Two-well recirculation tracer tests at the H-2 hydropad, Waste Isolation Pilot Plant (WIPP), southeastern New Mexico

Two recirculation tracer tests were performed on the Culebra Dolomite Member of the Rustler Formation at the H-2 hydropad at the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico. The first test, which used pentafluorobenzoate (PFB), sodium benzoate, and a suite of halocarbons for tracers, was terminated before breakthrough at the pumping well because of equipment failure. The second test, which used sodium thiocyanate (SCN) and difluorochlorobromomethane (BCF) as tracers, proceeded normally and lasted 270 days. During the second test, the tracer injected during the first test was recovered. Tracer test analyses for the two tests were performed only for the PFB tracer injected during the first test and recovered during the second, and for SCN. Analysis of the PFB recovery was crudely modeled as a one-dimensional pulse-injection test. The SCN tracer used in the second test was analyzed with the homogeneous, isotropic Grove and Beetem one-dimensional porous-medium recirculating flow test model. In general, model predictions are in poor agreement with the field measurements. Discrepancies could be produced by the combined effects of local nonhomogeneities, matrix diffusion, and possible sorption or degradation of the tracer during the test. The best overall match between the SCN breakthrough curve predicted by the semianalytical model and that observed in measurements was achieved using a porosity that ranged from 17% to 19% and a dispersivity of 16 to 18 ft. A match based only on the early part of the breakthrough curve, however, yielded a porosity of 11.5% and a dispersivity of 8 ft. The latter porosity might more closely represent the effective porosity of the Culebra, if the SCN degraded during the test