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The unsaturated (vadose) zone in arid and semiarid regions can be >100 m thick and may receive little or no moisture recharge from the surface. The microbiological properties of the vadose zone are of interest because of the potential for microorganisms to impact the fate and transport of contaminants in these environments. At numerous sites in the western United States, large volumes of wastewater or process water have been disposed of directly to the surface or shallow subsurface and have subsequently migrated through the vadose zone to the groundwater. The purpose of this study was to determine and compare the microbial properties of pristine and impacted vadose zone sediments. Vadose zone sediments from depths ranging from 24 to 90 m were obtained from 3 bore-holes drilled on the Hanford site in south-central Washington State. One bore-hole was located in a pristine area and the other two were located in areas where wastewater had been disposed of directly to the surface and near subsurface. An open-flow centrifugation method was used to determine unsaturated hydraulic conductivity for the sediments, allowing construction of characteristic curves relating the hydraulic conductivity (K) to the volumetric water content (theta). By comparing the volumetric water content of sediments obtained prior to centrifugation to their water content over a range of K(theta), it could be determined which vadose zone samples had been subjected to artificial recharge. Elevated concentrations of nitrate and the presence of carbon tetrachloride in vadose sediments were also used as indicators that sediments had been impacted by past waste disposal practices. Those vadose zone paleosol samples receiving artificial recharge had higher populations of culturable bacteria (log 1.0-6.6 CFU g-1) and were able to mineralize organic substrates more rapidly than a pristine paleosol. Thus, the in situ microbial population was stimulated by increased moisture from artificial recharge and/or from contaminants. In contrast, culturable bacteria were near or below detection in unimpacted fluvial and flood sands. The few culturable microorganisms that were present in unimpacted vadose sediments were either associated with the sediments at the time of deposition or transported from the surface during the last major proglacial flood, approximately 13,000 years ago.