Tracer tests in a fractured dolomite 3. Double-porosity, multiple-rate mass transfer processes in convergent flow tracer tests

Convergent flow tracer tests conducted in the Culebra dolomite (Rustler For mation, New Mexico) are analyzed with both single- and multiple-rate, doubl e-porosity models. Parameter estimation is used to determine the mean and s tandard deviation of a lognormal distribution of diffusion rate coefficient s as well as the advective porosity and longitudinal dispersivity. At two d ifferent test sites both multirate and single-rate models are capable of ac curately modeling the observed data. The single-well injection-withdrawal t est provides more precise estimates of the mass transfer parameters than th e convergent flow trace; tests. Estimation of the multirate distribution pa rameters is consistent across locations for the two types of tests. Limits of resolution are calculated for the multirate distribution, and these limi ts explain the precision with which the standard deviation of the multirate distribution can be estimated. These limits also explain the necessary inc rease in the advective porosity for the single-rate model at one location a nd not the other. Implications of the multirate mass transfer model at lime and length scales greater than those of the tracer tests include the insta ntaneous equilibrium of a significant fraction of the matrix and the possib ility of a fraction of the diffusive porosity not reaching an equilibrium s olute concentration at long times.