1.4 MTBE and the environment
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| 1.4.1 Air
1.4.1.1 Behaviour of MTBE in air
MTBE is volatile and may be emitted to the atmosphere through evaporation during the distribution and use of gasoline containing MTBE.
When improperly handled this may lead temporarily and locally to very low concentrations in the air. Due to the solubility of MTBE in water, some of this atmospheric MTBE will tend to be washed out by rain and enter surface water and shallow groundwaters. As a result, it is sometimes possible to detect MTBE in shallow groundwaters at very low levels (below 1 µg/l).
Liquid MTBE and MTBE vapours are highly flammable, so all the appropriate safety precautions should be strictly observed during handling. An MTBE spill releases vapours below normal ambient temperatures. These vapours may travel long distances along the ground and are explosive in air at concentrations of between 1.3 and 8% by volume.
Any MTBE in the atmosphere is destroyed within days by photo-oxidation reactions.
1.4.1.2 Impact of MTBE in air
The highest exposure to air-borne MTBE experienced by the public is during vehicle refuelling. However, exposures are generally less than 35 mg/m³ and are only at such levels for very short periods. Very small quantities may also be discharged unburned from vehicle exhausts: levels of approximately 0.05g/km have been reported.
Higher exposures for longer periods may be experienced by workers during the production, storage and distribution of MTBE itself, and also of gasoline containing MTBE. Typical occupational exposures are 4 to 45 mg/m³ during MTBE handling, and 0.3 to 20 mg/m³ during gasoline handling. These can be compared with the occupational exposure limit for MTBE of 92 mg/m³ in the UK (8 hour TWA). In most other European countries the limit is higher: up to 180 mg/m3.
Although MTBE's distinctive smell makes the vapours easy to detect, there is no indication that they persist long enough in the air at a level which would, according to latest scientific information, cause harm to the environment or to human health.
1.4.1.3 Background levels of MTBE in air / water
(Germany, Switzerland & The Netherlands)
Most amounts of MTBE released into the environment will be distributed in the atmosphere. Due to photochemical degradation the half life time in the atmosphere is 3-6 days. However MTBE background concentrations can be measured there. The EU-Risk Assessment report mentioned background levels in the US between 0.7 and 2.7 µg/m3. The Swiss Department for Environment, Transport, Energy and Communication (BUWAL) have reported levels between 0.04 and 2.5 µg/m3 for Germany and Switzerland. (Report " Abschätzung der Altlastenrelevanz von Methyl-tert-butylether (MTBE) ", including an English summary, to be downloaded from http://www.umwelt-schweiz.ch/buwal/de/fachgebiete/fg_altlasten/service/mtbe/index.html
In the same report BUWAL stated the level in lakes and rivers for Germany and Switzerland was 0.05 µg/l. See appendix no. 4.pdf. There seems to be already an equilibrium between atmosphere and groundwater concentrations in many areas. However near filling stations the levels may be up to 50 times higher than the general background level.
A survey about the levels in drinking water wells in some parts of Germany is given by Klinger et al. In 91% of the measurements in rural area the MTBE concentration was lower than the detection limit of 0.05 µg/l and also in 51% of urban areas. The medium concentration in urban area groundwater was calculated to be 0.17 µg/l.
The German Federal Environment Agency (UBA) has published in 2000 a report about " Possible Impacts on the Environment Caused by the Use of MTBE as a Fuel Additive in Germany and Western Europe", which mentioned only a very few cases of groundwater concentrations up to some hundred µg/l. See appendix no. 5.pdf. They have evaluated the situation with the conclusion that the contamination of aquifers in Germany is so small that no harmful effects are anticipated.
The figures mentioned show that the concentrations in surface water and groundwater are lower than the reported values in the US.
Lake Zürich (Switzerland)
From 2002 – 2004 Lake Zürich in Switzerland was monitored for the presence of fuel components. This study (“Occurrence and Fate of Methyl tert-Butyl ether (MTBE) and Aromatic Hydrocarbons in a Holomictic Lake used as a drinking water supply (Lake Zurich)”) by Schmidt, T.C. et al., was published in Water Research 38, 1520-1529.
From this study table 1:
|
Compound |
Max./mean concentration in Epilimnion (boating season) [ug/l] |
Max./mean concentration in Epilimnion (off-season) [ug/l] |
Max./mean concentration in Hypolimnion [ug/l] |
|
MTBE |
1.4 / 0.20 |
0.10 / 0.058 |
0.048 / 0.037 |
|
Benzene |
0.16 / 0.046 |
0.10 / <0.02 |
<0.02 |
|
Toluene |
0.40 / <0.1 |
<0.1 / <0.1 |
<0.1 |
|
o-Xylene |
0.18 / 0.054 |
0.035 / <0.03 |
<0.03 |
|
m-/p-Xylene |
0.46 / 0.11 |
0.067 / <0.03 |
<0.03 |
The study concludes that MTBE and BTEX are nearly completely volatilised before vertical lake mixing occurred in winter. “Due to the density stratification of holomictic lakes there is hardly any water exchange in summer and thus transport of dissolved contaminants between epi- and hypolimnion. If contaminants are almost completely eliminated during the stratification period, their concentration in the hypolimnion will remain very low even over longer periods. Drinking water is typically extracted well below the thermocline, therefore no risk is expected for the drinking water supply in the lakes.”
