A LASER ABLATION ICP-MS STUDY OF TREE RINGS IN AN AREA IMPACTED BY METAL MINING, WALDORF MINE, COLORADO, USA

Engelmann spruce (picea engelmannii) is the dominant tree species in many abandoned mine areas of the Rocky Mountains. It is long-lived, therefore may act as a temporal monitor of changes in soil chemistry caused by past metal mining activity. In this study, a cellulose-based trace element standard was developed for laser ablation ICP-MS analysis of individual tree rings of Engelmann spruce, to provide quantitative compositional data. The elements analyzed include Fe, Zn, Cu, Cd, Mn, Pb, and Sr. Horizontal cores were obtained from trees growing at tailings-impacted and control (unimpacted) sites near the Waldorf Mine (base- and precious-metal mineralization) in Colorado, USA. Counting of tree rings provided a temporal framework for the chemistry study. Zn, Cu, Fe, Cd, Pb and Sr concentrations remained consistently low through time in the control samples, whereas Zn, Cu, Fe, and Cd concentrations increased in post-mining rings (1900 to 1920) in tailings-impacted samples. Zn, Cu, Fe, and Cd levels are similar in pre-mining rings from control and tailings samples, indicating that present-day soil concentrations of these elements in the control area reasonably reflects local background. Mn concentrations in both sets of cores increased over time, making background values difficult to estimate. Pb and Sr concentrations remained constant with time and showed no changes between control and tailings-impacted sites. Thus, these elements are not definitive in determining changes in chemistry due to past mining activity. Based on the success of this study, the USGS has developed a new organic standard (MAOS) for laser ablation trace element studies involving a mixed cation/anion exchange resin. This new standard provides broader element coverage than is possible with the cellulose-based standard.