Tracer tests in a fractured dolomite 2. Analysis of mass transfer in single-well injection-withdrawal tests

We investigated multiple-rate diffusion as a possible explanation for obser ved behavior in a suite of single-well injection-withdrawal (SWIW) tests co nducted in a fractured dolomite. We first investigated the ability of a con ventional double-porosity model and a multirate diffusion model to explain the data. This revealed that the multirate diffusion hypothesis/model is co nsistent with available data and is capable of matching all of the recovery curves. Second, we studied the sensitivity of the SWIW curves to the distr ibution of diffusion rate coefficients and other parameters. We concluded t hat the SWIW test is very sensitive to the distribution of rate coefficient s but is relatively insensitive to other flow and transport parameters such as advective porosity and dispersivity. Third, we examined the significanc e of the constant double-log late time slopes (-2.1 to -2.8), which are pre sent in several data sets. The observed late time slopes are significantly different than would be predicted by either conventional double-porosity or single-porosity models and are believed to be a distinctive feature of mul tirate diffusion. Fourth, we found that the estimated distributions of diff usion rate coefficients are very broad, with the distributions spanning a r ange of up to 3.6 orders of magnitude. Fifth, when both heterogeneity and s olute drift are present, late time behavior similar to multirate mass trans fer can occur. Although it is clear that multirate diffusion occurs in the Culebra, the number of orders of magnitude of variability may be overestima ted because of the combined effects of drift and heterogeneity.