Sufficiency and Effectiveness Review of the HM Protocol, Chapter A3

Sufficiency and Effectiveness Review of the HM Protocol, Chapter A3

Draft, 12 April 2006

Results of modelling and mapping of critical loads of lead, cadmium and mercury and critical concentrations of mercury in precipitation and their exceedances in Europe

Gudrun Schütze

Jean-Paul Hettelingh

1

1 Introduction

The development of the critical load approach for heavy metals within the framework of the Working Group on Effects was inspired by Article 6 (g) of the Protocol, which encourages work on an effects-based approach for the purpose of future optimised emission control strategies. The critical loads approach was deemed to be an appropriate way to link depositions of metals with effects on human health and the environment. The reason included the fact that optimised control strategies for acidifying and eutrophying air pollution had been successfully applied in support of abatement policies in Europe.

The scientific basis of the critical loads approach for Europe was developed over a period of 11 years with stepwise improvements of the methodologies according to the state of knowledge. It started with first exercises in the framework of the ESQUAD Project (Van den Hout (ed.) 1994) followed by meetings organised in the framework of the Convention’s International Cooperative Programme on Modelling and Mapping Critical Levels and Loads and Air Pollution Effects, Risks and Trends (ICP Modelling and Mapping). These meetings included heavy metal sessions of workshops of the Coordination Center for Effects (CCE) of ICP Modelling and Mapping from 1995 until 2005 and a series of scientific workshops and expert meetings (Gregor et al. 1998, Gregor et al. 1999, Curlík et al. 2000, Schütze et al. 2003). The first manuals for modelling and mapping of critical loads of heavy metals were published at the end of the last century (De Vries and Bakker 1996/1998, De Vries et al. 1996/1998). A first European wide dissemination and voluntary application by the network of National Focal Centres (this network currently covers European countries only) of the preliminary methodology was requested by the Working Group on Effects to the Coordination Center for Effects at its 20th session. Results are described in a collaborative report (Hettelingh et al., 2002) of the CCE and the EMEP Meteorological Synthesizing Centre – East. At its twenty-third session the Working Group on Effects then requested the Coordination Center for Effects to issue a call for data to its National Focal Centres on critical loads of cadmium, lead and mercury and Hg. The result of this call for data was reported to the 21st meeting of the Task Force ICP Modelling and Mapping, to the 24th session of the Working Group on Effects (EB.AIR/WG.1/2005/10/Add.1), and documented in a second collaborative report of the Coordination Center for Effects and the EMEP Meteorological Synthesizing Centre East (Slootweg et al, 2005).

EMEP Meteorological Synthesizing Centre East modelled depositions of cadmium (Cd), lead (Pb) and mercury (Hg) for 1990 and 2000. By comparing maps from the Meteorological Synthesizing Centre East of depositions with CCE maps of critical loads it has been possible to identify geographic locations in Europe where critical loads are exceeded.

In Slootweg et al. (2005) the response by the National Focal Centres, of European maps on critical loads of cadmium, lead, and mercury as well as of preliminary exceedance maps is described while updates received after the 21st meeting of the Task Force on ICP Modelling and Mapping are summarized in Hettelingh et al (2005). An evaluation to which extent the critical loads approach provides a satisfactory scientific basis for application in air pollution policy will be provided in a chapter of the Chairman’s Report to the Working Group on Strategies and Review (EB. AIR.WG.5/2006/XXX).

2 Main principles of critical loads of heavy metals calculations

The methodology to calculate critical loads of Pb, Cd, and Hg is described in detail in the Convention’s Manual on Modelling and Mapping Critical Levels and Loads and Air Pollution Effects, Risks and Trends (Modelling and Mapping Manual 2004) and a related background document (De Vries et al. 2005). The description here is limited to the main principles and key variables and is excerpted from the Manual.

The critical load of a metal is the highest total metal input rate (g ha-1 a-1) below which harmful effects on human health and ecosystems will not occur in an infinite time perspective, according to present knowledge. While critical loads explore the sensitivity of ecosystems against metal inputs, the risk of effects can only be described by the exceedances, i.e. by comparison of critical loads with the actual inputs.

