Arsenic concentration in the soils of the Brenta Plain (Northern Italy): Mapping the probability of exceeding contamination thresholds

Traditional geochemical mapping is of little help when the uncertainty associated with the estimated values at unsampled locations is required to support decision making. In the Brenta Plain in Northeastern Italy (1350 km2), soils are characterized by a pedo-geochemical background value for arsenic which is higher (36 mg kg− 1) than the regulatory threshold (20 mg kg− 1), and it can prove difficult to distinguish between geogenic enrichment and anthropogenic pollution. A simple indicator kriging with varying local means calibrated on the soil map 1:50,000 was used to infer the local conditional cumulative distribution function (ccdf) of As concentration in topsoil (0–40 cm) and subsoil (70–120 cm). The use of local uncertainty models based on the estimated ccdf allowed the assessment of the probabilities to exceed critical thresholds. At the probability levels corresponding to the observed marginal probabilities of values above the regulatory threshold, and using the local background values as reference thresholds, it was found that As concentrations exceed these values at 9 and 7% of the interpolated locations, respectively, for topsoil and subsoil. The computation of the top enrichment factor (TEF), calculated as the ratio between topsoil and subsoil estimated concentrations, and its combination with the probability map of exceeding the usual background value for topsoil, allowed the identification of areas affected by anthropogenic As enrichment, which characterize about 6% of the study area.