Passive diffusion bag samplers were tested in 11 wells at the Naval Surface Warfare Center, Louisville, Kentucky, by comparing the volatile organic compound concentrations obtained from passive diffusion bag samplers to volatile organic compound concentrations obtained by pumping the wells. The wells were screened in poorly permeable formations, including overburden, shale, and limestone. In five of the tested wells containing detectable volatile organic compound contamination, the data suggest that the diffusion samplers accurately reflected ambient contaminant concentrations (wells 1-NEC-15-P, 1-NEC-MW17-S, 1-NEMW23-S, 1-NW-MW24-S, and 1-NW-MW24-P). Comparison of a third well (1-NEC-MW34-S), in which the passive diffusion bag samplers produced higher concentrations than the pumped sample, is less certain because the passive diffusion bag sampler had passed through a layer of light non-aqueous phase liquid during deployment, suggesting the possibility of carryover contamination. In two wells (1-NE-MW23-P and 1-NEC-MW15-S), it was unclear whether concentrations obtained by using the passive diffusion bag samplers adequately represented in situ concentrations because the comparison of concentrations obtained by using the samplers and the pump was inconsistent between sampling events and/or between volatile organic compounds. In one well (1-NEC-MW34-P), the methodologies matched poorly, with volatile organic compound concentrations obtained by using the passive diffusion bag sampler substantially lower than those obtained by using the pump. Two of the tested wells (1-NW-MW6-I and 1-SE-MW13-I) contained no detectable contaminants in water obtained from either method. Data from wells where multiple passive diffusion bag samplers were deployed showed the lowest volatile organic compound concentrations adjacent to the vuggy limestone and higher volatile organic compound concentrations deeper in the limestone, supporting colloidal-borescope data that indicate the vuggy limestone is not a zone that supplies water to the wells.