Shale deposits in the UK occur under nearly half of the area containing the principal natural stores of water – a study carried out by the British Geological Survey and the Environment Agency revealed.
BBC News reports that the Bowland Shale in northern England – the first to be investigated for shale gas potential – runs below no fewer than six major aquifers.
The risk of fugitive methane emissions contaminating water aquifers is one of the reasons for opposing hydraulic fracturing – a procedure necessary to extract gas and oil from shale. Supporters of fracking point out that fractured shale deposits lie many miles beneath water aquifers and therefore the risk of contamination is minimal. This seems to be confirmed by the recent study, which found that almost all of this geological formation – 92% of it – is at least 800m below the water-bearing rocks. In the case of the Weald Basin in southern England the uppermost layer of oil-bearing shale is at least 650m below a major aquifer.
So far, no definitive distance for separation between shale and aquifers has been set but a limit of 400m has been suggested because water from below that depth is rarely considered drinkable.
In the U.S. methane is known to be present in drinking water in some areas where shale exploration has occurred. It is difficult, however, to determine for certain whether this co-occurrence is the result of shale gas drilling, as no baseline tests were carried out before extraction commenced. Low levels of methane gas can occur either naturally – from bacterial activity – or from man-made sources such as landfill sites. Tests should be able to determine whether the gas is thermogenic (coming from mineral deposits) or biogenic in origin, which means that it is the consequence of bacterial activity. The trouble is, however, that some thermogenic methane emissions may occur naturally and not be caused by fracking.
Dr Rob Ward, director of groundwater science at the BGS, said “In the United States, they didn’t carry out a baseline survey before the industry took place and that has resulted in controversy and uncertainty about the source of methane in drinking water.”
Proponents of fracking say that wells are separated from layers of rocks with several layers of steel and concrete casing which should prevent any leakage of either methane or fracking fluids. However, a recent study from Cornell University demonstrated that shale wells are more likely to leak methane than traditional wells, and problems with casing were given as the main reason for the leakages. For shale gas wells drilled in Pennsylvania, in 2009, the risk of a cement/casing impairment was 1.57-fold higher than in a conventional well drilled within the same time period .
Another study, carried out by Princeton University found that all of the nineteen abandoned wells in Pennsylvania that were measured in the study, leaked methane. The methane found in these wells was of thermogenic origin, indicating that the gas was associated with hydrocarbon production.
The study carried out by the British Geological Survey should make it possible to avoid the mistakes made in Pennsylvania and make fracking in the UK safer for all concerned. Dr John Bloomfield, of the British Geological Survey, said the maps could serve as a guide for regulators and planners.
“We’ve identified areas where aquifers are in relatively close proximity to shale units and any developments would have to be looked at particularly carefully,” he said.
Dr Rob Ward added: “We now have a window of opportunity to collect data on methane before any industry goes ahead. If we see increases in methane in groundwater which may be attributed to shale gas, we’ll be able to spot those.”
Alwyn Hart, director of air, land and water research at the agency, said drinking water is an important British natural resource that deserves protection.
“We have strong regulatory controls in place to protect groundwater, and will not permit activity that threatens groundwater and drinking water supplies,” he said in a statement.
Interactive maps showing the relative proximity of shale layers and aquifers are available on the British Geological Survey website
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