- Email: [email protected]
Polychlorinated biphenyls in UK and Norwegian soils: spatial and temporal trends
ELSEVIER
The Scienceof the Total Environment 193 (1997) 229-236
Polychlorinated
biphenyls in UK and Norwegian soils: spatial and temporal trends
Wendy A. Lead”.*, Eiliv Steinnesb, Jeffrey R. Bacon”, Kevin C. Jones” “Institute
of Environmental and Biological Sciences, Lancaster University, Lancaster, LA 1 4YQ, bDepartmenf of Chemistry, University of Trondheim. N7055 Dragvoll, Norma) ‘MLURI, Craigiebuckler, Aberdeen, AB15 SQH. UK
UK
Received26 July 1996; accepted4 October 1996
Abstract Contemporary soil samples from 46 sites across the UK and 12 sites in Norway have been analysed for a range of PCB congeners. Results show spatial differences, in terms of concentration and congener profile. The difference is partly caused by an increased proportion of the mid-molecular weight congeners in the samples from Norway. The soils from southern Norway and the UK contained similar amounts of PCBs per unit area: those from northern Norway contained lesser amounts. The possible influence of long-term air-soil exchange, latitudinal fractionation processes and differences in land management practices on the observed patterns is discussed. Archived soils (1951- 1974) from the UK sites have also been analysed and the results show increasing concentrations of these compounds up to the late 1960s/early 197Os, after which there has been a substantial decline. This temporal trend is in accordance with that reported in previous studies. However, it is possible that some of the archived samples were contaminated in the process of air-drying. Due to this contamination artifact, it is not possible to ascertain whether the scale of the observed temporal differences truly reflect changes in the environment. The data are discussed in terms of possible spatial/temporal trends and the potential for air-soil exchange of these compounds. Copyright 0 1997 Elsevier Science B.V. Keywords:
Global fractionation; Polychlorinated biphenyls; Soils; Spatial and temporal trends
1. Introduction The ubiquity of polychlorinated biphenyls (PCBs) in the modern environment is well docu--.__ * Corresponding author.
mented, but fate processes of these compounds are not yet fully understood. It has been hypothesised by various groups that semi-volatile compounds, such as PCBs, will volatilise from terrestrial and aquatic environmental compartments in warm and temperate regions, will un-
0048-9697/97/$17.00Copyright 0 1997Elsevier ScienceB.V. All rights reserved PII SOO48-9697(96)05345-4
230
W’.A. Lead et (11. :I The Science
of the Total
dergo long-range atmospheric transport (LRT) and will then recondense when they reach colder circum-polar zones (Calamari et al., 1991; Wania and Mackay, 1993). In order to test this theory, it is necessary either to obtain spatial and temporal data for changes in concentrations of such compounds in samples of the same environmental matrix or to assemble data on the rates of change in concentrations at different locations over time. In an attempt to further our understanding of the transport and fate processes of semi-volatile organic compounds we have analysed a collection of contemporary surface soil samples from across the UK and Norway for a range of PCB congeners. The UK samples were all collected from sites where soil profiles had previously (1951- 1974) been collected. These archived samples have also been analysed for PCBs with the hope that any observed changes in congener concentrations and/ or profiles would give evidence to support/disprove the global fractionation hypothesis.
2. Materials
Environment
193 (1997)
229-236
for surveys of soil type. The archived samples dated from between 1951 and 1974. To reduce the risk of post-collection changes in the PCB concentration of the archived soils, they needed to have been sealed as soon as possible after collection with minimal air-drying exposure time and stored unopened and undisturbed. Sample sites were also chosen where land management would have been kept to a minimum, to help ensure that changes in contaminant composition could be attributed to changes in the field (such as air-soil exchange, biodegradation and other processes) rather than to agrochemical or sewage sludge amendments, In the summer of 1990, samples of surface soil from podzol profiles from 12 remote woodland areas in Norway were collected (Fig. 1). The samples were taken from the upper 5 cm of the humus layer after removal of litter, and in most cases represented the F-horizon. Each sample was a composite of several sub-samples taken within a 5 x 5 m area. The predominantly organic soil (typically around 90% organic matter) was sealed in an aluminium box and stored frozen.
