INTERPRETING BREAKTHROUGH TAILING AND MATRIX DIFFUSION FROM DIFFERENT FORCED-GRADIENT TRACER TEST CONFIGURATIONS IN FRACTURED BEDROCK

Breakthrough tailing observed in tracer tests conducted in fractured rock are usually interpreted to be a result of matrix diffusion. Forced-gradient tracer tests recently conducted in fracture crystalline bedrock, have demonstrated that extensive breakthrough tailing can also occur in the absence of significant diffusive transport. It appears, therefore, that highly variable velocity fields can cause breakthrough tailing that closely resembles tailing attributed to matrix diffusion. This advection-derived tailing may be typical of transport in fractured rocks, but is masked by matrix diffusion in certain formations and under certain tracer test configurations. If matrix diffusion is to be measured using forced gradient tracer experiments, it is important that advective tailing be understood such that it can be separated from tailing resulting from true matrix diffusion. One interpretation of advective breakthrough tailing is that tracer mass travels along largely independent transport paths (channels) from the injection well to the extraction well. If it is assumed that tracer mass is distributed according to the cube of the local fracture aperture, and that velocity is distributed according the square of the local fracture aperture, it can be shown that the composite arrival at the extraction well produces different signature breakthrough tails for different tracer test configurations. The slope of these signature breakthrough tails are independent of the statistical distribution of the hydraulic conductivity field, but dependent upon tracer experiment design. For example, different power-law tail slopes are predicted from push-pull and radially convergent tests. A semi-analytic model based upon the cubic law concept, was compared to field tracer tests conducted in a fractured crystalline bedrock. The model adequately predicted breakthrough power-law slopes in both push-pull and radially convergent tests.