- MTBE Water Contamination: Key Considerations for
Remediation,Risk Assessment, and Risk Management
- Paper presented at:
- Pacific Conference on Chemistry and Spectroscopy
- American Chemical Society
- Society for Applied Spectroscopy
- October 21 - 25, 1997
- Irvine, California
- Robert P. Ghirelli**, Hassan Amini, Brent D. Kerger, Alexis
Hillman, and Richard O. Richter
- McLaren/Hart Environmental Engineering
- 515 S. Figueroa Street
- Los Angeles, CA 90071
- [**Dr. Ghirelli is currently the Technical Director
of The [California] Orange County Sanitation District]
The discovery of the fuel additive methyl-tertiary butyl ether (MTBE) in ground water
and drinking water reservoirs in California and elsewhere has fueled public debate about
the health risks associated with this oxygenate. MTBE has been used since the mid-1970's
as a fuel additive to reduce carbon monoxide emissions. The City of Santa Monica in 1996
shut down the majority of the Citys drinking water wells due to MTBE contamination.
That event has focused much attention on the chemical and prompted public health
officials, water quality regulators, the state legislature, and the regulated community to
seek more information about the health effects and risk management options associated with
the continued use of MTBE in California. What does the future hold for this chemical and
how should it be dealt with in the regulatory arena? This paper examines some of the key
parameters concerning health risks from exposure to MTBE, the management of risks
associated with MTBE leaks from underground fuel tanks, and remediation options for
different spill scenarios.
At what levels is MTBE being detected in California waters?
As a result of its widespread use in reformulated gasoline over the last several years,
MTBE has been detected in ground water and surface water throughout the United States. In
order to measure the presence of MTBE in California drinking water supplies, the
California Department of Health Services in February 1997 required public water utilities
to begin testing their sources of drinking water for MTBE.
As of August 1997, approximately 2,200 water sources--20 percent of the total number of
sources in the state--had been sampled. Twenty seven of the sources (1.2 percent of the
total) reported MTBE detections: 15 ground water and 12 surface water sources (Figure 1).
In ground water, MTBE concentrations generally ranged from about 1 µg/l at the low end to
less than 35 µg/l at the high end, with 7 detections exceeding the interim drinking water
action level of 35 µg/l. Surface water concentrations were all below 5 µg/l except for
two raw lake water detections at 9.5 and 14.0 µg/l (1).
In February 1997, the Los Angeles Regional Water Quality Control Board (RWQCB) reported
that MTBE had been discovered in 10 of the approximately 436 drinking water supply wells
in the region. The City of Santa Monica closed down 7 wells in two well fields due to
elevated MTBE levels, the City of Los Angeles Department of Water and Power detected MTBE
in two wells in the Burbank area, and California Polytechnic University, in Pomona,
detected MTBE in one well (2).
Based on the findings so far, ground water appears to be more vulnerable to MTBE
contamination than surface water (Figure 2). At the present rate, less than 10% of the
states drinking water sources will ultimately show detections of MTBE and far fewer
will exceed the interim drinking water action level. Are we just now seeing the tip of the
iceberg? Are more water sources at risk? Or, will the regulatory controls now in force and
the requirements to test and upgrade underground storage tanks prevent MTBE from becoming
the "benzene" of the future? These are questions for which there are no quick
and easy answers. Improvements in underground storage tank (UST) technology and research
into effective treatment technologies, coupled with efforts to identify vulnerable sites
and monitor drinking water supplies, are essential elements of an aggressive strategy to
tackle the MTBE threat head-on.
Is MTBE a problem at these levels? What are the health risks?
MTBE exhibits a relatively low acute and chronic toxicity when considering its
potential caner and noncancer effects. The California State Action Level of 35 µg/l and
the U.S. EPA Health Advisory of 70 µg/l for MTBE are based on currently available
noncancer toxicity criteria developed by U.S. EPA (3). The U.S. EPA health advisory is
based on an oral reference dose of 0.005 mg/kg-day, which corresponds to safe lifetime
ingestion exposures of 180 µg/l for an average adult and 70 µg/l for a small child.
