The rapid expansion of unconventional shale oil & gas development, made possible by advances in hydraulic fracturing (fracking), has triggered concerns over potential risks it poses to water resources and public health. This is particularly evident in areas of active shale gas extraction where local residents rely heavily on shallow aquifers for drinking water. This research aims to develop a quantitative framework to evaluate the vulnerability of drinking water wells to contamination from unconventional oil & gas development activities. The concept of intrinsic well vulnerability is explored through the application of backwards travel time probability modeling to delineate capture zones and quantify various characteristics of contaminant breakthrough curves at the well. The model can be used in conjunction with measurements of well water quality over time, along with public records of shale gas-related activities to characterize risk. The approach is illustrated in a study site within Susquehanna County, Pennsylvania, a region which has experienced a dramatic increase in shale gas extraction in the last few years, and which is projected to continue to be an area of active expansion of the industry. The well vulnerability concept is proposed as a physically based quantitative tool for policy-making regarding the management of the contamination risks of drinking water wells.