Evaluation of the dutch environmental risk limits for metals by application of the added risk approach

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AbstractRecently, environmental quality standards (EQSs) that include background concentrations for metals have been set in The Netherlands. These EQSs are based on environmental risk limits and applying the added risk approach. The added risk approach was developed to incorporate the background concentration of naturally occurring substances such as metals in environmental risk limits. The original approach discriminates between the bioavailable and unavailable fraction and calculates a permissible concentration that can be added to a background concentration, i.e., the maximum permissible addition (MPA). This MPA includes the effects caused by the bioavailable fraction of the background concentration. The new EQSs in The Netherlands assume the bioavailable fraction of background metals is mathematically equal to zero. The rationale for this assumption is that, from an environmental policy perspective, background‐related effects that in reality do exist, are desirable because they may in theory lead to increased ecosystem variability or biodiversity. Moreover, there is little information on the bioavailability of background concentrations of metals. This paper presents a theoretical exercise in which the effects of varying background bioavailability on MPA values for cadmium, copper, lead, and zinc in water and soil are evaluated. The results show that, for these metals, bioavailability does not affect or only slightly affects the MPAs. Only the terrestrial MPAs for copper, lead, and zinc are sensitive to bioavailability. This minor influence of assumed percentage bioavailability on the MPAs, coupled with a policy‐driven assumption that effects of background concentrations should be ignored, explain the current policy in The Netherlands that the bioavailability of background concentrations of metals should be assumed to be zero.