CHAPTER 15 CHAPTER 15 Lead in the Environment
15.2 SOURCES OF LEAD IN THE ENVIRONMENT
Anthropogenic emissions of lead into the environment may be directly into air, water, and soil.
Although emissions into these media can be easily quantified, there is a continuous intercompartmental exchange of lead among them. The geographical distribution of atmospheric lead, relative to the point of emission, is largely dependent upon particle size. Most airborne lead is eventually deposited onto some surface, including plants, soil, bodies of water, artificial surfaces, and the respiratory tracts of animals, by dry or wet deposition processes. Dry deposition is either by gravitational settling of larger particles (> 10 àm)20 or impaction of particles of all sizes (especially smaller particles). Wet deposition results either from the incorporation of particles into water droplets within clouds or from the accu- mulation of particles by falling precipitation. Industrial discharges, highway runoff, and sewage effluent are responsible for much of the lead in water, with some wet deposition of atmospheric lead, and direct dry deposition, which is more significant for large bodies of water. The distribution of lead in water depends upon the chemical form of the lead. Lead in soils results mainly from dry and wet deposition of atmospheric lead, particularly close to emission sources, and the disposal of sewage sludge, often onto agricultural land. Lead in soils is relatively immobile compared to that in other environmental media, and soils and, ultimately, ocean bed sediments are the “sinks” for anthropogenically emitted lead.
Metallic lead also enters the environment from human activities. Hunting, fishing, and target shooting produces “hot spots” of metallic lead that have been responsible for the deaths of a number of species, ranging from ducks to eagles.21,22 Using stable lead isotope ratios, it is possible to determine the source of the lead in a variety of matrices, including avian bone. Juvenile herring gulls (Larus argentatus) from the Canadian Great Lakes had lead isotope ratios similar to that of lead originating from combustion of Canadian gasoline, whereas waterfowl and eagles had lead ratios most similar to the ratios found in lead shot and sinkers.23 Additionally, the isotope ratios for all species were higher than those derived from lead from mining or smelting.
15.2.1 Atmospheric Emissions
Estimated total lead emissions into the atmosphere from natural sources (1979 to 1984) ranged from (Table 15.2) 8.6 to 54.1 x 109 gm/year.24 Atmospheric lead emissions resulting from anthro- pogenic activities are estimated to be at least one to two orders of magnitude greater than from natural sources (Table 15.2). The most important source of atmospheric lead is the combustion of fuels containing tetraalkyl lead antiknock agents. This category by itself exceeds all of the natural emissions of lead. The lead from automobile exhaust is typically in the form of fine particulates.
However, lead profiles in dated sediment cores and atmospheric aerosols show a decline since 1970, presumably reflecting the reduction in the use of lead in gasoline.25 Other than gasoline-derived lead, most sources are point sources, resulting in locally elevated atmospheric lead concentrations.
15.2.1.1 Vehicular Emissions
The useful properties of tetraalkyl leads as antiknock agents (i.e., agents that allow for high engine compression ratios without spontaneous air/gasoline ignition in the cylinder) were first discovered in 1923.26 Lead is added to gasoline in organic tetraalkyl forms, as tetraethyl lead, tertramethyl lead, and mixed alkyls of triethylmethyl lead, diethyldimethyl lead, and ethyltrimethyl lead. During fuel combustion in the cylinder lead alkyls are converted to oxides, which foul spark plugs and remain in the cylinder. To prevent this, ethylene dichloride and ethylene dibromide are added to scavenge the lead. The lead chlorides and bromides consequently formed are relatively volatile and allow for approximately 70 to 75% of gasoline lead to be emitted into the atmosphere with exhaust fumes. These inorganic lead salts are emitted as very small particles, of approximately 0.015 àm aerodynamic diameter, that are considered to increase in size rapidly in the atmosphere by coagulation with other particles.27 A small amount of lead may also be emitted as larger particles
(5 to 50 àm) resuspended from the exhaust system.28 The atmospheric lifetime of emitted lead depends upon particle size, topography, and weather conditions. Particles > 10 àm are usually deposited rapidly close to the emission source (e.g., within 30 m of roads), either through gravi- tational settling or turbulent deposition. Due to their longer atmospheric residence time smaller particles can be carried by winds and deposited over very large areas.
