Mapping Preferential Flow Paths in Fractured Argillaceous Rock with a Nanoscale Zero-Valent Iron Tracer Test

A novel approach using nanoscale zero-valent iron (nZVI) as a tracer was developed for detecting fracture flow paths directly. This approach was examined at a hydrogeological research station in central Taiwan. Heat-pulse flowmeter tests were performed to delineate the vertical distribution of permeable fractures in two boreholes, providing the design basis of tracer test. A magnet array was placed in the observation well to attract arriving nZVI particles for identifying the location of incoming tracer. Then, the nZVI slurry was released in the injection well. The arrival of the slurry in the observation well was detected by an increase in electrical conductivity. The position where the maximum weight of attracted nZVI particles coincides with the depth of a permeable fracture zone delineated by the heat-pulse flowmeter. In addition, a saline tracer test produced comparable results with the nZVI tracer test. Numerical simulation was performed using multi-porosity approaches to estimate the hydraulic properties of the connected fracture zones between the two wells. The study results indicate that the nZVI particle could be a promising tracer for the characterization of flow paths in fractured rock.