Evaluating the probability of exceeding a site-specific soil cadmium contamination threshold

A non-parametric approach for assessing the probability that heavy metal co ncentrations in soil exceed a location-specific environmental threshold is presented. The methodology is illustrated for an airborne Cd-contaminated a rea in Belgium. Non-stationary simple indicator kriging, using a soft indic ator coding to account for analytical uncertainty, was used in combination with declustering weights to construct the local conditional cumulative dis tribution function (ccdf) of Cd. The regulatory Cd contamination threshold (CT) depends on soil organic matter and clay content, which entails that it s value is not constant across the study area and also is uncertain. Theref ore, soft indicator kriging was used to construct the ccdfs of organic matt er and clay. Latin hypercube sampling of the ccdfs of Cd, soil organic matt er and clay yielded a map cf the probability that Cd concentrations exceed the site-specific CT. Cross-validation showed that the ccdfs provide accura te models of the uncertainty about these variables. At a probability level of 80% we found that the CT was exceeded at 27.3% of the interpolated locat ions, covering 3192 ha of the study area, illustrating the extent of the po llution. Additionally, a new methodology is proposed to sample preferential ly the locations where the uncertainty about the probability of exceeding t he CT, instead of the uncertainty about the pollutant itself is at a maximu m. This methodology was applied in a two-stage sampling campaign to identif y locations where additional Cd samples should be collected in order to imp rove the classification into safe and contaminated locations. (C) 2001 Else vier Science B.V. All rights reserved.