Although U.S. EPA does not currently list any cancer potency values for MTBE, a screening
evaluation by the Office of Science and Technology Policy has proposed oral and inhalation
cancer potency values based on available data (4).
At the California State Action Level of 35 µg/l, the preliminary potency estimates
lead to calculation of lifetime incremental cancer risks for 30 years of continuous
exposure of less than 2 per million for MTBE ingestion exposures and less than 5 per
million for inhalation exposures related to indoor air pollution from tap water use.
Exposure modeling has shown that 76% of MTBE intake is attributable to inhalation of house
air. However, the upper bound estimates of exposures and cancer risks from inhalation
exposure are not plausible because predicted indoor air concentrations of MTBE are well
above the reported odor threshold using the McKone indoor air model (5). The odor nuisance
at these predicted indoor air concentrations would likely preclude long-term exposures to
the contaminated water source. The distribution of upper bound daily MTBE exposures
contributed by ingestion and three indoor inhalation scenarios is illustrated in Figure 3.
How are the regulatory agencies responding?
Several bills dealing with MTBE made their way through the California Legislature
during the 1997 session. A bill by State Senator Richard Mountjoy (SB 521) was scaled back
from a mandated phaseout of MTBE to instead require a health effects study of the impacts
of this and other oxygenates. The bill also prohibits closure letters at underground tank
sites, unless soil and ground water have been tested for the chemical. Two other bills
require a new drinking water standard for MTBE, new controls on pipelines carrying
products containing MTBE, and a determination whether MTBE should be listed under
In a settlement with the City of Santa Monica, two oil companies have agreed to clean
up a MTBE-contaminated site to a standard that is the lesser of 20 µg/l MTBE or any state
or federal regulatory standard. The current state action level is 35 µg/l while the U.S.
EPA advisory standard is 70 µg/l. The companies also agree that once they meet the 20
µg/l limit, it will be maintained for a year before they are given a release (6).
For two years the California State Water Resources Control Board (SWRCB) has been
considering adopting a policy that would establish statewide standards for the cleanup of
petroleum discharges from leaking underground storage tanks (UST). In October 1996 State
Water Board UST staff released an internal draft of the cleanup policy that would have
allowed the use of passive bioremediation of petroleum hydrocarbons for "low
risk" sites. Following criticism from staff at the California Regional Water Quality
Control Boards the State Board staff pulled back its proposal.
The recent findings of MTBE in soil and ground water prompted the State Water Board to
fund a study of MTBE by the Lawrence Livermore National Laboratory (LLNL) and results are
expected in 1998. New legislation now requires all major agency proposals that rely on
scientific concepts to be subjected to peer review. These events will likely delay the
release of the draft statewide policy even further.
In the meantime, the Los Angeles RWQCB staff follows an interim review procedure for
dealing with UST sites with MTBE contamination. Pending release of the LLNL MTBE report
and issuance of further guidance from the SWRCB, the RWQCB uses a 35 µg/l trigger level
for MTBE. UST sites are evaluated for closure using a tiered risk-based approach (Figure
Sites where the maximum concentration of MTBE in ground water affected by the discharge
is less than 35 µg/l may be eligible for closure if other specified criteria are met (2).
Those criteria include extent of contamination by other gasoline constituents,
specifically benzene, distance to the nearest drinking water well or surface water body,
and whether petroleum-saturated soil is in contact with ground water at the site.
Sites within the Los Angeles region where MTBE in the ground water is greater than 35
µg/l are further reviewed and must meet additional criteria prior to being considered for
closure. In all cases, the assessment assumes the site has been fully characterized and
the contamination delineated and sources of pollution (e.g., saturated soil) have been
removed. Cases that were formerly closed may be reopened in areas where drinking water
supply wells contain MTBE concentrations exceeding the 35 µg/l trigger. Reviews are
typically done on sites within a one-half mile to one mile radius of the affected water
supply well but in certain cases sites may be investigated that are located more than one
mile away from the affected well.
