DENDROCHEMICAL SIGNATURES OF ANTHROPOGENIC METAL POLLUTION IN ENGELMANN SPRUCE

The Upper Animas River Watershed, in southwestern Colorado, is a region with streams highly polluted by heavy metals, affected by both regional tectonic volcanism and historical mining activity. Distinguishing between anthropogenic and natural metal-sources is problematic. This study evaluated the utility and application of dendrochemistry to this problem in a highly mineralized sub-basin of the watershed, Prospect Gulch. The tree-ring record from six Engelmann Spruce trees in Prospect Gulch was analyzed. The tree-rings were considered to be passive recorders of external geochemical influences. Natural, background concentrations varied significantly but impact from historic mining activities was documented.

Two significant pulses of mining-induced metal-loading appear to have exceeded the geochemical baseline for several decades, especially with regards to Ag and Cd. For Ba, Cu, Mn and Zn, natural sources appear to dominate the total metal-loading. Previous work, by other researchers using different methods, has indicated that historical mining at the study site had a relatively insignificant impact comparative to natural metal-sources; this dendrochemistry study yielded similar results.

A major pulse of metal-loading beginning in the early 19^th century, well before any documented mining, indicates that concentrations of toxic elements at the study site have a wide range of natural variation, and that mining cannot be shown to have caused a majority of the observed toxicity in the region. Metals present in low concentrations evidently dominate the mining-related impact at the study site, while naturally-sourced metals found in relatively high concentrations seem to dominate the total metal-loading in the studied stream.

Based on results from this study, the dendrochemical technique successfully distinguished between the relative contributions of natural and mining-related metal sources to a stream in the San Juan Mountains.