High variability in geochemical partitioning of iron, manganese and harmful trace metals in sediments of the mining port of Santa Rosalia, Baja California Sur, Mexico

Abstract

Total trace metal concentrations and the HCl-soluble (MeHCl) and pyrite (Mepyr) fractions were evaluated in two sediment cores (SR1 and SR2) and mine tailings from the copper mining town of Santa Rosalia (SR), Baja California Sur, Mexico. MeHCl were significantly different in the two cores (p≤0.001), in particular Cd and Fe in core SR2 were 29 and 1.0x104 times higher than concentrations found in SR1, respectively. These higher concentrations in core SR2 are likely due to the fact that the core was collected from a navigational channel last dredged between 1931 and 1948. Assuming that the trace metal (Me) reactive fraction (MeRF=MeHCl+Mepyr) is the most labile, then for each ton of dredged harbor sediment, up to 4.5±3.6kg of CuRF (equivalent to almost 0.5% of Cu on a weight basis) could potentially be exported into the ocean. Since an important portion of the reactive trace metals are in the Mepyr fraction, as indicated by their high degree of trace metal pyritization (DTMP=100Mepyr/MeRF) values (>98% for Cu, Fe, Pb, Zn), export outside of the harbor will oxidatively dissolve trace metals and release them into the marine environment. Trace metal enrichments (EFcrust), relative to Al-normalized Earth's crust values (ECV) ranged from 0.94±0.14 to (11.5±5.6)x102 times the ECV for Ni and Cd, respectively. These EFcrust values were differentiated into three groups: EF≈1 (Ni, Fe), 10<EF<100 (Mn, Co, Pb, Zn), and EF>100 (Cu, Cd). Copper enrichments are not surprising considering its high concentrations in the SR mine tailings; however, the higher Cd enrichments (6 to 11 times higher than Cu) were unexpected. Further analyses of mine tailings showed that the Cd enrichments increased considerably through ore processing. Concentrations for a number of trace metals in SR sediments are high even when compared with wastes from mines of other countries, indicating that these sediments essentially should be treated as mine tailings. Finally, our results show that the distribution of the different trace metals is not the result of diagenetic processes, but rather the result of input from ore-derived sources.