IDENTIFICATION OF DEWATERING TARGETS FOR GRASBERG PIT USING HYDROGEOCHEMICAL FINGERPRINT APPROACH

The Grasberg mine is one of the largest copper-gold-silver mines in the world. It is located at the very high alpine elevations (above 4000m) on the island of New Guinea in the province of Papua, Eastern of Indonesia. About 60% of the annual rainfall of 4,000mm recharges to the groundwater system in the mining area. A dewatering system is required to dewater ground-water inflow into the Grasberg open since the current bottom of the pit is below the water table. The identification of the sources of ground water inflow into the pit is critical to the design of such a dewatering system.

The hydrogeological setting at the mine site is very complicated. The steep topography, the karstic geologic condition, and the deep water table (200-300 m below the ground surface) significantly complicate the identification of the pattern of ground-water flow. Hydrogeochemical fingerprinting approach, in combination with hydrogeologic interpretation of the data collected from drillholes installed from underground developments below the pit, appears to be more effective in identifying the source of groundwater as dewatering target.

A “fingerprint” of water chemistry comprised of pH, the concentrations of Ca, Mg, Na, and SO4, and the ratio of Ca/Mg are used in this study. Water samples are considered to be originated from the same source if their fingerprints are similar to each other. By comparing the fingerprints of water samples from different areas of the mine, in conjunction with geologic and structural information, the lateral and vertical ground-water flow pattern was identified. The derived flow pattern not only was used to design the controls on location of groundwater inflows and to select drilling and interception of water, but also used to evaluate the conceptual hydrogeologic model of the Grasberg pit area.