This study evaluated the effect of contaminant source width on the detection efficiency of five, six, and seven well monitoring networks for a range of groundwater seepage velocities. Wells were positioned downgradient of a rectangular landfill oriented obliquely to groundwater flow. A buffer zone boundary, establishing a distance limit within which contaminant plumes should be detected, was located 100 m from the landfill's downgradient corner. Various combinations of seepage velocity, from 0.01 to 10.0 m/d, and contaminant source width, from 0.1 to 5.0 m, were input to a computer model, which computed the detection capability of alternative monitoring networks. Smaller networks were more sensitive to changes in contaminant source width and seepage velocity. The smallest network showed a 21.6% greater range in detection efficiency than the largest network over the range of input values. Each network was considerably more sensitive to contaminant source width than seepage velocity, suggesting uncertainty in contaminant source width warrants attention in field applications. The type of analysis illustrated in this study may be useful for identifying appropriate monitoring networks given uncertainty in contaminant source width and seepage velocity at field settings.