Remediation options for MTBE soil and ground water contamination
Generally, the techniques used to remediate the BTEX (benzene-toluene-ethyl
benzene-xylene) components in gasoline are also applicable for MTBE. Remediation costs for
MTBE are highly dependent on the hydrogeological characteristics of the site, the history
of the spill, and the residence time of the contaminants. Almost any technique that
removes BTEX from water can also remove MTBE (7). However, due to its high water
solubility, lower volatility, lower adsorption to granulated activated carbon, and poor
biodegradability, removing MTBE from water is typically more expensive than removing BTEX
components. Figure 5 highlights some of the key characteristics of MTBE and their impact
A MTBE plume is often larger than the BTEX plume and this greater areal extent
translates to more wells, pumps, and pipelines, and treatment systems with greater
capacities and efficiencies. While the cost of remediation is dependent on site-specific
factors it is anticipated that the cost multiplier for remediating MTBE spills will be 1.1
to 2 times in many cases and possibly much higher for municipal water supplies or other
special cases (8).
Selection of the correct remediation technology for a MTBE-impacted site, as for most
sites, is dependent on a thorough understanding of the site characteristics, history of
the spill, and the limitations of the technology. Compared to the main components of
gasoline (i.e., BTEX), relatively little data exists for the successful remediation of
MTBE-impacted ground water. Most MTBE remediation projects seem to be in the research and
development state. A number of investigators have reported the successful application of
soil vapor extraction (SVE) to remove MTBE from soil (9). Figure 6 presents some key
considerations in the application of typical remediation technologies used for MTBE.
Removal of MTBE from aqueous solution is possible by the application of air-flow
enhancement techniques such as air sparging, high vacuum suction, and dual-phase
extraction techniques. Natural attenuation of MTBE is limited due to its high solubility,
low biodegradability, and poor adsorption to soil particles and organic matter.
Experiments conducted on oxidation of a mixture of MTBE and ethyl tert-butyl ether (ETBE)
in aqueous solution demonstrated that ozone and ozone/hydrogen peroxide treatment is more
effective in eliminating ETBE than MTBE (10).
Pump and treat and well-head treatment systems seem to be successful approaches for
containing and removing MTBE plumes in transmissive aquifer conditions. Removal of MTBE
from solution seems to be possible in an environment of high air to water volume ratios.
These conditions are created by the use of large air blowers and air strippers, or
multiple strippers in series. The introduction of heat appears to enhance removal of MTBE
Risk management considerations
Compared to benzene, MTBE is highly soluble and mobile in both air and water. Unlike
benzene, MTBE is fairly persistent in ground water and sediments since it is resistant to
most biodegradation processes. Also unlike benzene, MTBE is a more volatile ether compound
with an odor that can be readily detected in water at concentrations of 40 to 134 µg/l.
Peak shower air concentrations of 288 to 451 µg/m3, corresponding to the range of
reported odor recognition thresholds for MTBE, were generated by MTBE water concentrations
in the range of 34 to 53 µg/l (11). The apparently low odor detection thresholds for MTBE
would therefore provide a very conservative basis for an aesthetic water quality-based
standard to prevent odor nuisance complaints.
The physical and chemical properties of MTBE result in early warning of
potential health risks associated with groundwater transport of MTBE-containing gasoline
from leaking underground fuel tanks (12). In differing soil environments with MTBE at 5%
to 11% by weight in spilled gasoline it was shown that MTBE arrives at a residential water
well 100 meters down gradient anywhere from 54 to 59 days before benzene and the MTBE
concentration reaches 100 µg/l when the benzene concentration is about 0.1 to 0.3 µg/l
at the well. In each case, the upper bound cancer risk estimates associated with MTBE and
benzene are below 1 per billion incremental risk for exposures prior to water
concentrations reaching 100 µg/l.
Figure 7 provides estimates of peak indoor air concentrations of MTBE versus water
concentration using a conservative application of the McKone model for predicting exposure
to volatiles in tap water. Reported odor detection limits for MTBE vary in the literature
from air concentrations of 190 µg/m3 to 690 µg/m3. Figure 7 illustrates that MTBE water
concentrations in the range of 30 µg/l to 60 µg/l are necessary to produce a detectable
odor during showering. If MTBE remains below the odor detection threshold during
showering, it is unlikely that the resident will detect any adverse aesthetic qualities
(odor and taste) of MTBE during tap water use. Because water agencies will not serve water
with an objectionable taste or odor to their customers, the odor threshold appears to be
more limiting than the U.S. EPA health advisory level.