The study concludes: ‘In order to further reduce emissions of unburned fuel into surface water, limitations in the use of high-emitting 2-stroke engine types in motorboats should be considered.”
The Netherlands Case
In 2001 the National Institute for Public Health and the Environment (RIVM) in the Netherlands conducted a drinking water measurement programme in co-operation with the Netherlands Waterworks Association (VEWIN) for MTBE in drinking water and the corresponding sources.
From the abstract of this report:
“This study, consisting of two sampling periods, shows a generally low concentration of MTBE in drinking water at the selected drinking water plants. The selection of sampling locations was based on the vulnerability of the water catchment area. Measurements in the June/July period showed a concentration of <0.01 µg/l in 22 samples of raw water; the average concentration was 0.07 µg/l and the highest 0.42 µg/l. The average concentration in drinking water in September/October was 0.09 ug/l and the maximum 2.9 µg/l. This maximum concentration was unusual, considering that the second highest value was 0.14 µg/l MTBE. The raw water (both groundwater and surface water) samples registered a concentration of <0.5 µg/l; the highest concentration in surface water was 3.2 µg/l. However, at one location a relatively high concentration (11.9 µg/l) was found in an individual groundwater well. This contamination could be attributed to a local source.
The main conclusion is that MTBE occurs in drinking water, although the concentrations are generally very low (<0.14 µg/l), with a maximum value of 2.9 µg/l. No effects on health are expected. It is, however, recommended to screen for MTBE in groundwater at locations with a history or experience of soil contamination. Taking precautions for future spills at petrol stations remains priority number one.”
The RIVM report is available at : http://www.rivm.nl/bibliotheek/rapporten/703719001.html
In 2004 the Dutch Ministry of the Environment VROM prepared a project in which a number of sites will be examined for occurrence of MTBE in groundwater close to petrol stations. This project, which is planned to take place in 2005, consists of three phases:
1) Approx. 100 – 150 sites will be prepared, distributed evenly between urban and rural areas, different soil types and inside / outside water extraction areas.
2) Research of available dossiers of these sites, to optimise a boring and sampling strategy.
3) Fieldwork and reporting.
The project is expected to be finalised by mid-2005.
USA / California
MTBE was first found in drinking water sources in the early and mid 1990s in the counties of San Francisco and Los Angeles, respectively. In February 1997 the Department of Health Services (DHS) adopted a regulation that included MTBE as an unregulated chemical for which monitoring was required by certain public water systems. Subsequently, required monitoring has been associated with compliance with MTBE's Maximum Contaminant Levels (MCLs).
As of March 1, 2005, the DHS database includes MTBE analytical results reported for ~13,200 sources, where "sources" may include both raw and treated drinking water wells and surface water sources, distribution systems, blending reservoirs, and other sampled entities. Nearly all of the results are non-detects.
The DHS database contains 116 sources that have two or more reported MTBE detections at any concentration. These occurred in 31 counties. Counties with the greatest number of sources reporting MTBE detections are Los Angeles (28 sources), El Dorado (12), San Diego (11), Kern (7), Monterey (6), Lake (5), Alameda (4), San Francisco (4), Orange (3), Merced (3), and San Mateo (3). Ten counties had two sources with MTBE detections, and ten had single sources.
Of the 116 sources, 87 reported a peak detection > 3-µg/L (the DLR), distributed as follows:
- 31 sources in 13 counties reported a peak detection > 13 µg/L. These were in the counties of Los Angeles (6 sources), Kern (5), San Diego (5), Monterey (3), Riverside (2), Sacramento (2), and San Francisco (2). Six counties each had one source.
13 ugl/l is the primary maximum contaminant level for MTBE which is the enforceable regulatory standard under the State's Safe Drinking Water Act.
- 28 sources in 15 counties reported a peak detection > 5 µg/L but < 13 µg/L. These were in the counties of Los Angeles (11) and San Diego (3), and Monterey (2). Twelve counties each had one source.>
5 ugl/l is the secondary maximum contaminant level for MTBE designed to address taste and odour concerns.
- 28 sources in 19 counties reported a peak detection > 3 µg/L but < 5 µg/L. These were in the counties of Los Angeles (5), El Dorado (4), Orange (2), and San Diego (2). Fifteen counties each had one source.
3 ug/l is the detection limit for reporting purposes. This is the level at which the DHS is confident about the quantification of the level of MTBE.
According to the source the numbers presented above should be considered draft, since they may change with subsequent updates.
