7 Abstract A laboratory study was undertaken using mine 8 tailings and soil columns to evaluate some of the natural 9 processes that can control the mobility of metals at Pb-Ag 10 mine tailings impoundments. The effects of buffering, pH, 11 and salinity were examined with tailings from the El Arteal 12 deposit. Al, Ba, Cd, Cu, Fe, Mn, Ni, Pb, Sr, and Zn were 13 mobilized when the tailings were leached. However, when 14 the mine tailings were placed above alluvial soils, Al, Ba, 15 Cd, Cu, Mn, Pb, and Zn were retained, although Fe and Sr 16 clearly remained mobile. Most of the metal retention 17 appears to be associated with the increase in pH caused by 18 calcite dissolution. The sorption of some metals (Cu, Pb, 19 and Zn) onto oxyhydroxides of Fe and Mn, sulphates, clay 20 materials, and organic matter may also explain the removal 21 of these metals from the leachate. 22 23