THE ENVIRONMENTAL IMPACTS OF MINING IN THE EAST MANCOS RIVER BASIN AS REVEALED BY DENDROCHEMICAL SIGNATURES FROM TREE-RING ANALYSES

The East Mancos River in southwestern Colorado is listed as an impaired water body by the State of Colorado. Within the watershed, there are approximately 40 waste-rock piles from abandoned mines throughout the middle and upper reaches of the river. The Colorado department of Mining, Safety and Reclamation is interested in determining the dominant source of pollution of certain metals into the river. It is uncertain what contribution the mining activities have made to the metal pollution over time. To address this issue, this investigation examined the concentrations of selected metals (Cu, Fe, Zn, Al) preserved in tree-rings to gain insight into periods of time marked by high metal concentrations, and how these compared to mining operations.

Multiple tree-cores from Douglas Firs and Blue Spruces were sampled at three different locations along the East Mancos River. Standard dendrochronological techniques were used to determine accurate time records preserved in the samples. ICP-MS analyses on core samples constrained the dendrochemical signatures of specified tree-rings.

There were no significant time increments where polluting events related directly to mining activities. Prior to 2000 there were only minor increases in some chemical signatures above background levels, with a few periods marked by 14.78 to 49.60 parts per million (ppm) increases from previous time increments. Dendrochemical data displayed a significant increase ranging from 76.79 to 282.98 ppm in metal concentrations during the increment from 2000-2019 for each site location.

The lack of direct correlation between the period of most active mining from 1890-1920, and the dendrochemical data, reveal that mining did not have an immediate impact on the system or the effects were not detected in the tree-rings. Other possibilities to explain the observed patterns include factors related to trees-growth and chemical uptake, climatic effects, timing of decomposition of minerals and rocks, and the effect of sealing the mines. A combination of all four factors is likely.