Heavy metals

Activities

 

European surveys of heavy metal accumulation in mosses

The heavy metals in mosses survey was originally established in 1980 as a Swedish initiative under the leadership of Åke Rühling, Sweden. During 2001, responsibility for the coordination of the project was handed over from the Nordic Working Group on Monitoring and Data, Nordic Council of Ministers, to the ICP Vegetation Programme Coordination Centre at CEH Bangor.  Surveys are conducted every 5 years. By 2005, the study had expanded to include 28 European countries and 6,000 samples of mosses .

Moss sampling sites 2005/6 European survey.

 

The idea of using mosses to measure atmospheric heavy metal deposition had been developed in the late 1960s. It is based on the fact that mosses, especially the carpet-forming species, obtain most of their nutrients directly from precipitation and dry deposition; there is little uptake of metals from the soil. Heavy metals deposited from the atmosphere tend to be retained by the mosses thereby making sampling and chemical analysis more robust. It is easier and cheaper than conventional precipitation analysis as it avoids the need for deploying large numbers of precipitation collectors with an associated long-term programme of routine sample collection and analysis.

 

Data on the concentrations in mosses of the following heavy metals are collated, analysed and mapped by the ICP Vegetation Programme Centre: arsenic, cadmium, chromium, copper, iron, lead, mercury, nickel, vanadium, zinc; aluminium and antimony since 2005. Data show that in general the lowest concentrations are observed in (north) Scandinavia, the Baltic States and northern parts of the United Kingdom and the higher concentrations in Belgium and eastern European countries (see maps temporal trends below). However, spatial trends are metal-specific. The next moss survey is planned for 2010.

 

Bivariate analysis of the data showed the highest correlations between the cadmium and lead concentration in mosses and i) modelled EMEP depositions, ii) EMEP total emissions and iii) the ratio of urban land use in a 100 km radius. Correlations between the mercury concentration in mosses and modelled EMEP depositions or anthropogenic emissions were low. Multivariate analysis identified total modelled deposition to be statistically the most significant factor influencing the cadmium and lead concentration in mosses, whereas for mercury moss species was the most significant factor determining its concentration in mosses.

 

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Temporal trends in heavy metal accumulation in mosses (1990 – 2005)

An important part of the work at the Programme Coordination Centre has been to analyse the trends in the database. Europe-wide a dramatic decline in the concentrations of arsenic (72%), lead (72%), vanadium (60%) cadmium (52%) and iron (45%) was observed between 1990 and 2005, with a smaller decline being reported for zinc (29%), copper (20%) and nickel (20%). The decline for mercury (12%) and chromium (2%) was not significant. The temporal trends in heavy metal concentrations in mosses are in agreement with trends in emission and/or modelled deposition data reported for Europe (see annual report 2008/9 - to follow). However, temporal trends were country or region-specific, with no changes or even increases being observed. Therefore, even in times of generally decreasing metal depositions across Europe, temporal trends are different for different geographical scales.

 

Mean cadmium concentration in mosses per 50 x 50 km2 grid in 1990 and 2005 respectively
Mean lead concentration in mosses per 50 x 50 km2 grid in 1990 and 2005 respectively.

 

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Studying heavy metal deposition to white clover

In 1998, 2000, 2002 and 2004 participants in the ozone biomonitoring programme with white clover (see ozone) took part in a study to investigate the heavy metal concentration in white clover  in order to provide information on ‘normal’ background concentrations and pollution thresholds. The background concentrations were 0.065 µg Cd g-1 dry matter, 3.50 µg Cu g-1 DM and 0.15 µg Pb g-1 DM and the pollution thresholds were 4.15 µg Cu g-1 DM and 0.36 µg Pb g-1 DM, respectively. For cadmium only a preliminary pollution threshold of 0.155 µg g-1 could be established. For copper the background concentration and the pollution threshold are not far apart. At about half of the participating sites in Europe the pollution thresholds for copper and lead were exceeded at some time during the growth period. Although the white clover biomonitoring network was working well for the trace elements lead and copper, more data from polluted sites are needed to establish the pollution threshold for cadmium. The use of a standard soil low in cadmium is required to prevent confounding effects of the uptake of cadmium by the roots.

 

Cadmium (left) and lead (right) bulk deposition rates plotted against their concentration in white clover foliage

 

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Heavy metal deposition and potential contamination of crops

The ICP Vegetation has contributed to a review by the Task Force on Health regarding ‘Health risks of heavy metals from long-range transboundary air pollution’. Atmospheric deposition of metals has a direct effect on the contamination of crops used for human and animal consumption. Leafy vegetables are particularly vulnerable to the atmospheric impact of arsenic, lead and mercury. Away from pollution sources the deposition of arsenic is rather low but atmospheric deposition remains important as a main source of contamination of leafy vegetables. The contribution of atmospheric cadmium to the accumulated level in crops is often less important than that of soil-borne cadmium. There is limited transfer of cadmium to humans via consumption of animal products, except for offal. Lead is a widespread pollutant having a direct impact on above-ground plant parts. Even at remote sites there is still a clear impact of lead deposition on its concentration in leaves and wheat grain. A decrease in atmospheric deposition will have a beneficial effect on the daily intake by humans. In crops, atmospheric deposition is the main source of inorganic mercury. However, most crops have a rather low accumulation rate although some crops (curly kale, some herbs) are able to accumulate considerably more. A decrease of atmospheric mercury would result in a decreased mercury load for humans.

 

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