COMBINED IDTIMS AND LAM-ICPMS DENDROCHEMICAL STUDY OF A DEPLETED URANIUM AND HEAVY METAL CONTAMINATED BOG NEAR CONCORD, MASSACHUSETTS

Previously, we have demonstrated the uptake of depleted uranium (DU) in black oak (Quercus velutina) by isotope-dilution thermal ionization mass spectrometry (IDTIMS). The observed DU concentrations in sapwood from trees sampled adjacent to a DU warhead manufacturing facility correlate with measured aqueous DU concentrations in the nearby up-gradient wastewater holding basin, suggesting that IDTIMS analysis of sapwood is a viable method for determining the presence of DU contaminated groundwater. The existing data also demonstrate that once taken up, uranium is mobile within the xylem tissue of black oaks.

Laser ablation microprobe inductively coupled plasma mass spectrometry (LAM-ICPMS) in situ analysis of the same tree core further demonstrated the mobility of uranium within the tree, and also identified Cd and Pd as immobile elements. The usage of LAM-ICPMS permits analysis at the sub-annual growth increment scale, thus allowing identification of mobile versus immobile elements by measuring the relative concentrations of each element. The historic records for Pb and Cd uptake show an increase in concentration concurrent with an increase in the use of leaded gas in the mid-1960's, and a decrease in concentration after leaded gas was phased out of usage in the 1980's. Pb and Cd also show a distinct increase in concentrations at the same time as remediation efforts, including the excavation of contaminated soil, began at the subject site.

The combined usage of IDTIMS and LAM-ICPMS provides a method for identifying isotopically unique contaminants (DU) in various tree tissues, evaluating contaminant uptake pathways (groundwater versus foliar), and deciphering of historic records of uptake for other heavy metals (Cd and Pb) from the analysis of tree cores. This approach is being proposed as a viable alternative to expensive well drilling with a potential to offer similar spatial resolution for the identification and monitoring of contaminant plumes containing munitions related pollutants in shallow groundwater systems.