Source: www.dhs.ca.gov/ps/ddwem/chemicals/MTBE/mtbeindex.htm
A 2002 study by US-based law firm White Environmental Associates using data from the California’s SWRCB (State Water Resources Control Board) and Department of Health Services (DHS) revealed that out of 16,000 wells in California 4,227 are ADI (abandoned, destroyed or inactive), of which 2,028 were ADI due to exceedance of the MCL (Maximum Contaminant Limits) for
- Natural Constituents 1,162 wells
- Solvents 329 wells
- Nitrates 313 wells
- Pesticides 196 wells
- Benzene 14 wells
- MTBE 14 wells +
Total 2,028 wells
Source: Sheet 39 in http://www.calgasoline.com/MEA_000E.PDF
1.4.2 Soil and groundwater
1.4.2.1 Behaviour of MTBE in soil and groundwater
In practice, when a gasoline release occurs on the ground or subsurface, then, depending of the release rate and magnitude and site characteristics, it stays in the form of a Non-Aqueous Phase Liquid (NAPL) for a shorter or longer time.
If the release was subsurface, it immediately begins to migrate sideways and downwards into the soil pores and also begins to dissolve into soil water gradually forming a subsurface plume, which may contain a NAPL phase and a contaminated water phase. It also begins to volatilise into soil gas.
In the case where the release was on the ground, then the events are slower as the released gasoline has to penetrate through the soil surface and maybe also a tarmac or another type of pavement. In this case, the volatilisation into air may considerably reduce the volume that finally enters the subsurface. A few litres' release may disappear completely into air.
If the release was large, then a situation similar to the subsurface release case will develop.
The behaviour of any fuel component in soil, soil pores and ground water depends on a few physicochemical characteristics of the component, i.e.:
Water solubility. MTBE's and other ether oxygenates' solubility in water is very much higher than that of the hydrocarbon components of gasoline, i.e the potential to dissolve into ground water is higher than that of the hydrocarbon components, such as Benzene, Toluene, Ethylbenzene and Xylene.
Solids - water partition coefficient (Koc). Koc is a measure of a dissolved component's tendency to adsorb into soil particles from water. High adsorption slows the travelling of the component in the ground water flow.
Vapour pressure, is a measure of the components' ability to vaporise from its liquid form into the gas phase.
Henry's law constant (KH) , is a measure of a component's characteristic to partition between the dissolved phase and gas phase. High KH values facilitate a component's volatilisation from ground water into soil gas. Consequently, MTBE has a relatively low tendency to volatilise out of water.
Retardation factor describes a component's relative retardation in soil and ground water due to its physicochemical properties and various soil and ground water related characteristics. In favourable conditions, retardation slows the migration of the contamination in the subsurface soil and ground water.
Biodegradability describes the capability of the soil and ground water microbes to break down a component. In general, gasoline hydrocarbons and alcohols are relatively easily biodegraded, whereas ether oxygenates' biodegradation rates in natural conditions tend to be lower.
The following table is an excerpt from an API publication n° API 4699, "Strategies for Characterising Subsurface releases of Gasoline Containing MTBE.
Table A-1. Comparison of Physical Properties of BTEX and Oxygenates
While the current understanding of the transport and fate of MTBE in groundwater is generally based on laboratory and field studies undertaken in North America, the University of Sheffield was the first to report on the transport and fate of MTBE in a European dual-porosity aquifer: http://www.solinst.com/Res/cmt/UKChalk/UKChalk.html. The case described is an urban retail petroleum filling station in southern England overlying the Chalk aquifer, the most important aquifer system in the UK.
1.4.2.2 Impact of MTBE in soil and groundwater
MTBE's subsurface behaviour has been extensively studied and documentation on empirical experiences and theoretical approaches is extensive. Therefore, it is recommended to open the API Publication No: 4699, February 2000 via the following Internet link: http://api-ep.api.org/filelibrary/4699c.pdf.
This very comprehensive document, "Strategies for Characterising Subsurface Releases of Gasoline Containing MTBE" is free for downloading. It covers all essential areas of the topic and is a respected reference. Note: This document is 1.7 MB and contains 120 pages - it is advisable to study the Table of Contents first in appendix no. 6.pdf
The API website is a major source of useful MTBE-related information, documents and reports covering the most important issues related to soil and groundwater contamination by MTBE and gasoline blended with MTBE and other ether oxygenates, and measures needed to mitigate and remediate the situation.
Another useful source of information is the MTBE Remediation Handbook. Published in the United States (2003), this book documents the technology to clean up MTBE in a rational and economic manner. Published in the United States but useful around the world, and based on extensive experience in managing and cleaning up spills of gasoline, this new book documents the technology to clean up MTBE in a rational and economic manner.
The MTBE Remediation Handbook will ensure a comprehensive understanding of the cleanup approach, including a careful and adequate site characterisation, the selection of an appropriate technology or sequence of technologies, and sound engineering design. You will find EFOA's contribution in Section III - Remediation Case Studies, "Remediation Experiences in Finland".
The book is edited by Ellen E. Moyer and Paul T. Kostecki, and published by Amherst Scientific Publishers.
Both the Table of Contents and the Book Order Form are available at
http://www.aehs.com/publications/catalog/remediation.htm
Other books, reports, articles, summaries and conference proceedings are available in abundance, largely also via the Internet. See also the Website Directory section included in this guide.
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