Since the primary impact of MTBE on water quality is aesthetic rather than health-based
at concentrations approaching current advisory levels, risk management decisions regarding
cleanup of MTBE in soil and groundwater should be keyed to potability and use of the water
supply and fate and transport considerations at potential points of impact on potable
aquifers. For example, if an initial spill cleanup has resulted in limited MTBE
contamination such that ground water reaching a downgradient well will be below action
levels, no further remediation should be required. Similarly, if a large production well
has MTBE contamination near or below advisory levels, blending with less contaminated
water is a viable option to maintaining both acceptable aesthetic water quality and
preventing any appreciable health risks from MTBE.
Risk Management Considerations for Two Hypothetical Case Studies
The purpose of presenting these hypothetical case studies is to illustrate how the
approach to managing the problem and selecting a remedial option may vary with site
specific factors. Two scenarios are presented: Scenario 1 - high levels of MTBE present in
a domestic drinking water aquifer, and Scenario 2 - high MTBE levels in shallow ground
water at a corner gas station UST leak site (Figure 8). While there are no guarantees, it
may be possible to predict with some certainty the response from the regulatory agencies
to these situations. In both cases, MTBE is detected at high levels, well above regulatory
thresholds, yet the regulatory response and remediation approach for each case is markedly
In the first scenario, a domestic water supply well is contaminated with high levels of
MTBE exceeding state and federal regulatory guidelines. All UST sites and pipelines within
about one mile of the well will come under close scrutiny by the lead regulatory agency.
Under this scenario a full investigation and cleanup of UST sites will be required
including an evaluation of the hydrogeological characteristics of the basin and migration
pathways from sources in the basin to the production well. Enforcement orders may be
issued to ensure the assessment and cleanup work is completed, and periodic monitoring to
determine the progress of cleanup. As responsible parties are identified and their
contributions to the problem confirmed they may be required to contribute a share of the
cost for well-head treatment, or blending, or an alternative water supply until the
contaminated well is brought back on line.
The second scenario is at the other end of the spectrum: a single, isolated UST site
with MTBE contamination limited to an unuseable, shallow aquifer. This site would appear
to be a candidate for closure or at least a monitor-only containment approach under the
Los Angeles RWQCBs interim procedure if:
- there is no free product or free product is removed;
- there is no MTBE in any production well within a mile of the site;
- there is no saturated soil or saturated soil is removed; and,
- depth to useable ground water is greater than 20 feet.
For sites that fall somewhere in between these two cases the regulatory agencies seem
to be open to considering closure only after it can be shown either through investigation
or fate and transport modeling that the site has been stabilized and is no longer a source
of MTBE to underlying useable ground water or nearby production wells. Each regulatory
agency takes a different approach and until there is uniform guidance from the U. S. EPA
or State Water Resources Control Board it is best to consult with the RWQCB or local
agency to determine its policy with respect to MTBE and UST cleanup.
The detections of MTBE in California waters appear to be random, low-level occurrences
associated with leaking USTs in ground water and boating on surface waters. Remediation of
leaking fuel USTs will continue to be driven by the toxicity of BTEX, not MTBE, although
the presence of MTBE will typically increase the cost of cleanup by 20% or more. For older
spills at sites where contamination is limited to shallow ground water and is not
threatening a drinking water well a containment approach or monitoring-only scenario may
The taste and odor threshold values for MTBE will drive the regulatory cleanup
requirements for MTBE as they are roughly one/half the health-based levels. If agencies
will accept that MTBE at low levels (<100 µg/l) is an aesthetic rather than
health-based water quality problem, blending of water to maintain MTBE concentrations
below the California action level should be sufficient to achieve safe and nuisance free
water quality. MTBE will remain the focus of intense political and regulatory scrutiny as
the impact of recent legislation and continuing research expand our knowledge of the
impact of this chemical on public health and the environment.
