Are threshold values for metals in soils effective for protecting the soil ecosystem?

Regional geochemical data of heavy metals are commonly used for environmental risk assessment and management. The data used are often (near) total concentrations of the elements (often determined with Aqua Regia), whereas the exposure of the ecosystem is determined by the available or reactive fraction. The objective of our research was to develop a wider applicable method for quantitative hazard assessment of anthropogenic soil metal contamination, based on and illustrated with data for concentrations of Cd, Cu, Pb, and Zn from 360 locations in the Netherlands covering all major Dutch soil types. Extraction of soil samples with 0.43 M HNO3 gives a good approximation of the chemical availability of metals; the metal concentration in these extracts show a strong relation to the estimated anthropogenic enrichment, so we used them to assess the hazard of human-induced enrichment of these metals. To do this we used the toxic pressure concept, which estimates the fraction of biological species that is potentially affected due to the exposure to single metals or mixtures of metals. This is done using logistic concentration/response models parameterized with ecotoxicological effect data from toxicity tests and mixture models. Hazards varied from very low toxic pressures (lower than 0.01) to toxic pressures just below 0.05 (just staying within the so-called 95%-protection criterion used in some soil protection legislations). In rare cases, the toxic pressure exceeded the value of 0.05, to an upper limit of 0.054 for Cd. Ranking the metals according to toxic pressure suggests that Cd enrichment induces the largest hazard increase. Ranking the soil types according to their susceptibility for toxic pressure by metals only yielded minor differences in enrichment hazards between soil types. Comparing the judgement of soils based on the current soil screening levels and based on toxic pressures that we estimated in this study showed that the soil screening values tend to be on the conservative side. Conservative soil screening values do indeed protect the soil ecosystem properly, but they do not always indicate an actual hazard or risk. When screening values are exceeded, refined hazard insights can be obtained, as illustrated in this study. These insights in the ecotoxic implications of metal concentrations in soils can provide a more refined basis for risk management decisions.