Mass balance approaches to assess critical loads and target loads of heavy metals for terrestrial and aquatic ecosystems

Critical loads of heavy metals address not only ecotoxicological effects on organisms in soils and surface waters, but also food quality in view of public health. A critical load for metals is the load resulting at steady state in a metal concentration in a compartment (e.g. soil solution, surface water) that equals the critical limit for that compartment. This chapter presents an overview of methods to derive critical loads of metals, with a focus on cadmium (Cd), lead (Pb), mercury (Hg), copper (Cu) and zinc (Zn), using critical limits for dissolved total metal concentrations based on impacts on food crops, soil organisms and aquatic organisms. Unlike nitrogen and sulphur, the time to reach steady state can be very long and therefore dynamic models are needed to estimate the times involved in attaining a certain chemical state in response to heavy metal inputs. Therefore, simple approaches are also presented to calculate target loads, i.e. deposition levels at which a critical limit is attained within a predefined time horizon. Results are presented for critical loads of Cd in view of food quality, critical loads and target loads of Cd, Pb, Cu and Zn in view of soil biodiversity and a critical limit for Hg in precipitation in view of impacts on fish and soil organisms. Results are discussed in view of the uncertainty and potential applicability for policy support