Fracture Identification Using Low-Cost CR-39 Detectors
It has long been recognized that preferential flow, primarily through interconnected fracture networks, is critical to understanding the exchange of contaminants between bedrock and the shallow subsurface in water and gas phases. However, the difficulty in identifying discrete fracture-controlled transport pathways adversely impacts all phases of site characterization, modeling, and remediation at fractured bedrock sites. Boreholes do not always penetrate critical fractures, and fracture connections between boreholes are difficult to determine. There is, therefore, a critical need to develop inexpensive yet robust tools to better understand the location, geometry, and extent of significant fractures. Utilizing 222Rn as an indicator of fracture-controlled transport pathways may represent an inexpensive assessment tool for identifying fractures in some bedrock settings. Our research evaluated the applicability of using low-cost readily-available CR-39 222Rn detectors to locate and map conductive pathways (i.e., active fractures and fracture networks) in the subsurface at a shallowly buried granitic bedrock upland located at Fort Devens, Massachusetts. Unique to this approach is the development of a new rapid deployment port that allows the CR-39 detector to be placed within or above bedrock, while restricting atmospheric 222Rn from reaching the detector. For this study, a power auger was used to penetrate thin overburden deposits in order to place the detector-casings just above the top-of-bedrock surface, generally less than 5 feet below grade. Our research demonstrated that by deploying a series of these inexpensive detector-casing combinations statistically significant measurements of the 222Rn flux could be collected. These measurements allowed for accurate delineation of the most communicative fractures (i.e., actively transporting water and gasses) at the study site. The CR-39 data was then used to validate the existing site conceptual model. In contrast to other available fracture-locating techniques, deploying our technique, in some cases, potentially offers improved X-Y spatial resolution at significant cost savings.