HEAVY-METAL HORIZONTAL DISTRIBUTION IN SURFACE SEDIMENTS FROM A SMALL ESTUARY (PONTEDEUME, SPAIN)

Horizontal distributions of Fe, Mn, Co, Cu, Ni, Pb, and Zn were studied after a spatially intensive sampling campaign. Special attention was devoted to the head of the estuary where metal partitioning as well as the representative nature of the sediment data were examined. Sediments showed a clear seaward decrease in all elements and variables; the Eume river appeared to be the main source of particulate metals. Heavy metal functional relationships and non-residual concentrations suggested that different mechanisms could explain the trace element gradient found in different zones of the estuary. Fluvial and marine particle mixing could account for the constant extractability observed along the gradient in the outer zone of the estuary. A change in extractability parallels the upstream metal enrichment recorded in the semi-enclosed inner zone where sediment particles seemed to be mostly of fluvial origin. The fractionation of fluvial particulate matter through settling is proposed as a way to explain the metal concentration changes registered in the inner estuary. Inner zone sediment samples were highly representative of the average situation for the surrounding area. Five samples were sufficient to detect differences between total concentration means of 20% for different sites for all metals considered except Mn. Analytical error explained most of the total variation observed, Mn being the only metal showing a remarkable field error. The higher field variability of Manganese made up 50% of the minimum detectable difference using a sample size of five.