Introduction to Heavy Metal Monitoring

What are heavy metals?

A heavy metal is a member of an ill-defined subset of chemical elements that exhibit metallic properties. Many different definitions of the term heavy metal have been proposed, based on either density, atomic number, atomic weight, chemical properties or toxicity. The heavy metals which are monitored by CEH include:

Aluminium (Al), Antimony (Sb), Arsenic (As), Barium (Ba), Cadmium (Cd), Cobalt (Co), Chromium (Cr), Copper (Cu), Iron (Fe), Nickel (Ni), Lead (Pb), Manganese (Mn), Molybdenum (Mo), Rubidium (Rb), Scandium (Sc), Selenium (Se), Strontium (Sr), Tin (Sn), Titanium (Ti), Tungsten (W), Vanadium (V), Zinc (Zn).
 

Heavy metals are natural constituents of the Earth's crust. They are stable and cannot be degraded or destroyed, and therefore they tend to accumulate in soils and sediments. However, human activities have drastically altered the biochemical and geochemical cycles and balance of some heavy metals. The principal man-made sources of heavy metals are industrial point sources, e.g. mines, foundries and smelters, and diffuse sources such as combustion by-products, traffic, etc. Relatively volatile heavy metals and those that become attached to air-borne particles (particulates) can be widely dispersed throughout the atmosphere, often being deposited thousands of miles from the site of initial release. In general, the smaller and lighter a particle is, the longer it will stay in the air. Larger particles (greater than 10 micrometers (µm) in diameter) tend to settle to the ground by gravity in a matter of hours whereas the smallest particles (less than 1µm in diameter ) can stay in the atmosphere for weeks and are mostly removed by precipitation.

How do we measure Heavy Metal Concentrations and Deposition ?

The monitoring network set up for this work consists of 15 sites around the UK, as shown in accompanying map.  Some of these sites were set-up by kind permission of external organisations including The National Trust, Natural England and ADAS. In addition to the sites listed on the map, a new site at Harwell in Oxfordshire is currently being set up (December 2008).

The aim of the network is to measure the background concentrations and deposition of heavy metals. The sites in the network were specifically chosen as they are rural locations which are not influenced by nearby emission sources such as industrial plants or major roads. Note that there is a separate Defra-funded network (managed by NPL) which monitors heavy metals in urban and industrial areas.

At each of the 15 sites in the rural network, rain collectors collect rain water for analysis back at the laboratory. Since these open collectors can lead to evaporation of the sample, tipping buckets are used to measure the total volume of precipitation in the measurement period. The chemical analysis of the rain water is combined with the total rainfall from the tipping bucket to calculate the heavy metal concentration in rainfall at each of the sites. There are a further 2 sites where both rain water and cloud water are collected.

In addition to the rain collection, 10 of the sites operate Partisol 2025 sequential particulate samplers. These instruments collect particulates with a diameter of less than 10µm from the atmosphere by drawing the air in to the sampler and collecting the particulates on a filter. The filters are changed every week and sent back to the CEH laboratory for analysis of heavy metals.

The concentration of metal in the atmosphere is calculated from the total amount of each metal on the filter and the volume of air which has passed through the sampler.

The metal concentration data are then combined with the local meteorological data (rainfall etc) to allow the amount of each metal that is deposited onto the ground each year to be calculated. Separate values for wet deposition (from rain and snow etc), dry deposition (from dust settling etc) and cloud deposition (condensation of cloud droplets) are used to calculate the total deposition.

Data obtained from the monitoring sites are analysed and used to create maps of the UK showing the concentrations and deposition of heavy metals in both air and precipitation. These maps are used to identify the areas where the metal deposition is most likely to cause a pollution effect.

What legislation exists to regulate heavy metals?

European Legislation
The EC Framework Directive 96/62/EC set-out a common strategy to define and set objectives for ambient air quality including:

• avoiding, preventing or reducing harmful effects on human health and the environment
• assessing the ambient air quality in Member States on the basis of common methods and criteria
• obtaining adequate information on ambient air quality and ensure that it is made available to the public, inter alia by means of alert thresholds
• maintaining ambient air quality where it is good and improve it in other cases

Following on from the Framework Directive, subsequent Daughter Directives specifying limit and target values for individual pollutants have been published (Table 1). These directives also specify details of actions required should concentrations of the relevant metals exceed either, the limit / target values or their designated assessment thresholds.

Table 1. Limit and Target values for heavy metals in the PM10 particulate fraction of ambient air.

The First Daughter Directive (1999/30/EC) defined an atmospheric limit value for lead of 0.5 µg/m3, expressed as an annual mean. This was to be achieved by 1 January 2005 (or 1 January 2010 in the immediate vicinity of specific point sources).

The Fourth Daughter Directive (2004/107/EC) defined 'target values' for arsenic, cadmium, and nickel in the PM10 particulate fraction of ambient air (Table 1).  The Fourth Daughter Directive also requires monitoring of mercury although no limit or target values have been set. Member States had to transpose the 4th Daughter Directive into national law by 15th February 2007. The European Commission will report on its implementation by 31st December 2010. Governments must report to the Commission on zones and agglomerations where the target values are exceeded with the first such reports being required by 30th September 2008.

Data

CEH has been measuring annual average concentrations for heavy metals at rural sites since 2004. The data are presented as annual averages at each of the sampling points (Table 2). The data are also interpolated to produce UK maps of concentrations and deposition.

For each metal, we produce the following annual maps (2004-2007):

• Concentration in air (ng/m3)
• Concentration in rainwater (µg/l)
• Total deposition (g/ha/year)
• Dry deposition (g/ha/year)
• Wet deposition (g/ha/year)
• Cloud deposition (g/ha/year)
• Total deposition to forest (g/ha/year)
• Total deposition to grass (g/ha/year)

The UK spatial patterns of concentration in rain and as aerosol for most metals in the network show the smallest values in North West Britain with the larger values in the South East, typically by factors of 2 to 4 depending on the metal. For most of the metals, current concentration and deposition values are smaller than those in earlier measurements, by between 20% to 60% relative to values in the mid 1990s.

The fractionation of total deposition of metals in the UK between wet and dry deposition varies between the different metals but dry deposition is typically 30% of the total. These data show that dry deposition is a very important fraction of the total and needs to be measured to quantify the total deposition.

Table 2. CEH annual mean concentrations of heavy metals in ambient air.