THE GEOCHEMICAL IMPACT OF WILDFIRE AND MINING ON STREAM AND SEDIMENT CHEMISTRY OF THE FOURMILE CREEK WATERSHED, COLORADO

Abandoned mines and wildfire can both have profound geochemical and geomorphic effects on a watershed. However, the combined impact of these two disturbances is not well-studied. In September 2010, a severe wildfire burned 26 km² of the Fourmile Creek catchment in the Colorado Front Range, followed several months later by a large storm event. Located in the Colorado Mineral Belt, the area was historically mined for gold and is still affected by mine drainage and the erosion of tailings. This study examines how the combined disturbances of wildfire and mining affect sediment and water chemistry in this watershed.

This past summer, water, channel sediment, and flood deposit samples were collected from ten locations along Fourmile Creek and from eighteen of its tributaries. Stream conductivity and discharge were measured at each sampling site. Initial field results show that conductivity in Fourmile Creek ranges from 37 to 272 µS/cm, while burned and mined tributaries have values as high as 1305 µS/cm. Water samples are being geochemically characterized using alkalinity titrations, IC, and ICP-MS; sediment samples are being analyzed using LOI, XRF, and Hydra (DMR) mercury analysis. A GIS analysis of the area includes delineation of the Fourmile Creek watershed and all sampled subwatersheds. An up-to-date map of mines in the area will also be produced in GIS. Using this information, the burn severity and degree of mining in each subwatershed will be quantified and correlated with chemical results.

The general impact of wildfire on streamwater chemistry is to increase concentrations of major cations and anions, notably Ca, Mg, K, Cl, NO₃, and SO₄, as well as alkalinity. Mine drainage is characterized by low pH and high concentrations of metals, SO₄, and ferric oxides. It might be expected that in waters affected by both mining and wildfire, the concentrations of many constituents, particularly SO₄, would be elevated. Initial water chemistry results indicate that burned and mined areas have high SO₄ concentrations and alkalinity. Sediment chemistry, especially that of flood deposits, likely will reflect increased concentrations of trace metals, nutrients, and other constituents. Initial sediment chemistry results indicate low levels of organic C, especially in burned areas, and a range of mercury concentrations from less than 30 ppb to 349 ppb.