Metal attenuation at the abandoned Spenceville copper mine

Acid mine drainage from the abandoned Spenceville Cu mine contributes toxic levels of metals to nearby streams. The majority of metal contamination, however, is removed from the system over a short spatial distance. The primary mechanism responsible for this geochemical scavenging is adsorption onto Fe, Mn and Al-hydroxide precipitates that are clearly visible in the streambed. A 5-step sequential extraction procedure was performed on the streambed sediments in order to characterize solid phase speciation of metals in this area. Two fractions (carbonate and amorphous Fe-Mn hydroxide) were considered to be bioavailable, with the possibility of the metals in these phases being remobilized under changing ambient conditions. Copper and Zn were found to have the highest bioavailability, followed by Cd, Pb and Al. Although most metal in the acid mine drainage was rapidly removed from the system, aqueous concentrations of Cu and Zn exceeded California water quality criteria 0.5 km downstream of the mine site.