ABSTRACT:This paper presents a parameter sensitivity study of a two‐dimensional flow and transport model of a contaminated site. Hydrogeological and site data from previous investigations were used for calibration. The USGS contaminant transport model (MOC) was used.After flow calibration to establish a reference model, parameters were varied to examine the effect each had on predictions of a contaminant plume. Hydrogeological parameters and a step size parameter were incrementally varied individually. Each result was compared to the reference model output to evaluate changes in concentration values and contaminant plume configuration.The study indicated that a generally predictable trend can be established for some parameters not affected by pumping or similar high stresses. Ranges were identified to relate concentration error or plume change to the amount of parameter error. Some parameter perturbations produced distorted model responses at high stress locations. Porosity and anisotropy were found to be the most influential of the model parameters studied on the plume predictions. (KEY TERMS: ground water hydrology; hydrogeology; pollution modeling; water quality; model calibration