The use of a Cs-137 vertical migration model to study the temporal evolution of heavy metals in coastal sediments of the Bay of Cadiz (Spain)

The evolution of heavy metals (Zn, Cd, Pb, Hg) in sea bed sediments of the Bay of Cadiz (Spain) has been studied. Four sediment cores were collected from the Inner Bay zone where sediment textural composition is mainly formed by silt and clay with high organic content, showing a very high absorption capacity for the substances solved in the aquatic medium. The dating of the sediments has been performed using the fallout radionuclide 137Cs as a tracer. Due to the high vertical mobility of this radionuclide in the sediment column, as a consequence of its molecular diffusion and the bioturbation existing in the area, the observed profiles are continuous and the 1963 maximum in fallout activity could not be assigned. The one dimensional diffusion-advection equation has been applied, considering the residence time of the radionuclide in the marshes zone, to develop a model that permits to interpret the profiles and to estimate the sedimentation rate, in order to infer the recent chronology of the sediment layers. By using the 210Pb dating method with the CRS model in one station, it can be showed that the sedimentation rate could be assumed to be constant during a longer period of time. In consequence, the sediment dating has been extended up to 100 years backwards, allowing the study of the evolution of heavy metals during the last century. The sediment dating shows that the heavy metal pollution of the Inner Bay has been produced simultaneously in the whole zone, during the industrial development of the environment. The increase of heavy metals Zn and Cd started in the second half of the XX century, while the enhancement of Pb and Hg concentration started at the beginning of the XX century. Heavy metal concentrations reached maximum levels during the eighties decade, afterward a remarkable decrease in surface sediments has been observed, which could be attributed to the restrictive environmental measurements undertaken at the zone, in particular the control of industrial effluents and the decreasing use of leaded fuels