Arsenic from gold mining in marine and stream sediments in Baja California Sur, Mexico

The environmental impacts associated with arsenic (As) from historical mining areas may persist for many years. We evaluated the influence of As in smelter and tailing wastes from a historic gold mine near San Antonio in the south-east of the Baja California, Mexico, on fluvial and marine sediments in the San Juan de Los Planes Basin and La Ventana Bay, respectively. Arsenic levels in the smelter and in the surrounding tailing wastes ranged from 82,000 to 208,000 mg kg⁻¹, and from 4220 to 92,000 mg kg⁻¹, respectively. The As concentrations in fluvial sediments were much lower, as these derived from wastes that had been scattered and had been subject to weathering for some time. As concentrations in surface sediments in the San Antonio stream close to the smelter and further downstream ranged from 477 to 1100 mg kg⁻¹, and from 64 to 613 mg kg⁻¹, respectively. As concentrations were between 0.05 and 9 mg kg⁻¹ in surface shelf sediments from the coast adjacent to La Ventana Bay; the concentration was highest in sediment collected close to the outlet of the main stream (La Bocana) of the Los Planes Basin. The As concentrations in all the solid waste samples from near San Antonio exceeded the Effect Range Medium (ERM) of the Sediment Quality Guidelines (70 mg kg⁻¹). Overall, 71 and 43 % of the stream samples exceeded the Effect Range Low (ERL; 8.2 mg kg⁻¹), and the ERM, respectively, indicating potential risks for biota and humans.