Bacterial-facilitated uranium transport in the presence of phytate at Savannah River Site.

At the Department of Energy (DOE) managed Savannah River Site (SRS), uranium and other heavy metals continue to pose threats to the ecosystem health and processes. In the oxic soil of this site, uranium is present primarily as soluble salts of the uranyl ion (i.e., U(VI) or UO22+). Although UO22+ has a strong sorption to the soil, the mobile indigenous bacteria may facilitate its transport. On the contrary, precipitation of UO22+ with phosphate has been found to be an alternative remediation strategy. This research investigated the effects of mobile bacteria and phytate on UO22+ transport at SRS in column experiments. It was discovered that UO22+ can barely be mobilized by de-ionized water but can be significantly transported with the aid of mobile indigenous bacteria. UO22+ had the most facilitated transport observation when it reached equilibrium with the bacteria before the transport. When UO22+ and bacterial were introduced to the soil at the same time or UO22+ was pre-deposited in the soil, the facilitated transport was less pronounced. In the presence of phytate, bacterial-facilitated UO22+ transport was hindered. pH was found to play the key role for UO22+ immobilization in the presence of phytate. The immobilization of UO22+ with the addition of phytate increased with the increase of pH within the pH range of this study because of the impact of pH on the solubility of UO2(OH)2. Phytate promoted UO2--PO43- complex and/or [Ca(UO2)2(PO4)2] formation, leading to enhanced UO22+ immobilization in the SRS soil.

Factors affecting public-supply well vulnerability in two karst aquifers.

Karst aquifers occur in a range of climatic and geologic settings. Nonetheless, they are commonly characterized by their vulnerability to water-quality impairment. Two karst aquifers, the Edwards aquifer in south-central Texas and the Upper Floridan aquifer in western Florida, were investigated to assess factors that control the movement of contaminants to public-supply wells (PSWs). The geochemistry of samples from a selected PSW or wellfield in each aquifer was compared with that from nearby monitoring wells and regional PSWs. Geochemistry results were integrated with age tracers, flow modeling, and depth-dependent data to refine aquifer conceptual models and to identify factors that affect contaminant movement to PSWs. The oxic Edwards aquifer is vertically well mixed at the selected PSW/wellfield, although regionally the aquifer is geochemically variable downdip. The mostly anoxic Upper Floridan aquifer is affected by denitrification and also is geochemically variable with depth. In spite of considerable differences in geology and hydrogeology, the two aquifers are similarly vulnerable to anthropogenic contamination. Vulnerability in studied PSWs in both aquifers is strongly influenced by rapid karst flowpaths and the dominance of young (<10 years) groundwater. Vulnerability was demonstrated by the frequent detection of similar constituents of concern in both aquifers (nitrate, atrazine, deethylatrazine, tetrachloroethene, and chloroform). Specific consideration of water-quality protection efforts, well construction and placement, and aquifer response times to land-use changes and contaminant loading are discussed, with implications for karst groundwater management.