MODELING CADMIUM ACCUMULATION AT A REGIONAL-SCALE IN THE NETHERLANDS

The cadmium content in soils in the rural environment in the Netherlan ds may increase towards an exceedance of quality standards due to atmo spheric deposition and the use of fertilizers and animal manure. To ev aluate this problem, a simple dynamic, process-oriented model SOACAS h as been developed which is aimed at predicting the accumulation of hea vy metals in the topsoil in a regional context. SOACAS describes the f ate of a metal in one completely mixed soil compartment using a Freund lich isotherm and analytical equations to solve the mass balance. We t ested if it was possible to reconstruct the soil's present cadmium con tents, using independent estimates of historical cadmium loads ('hind- cast simulation'). About 2500 recent point observations of cadmium con tents in rural areas were available. Before comparison, a map was crea ted to translate the point information obtained from the field data to areal average information (resolution 500 x 500 m(2)), required for c omparison with results of SOACAS. A regression model in combination wi th a locally-weighted smoother within the framework of Generalized Add itive Modelling (GAM) was used for this purpose. A realistic geographi cal pattern could be obtained with very few a priori assumptions. Comp arison of the map obtained by the GAM and the map obtained by hind-cas t simulation showed that, despite the large uncertainties about histor ical cadmium loadings, the current cadmium contents were only slightly underestimated by SOACAS. Moreover, the geographical pattern for the observed and simulated contents compared reasonably well, On this basi s of this exercise we believe that the model can be used to predict tr ends of future metal contents as a function of emission/immission scen ario's. Simulations showed that cadmium contents currently decrease in highly polluted areas around industrial plants in the South-Eastern p art of the country, and still increase in arable land.