Chapter 6 - Sources, Levels and Movements of Lead in the Environment
NATURAL OCCURENCE
Lead occurs naturally in low concentrations in all rocks, soils and dusts, usually ranging from 2 to 200 parts per million. The total amount of lead in the earth’s crust is estimated to be 3.1 x 1014 tonnes. Some soils have relatively high concentrations of lead, where the underlying parent rock has significant lead content. Lead contents of waters are generally low, but significant amounts of lead-rich dusts and vapours are carried in the air, from windblown material and volcanoes. However, these natural emissions are small in comparison with those resulting from human activity.
ANTHROPOGENIC SOURCES OF LEAD
Production of lead – Mining, smelting and refining of lead and other metals have in former times caused large emissions. Most of this is solid waste material, but sizeable emissions have also occurred to the atmosphere and to water. Modern techniques have minimized emissions to meet statutory requirements, including employment of best available technology.
Use of lead – Mobile sources (i.e. vehicles running on leaded petrol) continue to be a major contributor of lead to atmosphere in some countries, and this gives rise to elevated lead levels in soils, dusts and surface waters. Emissions are declining in many countries with the phasing out of leaded petrol, but lead deposited from petrol in the past remains in the environment. Residues from leaded paints, though now not used except for a few specialised outdoor applications, still continue to present a significant source of lead in house dusts and garden soils. Emissions from direct use of lead in other forms are small, resulting from abrasion and corrosion of lead or its compounds in some applications. Much of this material ends up in the sewage system, and contributes to levels of lead in discharged water and sewage sludge.
End-of-life of lead products – Most of the lead used at present is in products, such as batteries and lead sheet, which are largely recycled. Emissions can occur from collecting and processing operations, but these are small in modern well-run installations; legislation in all EU countries limits permissible industrial emissions. Recycling is certainly the preferred end-of-life option. For items which are not recycled, releases to the environment are much greater at the end of their lives than during use. For example, lead shot from hunting and firing ranges can locally result in high levels of lead in the soil, and lost lead weights add to the burden of lead in aquatic systems. These applications together can be the prime sources of lead inputs to waters and soils, in countries where there are no significant emissions from industry or leaded petrol. However, in general these inputs are relatively small and local.
Lead in the waste stream: landfill, incineration and compost – Many lead-containing products (such as leaded solder, glass, PVC, and small lead items) are disposed of as waste. Lead in most forms is fairly inert, and if buried in a modern well-maintained landfill, any releases should be very small. However, in the long term, some small losses of lead and other metals can be expected in leachate.
Disposal by incineration can result in emissions. However, EU legislation requires that exhaust gases are thoroughly cleaned, and the lead-rich dusts and vapours are trapped and must be disposed of, usually to landfill. Several countries in Europe use incineration (sometimes with energy recovery) as a preferred disposal method for municipal solid waste; it is also the recommended route for the non-metallic fraction from end-of-life vehicles, even though this fraction contains up to about 0.5kg of lead per vehicle. Lead in incinerator ash is usually subject to final disposal in controlled landfills to prevent releases to the environment.
Some countries in Europe compost significant amounts of biodegradable waste. In a few cases, lead items in the waste stream can enter this fraction if not separated fully; if resulting composts have levels of lead or other metals above agreed standards, this cannot be used for agriculture or gardens.
OTHER SOURCES OF LEAD
Coal and oil combustion results in the emission of small amounts of lead, along with many other metals. Sewage sludge often contains lead and other metals, from various sources, though inputs to farmland are strictly controlled by EU legislation. Application of sludge to land continues to be a source of lead input at low levels to agricultural soils.
CHEMICAL SPECIES OF LEAD
The behaviour of lead in the environment depends upon the chemical form it is in. Natural weathering processes usually turn metallic lead and its compounds into compounds which are relatively stable and insoluble. However, under acid conditions soluble compounds can also result in increasing mobility and potential bioavailability.
TRANSPORT OF LEAD IN THE ENVIRONMENT
Small lead particles emitted to air can remain in the atmosphere for over three weeks and in that time they may travel many hundreds of kilometers, though larger particles, which may constitute up to 95% of the emission, settle out within very short distances of the source. Deposition from atmosphere is a major contributor to lead inputs to water and to land but this continues to fall as the use of leaded gasoline is phased out. Lead can be carried in water, either dissolved or as waterborne particles. However, few compounds of lead dissolve readily in water, though most of this lead is then precipitated as a solid and becomes incorporated in the sediments at the base of the watercourse or ocean. In most cases lead in soil is relatively insoluble and has a low mobility. Thus, soils contaminated with lead retain high lead contents for many hundreds, even thousands, of years. Lead compounds are more mobile under acidic conditions, which can occur in mine wastes or from landfill leachate.
BIOAVAILABILITY OF LEAD
Knowledge of the total lead content of a surface soil is not very helpful in assessing the potential risk to humans or other organisms, because the degree of exposure to lead depends very much on the chemical and mineral form in which the lead occurs. The majority of lead compounds are relatively insoluble, though the small amounts of lead passing into the soil solution are easily taken up by biota. Some very insoluble lead compounds have little or no effect on living organisms. However, there is no single test for bioavailability: a compound which is unavailable to plants because it does not dissolve in soil water, may dissolve in the acidic stomach of an animal which ingests it. The development of simple cost-effective tests for bioavailability remains an urgent research requirement.
