A Yale University Energy Study Group has done a cost-benefit analysis of shale gas extraction and has determined that at current levels, the extraction of shale gas has generated a consumer surplus of slightly more then $100 billion for the American economy:

Gas production in 2008 was 25.6 tcf so that the surplus to consumers by the price reduction from shale gas equaled $102.9 billion.... [It] is startling to acknowledge that consumer benefits from the technology of shale gas drilling and new gas production can be expected to exceed $100 billion per year, year in and year out as long as present production rates are maintained.

Yet the whole point of the analysis is to weigh the benefits of the new and cheaper fuel source against the costs of extraction, especially the potential environmental accidents that can occur.

To determine how significant the environmental costs were, the authors used an EPA report to determine the scale of accidents associated with shale extraction, with a focus on incidents of groundwater contamination. The number of times this happened was surprisingly few:

To undertake such an assessment of costs, we have reviewed current studies and reports on accidents, misuse of technology and poor well design and installation. A 2011 report for the Secretary of Energy (“Deutch Report”) counted 19 instances of problems with frackwater over the previous few years, amid thousands of wells drilled.

(The report also noted that the Oklahoma Corporations Commission has found no documented instances of groundwater contamination in that state. It did acknowledge that the EPA also identified groundwater contamination occurring at a couple of salt water aquifers in Wyoming.)

Since the paper claimed there were only 19 reported instances of problems with frackwater contamination I had to read the original EPA report to determine where this number came from. The EPA report itself does not give a number, instead stating that opponents of fracking tend to lack a lot of solid cases to point to:

Advocates state that fracturing has been performed safety without significant incident for over 60 years, although modern shale gas fracturing of two mile long laterals has only been done for something less than a decade. Opponents point to failures and accidents and other environmental impacts, but these incidents are typically unrelated to hydraulic fracturing per se and sometimes lack supporting data about the relationship of shale gas development to incidence and consequences.

Which in turn, lead to a footnote that cited an MIT study on shale gas extraction which had an appendix entitled "Publicly Reported Incidents Related to Gas Well Drilling". It found 43 publicly reported incidents. Of those 43 incidents, half were incidents of water contamination:

Of the 43 incidents reviewed, almost 50% were related to the contamination of groundwater with natural gas, as the result of drilling operations. Most frequently, this appears to be related to inadequate cementing of casing into wellbores, allowing natural gas to migrate from lower formations into groundwater zones. Properly implemented cementing procedures should prevent this from occurring. The second major category is on-site surface spills, which can arise from a variety of causes — from hose leaks to overflowing pits to breaching of pit linings.

The authors of the MIT report note that this is not a "definitive analysis of all known incidents" but this is still fewer then I expected to find given the size of the shale gas industry.

There are hundreds of thousands of shale gas wells across America, and while there are many horror stories in the media, it seems there are surprisingly few documented and confirmed cases of accidents occurring, especially with contaminated water.