Critical loads and critical limits of cadmium, copper, lead and zinc and their exceedances for terrestrial ecosystems in the United Kingdom

Critical loads have been calculated and mapped for cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) for six habitat types in the UK. This chapter focuses on updates to the methods since 2006 and the results of critical load and critical limit exceedance. The critical limits are expressed as free-metal ion concentrations in soil solution. Transfer functions are used to convert free-ion critical limits to total soil metal concentrations for comparison with current observed values. Critical limit exceedance provides an indication of current soil condition while critical load exceedance provides an indication of areas potentially at risk when steady-state is reached. As it is possible for the critical load to be exceeded but not the critical limit, it is important to look at the exceedance of both, because if deposition continues at current levels, critical limit exceedance may occur in the future. Results for Cu and Zn are similar, with exceedance of the critical load and critical limit occurring across ~ 20 % of the same habitat areas for managed broadleaved woodland and unmanaged woodland. There are few areas with critical limit exceedance for Pb for any habitat, but significant areas of woodland habitats with critical load exceedance, due to higher deposition scavenging by trees. For Cd there are no habitat areas with critical load exceedance and only small areas with critical limit exceedance. The main reason for the different patterns of critical limit and critical load exceedance is the different rates of change in the soil metal pools in relation to changes in metal deposition.