Environ. Sci. Technol. 1996, 30, 2511-2518
Isotopic Character of Lead Deposited from the Atmosphere at a Grassland Site in the United Kingdom Since 1860
higher ratios. The ratios in the sections did not reach however that measured in bulk soil samples collected in 1876, indicating that lower ratio lead had reached a depth of at least 16 cm in the soil. Up to 50% or more of the lead at a depth of 16 cm could be anthropogenic in origin.
J . R . B A C O N , * ,† K . C . J O N E S , ‡ S. P. MCGRATH,§ AND A. E. JOHNSTON§
Introduction
Soils and Soil Microbiology Division, Macaulay Land Use Research Institute, Aberdeen AB15 8QH, U.K., Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ, U.K., and IACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, U.K.
Lead isotope analysis, using high-precision thermal ionization mass spectrometry, of herbage samples collected annually since the 1860s has revealed a steady reduction in the 206Pb/207Pb ratio from about 1.170 in 1880 to about 1.098 in the period 1980-1985. The value of 1.170 is very close to that (1.171) found previously in Scottish lake sediments for anthropogenic lead deposited prior to the introduction of leaded petrol. Analysis of the data suggests two dominant components in the lead deposited from the atmosphere over the period of 130 years studied. Prior to the turn of the century, lead of industrial or coal origin had a typical 206Pb/207Pb ratio of 1.170 following which a nonradiogenic component with a lower 206Pb/ 207Pb ratio (1.09 or less) made an increasingly more significant contribution. The decline in ratio commenced well before the introduction of tetraethyllead in petrol, indicating either changes in the source of lead-containing ores or coal or changes in the relative contributions of different sources. The generally smooth change in the isotopic character of the deposited lead suggests a slow change in the relative proportions of the dominant sources. The trend appears however to have been reversed following the declining use of lead additives in petrol. The deposition, in particular that with the low ratios associated in Britain with petrol additives, has had relatively little effect on the bulk soil. The 206Pb/207Pb ratio in bulk soil samples (to 23 cm) from the site, which had been undisturbed for at least 200 years, decreased from 1.187 in 1876 to 1.180 in 1984 but at no time reached the ratio found in the deposition on herbage. Analysis of sections taken down a soil core showed a steady increase in the 206Pb/207Pb ratio from 1.159 at the surface to 1.181 at 15 cm depth. Recent deposition with low 206Pb/ 207Pb ratios had a significant but not dominant contribution to the lead in the surface soil, which also contained a substantial proportion of older deposition with
S0013-936X(95)00839-X CCC: $12.00
1996 American Chemical Society
Estimates of the total atmospheric inputs of anthropogenic lead in the U.K. since 1700 A.D. (1) suggest that these inputs could have been a major source of the total soil lead content even in rural areas. In 1986, Hutton and Symon (2) identified atmospheric deposition as the major contemporary source of lead in agricultural soils and calculated that at that time about 1450 t of lead was entering U.K. arable land each year from the atmosphere. Jones and Johnston analyzed herbage samples collected from one site on an annual basis (3) and showed continuous deposition of lead from the atmosphere since the 1860s. The concentrations of airborne lead in the U.K. have been declining following the reduction on January 1, 1986, of permissible lead concentrations in petrol from 0.4 to 0.15 g L-1 and the subsequent greater use of lead-free petrol (4, 5), but the continuous deposition over centuries has resulted in widespread accumulations of lead in surface soils, not only close to centers of population and industry but also in remote areas (6). The presence of these accumulations indicates the persistent nature of lead in soils and its relative immobility. It also means, however, that considerable stores of this toxic metal are held in the environment and are not being dissipated in any substantial manner by natural processes. With the current concern over changes in climate and changes in land use, for example, the increasing application of sewage sludges to arable and grasslands, it becomes imperative to understand fully the consequences of such changes in terms of release of accumulated metals from soils. This involves not only the chemistry and chemical association of metals in soils but also an assessment of their availability for uptake by plants and transfer to herbivores and humans and into the aquatic environment. It is possible to use isotopic composition to identify the source of lead in individual samples and to trace the lead through the environment. Variations in the relative abundances of the isotopes of common lead, first measured by Nier in 1938 (7), result from the radioactive decay of thorium and uranium (8). The feasibility of using isotope ratios to trace lead pollutants was first demonstrated by Chow and Johnstone in 1965 (9), and in other early studies, the source of lead in soil and plants was identified as being petrol (10, 11), coal (12), or smelter emissions (13). The introduction of improved and commercial instrumentation has seen an increasing number of studies that use the isotopic composition of lead as an indicator of its source (14-20). The * Corresponding author fax: 01224-311556; e-mail address: mi038@ mluri.sari.ac.uk. † Macaulay Land Use Research Institute. ‡ Lancaster University. § IACR-Rothamsted.