Although most organic leads in gasoline are converted to inorganic forms during combustion, evaporation and incomplete combustion can release organic leads in the vapor phase into the atmosphere. Tetraalkyllead is relatively stable in the atmosphere, since it is not readily attacked by oxygen or light, and probably accounts for 1 to 6% of total lead at urban sites and possibly a similar percentage at inland rural sites.28 Most organic lead in the environment results from gasoline additives. Leaded gasoline usually contains 0.15 to 0.40 g Pb/L, and unleaded < 0.01 g Pb/L.
Exhaust fumes may contain 2000 to 10,000 àg Pb/m3 when leaded gasoline is used.28 These fine particles tend to persist in the atmosphere and can be transported long distances; their small diameter further enhances their absorption into the bloodstream.29
15.2.1.2 Fossil Fuel Combustion
The combustion of fossil fuels contributes only a small percentage of total lead emissions to the atmosphere, even relative to natural emissions (Table 15.2). However, this source of lead may be important locally. Coal contains, on average, approximately 25 ppm lead, with 17 ppm lead in British coal compared with 35 to 44 ppm in the United States.30,31 During combustion most lead in coal becomes deposited in fly ash, which may have lead concentrations an order of magnitude higher than the initial fuel. Little fly ash reaches the atmosphere during domestic coal combustion, although high chimney gas velocities may result in some loss of fly ash during industrial combustion.
The amount lost depends upon the efficiency of control devices used. Modern electrostatic precip- itators have greatly reduced atmospheric lead emissions from this source. However, other environ- mental media may become contaminated through the disposal of fly ash. Crude petroleum oil contains relatively less lead than coal, generally 0.001 to 0.2 ppm, with maximum concentrations reaching 2 ppm.31 During fractionation, most of the lead becomes concentrated in heavy fuel oils or residues, and total lead emissions are low.
Table 15.2 Global Estimates of Anthropogenic Emissions of Lead to the Atmosphere from Production and/or Combustion in 1979/1980 (109gm/year)24
Source Nriagu United States × 5a Europe × 5a
Gasoline/waste oil 177b 176 176 (372)c
Waste incineration 8.9 4.2 4.0
Coal combustion 14 4.8 4.0
Primary nonferrous metal 85 9.8 143.0
Secondary nonferrous metal 0.8 — 2.2
Iron/steel 50 4.1 73
Industrial applications 7.4 1.6 —
Wood combustion 4.5 — 2.8
Phosphate fertilizers 0.1 0.4 0.03
Miscellaneous 5.9 1.2 3.8
Total 354 202 419
aFigures for the United States and for Europe multiplied by a factor of 5 to give a global estimate.
bAssumes a 35% reduction in leaded gasoline use between 1975 (the base year used in the original calculations) and 1979/1980.
cThe figure in brackets, which corresponds to the reported estimate for Europe, seems to be too high.
Source: From Hutchinson, T.C. and Meemia, K.M., Eds., SCOPE31, Lead Mer- cury, Cadmium, and Arsenic in the Environment, John Wiley and Sons, Chich- ester, U.K., 1987. With permission.
15.2.1.3 Industrial Emissions
Elevated lead emissions may be recorded in the vicinity of lead and other metal mines, primary and secondary smelters, and many industries that produce, use, recycle, or dispose of lead-containing products. These include the pigment and dye industry,32 car battery manufacture,33 the manufacture of tetraalkyl additives for gasoline,34 and municipal waste incinerators.
15.2.1.4 Dusts
Many sources contribute to lead in dusts, and concentrations may become elevated in urban environments, including within dwellings. A large proportion of lead in dusts probably results from aerial deposition, although paint (sanding and scraping of buildings) or other sources may be the major contributors. Household dusts usually contain < 1000 ppm dry weight (d.w.48) Pb35 and urban and street dusts 500 to 5000 ppm d.w.36
15.2.2 Lead Emissions into Water
Present-day lead concentrations in marine water are probably up to an order of magnitude greater than natural concentrations. Table 15.3 illustrates industrial inputs of lead annually into all oceans. These inputs are some 2.6 times higher than estimated preindustrial inputs. However, most industrial and domestic lead inputs to waterways occur in urban or coastal areas, and in rural areas the natural mineralization of parent rock may provide the most significant contribution of lead into the aquatic environment.
