English Abstract Chloride and nitrate concentrations in groundwater are
mapped on a national scale using classification, point kriging, stratified
point kriging and stratified block kriging. A soil map aggregated to six
main soil types (peat, sand, marine clay, fluvial clay, old clay and loam)
is used as classification criterion. Zinc and cadmium concentrations in
groundwater are mapped on a national scale using stratified block kriging.
The results are presented as 95% confidence intervals of the predictions.
When spatial dependence between measurements is observed, kriging
interpolation results in more spatially differentiated maps than those
obtained by classification. For mapping on a national scale block kriging
seems more appropriate than point kriging because of the large uncertainties
that result from the latter. Stratification preceding kriging is needed
because of important differences between the strata. High concentrations
along the coast characterise the chloride concentration maps. Block
averages indicate a difference between high sand areas with chloride
concentrations below 50 mg/l and areas near the coast with chloride
concentrations above 50 mg/l. All methods used for mapping nitrate produce
overlapping confidence intervals. The upper side of the confidence
intervals show high tendencies on areas with high sand soils. In the
provinces Noord-Brabant and Limburg (South East Holland) some areas have
block averaged zinc and cadmium concentrations higher than the background
value (a-level). The confidence intervals do not exclude b- or c-levels
(signal and danger levels, respectively).