To enable a precautionary risk assessment for future inputs of metals, steady-state methods have been developed to assess critical loads of metals avoiding long-term risks to food quality and eco-toxicological effects on organisms in soils and surface waters. A critical load for metals equals the load resulting at steady state in a concentration in a compartment (e.g. soil solution, plant, fish) that equals the critical limit for that compartment. This chapter presents an overview of methods to assess critical limits and critical loads of metals, with a focus on cadmium (Cd), lead (Pb), copper (Cu) and zinc (Zn) in soils in relation to impacts on: (i) agriculture (food quality and crop health) and (ii) ecology (plants, invertebrates and soil organisms involved in nutrient cycling processes). Results are presented using generic input data. Furthermore, examples of national and European applications are shown. Results are discussed in view of the uncertainty and applicability of the critical load concept for heavy metals in future agreements on the reduction of metal emissions. It is concluded that for policy applications, dynamic models are also needed to estimate the times involved in attaining a certain chemical state in response to input (deposition, fertilisers or manure) scenarios.
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