Unconventional shale gas production can greatly increase the risk of methane contamination of private domestic drinking water wells. The casing failure rate of unconventional wells has been investigated in various studies and shown to range between 1.9% and 6.3%. The casing and/or cement failure rate of unconventional wells versus conventional wells is shown to be 6-fold higher in Pennsylvania’s Marcellus Shale play, for example. In the United States, 17% of the population obtains their domestic water supply from privately owned groundwater wells. These wells are unregulated by the Safe Drinking Water Act and therefore are not subject to water quality testing. While the locations of hydraulic fracturing wells are well-known, estimates of private well locations by census block were only recently developed. I present a sensitivity analysis using the range of identified casing failure rates to quantify the number of domestic wells potentially at risk of contamination from methane migration from unconventional shale gas development. To estimate exact well locations from census block aggregations , a Monte Carlo simulation is used to approximate the likelihood of specific well locations within a 1 km radius of a well pad. Using lower and upper bounds of failure rates combined with a national database of unconventional gas well operations, potential impacts are quantified and shown as geographical hot spots to identify areas of possible concern relating to the contamination of domestic water supplies.