Comparing flux-averaged and resident concentration in a fractured bedrock using ground penetrating radar

Saline tracer transport experiments were performed to compare flux-averaged and resident concentration in a single subhorizontal fracture in sandstone bedrock. Tracer migration over a 14 m distance was monitored at an extraction well and imaged within the rock as it passed below a ground penetrating radar (GPR) positioned at the surface. Reflected radar amplitude was calibrated to tracer concentration by circulating saline fluid of known concentration through the fracture. Saline breakthrough curves measured at the well and within the rock were comparable but showed differences in both magnitude and shape. Transport differences were explored using flux-averaged and resident concentration first-passage-time models combined with streamline advective tracking. Application of the appropriate transport model to the two breakthrough curves produced identical estimates of dispersivity and similar estimates of effective fracture aperture. The tracer-derived fracture aperture also agreed reasonably well with hydraulic aperture derived from cross-hole pump tests. The availability of both flux and resident concentrations helped constrain the interpretation of the flow and transport behavior in the fracture. Flow appeared to be highly channelized with less than half the hydraulically swept area of the fracture contributing to efficient tracer transport.