa Monica, which had been affected by a nearby leaking underground storage tank. It should be noted that California's drinking water standards are very conservative, and the secondary standard in particular is 4 to 8 times lower than the USEPA advisory level of 20-40 ppb, which is based on taste and odor effects. These findings suggest that current levels of MTBE in drinking water are unlikely to pose a health or, in most cases, even an aesthetic concern. In our own independent analyses, we have found that current levels of MTBE in California drinking water pose a negligible health risk to water consumers. According to recent statements
made by the U.S. Geological Survey, similar findings for MTBE have been observed
in other regions (see Statement by Robert M. Hirsh to the House Committee on
Energy and Commerce on November 1, 2001).
Specifically, various national assessments by USGS have indicated that
MTBE levels do not appear to be increasing over time, that MTBE is typically
present at very low concentrations in shallow ground water within areas where
MTBE is used, and that MTBE levels are almost always below those of concern from
aesthetic and public health standpoints. Recent
findings from USGS, based on an evaluation of 954 randomly selected community
water systems nationwide, also found that the median concentration of MTBE
detected was only 0.54 ppb. Perhaps of greater interest is
that, besides MTBE, many other VOCs have been detected in California's
drinking water. For example,
chloroform, tetrachloroethylene (PCE), and trichloroethylene (TCE) were found in
approximately 10-14% of sampled drinking water sources in California from 1996
to 2001. In addition, many of the
drinking water sources in which PCE and TCE were detected (i.e., about 18-22%
of sources from 1995 to 2001) had concentrations that exceeded California's
primary standard of 5 ppb for these chemicals.
These findings suggest that the intense efforts to regulate or decrease
exposures to certain chemicals, such as MTBE, may be misguided from a public
health perspective, given the presence of other chemicals in drinking water that
may pose a greater risk. Toxicity of MTBE
Another common perception-one
that is often fueled by the media-is that MTBE is a human carcinogen.
This belief stems from reports that MTBE has been found to be
carcinogenic to laboratory animals at very high doses.
However, these animal studies have several important limitations with
respect to understanding the carcinogenic potential of MTBE in humans, and do
not provide any clear evidence of human cancer potential.
In fact, no national or international regulatory agency has classified
MTBE as a known human carcinogen. Although
few national (or international) regulatory guidelines exist for MTBE, the USEPA
believes that its aesthetic standard (20-40 ppb) is at least 20,000 to 100,000
(or more) times lower than the range of exposure levels in which cancer or
non-cancer effects have been observed in rodent tests. In California, the Office of
Environmental Health Hazard Assessment (OEHHA) considers MTBE to be an animal
carcinogen and a possible human carcinogen.
OEHHA is the only state or national agency in the country that has
derived a cancer potency value for MTBE. A
comparison of their value for MTBE with those for benzene, TCE, and PCE
indicates that OEHHA considers the latter three to be about 5 to 50 times more
potent than MTBE (when considering oral exposures).[2]
Several aspects of OEHHA's approach for evaluating MTBE's cancer
potential in humans have been criticized, including their use of unvalidated
non-human models and reliance on animal tumor data that may not be relevant to
humans. In short, the USEPA has not
conducted a cancer risk assessment for MTBE.
The analysis by OEHHA, which is very controversial, is the only one that
is currently available. Risks and Benefits of MTBEDespite perceptions to the
contrary, developing and using any material or technology entails some degree of
risk. Decision makers are therefore
faced with the challenge of a "risk/benefit balancing act," in which they
must decide whether the benefits achieved by a particular technology or material
are greater than the associated risks. The
risks and benefits of alternative technologies or materials must also be
evaluated, with the same level of thoroughness, to ensure that decisions
ultimately provide the greatest benefits (and fewest risks) to society. In the case of MTBE, the
tradeoff is clearly between air quality benefits and potential threats to water
quality. While the air quality
benefits of gasoline containing MTBE have been documented in California and
elsewhere, claims of widespread MTBE contamination of drinking water supplies
have not been proven. In addition,
preliminary data suggest that alternatives to MTBE, such as ethanol, may result
in increased health risks to the public, while providing few additional
benefits. Of particular concern is the potential for increased air
emissions and greater water contamination by other gasoline constituents (e.g.,
benzene) if ethanol is substituted for MTBE.
Other life-cycle impacts may occur from the production and transportation
of alternative fuels. Concluding Comments
The decision about whether or
not to ban or diminish the use of MTBE, or to require a specific replacement for
MTBE, requires the consideration of many factors.
Although public perceptions are certainly important to policy makers,
decisions about whether a chemical poses a significant threat to human health or
the environment should be based on a review of the scientific data. MTBE has clearly impacted a few
drinking water sources in the U.S. (most notably the Santa Monica wells in the
mid-1990's), and these incidents have raised legitimate concerns about the
longer-term use of this oxygenate in gasoline.
However, the assertion that there is widespread or growing contamination
of MTBE in drinking is not supported by either historical or more recent
drinking water data. In most cases,
detected concentrations of MTBE are also significantly below the USEPA advisory
level for MTBE. The benefits from
banning or diminishing the use of MTBE are therefore likely to be
inconsequential in terms of reducing human exposures or health risk,
particularly when evaluated in the broader context of other drinking water
contaminants. A more comprehensive
(life-cycle) analysis of alternative oxygenates or fuels is required to better
inform decision makers about the potential risks, costs, and benefits of these
alternatives. I hope that the information I have presented here today helps clarify some of the common misperceptions and factual data about MTBE. Again, I appreciate the opportunity to testify about my knowledge and research on MTBE. I would be pleased to respond to any questions that you may have. [1] Maximum contaminant levels (MCLs) are enforceable and represent the maximum permissible level of a contaminant in water delivered to users of a public water system. In California, the primary MCL (13 ppb) was established to be protective of cancer effects, while the secondary MCL (5 ppb) was established to address taste and odor concerns [2]
Cancer potencies (often expressed as a cancer slope factor or CSF)
are defined as an upper bound, approximating a 95% confidence limit, on the
increased cancer risk from a lifetime exposure to an agent.
OEHHA's CSF for MTBE is 1.8 (mg/kg-day)-1.
OEHHA's CSFs for benzene, TCE, and PCE are 100, 21, and 10
(mg/kg-day)-1, respectively.
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