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)

Abstract

The geochemical mobility of arsenic and somemetals in the fine fraction of 20 alluvial sediment samplescollected along the thalweg of the San Antonio arroyo wasassessed in the abandoned gold mining district of the semi-arid southeastern Baja California peninsula. Acid-digestedelement concentrations were determined by treating thefine fraction of the sediments with concentrated HNO3viamicrowave heating. A separate leaching of the diluted acid-soluble As and some metals from the samples was carriedout using 1 M HCl. The most reactive As phase wasextracted from the subsamples using an ascorbic solutionof Na citrate buffered at pH 8. The high abundance ofsulphosalts (tennantite Cu12As4S13and proustite Ag3AsS3)in the sediments from the head of the arroyo changesdownstream to the predominance of arsenic trioxide (ar-senolite As2O3) and arsenate (scorodite FeAsO42H2O).Acid-digested As concentrations were always found to behigh (230–270 mg kg-1) from the San Antonio villageuntil the end of the arroyo. The concentration of reactiveAs and its relative contribution increased from low valuesof 2.3 mg kg-1and 1 %, respectively, at the upstream ofthe arroyo, to 191–220 mg kg-1and 75–89 % in themiddle and lower parts, and is probably a result of Asrelease from the mineral sulfide fraction of the tailings.Metallic contaminants exhibited different behavior, with apeak of acid-digested and diluted acid-leached concentra-tions observed in the central part of the arroyo and analmost permanent geochemical mobility.