Ja. Villaescusacelaya et al., HEAVY-METALS IN GEOCHEMICAL SEDIMENT FRACTIONS OF THE BORDER REGION BETWEEN BAJA-CALIFORNIA, MEXICO, AND CALIFORNIA, USA, Ciencias marinas, 23(1), 1997, pp. 43-70
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.