EVALUATING THE RELIABILITY OF TREE RINGS FOR MONITORING PAST ENVIRONMENTAL U CONTAMINATION

Uranium (U) concentrations and isotopic ratios were measured in sugar maple (Acer saccharum), slippery elm (Ulmus rubra), and black walnut (Juglans nigra) tree cores to evaluate the reliability of tree rings for monitoring environmental U contamination. The mobility of metals in tree rings has been shown to be both element- and tree species-dependent but few studies have investigated the behavior of U. Understanding the behavior of U in annual growth rings of trees has potential applications to retroactively monitor (1) poorly documented contamination events and (2) the declared and undeclared production of nuclear fuels and weapons. The former Fernald Feed Materials Production Center (FFMPC), SW Ohio, provides a test site for this study by comparing its known U releases to the dendrochemical record. Production of purified U metal from ores and recycled materials began in 1951 and ceased in 1989 with 90 - 450 Mg U discharged into the environment during this time. Site remediation continues to the present. Tree cores (1 cm diameter) were dated by either ring counting or cross correlations and sectioned into segments containing 3 – 5 rings each to be analyzed by TIMS. All of the cores have rings that pre-date the beginning of production at FFMPC, spanning 1936 – 2014. Tree ring sections have 0.06 – 6.51 ppm U, ²³⁵U/²³⁸U with natural and depleted compositions, variable ²³⁴U/²³⁸U, and detectable ²³⁶U, consistent with the presence of anthropogenic contamination. Both the slippery elm and black walnut cores have non-natural U isotopic compositions pre-dating production at FFMPC, illustrating that U is mobile across annual growth rings and that these species are unreliable for use as retroactive biomonitors. However, the sugar maple core has natural U isotopic compositions in pre-production years and a peak U concentration during remediation, consistent with known site history and limited mobility of U in this species. The outermost rings of the sugar maple and slippery elm have isotopic compositions consistent with contemporary soil indicating that these species may be useful for monitoring current environmental contamination. These results suggest that sugar maple may be a robust archive of U contamination whereas application of slippery elm is limited to monitoring present contamination.