The method to calculate critical loads of heavy metals is based on a steady state balance of ecosystem inputs and outputs of heavy metals. The underlying assumption of steady state for the fluxes as well as chemical equilibrium in an undetermined future is consistent with concepts of sustainability. Implications of this on the interpretation of critical loads and their exceedances are explained later in this chapter.

2.1 Effects, indicators and critical limits

Receptor dependent critical loads of Pb, Cd, and Hg as well as the critical concentration of Hg in precipitation can be calculated when a representative effect indicator is identified. The metal concentration in an environmental compartment is such an indicator. The critical limit is the maximum of this indicator concentration that will not cause harmful effects in the long-term. Approaches have been designed to assess critical limits addressing either ecotoxicological effects or human health effects. Not all effects have the same relevance for every metal. Table 1 lists effects and their indicators that have been addressed in the 2004 call for data of the CCE.

Table 1. Overview of effects indicators used in the calculation of critical thresholds.

Effect_no / Effects (indicators) / Ecosystems / Metals
1 / Human health effects (ground water quality in view of use for drinking water supply) / Terrestrial ecosystems / Pb, Cd, Hg
2 / Human health effects (food quality) / Terrestrial ecosystems
(arable land only) / Cd
3 / Ecotoxicological effects / Terrestrial ecosystems / Pb, Cd, Hg
4 / Ecotoxicological effects / Fresh water ecosystems / Pb, Cd
5 / Human health effects (food quality) / Fresh water ecosystems / Hg

Oral uptake is the main pathway for human health effects of environmental Cd, Pb as well as Hg. Critical loads for terrestrial ecosystems addressing human health effects can be calculated, either in view of not violating food quality criteria in crops or in view of ground water protection regarding its potential use as drinking water. The derivation of critical limits used for critical loads to protect human health requires complex analyses including pathways of metal intake independent from the environmental situation (Cd in cigarette smoke, Hg in dental amalgam etc.). In agreement with the Joint Task Force on Health Aspects of Air Pollution it was decided to use internationally accepted critical limits. Therefore, critical limits to protect ground water quality (effect 1) were set to the recommendations for maximum metal concentration in drinking water of WHO (2004) Cd: 3 mg m-3; Pb: 10 mg m-3, Hg: 1 mg m-3.

An appropriate indicator for critical load calculations addressing human health effects via food intake (effect 2) is the Cd concentration in wheat. The EU regulation (EG) No.466/2001 uses a limit for Cd of 0.2 mg kg-1 fresh weight in wheat grains. This limit is, however, not based on effects (it was derived with the principle “As Low As Reasonably Achievable – ALARA). In the critical loads calculation a conservative effects-based food quality criterion for wheat of 0.1 mg/kg fresh weight was used (see De Vries et al. 2005, Appendix 4). Using this criterion the pathway of Cd to wheat leads to the lowest critical Cd content in soils therefore protecting also against effects on human health via other food and fodder crops (including also the quality of animal products, De Vries et al. 2003).

The quantification of the risk of human health effects of Hg through fish consumption (effect 5) is not related to a critical load (g ha-1 a-1) but to a critical concentration of Hg in precipitation (ng L-1). This critical Hg concentration in precipitation can be linked with a simple model to the Hg concentration in fish assuming a steady-state situation in the catchment. A limit of 0.3 mg kg-1 fresh weight on total Hg in fish is consistent with recommendations by the USEPA (2001) and the WHO/FAO (2003) and therefore used in the calculation. This limit is frequently exceeded in Nordic surface waters already now.

