Pump and treat is widely used for containment and remediation of groundwater contaminant plumes. However, it is commonly observed that pump-and-treat systems begin to exhibit reduced efficiency at some point. A number of factors and processes may contribute to the reduced efficiency, among them being those associated with nonideal transport behavior, such as subsurface heterogeneity, nonlinear, rate-limited sorption/desorption, and rate-limited dissolution of immiscible liquid. We use numerical modeling to analyze the regional-scale (~49 km2) nonideal transport behavior of trichloroethene in a contaminated aquifer undergoing pump and treat remediation. The pump-and-treat system has been in operation for ~12 years, during which time the composite concentrations of trichloroethene in the treatment plant influent have decreased from >300 to ~100 &mgr;g L-1. However, as is typically observed elsewhere, the system is exhibiting extensive concentration tailing, wherein the concentrations have remained relatively constant at ~100 &mgr;g L-1 for the past 8 years. Various factors that may be contributing to this tailing phenomenon are evaluated using a three-dimensional solute transport model specifically developed for the site, which is located in Tucson, Arizona. The values for almost all of the input parameters of the model were obtained independently of the historic concentration data being simulated. The hydraulic conductivity field was generated using information obtained from borehole logs and pumping tests, the sorption and local-scale mass transfer parameters were obtained from laboratory experiments conducted with aquifer material collected from the site, and the initial immiscible-liquid saturations were based on the results of partitioning tracer tests conducted in a representative source zone at the site. On the basis of our analyses we conclude that while rate-limited desorption and large-scale spatial variability of hydraulic conductivity have significant impacts on trichloroethene transport, the dissolution of immiscible-liquid saturation associated with the source zones is most likely the primary cause of the extensive concentration tailing observed at the site. The impact of nonlinear sorption and local-scale mass transfer on trichloroethene removal appears to be insignificant. ¿ 1999 American Geophysical Union