Sensitivity of Water Quality Model Results for the Dutch Coastal Zone to the Distribution Coefficient of Cadmium

Results of a water quality model of the Dutch coastal zone appeared to be highly sensitive for the distribution coefficient particulate/dissolved (K_d) of cadmium.Field data of the Dutch coastal zone were used to calculate the annual and seasonal trend in the distribution coefficient of cadmium over the years 1983–88. A strong seasonal and spatial gradient in the distribution coefficient was found with relatively high values in summer and lower values in winter (K_d=3.0–7.0 log l/kg). Near the coast (2 km) the K_d was lower than more offshore (70 km from the coast). In addition, values for the distribution coefficient for cadmium were extracted from the literature (K_d=2.9–4.7 log l/kg).The range of K_d values obtained from the field data was used to perform model simulations for cadmium, in order to determine the sensitivity of the model to the distribution coefficient. The modelled yearly averaged concentrations of dissolved cadmium at one location 10 km from the coast, ranged from 0.005–0.035 μg/l, depending on the magnitude of the K_d used in the simulation. These concentrations are low compared to measured values (0.053 μg/l) due to an underestimation of the cadmium input to the North Sea, or possibly the occurrence of bottom-water exchange processes which the model does not include.