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. This drainage potentially impacts flora and fauna that utilize affected ground and surface waters. Such is the case for the abandoned mine tailings at the 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 Two. A 50-ton mill was constructed and used by several local mines. The conifer, Engelmann spruce (Picea engelmannii), is the dominant tree species growing in many areas both covered with transported mill tailings and elsewhere. Ten Engelmannspruce 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. All cores were analyzed using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) to determine historic trends in trace metal concentrations related to mill tailings deposition/migration and acid mine drainage associated with the Waldorf mine. Tree cores analyzed using LA-ICPMS, indicate distinct differences in Zn, Fe, Mn, Cd, Pb and Sr uptake by trees growing in the area that has received transported mill tailings (5 cores) vs. control trees from upslope areas (5 cores). Several of the cores from trees in the impacted area indicate that the trees pre-date mining activities, thus affording a look at pre-mining baseline metal uptake. Aerial photos, dating back to 1938, show that the mill tailings have migrated over time. Metals concentrations in the impacted tree cores reflect the tailings migration and indicate that successive tailings breakthrough events may have occurred since the 1920?s.