Critical loads for heavy metals

The critical load for heavy metals depends on the acceptable total load of anthropogenic heavy metal inputs (deposition, fertilisers, other anthropogenic sources), below which ecosystem damage is unlikely.

Critical loads methods for toxic metals are currently being developed within the United Nations Economic Commission for Europe (UN/ECE) Convention on Trans-boundary Air Pollution (CLRTAP). In the UK, a research consortium currently contributes both to the development and the improvement of the critical loads method for application within the UN/ECE and to the development of improved tools to assess the effects of changing rates of atmospheric deposition on pools of metals in soils and freshwaters. Forest research is a sub-contractor to this consortium and have contributed to the calculation, evaluation and update of critical loads for heavy metals for forest ecosystems in the UK.

Effects-based steady-state and stand-still critical load approaches have been used for heavy metals critical loads calculations and mapping.

Effects-based steady-state critical loads: CLeffb(M)

The steady state equation for the calculation of heavy metal critical loads is as follows:

CL(M) = Mu – Mw + Mle(crit)


CL(M) is the critical load of heavy metal M.

Mu is the removal of heavy metals by biomass harvesting or net uptake in forest ecosystems from the mineral topsoil.

Mw is the weathering release of heavy metals from the mineral top soil,

Mle(crit) is the critical leaching of heavy metals from the mineral top soil, in which only the vertical drainage flux is considered.

Growth uptake values are calculated from estimates of average annual biomass increment multiplied by the content of the metal in the tree. Previous uptake values for woodland have been derived from a default range of metal concentrations in trees (UBA, 1998), and average yield data. New uptake values for heavy metals (Cu, Zn, Cd, Pb, Ni and Cr) are now available based on the measurements from the twenty Level II Intensive Monitoring sites in the UK and will be used in future modelling.

For the UK, the model has been simplified by removing the weathering term because the input of metals by weathering was considered negligible compared to metal deposition, while the uncertainties associated with such calculations are high. The approach described here implies that the critical load equals the net uptake by forest growth plus an acceptable metal leaching rate.

The ‘critical limit’ defines an acceptable maximum concentration of a metal below which long term deleterious effects to an ecosystem should not occur. Thus, defining the ‘critical limit’ is crucial to the critical load approach. For lead (Pb) and cadmium (Cd), ‘critical limits’ of 8 mg/m3 and 0.8 mg/m3 respectively have been adopted.

The depth considered in the calculation is 10 cm for forest soils or the depth of the A horizon, since impacts on plants and soil organisms, which are the main target groups considered are largely restricted to this depth.

Stand - still critical loads: CLstst(M)

The critical load equation used for the standstill approach is the same as for the effect based steady-state method, but with the ‘critical limit’ being replaced by the ‘concentration of metal’ in the soil solution, on the premise that no further metal accumulation will be allowed.

In brief an ‘effect based’ methodology, identifies atmospheric deposition (critical loads) that will not lead to concentrations of heavy metals above critical limits in defined compartments at steady-state. These critical loads can be derived using:

  • Critical limits of heavy metal concentrations in the soil solution which will not harm microbiota and plants and/or
  • Critical limits of (reactive) soil metal concentrations which will not lead to adverse impacts on soil functioning, such as soil invertebrates that ingest soil.

‘Stand-still’ critical loads equal the atmospheric deposition that will not lead to any further accumulation of heavy metals in the soil. In fact, stand-still critical loads should also include inputs other than atmospheric deposition.

A limitation of the ‘stand-still’ approach is that data on current concentrations of heavy metals in soil solution are scarce. Leaching can therefore only be mapped on the basis of ‘transfer functions’ which convert total content of metals in the upper soil horizon into concentrations in soil solution. ‘Transfer functions’  are available which describe the relationship between dissolved and adsorbed concentrations of heavy metals, accounting for the impact of soil properties such as pH and organic carbon content.

Previously, the critical loads models have only been applied to upland forests. Recently, the critical loads models have also been applied nationally to lowland forests,

Preliminary modelling and mapping of critical loads for cadmium and lead for upland forested areas suggest that the soil solution concentrations exceed the critical limits (ie, Pb 8 mg/m3, Cd 0.8 mg/m3) in some upland areas of England, Wales and Northern Ireland, with the critical limit for Pb exceeded in 41% of grid squares. For Cd, the critical limit is exceeded in 17% of grid squares.

However, there are still major uncertainties concerning the most appropriate methodological approaches to be used when applying the critical load concept to metals, as opposed to acid deposition. Further research will test the approaches to heavy metal critical loads mapping, applying site-specific soil water chemistry, heavy metal deposition and estimates of heavy metal uptake.