The Application of Radon for Mapping Open Fracture Networks in a Thin Vadose Zone

Core Ideas Locating preferential pathways for infiltration is critical for contaminated fractured rock aquifers. Mapping open fractures in buried bedrock formations has been marginally successful and costly. Mapping at greater resolution and lower cost can be valuable for many fractured‐rock sites. We used CR‐39 particle track 222Rn detector technology in a simple passive sampling design. Our methodology is characterized by low cost and ease of deployment. The use of naturally occurring tracers to elucidate subsurface flow and transport of water and gases through fractured vadose zones represents a powerful and inexpensive methodology for understanding geologic systems. Our study was conducted at Shepley's Hill, a highly fractured granite highland at the former Fort Devens military base. This study evaluated the applicability of using inexpensive, readily available passive Rn detectors to identify conductive structures (e.g., fractures and fracture networks) in fracture‐dominated vadose zones. Results from the Shepley's Hill study show a clear spatial correlation of elevated 222Rn concentrations emanating from fracture zones previously identified by independent geologic studies. At Shepley's Hill, the 222Rn concentrations measured from detector locations directly above bedrock fractures located within the Disc Golf Fracture Zone were almost exclusively the highest values measured in the study. Conversely, probes located above areas of less‐fractured bedrock and the Nona–Shep Fracture Zone showed relatively low 222Rn concentrations. These two observations provide strong supporting evidence that not all fractures are equal when it comes to their ability to transmit gas and, by inference, water into and out of the subsurface. Based on the results of this study, 222Rn is an environmental tracer capable of efficiently detecting major bedrock fracture sets in the vadose zone that readily transport gas and may be responsible for focusing water infiltration into contaminated aquifers.