Abstract
The Schelde watershed drains a densely populated and industrialized region in central Europe. The Zenne River, a tributary which flows through the center of Brussels industrial region, contributes most of the arsenic to the river-estuary system. Inputs of industrial and domestic effluents create a region of anoxic conditions in the water column of the upper estuary. A study of arsenic concentrations and speciation in the rivers of the Schelde watershed shows that the ratio of As(III) to As(V) is elevated in the anoxic part of the estuary and those tributaries that are depleted in oxygen. The combination of a near-constant anthropogenic arsenic emission and seasonally fluctuating water discharge creates a variable arsenic concentration in the river endmember. This variability can explain the non-linearity in the arsenic-salinity relationship in the estuary, where a pronounced arsenic maximum is seen in a region without local arsenic sources. The As(III)/As(V) ratio in the estuary has typical marine values in the lower estuary and increases sharply at the oxic/anoxic interface near the head of the estuary. A numerical model, incorporating fluctuating river composition and discharge tidal mixing, zero-order reduction of arsenate, and first-order oxidation of arsenite was tested to model the distribution and redox speciation of arsenic in the estuary under non-steady-state conditions. It is concluded that the limited data set available does not make it possible to rigorously verify the applicability of the model or the uniqueness of the solutions obtained. Further profiles measured under different flow conditions will be required before it can be considered that the major features of the biogeochemical arsenic cycle in the Schelde estuary have been conclusively established. However, the proposed model can serve as a useful tool in the interpretation of the distribution and speciation behavior of an element in the estuarine environment, when both the composition of the river endmember and the river discharge are allowed to fluctuate in a non-analytical fashion. (Friedmann-PTT)