MICROBIOLOGICAL CHARACTERISTICS OF PRISTINE AND CONTAMINATED DEEP VADOSE SEDIMENTS FROM AN ARID REGION

The unsaturated (vadose) zone in arid and semiarid regions can be >100 m thick and may receive little or no moisture recharge from the surfa ce. The microbiological properties of the vadose zone are of interest because of the potential for microorganisms to impact the fate and tra nsport of contaminants in these environments. At numerous sites in the western United States, large volumes of wastewater or process water h ave 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 zon e sediments from depths ranging from 24 to 90 m were obtained from 3 b ore-holes drilled on the Hanford site in south-central Washington Stat e. 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 th e 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 hydrauli c conductivity (K) to the volumetric water content (theta). By compari ng the volumetric water content of sediments obtained prior to centrif ugation to their water content over a range of K(theta), it could be d etermined which vadose zone samples had been subjected to artificial r echarge. Elevated concentrations of nitrate and the presence of carbon tetrachloride in vadose sediments were also used as indicators that s ediments had been impacted by past waste disposal practices. Those vad ose zone paleosol samples receiving artificial recharge had higher pop ulations 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 moi sture 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 sedimen ts at the time of deposition or transported from the surface during th e last major proglacial flood, approximately 13,000 years ago.