Among terrestrial ecosystems, critical loads of Cd and Pb are to be calculated from the viewpoint of ecotoxicology for areas covered by non-agricultural land (forests, semi-natural vegetation) or agricultural land (arable land and grassland). Soil toxicity data collated and accepted under the terms of current EU risk assessment procedures (Draft risk assessment report Cd (EC 2003), Voluntary risk assessment for Pb, draft report, status 2006, provided by the Lead Development Association (LDA International) were used as basis for the derivation of critical limit functions for free metal ion concentrations (Pb, Cd) in soil solution. The data covered chronic effects on plants, soil-dwelling invertebrates and microbial processes on the population-level. While the ecotoxicological database was harmonised with EU risk assessment, the critical limits derivation was done in a different way: The free metal ion approach was considered most appropriate for the assessment of the influence of the bioavailability of metals on the effects on related organism groups. The bioavailability of metals does, however, not only depend on the free metal ion concentration but also on the concentration of other cations, particularly H+. This was taken into account in deriving critical limits as a function of the pH in soil drainage water. The method of derivation of the critical limit function is described in detail in Lofts et al. (2004). Critical limits taking into account secondary poisoning were explored in the background paper for the Manual (DeVries et. al. 2005). They were however considered very uncertain and therefore not included in the Modelling and Mapping Manual (2004) and current critical loads calculations for Europe.

Organic forest (top)soils are considered as the best understood critical receptor with respect to atmospheric Hg pollution, based on knowledge on effects on microbial processes and invertebrates. The suggested critical limit for Hg is that the concentration in the humus layer of forest soils after normalization with respect to the organic matter content should not exceed 0.5 mg (kg org)-1 (Meili et al. 2003). The strong association of Hg with organic matter leaves virtually no free ions. Therefore the exposure of biota to Hg is controlled by the competition between biotic and other organic ligands. Furthermore the contamination of all types of organic matter is determined by the supply of organic matter relative to the supply of Hg at a given site (Meili 1991, 1997). As a result, the critical limit for Hg in soils is set for the organically bound Hg rather than for the free ion concentration, also in solution.

In order to describe ecotoxicological effects of Cd in surface waters the 5-percentile cut-off value of chronic toxicity from the Draft EU risk assessment report (EC 2003) was used as critical limit. Deviating from the draft EU risk assessment no additional assessment factor was applied. For Pb the critical limit is based on Crommentuijn et al. (1997). It represents the highest value of a range for critical limits (to be used in dependence on water chemistry) suggested by a workshop of ICP Waters on heavy metals, 2002, in Lillehammer, Norway (Skjelkvale and Ulstein, 2002) These critical limits of Cd, Pb are provided as total dissolved concentrations. The free metal ion approach could not be used to derive critical limits of Pb and Cd due to limitations in the effects database for aquatic systems.

2.2 Calculation of tolerable metal fluxes in terrestrial ecosystems

Critical loads of metals for terrestrial ecosystems are focussed on the top soil. The soil depth to be considered in the quantification of metal fluxes depends on the receptor type and effect that is addressed (see Table 1).

The internal metal cycling within an ecosystem is ignored, since its influence on critical load results is relatively small, at least for Pb, Cd, Hg (De Vries et al. 2005). Because weathering of Pb, Cd, Hg causes only a minor input flux to topsoils, while uncertainties of such calculations are high, this flux was also neglected. Re-emission (volatilization) of deposited Hg is ignored, because this flux is already treated as part of the atmospheric net deposition in the modelling by EMEP Meteorological Synthesizing Centre East (Ryaboshapko et al. 1999, Ilyin et al. 2001). In consequence the critical load of a Pb, Cd, Hg equals the sum of tolerable outputs from the considered system in terms of net metal uptake and metal leaching.

The metal net uptake (g ha-1 a-1) is calculated from the annual yield (removal or increment) of biomass (kg ha-1 a-1) times a metal concentration (g kg-1) in harvestable parts of plants. Information on annual yields can be obtained from agricultural statistics and forest growth tables. It can also be modelled by relating yields to site characteristics as soil quality, climate and land use. The site specific share of different crops on agricultural land has to be considered. There is hardly any close relationship between metal contents in soil or soil solution and metal concentrations in harvestable parts of plants (an exception is Cd in wheat grains). Therefore, in general the metal uptake is calculated independently from metal concentrations in soils using medians of measured metal concentrations from relatively unpolluted areas. These median values in general neither exceed limits for food and fodder nor phytotoxic limits. The related uptakes are therefore considered to be tolerable. However, a good relationship of Cd in wheat grains to Cd in the soil solution exists (R2 = 0.62 according to Roemkens et al. 2004). This relationship was used in the calculation of critical loads of Cd to protect food quality. In these calculations, which were only performed for arable land, it was assumed that hundred percent of the crop rotation is wheat.