New aquifer threat from petrol concerns water companies

An additive used in unleaded petrol, methyl butyl tertiary ether (MTBE), is becoming a common contaminant of aquifers in southern and eastern England. At least two serious incidents have occurred, and surveys are revealing low levels of the compound in many rural and urban areas.

Petrol leakages are a well known source of soil and groundwater contamination. In April, Shell admitted that one-third of its service stations have contamination problems (ENDS Report 219, pp 4-5 ). The figure is thought to be typical of all retail sites.

The introduction of unleaded petrol into the UK seven years ago was heralded as an environmental boon. However, there is growing concern in the water industry that the move has created a new threat to groundwaters.

MTBE is added to unleaded petrol as an octane booster, forming up to 5% of regular blends and as much as 15% of super blends. It is at least ten times more soluble in water than other constituents of petrol, and in contact with groundwater it dissolves and spreads rapidly. Research has shown that MTBE is not significantly retarded by organic substrates. These properties apparently account for its sudden appearance in aquifers.

Although the compound has only been in use for a few years, Anglian Water and the National Rivers Authority (NRA) acknowledge that MTBE contamination in East Anglia is now widespread. Bob Price, Anglian Water's Director of Quality, told ENDS that the company was aware that MTBE contamination was not limited to only one or two sites, although levels were generally not high enough to cause concern.

The NRA says it knows of several instances of contamination. In two cases MTBE levels were high enough to present a taste problem. The compound is not particularly toxic but has a low taste threshold of about 10µg/l. In one instance, the NRA said, site remediation proved necessary to protect a water supply.

The Water Research Centre (WRc) has conducted a survey of sites thought to be at risk of MTBE contamination for a water company in south-east England, rumoured to be Thames Water. The survey found contamination at a number of sites in both urban and rural areas. Levels were generally below 1µg/l, but in many cases the source was not immediately apparent. The WRc suggests that drains and soakaways, as well as petrol stations, may be significant sources.

MTBE's properties are likely to make it a nuisance for water companies, which will need to ensure that monitoring for the compound is carried out in vulnerable areas. However, MTBE is also regarded as a sensitive indicator of petrol contamination by both the oil and water industries. Because of its solubility and low taste threshold, it is likely to be identified more rapidly in groundwaters than more toxic petrol ingredients such as benzene.

Oil industry research in the USA has suggested that MTBE does not increase the solubility of other petrol constituents in groundwaters, but the NRA is planning its own research to check these findings.

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