Despite the favourable consensus of scientific opinion, MTBE is still the focus of public concern, particularly in the United States. The controversy centres on its presence in groundwater supplies, as a component of gasoline that has entered the environment through spills or leaking underground storage tanks. The strong, terpene-like odour of MTBE makes its presence easy to detect, even in minute quantities.
Unfortunately, groundwater contamination involving gasoline is not a new phenomenon. Leaking underground storage tanks and poor operational practices are well known, though not readily admitted problems. Although strict regulations exist in the USA and most European countries to prevent such situations, these laws have not always been efficiently enforced.
MTBE has attracted attention to groundwater contamination because it has become a widely used gasoline component, with several specific properties that distinguish it from other products:
it has a very low taste and odour threshold, which means
it is noticeable at low concentrations;
it is more soluble in water than other gasoline components, so in certain conditions it tends to spread faster and further through the soil, creating a "halo" effect around the spill;
it biodegrades more slowly and therefore may appear to persist in the environment for longer periods.
The incidence of serious water supply well contamination by MTBE, both in the USA and Europe, is in fact low. This has been demonstrated in official reports relating, for instance, to California, the UK, Germany, the Netherlands and Switzerland.
The Swiss Federal Office of Environment, Forest and Landscape established that MTBE emissions have decreased by 40% over the last 10 years due to the introduction of better operating standards at petrol stations (through improved delivery pumps and underground storage tanks) and better vehicle technology.
Trace levels of MTBE have been detected across Europe in surface water (lakes, rivers) and groundwater. Such background levels are typically below 1 ìg/l (the limit of determination varies between 0.05 and 0.10 ìg/l). Although the origin still remains unresolved, it is thought that such measured levels do not correspond with specific point sources or incidental releases. Background levels suggest the existence of other, yet unclear routes of MTBE in the environment. Exhaust emissions, either through run off from roads or via atmospheric deposition, are likely to play a role. In this context it is worth noting that the Swiss Federal Office of Environment, Forest and Landscape established that MTBE emissions have decreased by 40% over the last 10 years due to the introduction of better operating standards at petrol stations (through improved delivery pumps and underground storage tanks) and better vehicle technology. Such very low background levels pose no health risk. The values typically observed across Europe are more than one order of magnitude below the recommended maximum concentration of 5 µgl/l, a limit value which is based on organoleptic properties only. The toxicity threshold are several orders of magnitude above this recommended maximum concentration.
Nevertheless, concerns about MTBE contamination remain; these are based on three major issues:
the fact that the presence of MTBE makes drinking water unpalatable;
the fear that MTBE could pose a serious risk to human health
(see section Human health : a safe component)
the belief that remediating MTBE contamination could be difficult and therefore prohibitively expensive.
TOP
Taste and odour
Undoubtedly, MTBE has an unpleasant taste and smell, which is noticeable in very small quantities. In view of this, the US EPA has recommended a maximum MTBE concentration in drinking water of 20 to 40 parts per billion (ppb, µg/l) or below. These quantities are usually outside the taste and odour thresholds of most consumers. More importantly, they are 20,000 to 100,000 times lower than the lowest concentration that has caused observable health effects in animals, thus ensuring an exceptionally large margin of safety from any possible toxic effects.
Water suppliers have an array of proven options to protect consumers in the case of groundwater contamination, which are applicable in the case of MTBE.
TOP
Fuel ethers and human health
The human health impact of MTBE has been extensively studied. MTBE has never been classified as toxic or carcinogenic by any regulatory body anywhere in the world, nor does it induce adverse acute health effects in the general population under common exposure conditions. MTBE is not sensitising, it exhibits very low acute toxicity via oral, dermal and inhalation, it is not genotoxic, mutagenic or a reproductive toxicant. It can be a skin irritant at very high concentrations -- much higher than people are normally exposed to.
TOP
Remediation techniques
There is a large array of well-proven remediation technologies for treating soil and groundwater contaminated by conventional gasoline. In general, these methods are applicable to gasoline containing MTBE. The most common methods are air-stripping, adsorption using activated carbon, and biological degradation. Traditional pump-and-treat technologies available for water plants have been proven effective in remediation of gasoline-contaminated water, even if it contains MTBE. Emerging biological techniques have been demonstrated to further enhance the effectiveness of remediation.
TOP
Biodegradable substance
It is often alleged that MTBE is a persistent substance in soil and groundwater, yet research has demonstrated that aerobic decomposition occurs in natural conditions. Anaerobic decomposition of MTBE is admittedly slow, but in many cases the situation can be readily improved by oxygen feed, extra nutrients and microbe augmentation. Due to the mostly shallow nature of contamination at retail gasoline sites (generally 1 to 10 metres), these methods of remediation do not pose complex technical problems.
TOP
Preventive measures
With any form of pollution, the best defence is to prevent it happening in the first place. Gasoline containing MTBE will not leak into the soil and groundwater if storage tanks and pipelines are built to a high specification, properly installed, operated, inspected and maintained. In Europe, the major oil companies' strong commitment to improving their environmental and safety performance has led them to invest substantial sums in tank upgrading programmes. The introduction and enforcement of stringent safety regulations is the key to ensuring consistently high standards across the industry as a whole.
TOP
Putting things right
The problem of MTBE presence in groundwater is not inherent to the product itself, but has arisen from careless handling. It is an avoidable issue, which can be successfully resolved through co-operation between the oil industry, oxygenate producers, water companies and regulators. The steps that need to be taken include:
eliminating past contamination on a risk-based assessment basis, using proven remediation technologies;
upgrading retail sites to high environmental standards;
developing responsible attitudes and practices to prevent further contamination due to technical systems and operational failures;
minimising the impact of accidental releases by immediate intervention.
TOP
Benefits in the balance
Banning MTBE will not solve the underlying problem of gasoline leakage, and may create a false sense of security, as contamination could go undetected for far longer. MTBE has delivered significant air quality benefits by reducing harmful exhaust emissions. If all the organisations concerned work together to address the issue of groundwater contamination, the public need not be denied the proven benefits of this useful product.
TOP |
|