References [.....Figures appear after References]
1. California Department of Health Services. MTBE in Drinking Water. Summary of
Sampling of Public Drinking Water Systems for methyl tertiary butyl ether (MTBE). Internet
Site. August 20, 1997.
2. California Regional Water Quality Control Board, Los Angeles Region. "Review
Procedure for UST Sites with MTBE." April 29, 1997.
3. IRIS. 1996. MTBE. Washington, D.C.: Integrated Risk Information Service, Office of
Health and Environmental Assessment, U. S. Environmental Protection Agency. Down-loaded
from National Library of Medicine on-line service.
4. National Science and Technology Council. 1996. Interagency assessment of potential
health risks associated with oxygenated gasoline: National Science and Technology
Committee on Environment and Natural Resources, Office of Science and Technology Policy
5. McKone, T. E. 1987. Human exposure to volatile organic compounds in household tap
water: The indoor inhalation pathway. Environmental Science and Technology 21: 1194-1201.
6. California Environmental Insider. "Santa Monica MTBE Settlement May Set
Precedent." Volume 11, Number 3. July 15, 1997.
7. Davidson, James M. "MTBE in ground water and drinking water: a technical data
summary." Alpine Environmental, Inc., Fort Collins, CO. February 1997.
8. Davidson, James M. and R Parsons. "Remediating MTBE with current and emerging
technologies." Alpine Environmental, Inc., Fort Collins, CO. November 1996.
9. Davidson, James M. Existing and emerging MTBE remediation technologies. Alpine
Environmental, Inc., Fort Collins, CO. In Workshop on MTBE Ground Water Issues, sponsored
by Western States Petroleum Association, Oxygenated Fuels Association, American Petroleum
Institute, Los Angeles, CA, November 4, 1996.
10. Karpel Vel Leitner, N. A. L. Papilon, J. P. Crone, J. Peyrot, and M. Dore.
"Oxidation of Methyl tert-Butyl Ether (MTBE) and Ethyl tert-Butyl Ether (ETBE) by
Ozone and Combined Ozone/Hydrogen Peroxide. Ozone Science & Engineering. Vol 16, pp
41-54. July 21, 1993.
11. Abstract for Society of Toxicology Meeting, March, 1998. Use of odor threshold data
and shower air modeling to determine allowable limits of methyl-tertiary butyl ether
(MTBE) in drinking water. R O Richter1, D Suder2, and B D Kerger1, 1McLaren/Hart-Chemrisk,
Irvine, CA, and 2Precise Environmental Consultants, Davis, CA.
12. Kerger, B.D., D.G. Dodge, R. O. Richter. "Key considerations for risk
assessment and risk management of methyl-tertiary butyl ether (MTBE) regarding leaking
underground fuel tanks." The Toxicologist. Abstract Issue of Fundamental &
Applied Toxicology 36 (1, part 2): 283 [abstract 1438].
13. TRC Environmental Corporation. 1993. Odor threshold study performed with gasoline
and gasoline combined with MTBE, ETBE and TAME. API Publication No. 4592. American
Petroleum Institute, Washington, D.C.
14. API. 1994. Odor threshold studies performed with gasoline and gasoline combined
with MTBE, ETBE and TAME. API Publication No. 4592. American Petroleum Institute,
15. Prah, J.D., G.M. Goldstein, R. Devlin, D. Otto, D. Ashley, D. House, K.L. Cohen,
and T. Gerrity. Sensory, symptomatic, inflammatory, and ocular responses to and the
metabolism of methyl tertiary butyl ether in a controlled human exposure experiment.
Inhalation Toxicol. 6:521-538.
- Figure 1: Frequency of MTBE Detections
- Figure 2: Highest Reported Detections
- Figure 3: Dose Distribution for Ingestion and Inhalation of MTBE
- Figure 4: LARWQCB Review of UST Sites with MTBE
- Figure 5: MTBE Impact on Remediation
- Figure 6: MTBE Remediation Technology
- Figure 7: Maximum MTBE Concentration in Ambient Air
- Figure 8: Two Hypothetical MTBE Case Studies