GEOCHEMICAL IMPACTS OF LEGACY MINE WASTE CONTRIBUTIONS TO FOURMILE CANYON, COLORADO

The history of 19^th to mid-20^th century mining and milling in Fourmile Canyon, Colorado has left the hillslopes and valley bottoms scarred with potentially dangerous legacy sediment. Fourmile Creek is the primary source of drinking water for several communities; mine and mill waste sediment represent potential threats to these populations. The 2010 Fourmile fire, followed by floods in 2011, 2012, and catastrophic flooding in Sept. 2013, severely eroded some areas of legacy sediment, exposed them to future erosion, and transported significant volumes downstream. We present data on the chemical composition, potential mobility of heavy metals, eroded volumes and spatial distribution of waste rock and mill waste sediment from Fourmile Creek and four side tributaries.

To assess the potential danger of the mining legacy in relation to the above physical processes, we took bulk samples from over forty waste rock piles and over fifty layer-specific samples from sites containing mill waste sediment cut by Fourmile Creek. We determined the mobility of the chemicals within the waste rock piles and the mill tailings using leach tests, such as the Meteoric Water Mobility Procedure and the Toxicity Characteristic Leaching Procedure. For samples that had high toxicity, we determined the chemical composition (ICP-OES). Preliminary bulk sediment geochemical analysis indicates that select mill tailings exhibit consistently high levels of arsenic, lead, and other trace metals.

Pre- and post-flood LiDAR images were analyzed to find the mill waste sediment volume lost during the 2013 flood. Loss from the largest site, Salina, was at least 20% (3200m³) of the total volume. We used bulk and layer-specific sampling of the mill sediments to estimate chemical mass transport downstream. We tracked some of the mill waste deposition through sampling sediments downstream of the deposits.

The tailings do not appear to contribute enough to the stream chemistry during low flow to cause the water to exceed state and federal safe limits. However, the composition of the mining legacy sediment poses a separate danger to the soil, and the increased erosion of trace metals such as arsenic and lead is cause for concern for the local communities. If erosive events are increasing, the harmful influence of the legacy sediments in the watershed will only increase.