This paper describes the factors affecting arsenic (As) mobility from hydrothermally altered rock under in situ conditions. Four impoundments were built on site with rectangular base, truncated-pyramid structures. Impoundment 1 was composed solely of the hydrothermally altered rock while impoundments 2-4 were covered with different types of silty covering soil in order to minimize O(2) and water intrusion into the rock. The results indicate that seasonal variations in temperature, O(2) concentration and volumetric water content in the impoundments strongly influenced As leaching. When the temperature was high and the water content low, oxidation of sulfide minerals in the rock was enhanced because of a higher air-water-rock interaction. Concentration of As in the porewater increased when it rained after a specific period of dry weather. The use of a silty covering soil influenced the concentrations of As and SO(4)(2-) in the porewater although it did not affect the pH and Eh significantly. In this case, O(2) and water movement might be the rate controlling step of As leaching, that is, if there is no covering soil on the impoundment, more As will leach out from the rock. The use of a silty covering soil showed promise of reducing As leaching from the waste rock, but utilizing it alone was insufficient to effectively prevent As release from the rock. A combination of covering soil and bottom As-adsorption layer to incorporate any As released from the rock is therefore recommended. (C) 2009 Elsevier Ltd. All rights reserved.