There are many potential issues that have been associated with shale gas extraction. I think that by far the most emotive is that of water contamination. The idea that fracking could render groundwater supplies permanently contaminated and unusable is indeed a potent rallying point, making up the majority of the charge sheet in films such as Gasland.
Gasland shows a small handful of cases where gas drilling is imputed to have impinged on groundwater. There are now hundreds of thousands of shale gas wells in the US. Pointing to a handful of accusations can hardly be considered science (although it does make for good movies).
Heretofore this has been something of an issue, because there doesn't seem to have been a large amount of data available with which to go about addressing this issue. So, in this post I will explore two recent reports (one a scientific paper, one a newspaper article) that attempt to answer the question - will shale gas lead to water contamination?
The first report comes from the Scranton (Pennsylvania) Times-Tribune, via a freedom of information request to the Department of Environmental Protection to release its record pertaining to water contamination incidents. The original article can be found here.
The two statistics that initially leap out at me are that of 969 complaints drilling impacts on water quality, 77% were in fact unrelated to drilling. Perhaps this is not surprising when 40% of water wells tested by Penn State in 2011 failed at least one federal drinking water standard.
Almost everyone in the UK gets their water from a utility, almost noone maintains their own well. The utilities filter and treat the water for us, so we know we can take a glass right from the tap. Because the distances and infrastructure needed to reach rural parts of the US, it is much more common for people to have their own wells. Essentially, you go out into you back garden, drill a hole approximately 20-100m down, and drink whatever comes up, sometimes with minimal treatment or filtering. I don't know about you, but I wouldn't be prepared to drink what comes up, even in my parent's fairly rural, pleasant Hampshire pile, let alone in my own rather grubby back garden in Bristol.
There are many potential sources of contamination, both naturally occurring and manmade. Groundwater can contain naturally occurring salts and heavy metals leached from surrounding rocks, and naturally occurring hydrocarbons (methane, benzene etc) from both bacterial activity and from natural oil and gas seeps. Further human activity can add to contamination from, for example, prior industrial activity in a region, coal mining (which can be especially bad for water contamination, and there has been a LOT of coal mining in Pennsylvania, and run-off of agricultural products (fertiliser, pesticides etc).
We seem to have made the mistaken assumption that all groundwater, prior to drilling, is pure, virgin Evian-quality drinking water, meaning of course that any contamination must be due to drilling. I think this is a concept that must be addressed. In an area where 40% of the water already fails drinking water standards, differentiating a new source of contamination from the old and/or naturally occurring can be extremely challenging.
Moreover, drinking water quality may change over time: the amount and rate of water taken out of an aquifer, and the rate/amount supplied via rainfall, will affect the water composition. Bearing this in mind it's pretty easy to see how misconceptions can quickly arise: a local person notes a change in the water quality from his well. They look around, and see that gas drilling has been going on in the nearby area, and of course that is going to be the first and only thing that they blame, regardless of what any subsequent scientific tests show. This is not to in anyway accuse the 77% of unsubstantiated complainants as blaggers. However, equally it shows that we cannot simply take every complaint and assume that the little guy is right and that gas companies are nasty corporate bastards and that fracking is inherently evil. Rigorous scientific testing is necessary before blame (if any) can be ascribed.
What about the remaining cases? The vast majority are associated with stray methane leakage, although their are a couple of more mundane examples - sediment increases from road and pipeline construction, for example. It's worth keeping in context though that over 12,000 shale gas wells have been drilled in the area between 2008-2012, so these examples represent a small percentage of the wells drilled.
Moreover, the timing of the incidents is also interesting. The majority of incidents were in 2008 and 2009, when fewer wells were drilled, while in 2011-2012, when more wells were drilled, saw fewer cases. This almost certainly is associated with changes in regulations, and changes made by drillers, to ensure adequate cement casing for wells. The drop from 2008-09 to 2011-12 shows to me that shale gas extraction can be regulated.
The second report is an academic paper published in 'Applied Geochemistry' which examines water quality above the Fayetteville shale in Arkansas. This paper examined major ion chemistry, trace metals, methane content, and methane isotopic signature, in 127 drinking water wells, and compared them with flowback water from Fayetteville shale wells.
They found small amounts of methane in many of the wells. However, there was no relationship between cases near to and far from shale gas wells, both had equal amounts of methane. Isotopic testing confirms that the gas has a shallow, biogenic (i.e. produced by bacterial activity) source. Again, these results serve to show that small amounts on methane in groundwater is a common and naturally occurring, so cannot immediately be blamed on gas drilling. The chemical analyses did not reveal any other evidence for frack fluid chemicals in the sampled water wells.
I would describe the Pennsylvania report as encouraging, but showing room for improvement - there have been a small number of genuine contamination incidents that need to be addressed. Especially encouraging is the reduction in incidents seen from 2008 to 2012 - showing the need for effective regulation in order to ensure that there is no methane migration. The Arkansas report is even more encouraging, comprehensive testing showing no evidence for either methane or frack fluid contamination across the Fayetteville shale