VOL. 30, NO. 8, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2511
isotope method is particularly effective in those studies in which a single well-defined and dominant source of contaminant can be identified. Examples include tracing the trajectories of air masses through identification of the source of the lead in the air masses (16-19) and the studies of soils and sediments contaminated by a dominant source (19-23). Generally, however, lead in the soil is not from a single or dominant origin and is derived from multiple sources over long periods of time. Even lead in highly contaminated soils from beside major roads has been deposited and accumulated since the start of the use of petrol additives in the 1920s. The source of the lead in these additives has not been constant over the years, and so lead in the majority of soils is derived from many different sources (6). If we are to understand better the origin of lead in soil, we need to know how the isotopic nature of lead deposited from the atmosphere has changed with time. This will also allow us to interpret much more precisely the chemical association of lead in contaminated soils (24) and add to our understanding of the history of lead emissions to the atmosphere. Changes in the isotopic signature of atmospheric lead have only been recorded accurately for North America (2530). There is a scarcity of information on deposition in Europe with almost none on deposition prior to the last 30 years. One exception is the work of Sugden et al. (31) in which the lead isotopic composition was measured in dated peat cores from three sites in Scotland, and the authors were able to demonstrate, over a coarse scale, changes in deposition since the late 18th century. The lack of data was recognized by Grousset et al. (32), who attempted to reconstruct, from the data available in the literature and recent determinations, the pattern of atmospheric lead isotope distribution in western Europe. They could only do this over the last 15 years or so, and data prior to this period are still lacking. This study provides such information for a site in southern England from the 1860s to the present day. The Park Grass experiment at Rothamsted (Figure 1) started in 1856 as a study on the effect of plant nutrients and lime, in various combinations, on the yields of hay from permanent grassland. Samples of the hay and subsequent growth have been stored since the start of the experiment and therefore represent a valuable scientific resource. Lead is not readily taken up from soil by plant roots (33), and lead on plant shoots results primarily from atmospheric deposition. The lead stored on these herbage samples therefore can be considered as the lead deposited from the atmosphere at the time of the growth of the grass. Lead isotope analyses have been carried out on 5-year bulked samples of herbage from the start of the experiment, on soil samples taken periodically since 1876, and on sections from a soil core taken specifically for this study. The herbage samples represented real time indicators of atmospheric deposition; the soil samples represented the integrated accumulation over at least 300 years.
Samples and Methods Sampling. Rothamsted is an agricultural estate on the outskirts of Harpenden, a town some 40 km (25 mi) to the north of London. Herbage grown at the Park Grass experiment at Rothamsted has been cut twice yearly (in June and in October or November) since 1856 until the present day. Originally, herbage was cut by hand-held scythe; later, reciprocating mechanical mowers were used.
2512
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 8, 1996
FIGURE 1. Location of Rothamsted Experimental Station.
The cut herbage was allowed to dry on the individual plots with occasional turning with a wooden rake to aid drying. Once dry (about 85% dry matter), the herbage was sampled before being raked up on each plot for the weight to be taken. The samples were chopped into short lengths (2-5 cm), and two subsamples were taken. One was dried for the determination of dry matter; the other was oven-dried at 80 °C before being stored in sealed containers (mainly metal tins but also some glass jars). Since 1959, yields have been estimated by cutting at least two sample strips per plot with a flail mower, and the cut herbage was collected, weighed green, and sampled. One subsample was dried at 105 °C to determine percentage dry matter; a second sample was dried at 80 °C before storage in an air-tight tin. These containerized samples have been kept in a dedicated archive store, and only very infrequently have the containers been opened to remove subsamples for analysis. No stored samples have ever been washed to remove surface deposits. The samples of hay used in this study were from an untreated (control) plot (pH 5.3) that had received no manure or fertilizer treatment. For this study, samples were removed from the center of the storage containers to minimize possible contamination from the metal and were bulked, in the ratio of dry matter yields, for 5-year periods. Prior to analysis, grass samples were milled in an agate ball mill. Soil samples, taken at irregular intervals from the top 9 in. (0-23 cm) of the plot, were air-dried, passed through 2-mm sieves, and ground in iron pestle and mortars. The samples were stored in sealed glass jars. The total lead concentration in these soils was in the range of 63-74 mg kg-1 as measured by X-ray fluorescence (XRF) (34). An additional soil core sample (0-16 cm) was taken in 1991 using a stainless steel corer. The cores were sectioned (1cm sections to 6 cm depth and then 2-cm sections), and the sections were stored in plastic bags until analysis. All soil samples were ground in an agate Tema mill prior to analysis.
Sample Preparation. Chemicals used in the preparation of samples for analysis were of the highest grade available. Acids (HNO3, HCl, HF) were of commercial Aristar grade or redistilled in the laboratory. Deionized water was of 16 MΩ quality. All glassware was acid washed overnight and rinsed thoroughly with deionized water prior to air drying. Milled herbage samples (2-3 g was the preferred amount but less was available for the older samples) were ashed for 16 h at 450 °C. The ash was dissolved in aqua regia (6 M HCl-16 M HNO3, 3:1) (15 mL) by heating in a covered beaker on a steam bath. The solution was subsequently allowed to evaporate to dryness, and the residue was dissolved in 0.5 M HCl (25 mL). This solution was filtered and dried ready for isotope analysis. Ground soil samples (500 mg) were digested at 180 °C in 20 M HF-16 M HNO3 (5:1,v/v) (12 mL) in Teflon bombs for 4-5 h. The lids were removed after cooling, and the acid was removed by heating at no more than 120 °C. The residue was redissolved by heating in 16 M HNO3 (a few milliliters), and the acid was removed by evaporation. The residue was finally dissolved in 0.5 M HCl (25 mL), and this solution was filtered and dried ready for isotope analysis. Lead Isotope Ratios. The acid extracts of ashed herbage samples or acid digests of soil samples were prepared by anion-exchange chromatography for isotope analysis, and the lead isotope ratios were determined using a magneticsector thermal ionization mass spectrometer (35). No correction for blanks was considered necessary as lead in blank solutions was undetectable by graphite-furnace atomic absorption spectrometry and would be negligible in comparison to the several micrograms collected from samples. The precision of the isotope measurement was such that the 206Pb/207Pb ratio in the standard reference material NIST 981 was measured to be 1.09418 ( 0.00036 (2σ) (n ) 14). The slight bias observed in this ratio was used to correct measured ratios for mass fractionation. All isotope ratio determinations were carried out in duplicate, and agreement as measured by a single standard deviation was generally
