Overburden (cover rocks layer) is rocks that should be put aside in order to reach the mined ores and processed to obtain metal for commercial need. PTFI manages the overburden by referring to a comprehensive Overburden Management Plan approved by the Government of Indonesia.

A lot of metals are available in the free nature in form of sulfide minerals. When the ores are mined and the overburdenoverburden containing sulfide is exposed with the natural elements, the potential occurred for the formation of sulfide acid by activities of water, oxygen, and bacteria which naturally occurs. Such acid water can dissolve metals that are contained in the overburden and taken along in the water disposal system, which if it is not well managed, may cause negative impacts on the environment. This process is referred to as the mine acid water formation process.

PTFI manages and monitors the mine water acid produced by its activities. The independent audit is conducted on the environmental management system. PTFI concludes that the overburden management programs of PTFI are “well integrated” and “in accordance with the international practice”. In accordance with the overburden management plan, which is approved by the government, PTFI places the overburden at areas that are managed in the surrounding of the Grasberg open mine. The plan of PTFI to reduce the mine acid water includes the accommodation and processing of the available rocks acid water by blending with limestone and conducting the limestone capping on the overburden placement areas in the framework of the mine acid water formation in the future.