USE OF TREE RING CHEMISTRY TO EVALUATE NATURAL BASELINES AND HISTORIC METAL LOADS FROM A BASE-METAL MINE AND MILL, COLORADO, USA

Abandoned mines throughout the Rocky Mountains are potential sources of acid, metal-rich drainage. There is an on-going debate over what the extent of clean up should be, driven in part by the question of determining natural baseline metals loads. Tree ring chemistry provides insight on the historic metals load at the abandoned Waldorf mine, approximately 6 miles southwest of Georgetown, Colorado. Mining at Waldorf began around the turn of the 20th century and continued until World War II. The conifer, Engelmann spruce (Picea engelmannii), is the dominant tree species growing in many areas both covered with transported mill tailings and elsewhere. Ten Engelmann spruce trees were cored using an increment borer; five trees were selected from the area that received mill tailings, and five from an unaffected area slightly upslope from the tailings-impacted area. Of the ten cores taken, six cores came from trees that pre-date the onset of tailings deposition. Quantitative determinations of Fe, Mn, Zn, Cd, Cu, Pb, and Sr were obtained from individual rings of the cores using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). A spiked cellulose pellet was used as the standard for all analyses and 13C was used as the internal standard. The results indicate distinct differences exist in Mn, Fe, Zn and Cd uptake by trees growing in tailings impacted vs. control trees (Mn, Fe, Zn, and Cd post-mining metals values are greater by factors of 1.2, 2.6, 2.2, and 5 over control concentrations, respectively). In addition, distinct differences in metals concentrations occur between the pre-mining rings and the post-mining rings in the trees that are growing in tailings impacted areas. In the impacted trees, Mn and Fe increase by a factor of 2, while Cd increases by a factor of 3. This difference in metal uptake is not seen in the control trees with the exception of Mn, which increases over time in both sets of cores. In general, in all cores which pre-date mining, pre-mining concentrations of Fe, Cu, Zn, Sr, and Cd are constant and comparable. Finally, in the tailings impacted cores, the increased metals concentrations are correlated with a subsequent decline in ring width indicating a decrease in growth rate. Based on this study, tree ring chemistry holds great promise for defining geochemical baselines in mining impacted areas.