Using isotopic lead ratios and lead/calcium atomic ratios, the lead in teeth of California sea otters (Enhydra lutis) from Alaska have increased 2- to 15-fold over otters from 2000 years ago.
The lead in preindustrial otters was derived from natural sources, whereas the lead in recent animals is derived from aerosols or industrial lead waste deposits.37,38 Likewise in birds, lead burdens are derived from recent anthropogenic sources, primarily gasoline combustion or recently manufactured lead shot and lead sinkers.23
Anthropogenic inputs of lead into water include dry and wet deposition of atmospheric lead, which act globally, or at least hemispherically, and point sources of emission, which influence the environment more locally and often involve effluents with high lead concentrations. Point sources include discharges from sewage works and direct industrial and mine works discharges. In addition to such point sources, lead may be leached from soils and mine tailings, and highway runoff frequently reaches local waterways. Lead in freshwater often becomes adsorbed onto sediment and ultimately reaches oceans.
Table 15.3 Approximate Lead Input to All Oceans
Input Tons/Year
Industrial inputs
Aerosols (gasoline) 37,000
Aerosols (smelters, forest fires) 3,000 Rivers/sewers (soluble, mainly aerosols) 60,000
Rivers/sewers (solids) 200,000
Preindustrial inputs
Aerosols 1,000
Rivers (soluble) 13,000
Rivers (solids) 100,000
Source: From Harrison, R.M. and Laxen, D. P. H., Lead Pollution, Causes and Control, Chapman and Hall, London, 1981. With permission.
15.2.2.1 Vehicular Emissions
Although the contribution of gasoline lead in water is mainly indirect, through dry and wet deposition of atmospheric lead and highway runoff, it remains very important. Trefry et al.39 found that lead inputs into the Gulf of Mexico from the Mississippi River in the United States had decreased by 40% 10 years after the introduction of regulations reducing the lead content of gasoline. The Mississippi carries over half the total water and sediment carried by all U.S. rivers, and the 40% decrease in lead transported was comparable to the decrease in the use of leaded gasoline over that period.
Gasoline lead deposited onto roads constitutes the most important source of lead in urban runoff.40,41 The amounts of lead deposited on road surfaces from vehicles in urban areas depend upon a wide range of factors including traffic intensity, driving mode, weather and climatic factors, road surface area, etc. Runoff water is usually either directed to sewage treatment plants, in which case only a small proportion will reach waterways in the short term, or transported directly to local water courses. Small amounts of gasoline lead may also reach water directly, through combustion by boat engines, although only 9 to 31% of gasoline lead is found in boat exhaust water.42 15.2.2.2 Lead in Sewage Effluent
The amount of lead released into water from sewage treatment plants depends upon the range and nature of effluents received by the plant and the treatment process. Sewage treatment plants may receive, in addition to domestic wastewater, certain industrial discharges and highway runoff, partic- ularly following storms. A large proportion (80 to 100%) of lead present in the effluent is incorporated into sewage sludge during treatment,43 and the remaining effluent is discharged into waterways.
15.2.2.3 Leaching and Seepage
Lead may reach waterways by leaching from fly ash and from soils that have been subject to additions of lead (e.g., sewage sludge) and by leaching and seepage from wastedumps, mine tailings, and tailing pond decant. Lead is relatively immobile in soils; therefore, leaching is only likely to contribute a small amount of lead in water. A certain amount of lead may be leached from the fly ash resulting from fossil fuel combustion. It has been reported that distilled water can leach 3.4%
of the total lead in fly ash in 48 h.44 15.2.2.4 Other Sources
Certain industrial discharges directly entering waterways result in increased lead concentrations and local contamination, and accidental releases of lead from industrial and other sources can cause considerable water contamination. Lead also reaches water in less obvious ways such as through the use of lead plumbing systems. In areas of soft water, where no calcium layer coats the inside of plumbing systems, elevated lead concentrations can accumulate in static water, for example, overnight.
