Environmental assessment of the potential for arsenic leaching into groundwater from mine wastes in Baja Cali- fornia Sur, Mexico

Arsenolite (As203)was historically produced as a byproduct of gold and silver extraction in southernmost Baja California Peninsula. There are in the San Antonio-El Triunfo area more than 800,000 tonnes of mine waste material with an average content of 4% arsenic oxide.  The chemical reaction to produce arsenic trioxide (arsenolite) was the oxidation of arsenopyrite (AsFeS) with gold (ore) to produce iron oxides (Fe203) with gold and releasing SO2and As203 fumes. During the process the arsenic fumes sublimated onto the inner walls of old, mineral processing plants. When the chambers filled up, they were periodically emptied out on big piles of oxidized tailings. But once the plants were abandoned, the chambers remain half filled with approximately 600 tonnes of pure arsenolite. X-ray diffraction and scanning electron microscopy/energy dispersive X-ray spectrometry (EDS) indicate that the trace element content in the arsenolite and its banded structure may be due to the use of different ore for gold and silver extraction. A rough but conservative analysis of the potential leaching, based on results of the humidity cell test on mine waste material (arsenolite and arsenolite-bearing oxidized tailings) indicates that from 0.5 to 2.0 mg/l arsenic could be released into the groundwater system of the region. Real world data from the local aquifer show a close match with the calculations, indicating the close relation between arsenic content in tailings and groundwater.