Spatial variability and trigger values of arsenic in the surface urban soils of the cities of Tampere and Lahti, Finland

Mapping arsenic concentration of urban soils and defining the upper limit of regional geochemical baseline variation are preliminary steps in assessing arsenic contamination and deciding possible remediation needs. The surface urban soils of two Finnish cities (Tampere and Lahti) were studied with the aim of modelling the spatial variability of arsenic concentration, assessing the uncertainty associated with mapping and delineating areas having arsenic concentration higher than a defined trigger value. The cities of Tampere and Lahti are located in the natural geochemical arsenic provinces where most of the arsenic in the baseline mapping samples has a geogenic source. A geostatistical approach was used to assess the spatial variability of As and turning bands method to draw a series of stochastic realizations. The differences between realizations provided a measure of spatial uncertainty and allowed to assess the probability that arsenic concentration exceeds a critical threshold. Results showed a short range component in the variogram model for Lahti that may be related to anthropogenic sources of arsenic as well. Arsenic concentrations are generally higher in Tampere than in Lahti due to its high natural concentrations in bedrock and soil. The widespread anthropogenic short range component was not observed in Tampere while misting up with natural As concentration because of the natural level of arsenic in the soil. The upper limit of the baseline variation values for Tampere (15 mg kg-1) and Lahti (6 mg kg-1) are suggested as regional trigger values for assessment of soil contamination and remediation needs. The results show that geostatistical analysis with stochastic simulations could be adopted to delineate other areas where lower or higher trigger values could be used.