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

Recently, environmental quality standards (EQSs) that include background co ncentrations for metals have been set in The Netherlands. These EQSs are ba sed on environmental risk limits and applying the added risk approach. The added risk approach was developed to incorporate the background concentrati on of naturally occurring substances such as metals in environmental risk l imits. The original approach discriminates between the bioavailable and una vailable fraction and calculates a permissible concentration that can be ad ded 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 bi oavailable fraction of background metals is mathematically equal to zero. T he rationale for this assumption is that, from an environmental policy pers pective, background-related effects that in reality do exist, are desirable because they may in theory lead to increased ecosystem variability or biod iversity. Moreover, there is little information on the bioavailability of b ackground concentrations of metals. This paper presents a theoretical exerc ise in which the effects of varying background bioavailability on MPA value s for cadmium, copper, lead, and zinc in water and soil are evaluated. The results shaw that, for these metals, bioavailability does not affect or onl y slightly affects the MPAs. Only the terrestrial MPAs for copper, lead, an d zinc are sensitive to bioavailability. This minor influence of assumed pe rcentage bioavailability on the MPAs, coupled with a policy-driven assumpti on that effects of background concentrations should be ignored, explain the current policy in The Netherlands that the bioavailability of background c oncentrations of metals should be assumed to be zero.