Heavy metal concentrations in the topsoils of 'Achterhoek-Kreis Borken'. Mapping across the Duth-German border.

In the Dutch part of the area, heavy metal concentrations were measured at 113 locations and in the German part at 832 locations. For the Dutch part, spatially exhaustive ancillary information on clay content, organic matter content and pH is provided by the Dutch Soil Information System. For the German part, the Soil Map, 1:50,000, of Nordrhein-Westfalen provides spatially exhaustive information on clay content only. To obtain ancillary information on organic matter content and pH, point observations on organic matter content and pH are extrapolated by using the Soil Map, 1:50,000, of Nordrhein-Westfalen. Besides information on soil conditions, land use is an ancillary variable. The relationships between heavy metal concentrations and ancillary variables is described by multiple linear regression models, taking measurements below the detection limit into account by a maximum-likelihood procedure. Using these regression models, concentrations are predicted at a 25x25 m grid and next the regression residuals are interpolated spatially (Regression+Kriging). The accuracy of spatial predictions is evaluated by cross-validation