A single-well injection-withdrawal (SWIW) test is evaluated as a tool to assess a single-porosity conceptualization for a transport system. We have shown that there are cases in which matrix diffusion cannot be proven definitively from the results of SWIW tests owing to uncertainties of aquifer properties. Drift during the resting phase of the test can carry a plume to areas where the flux is lower during the withdrawal phase, leading to decreased mass recovery rates. Results of sensitivity studies on multiple, stochastically generated, heterogeneous transmissivity fields indicate that variance in transmissivity has a strong effect on drift and consequently mass recovery rates. In addition to impacting drift directly, the regional gradient has the potential to influence mass recovery rates by interfering with the pumping-induced fluxes toward the well during withdrawal. The Kolmogorov-Smirnov test is used to evaluate whether simulated data using a single-porosity conceptualization with plume drift significantly differ from SWIW tracer test data from a fractured dolomite unit at the Waste Isolation Pilot Plant. Given the uncertainty in the variance of transmissivity, it is unlikely that a single-porosity conceptualization can match the field data. Matrix diffusion is a likely explanation for the observed gradual mass recovery rates.