Increasing arsenic mobility in the fine fraction of the dry stream sediments of the semi-arid San Antonio gold mining district (Baja California peninsula, Mexico)

The geochemical mobility of arsenic and some metals in the fine fraction of 20 alluvial sediment samples collected along the thalweg of the San Antonio arroyo was assessed in the abandoned gold mining district of the semi-arid southeastern Baja California peninsula. Acid-digested element concentrations were determined by treating the fine fraction of the sediments with concentrated HNO₃ via microwave heating. A separate leaching of the diluted acid-soluble As and some metals from the samples was carried out using 1 M HCl. The most reactive As phase was extracted from the subsamples using an ascorbic solution of Na citrate buffered at pH 8. The high abundance of sulphosalts (tennantite Cu₁₂As₄S₁₃ and proustite Ag₃AsS₃) in the sediments from the head of the arroyo changes downstream to the predominance of arsenic trioxide (arsenolite As₂O₃) and arsenate (scorodite FeAsO₄·2H₂O). Acid-digested As concentrations were always found to be high (230–270 mg kg⁻¹) from the San Antonio village until the end of the arroyo. The concentration of reactive As and its relative contribution increased from low values of 2.3 mg kg⁻¹ and 1 %, respectively, at the upstream of the arroyo, to 191–220 mg kg⁻¹ and 75–89 % in the middle and lower parts, and is probably a result of As release from the mineral sulfide fraction of the tailings. Metallic contaminants exhibited different behavior, with a peak of acid-digested and diluted acid-leached concentrations observed in the central part of the arroyo and an almost permanent geochemical mobility.