HEAVY-METALS IN GEOCHEMICAL SEDIMENT FRACTIONS OF THE BORDER REGION BETWEEN BAJA-CALIFORNIA, MEXICO, AND CALIFORNIA, USA

The concentrations of the heavy metals Cu, Zn, Ni, Cr, Pb, Cd, Ag, Mn and Fe in the different geochemical sediment fractions are determined in this study, as a means of assessing the impact of residual wastewat er on the coastal sediments of the region. During an oceanographic cru ise in April 1992 (ECOBAC IV), 20 samples of surface sediments were co llected from the coastal shelf of the border region between Baja Calif ornia (Mexico) and California (LISA). The sediment samples were subjec ted to a sequential extraction procedure designed to obtain the geoche mical fractions defined as: exchangeable (F1), carbonates (F2), Fe and Mn oxyhydroxides (F3), organic matter/sulfides (F4) and residual or l ithogenic (F5). The operational speciation of the metals indicates tha t the principal phases in the sediments that accumulate metal are the detrital or lithogenic matrix, the Fe and Mn oxyhydroxides and the org anic matter/sulfides, in this order. In general, the spatial distribut ion of the heavy metals in the reactive phase (defined as the sum of F 1-F4) tended to increase in concentration in offshore sediments. The h eavy metals in the reactive phase are apparently controlled by the con centration of reactive iron in the sediments, except for Pb, Cd and Ag , suggesting that the latter elements are controlled by other processe s, such as particulate matter of biogenic origin. In contrast, the spa tial distribution of heavy metals in the detrital phase behaved invers ely, that is, the greatest concentrations of metals are found close to the coast off the wastewater treatment plant at Punta Bandera, Baja C alifornia. The average concentrations of total metals (sum of F1;F5) w ere 7.6, 40, 21, 68, 428, 11, 0.10 and 0.025 mu g g(-1) for Cu, Zn, Ni , Cr, Mn, Pb, Cd and Ag, respectively, and 2.3% for Fe. These concentr ations are similar to those reported for uncontaminated reference sedi ments from southern California, indicating that a large amount of poll utants from the effluent of residual water in the coastal zone are bei ng transported outside the region.