Uptake and mobility of uranium in black oaks: implications for biomonitoring depleted uranium-contaminated groundwater

In a preliminary study, the uptake and the mobility of uranium (U) by black oak trees (Quercus velutina) were assessed by measuring the isotopic compo sition of tree rings in two mature oak trees in a heavy metal contaminated bog in Concord, MA. The bog is adjacent to a nuclear industrial facility th at has been processing depleted uranium (DU) since 1959, Over the past 40 y ears, DU has been leaking from an onsite holding basin and cooling pond dow n gradient to the bog where the oaks are located. Because DU has no source outside the nuclear industry, contamination from the industrial facility is readily discernable from uptake of natural U by measuring isotopic composi tions. Isotope ratio analysis confirms the occurrence of DU in bark, sapwoo d and heartwood tree rings dating back to 1937, pre-dating the introduction of DU at the site by at least 20 years. Isotope dilution analysis indicate s high concentrations of U (>3 ppb) in sapwood that drop rapidly to relativ ely constant concentrations (0.3-0.4 ppb) in heartwood. These data indicate that once incorporated into tree cells, U is mobile, possibly by diffusion through the tree wood. Concentrations of U in sapwood are approximately eq ual to average U concentrations in groundwater onsite over the past 10 year s, suggesting that oak trees can be used as present-day bioindicators of U- contaminated groundwater. We suggest that regional sampling of oak bark and sapwood is a reasonable, inexpensive alternative to drilling wells to moni tor shallow groundwater U contamination. (C) 2001 Elsevier Science Ltd. All rights reserved.