Marine vs Fluvial Bottom Mud in the Scheldt Estuary

The ratio of marine to fluvial bottom mud in the Scheldt Estuary (south-west Netherlands) was calculated by applying factor analysis to a data set of 66 bottom samples. In each sample, concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn were measured in the mud fraction (<63 mu m). Three linear independent processes were found to explain the variability of the elemental concentrations: (a) the supply of fluvial mud to the mixing zone, (b) chemical precipitation of Ni on insoluble manganese oxides in the transition area between the anoxic upper estuary and oxic lower estuary, and (c) the formation of insoluble Cd-, Cu-, Hg- and Zn-sulphides in the anoxic upper estuary and a mobilization of Cd, Cu, Hg and Zn in the oxic lower estuary. Scores of the first factor were used to calculate the marine fraction of bottom mud. In most of the upper estuary, the marine fraction is less than 10%. Between Lillo and Saeftinge, the marine fraction increases sharply from 10% to 75%. Further seaward, in the lower estuary, the marine fraction gradually increases from 75% to 95%. Possible influences of the Zeebrugge dumpsites on the mixing ratio was not revealed by the data set.