15.2.3 Lead in Soils and Sediments
The major natural source of lead in soils is the weathering and mineralization of parent material, and in rural areas receiving little lead pollution, there is often a strong relationship between lead concentrations in soil and parent material. Naturally high soil lead concentrations can be found in local areas of high mineralization, but even these rarely approach those resulting from anthropogenic activities around urban areas. Emitted lead reaches soils via a wide range of pathways. This may occur on a local scale from point sources, such as the application of sewage sludge to land or land
disposal of mine wastes and fly ash, or universally through the deposition of atmospheric lead. By one pathway or another, the vast majority of atmospheric lead, and much lead in water, eventually becomes associated with soils or sediments, due to the strong binding capacities of many soil components for metals.
Lead mine tailings comprise a localized but significant source of lead-bearing sediments. These sediments wash far down exposed waterways, where they are ingested by wildlife, especially waterfowl.45 This venue constitutes one of the few instances where wild birds have died of lead poisoning without having ingested lead shot.46
15.2.3.1 Intercompartmental Lead Transfer to Soils
The extent of dry deposition of airborne lead onto soils from point emission sources is related to the atmospheric lead concentration, particle sizes, weather, wind direction, and soil surface characteristics. Gravitational and turbulent settling of large particles is high near the emission source, decreasing with distance. Deposition of smaller particles by impaction, or diffusion mech- anisms (e.g., Brownian diffusion), takes place over a much larger geographical area and is related to surface characteristics. Deposition tends to be lowest on smooth surfaces, with concentrations of lead up to eight times higher on rough or hairy leaf surfaces than on smooth leaf surfaces.47 Lead deposited onto plant surfaces may finally reach the soil when plants die, if they are not grazed or cropped. In addition, lead reaching plant and other natural and artificial surfaces via dry deposition may be washed off and reach the soil in water.
Water plays a very important role in the transport of lead to soils and sediments. Dry deposition of atmospheric lead gives rise to contamination close to emission sources, whereas wet deposition through precipitation is universal. Both are thought to deposit approximately similar amounts of lead in rural areas.28 Rain and dew also transport lead to soils by washing deposited lead off plant surfaces, roads, and other artificial surfaces. The movement of lead-contaminated leachates and seepage water contributes locally to soil contamination, as does accidental flooding with water contaminated by industrial processes, sewage processing plants, and waste dumps.
15.2.3.2 Sewage Sludge
As the sludge from sewage treatment facilities is rich in plant nutrients, particularly nitrogen and phosphorus, its addition to agricultural land as fertilizer is considered a convenient and useful means of disposal. Sewage sludge contains variable amounts of lead, depending upon the effluent types received by the treatment plants. Most sewage sludges contain within the range of 120 to 3000 ppm d.w., and surveys in the United Kingdom have found sludges to contain from as low as 19 to as high as 45,400 ppm d.w. of lead.49,50 Although heavy-metal concentrations are often high in sludge, agricultural use has been permitted under the premise that most lead becomes immobilized within soils and sediments and is unavailable for uptake by plants. However, in cases of surface sludge application, grazing animals may ingest sludge deposited on the surface of, or along with, plants. The U.K. government has regulated the maximum permissible concentrations of lead and other metals in sludge to be applied in this way (2000 ppm d.w. for Pb), along with maximum concentrations to be disposed over a given period of time.19
15.2.3.3 Other Sources
Local soil contamination may occur around storage battery reclamation plants, near mines and waste, dumps, and around mainly older buildings where leaded paint has been used and is flaking.
An additional source of lead in soils is lead ammunition.51,52 The production of lead ammunition, both for military purposes and for hunting, has continued to increase throughout the last three
centuries. Today, lead shot is regularly and widely distributed in the environment, mainly by hunters. Although lead shot is relatively stable in most soils, it has resulted in significant soil- pollution problems at specific sites, and the ingestion of spent ammunition causes large-scale avian mortality. However, the use of lead in ammunition has declined or, in the case of lead shot for waterfowl, been banned. After 6 years of the ban, an estimated 1.4 million ducks were spared from lead poisoning in 1997.53