Colloid formation and metal transport through two mixing zones affected byacid mine drainage near Silverton, Colorado

Stream discharges and concentrations of dissolved and colloidal metals (Al, Ca, Cu, Fe, Mg, Mn, Ph, and Zn), SO4, and dissolved silica were measured t o identify chemical transformations and determine mass transports through t wo mixing zones in the Animas River that receive the inflows from Cement an d Mineral Creeks. The creeks were the dominant sources of Al, Cu, Fe, and P b, whereas the upstream Animas River supplied about half of the Zn. With th e exception of Fe, which was present in dissolved and colloidal forms, the metals were dissolved in the acidic, high-SO4 waters of Cement Creek (pH 3. 8). Mixing of Cement Creek with the Animas River increased pH to near-neutr al values and transformed Al and some additional Fe into colloids which als o contained Cu and Pb. Aluminium and Fe colloids had already formed in the mildly acidic conditions in Mineral Creek (pH 6.6) upstream of the confluen ce with the Animas River. Colloidal Fe continued to form downstream of both mixing zones. The Fe- and Al-rich colloids were important for transport of Cu, Pb, and Zn, which appeared to have sorbed to them. Partitioning of Zn between dissolved and colloidal phases was dependent on pH and colloid conc entration. Mass balances showed conservative transports for Ca, Mg, Mn, SO4 , and dissolved silica through the two mixing zones and small losses (< 10% ) of colloidal Al, Fe and Zn from the water column. Published by Elsevier S cience Ltd.