The re-emergence of MTBE - What you need to know?

- Ken Kiefer

Methyl-tert-butyl ether (MTBE) historically has been used in gasoline at low levels as an octane enhancer and to reduce air pollution. Due to environmental concerns, MTBE has been phased out as an additive (e.g. Australia and U.S.), but is still widely used in Europe and Asia. MTBE used in countries that export fuel to Australia which results in cross-contamination in fuel supply. 

While, MTBE has not been used in refining within Australia since restrictions came into effect in the 1990s, there is new potential for the introduction of MTBE into the environment through the use of imported fuels. Fuel Quality Standards Regulations 2001 allows fuels to be imported up to 1% MTBE (0.1% WA; QLD)...

Hazard Potential

In the environmental management of fuel sites in Australia, it is relevant to consider that not only do current fuels (imported since 2001) have up to 1% MTBE but historical releases contained up to 15% MTBE. Also, relative to the other main petroleum hydrocarbon contaminants of concern (e.g. benzene, toluene, ethylbenzene, and xylene (BTEX)) at petrol sites, MTBE is highly soluble and mobile in aqueous environment and subject to minimal retardation/degradation in groundwater. MTBE groundwater plumes extend past petroleum hydrocarbon plumes. While volatile and a potential for vapour intrusion, it’s high solubility (~51,000 mg/L), MTBE is less volatile from water than BTEX with a Henry’s Law of 0.024 (unitless) an order of magnitude lower than BTEX which range from 0.227 to 0.322.

For human health MTBE can pose hazards from inhalation exposure affecting the respiratory, neurological and hepatic effects based on laboratory animal data. The inhalation Chronic Reference Concentrations (RFCs) range from 3.0 mg/m3 (USEPA IRIS) to 3.6 mg/m3 (ATSDR Draft. 2022). Health effects posed form oral exposures include hepatic, neurological, lymphoreticular, and male reproductive effects. An oral Intermediate Reference Dose (RFD) of 0.6 mg/kg has been derived by (ATSDR Draft. 2022). In addition to health effects poses aesthetics in drinking weater due to its low odour threshold. In drining water, the potential for aesthetic issues would occur at levels lower than levels that could pose health risks. MTBE has shown some evidence of cancer in rats however, it is not consider mutagenic. Environmentally MTBE poses aquatic toxicity to fish and aquatic organisms.

Screening levels are available to address the common groundwater related risks (see Table 1 below):

Australian Occurrence

To provide some perspective on occurrence in groundwater associated with petrol station sites, a review of groundwater monitoring was undertaken of 61 sites where MTBE was part of the analytical suite. This included a review of 1 474 samples from these sites. Of these sites, 31 (52%) results in detected concentrations. A summary of the results is presented in Table 2 below. The highest concentration measured was 7,900 µg/L. However, 95% of the concentrations were below 607 µg/L.

Table 2. MTBE Groundwater Results Summary

Of the sites, the following exceedances were observed:

• 19 of 41 sites (46%) exceed WHO Drinking Water Guideline (odour)
• 1 of 41 sites (2.5%) exceed derived health risk based drinking water value
• 0 of 41 sites exceed CRC CARE ecological guidelines values or vapour intrusion screening level

Discussion

The potential risks are driven potential use of groundwater or degradation of the benficial use of groundwater for drinking (or non-potable) for odour. There is also the potential for the MTBE to affect beneficial groundwater use beyond the petroleum hydrocarbon plume extent that typically would require management (e.g. use restrictions or remediation)

Some lessons learnt from these are:

• Assess MTBE early in the process;
• Late assessment of MTBE can identify un-delineated plumes requiring further assessment Remediation/Mitigation may be driven by MTBE is sensitive receptors near-by, mainly water use ; and
• Assessing remediation options that are suitable for the presence of MTBE, even if it is not the risk driver.

References

Agency for Toxic Substances and Disease Registry (ATSDR). 2022. Toxicological Profile for Methyl tert-Butyl Ether (MTBE). Draft for Public Comment. U.S. Department of Health and Human Services. January.

CRC CARE 2016, Guidance for the assessment, remediation and management of MTBE, CRC CARE Technical Report no. 36, CRC for Contamination Assessment and Remediation of the Environment, Newcastle, Australia.

WHO 1998, Methyl tertiary-butyl ether, Environmental Health Criteria 206, International Programme on Chemical Safety, World Health Organisation, Geneva, Switzerland

USEPA Integrated Risk Information System. https://www.epa.gov/iris

USEPA Vapor Intrusion Screening Levels (VISLs). https://www.epa.gov/vaporintrusion/vapor-intrusion-screening-level-calculator

Article Published on 20/04/2023

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