Pyrite-bearing rock is readily oxidized when exposed to the atmosphere. Effluent from the impoundment of the rock commonly becomes acidic and enhances the mobility of heavy metals that are fixed or adsorbed on solid phases. Clay-rich covering soil is expected to be effective in reducing the oxidation rate of pyrite in the crushed rock, since the soil restricts the supply of gaseous oxygen to the rock. In this study, we constructed a miniature impoundment of truncated-pyramid shape, consisting of crushed igneous rock covered with a 1.7 m thick layer of silty soil, in order to observe gaseous oxygen concentration profiles. The soil used has a hydraulic conductivity of 3×10^-8 m/s and a porosity of 0.53. Interstitial gaseous oxygen within the crushed rock decreased with time to-1.5% due to oxidation of pyrite within the rock. The silty soil interrupted oxygen transport to the extent that the oxidation rate of the crushed rock was reduced by 93% compared with equivalent rock without a soil cover.