Small scale mining in the area of the Choapa river, Central Chile: an environmental risk?

Abstract: The Choapa valley is typically for the northern part of Chile with agriculture based one hundred percent on irrigation influenced by serious variations of precepitation. It will also become one of the major copper mining districts of Chile due to the increase in production of the mine Los Pelambres. For a further development of the water resources esp. groundwater a baseline research was done, besides other studies, on the impact of former small scale mining of copper in the center part of the valley. The slightly alkalic river waters and groundwaters are uncontaminated with respect to As, Cd, Cu and Pb. The determination of the net acid producing potential, net acid generation tests and kinetic tests on waste rocks demonstrate that in most cases carbonates and esp. Cu-carbonates buffer the systems hindering the formation of acid rock drainage and heavy metal release. The tailing pond of the only modern middle scale mine in the area is seeping and a sulfate plume emerges from the tailing pond downstream. Due to the high pH metals are not transported in aqueous form. The risk of particulate transport of metals with fine silts washed out of the pond will be studied