DISTRIBUTION OF METALS BETWEEN WATER AND ENTRAINED SEDIMENT IN STREAMS IMPACTED BY ACID-MINE DISCHARGE, CLEAR-CREEK, COLORADO, USA

Acid mine drainage from the Central City and Idaho Springs mining districts has been identified as a source of metal contamination impacting surface water along Clear Creek and North Clear Creek, Colorado. Based on a mass balance calculation along North Clear Creek, Al, Cd, Cl, F and Zn were found to behave conservatively, while Cu, Fe, Mn and Zn were all depleted in the water column. The field data also demonstrated that Al, Cu and Fe were associated with the particulate fraction in the water column, while Cd, Mn and Zn occurred predominantly in the dissolved (< 0.45-mu-m) fraction. Microprobe analysis and geochemical modeling support the hypothesis that Fe precipitates as an amorphous hydroxide, bearing trace quantities of Mn. A series of batch studies using stream-bed sediments from North Clear Creek was used to assess the effect of thunderstorms on metal desorption from sediments entrained in the water column. Metal desorption increased as the pH decreased from 6.8 to 3.5. Mass balance constraints allowed the construction of desorption isotherms that demonstrated a metal-substrate affinity of Mn < Zn < Cu < Al, characterized by 50% desorption at pH 6.5, 5.5, 4.5 and 4.4, respectively. From these data, values of K(d) as a function of pH were generated for future use in modeling metal transport in the creek system.