Abstract

Wildfire and abandoned mines both have profound geochemical effects on a watershed. However, the combined impact of the two disturbances is not wellstudied. In September 2010, a severe wildfire burned 26 km² of the Fourmile Creek catchment in the Colorado Front Range. 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, which takes place soon after the first storm event following the fire, examines how the combined disturbances of wildfire and mining
affect water and sediment chemistry in this catchment.

Streamwater concentrations of most major and trace solutes increase in burned and mined watersheds relative to unburned and unmined watersheds. Mining
intensity correlates positively with SO4^2– and negatively with HCO3- in tributaries, while fire intensity correlates positively with SO4^2–, NO₃^–, and major cations, but notably not with HCO3– . Acidity from mine drainage and local carbonate deposits reduce the effect of wildfire on streamwater alkalinity. Sulfate concentrations are on average 6.2 times higher in burned than in unburned tributaries, and this exceptional increase is likely due to additional SO4^2- input from mining. Downstream changes in water chemistry along Fourmile Creek reflect substantial influence from burned and mined tributary input, with especially large increases in conductivity, SO₄^2-, and Ca2+through the disturbed area.

Stream sediment from burned watersheds has increased percentages of most major oxides, and less SiO2 and nutrients. Mining is not associated with substantial changes in major oxides or nutrients in stream sedimentbut supplies increased mercury. In the short-term, the Fourmile Fire, exacerbated by historical mining, dominates the chemistry of Fourmile Creek and its tributaries.

Citation

Beganskas, S. (2012): The Geochemical Impact of Wildfire and Mining on the Fourmile Creek Watershed, Colorado . Submitted to the Department of Geology of Amherst College in partial fulfillment of the requirements for the degree of Bachelor of Arts with honors..