and methods
2.1. Sampling
The contemporary UK soil samples were taken from 46 sites where soils had previously been collected (Fig. 1). Obviously, sampling depth is critical when examining contaminants introduced to the soil from the atmosphere. Therefore, a small hand held auger was used to collect the soils, with sample depth varying between O-2.5 and O---25 cm (depth was identical to that used in the collection of the archived soils). Samples were taken to represent an area of 100 rn’. Therefore, approximately 20 cores were taken from across the area, and the cores bulked together to give the sample. Samples were frozen immediately on return to the laboratory until required for extraction and analysis. The archived soil samples from the UK were obtained from collections held at the Soil Survey of England and Wales, Silsoe, and the Macaulay Land Use and Research Institute (MLURI), Aberdeen. They were originally collected as profiles
Fig. 1. Location of the 46 British and 12 Norwegian sites. Numbered sites referred to in Table 1.
sample
W.A. Lead
et al. / The Science
of‘ the Total
Environment
193 (1997)
231
229-236
congener basis are available from the authors on request. 3.1. Spatial differences
Fig. 2. Simplified PCB concentration
2.2. Extruction
conceptual diagram of the change on exposure to air.
in sample
and unalysis
The extraction, analysis and quality assurance methods used were identical to those given by Alcock et al. (1993), in order to ensure that results would be comparable with the earlier study, except that all contemporary samples were extracted wet, i.e. they were mixed with sodium sulphate prior to extraction to remove water. This is because recent studies (Alcock et al., 1994; Lead et al., 1996) have shown that exposure of samples to air can cause either contamination by or loss of PCBs depending on the equilibrium position of the sample with the air (Fig. 2). In this study, contamination or loss was minimised for the contemporary samples by avoiding air drying. For the archived soils, the artifact was minimised by selecting soils which were stored sealed and unopened. Although analysis of archived material is not ideal for the inference of temporal trends of volatile/semi-volatile compounds, it is one of the few techniques available (Lead et al., 1996).
in contemporary
soils
It was found that soil PCB concentrations in the south of Norway were significantly higher (99% confidence limit) than those in samples collected from the north of the country. It was also found that the PCB content of the Norwegian samples was higher than that of the UK samples. For example, the mean total PCB concentration (CPCB, see Table 1 for definition) in the UK soils was 4 pgg/kg; in samples from the south of Norway, it was 17.5 ,ug/kg and from the north of Norway, it was 9.5 ,ugg/kg. Principal components analyses (PCA) were used to further investigate this spatial difference. These analyses revealed differences between the UK and Norwegian samples when all factors having Eigen values greater than 1 were plotted against each other (e.g. Fig. 3). It is found that the clustering observed in Fig. 3 is due to mid-molecular weight PCBs, particularly congeners 101, 105, 110, 138, 149 and 187. It can therefore be concluded that it is these midmolecular weight congeners which are having the greatest influence on the difference in the patterns between UK and Norwegian samples. It is not
3. Results and discussion
Due to the quantity of data obtained (104 samples plus replicates: 37 PCB congeners) it is necessary to be selective in the presentation of results. Information on all the Norwegian sites and ten representative UK sites (archived and contemporary samples), together with concentrations of tri- to octachlorinated PCB congeners, are shown in Table 1. The location of these sites is highlighted in Fig. 1. Results on an individual
I
-41
-3
-2
I 0
-1
I
I
0 FACTOR
1
I
I
2
3
(1)
Fig. 3. PCA plot of factor 2 vs. factor 1 for contemporary UK (U) and Norwegian (N) soil samples. Woodland UK samples are highlighted (0). Factors 1 and 2 together explain approximately 25% of the variance of the data.
Clay
Clayloam
Peat
Clay
Loam
Sandyloam
Loam
Loam
3
4
5
6
I
8
9
10
Pasture
Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland
Grassland
Pasture
Grassland
Pasture
Pasture
Woodland
Grassland
Scrub
use
.~
Pasture
-. --~
Land
1990 1990 1990 1990 1990 1990 1990 1990 1990 1990 1990 1990
1951 1993 1953 1993 1955 1993 1956 1993 1959 1993 1961 1993 1963 1993 1965 1993 1966 1993 1968 1993
Sample
,md PCB concentration data
95.2 91.6 94.2 92.8 82.9 94.9 94.2 46.5 94.5 71.4 93.6 95.0
7.2 16.2 9.1 7.9 13.0 9.0 11.7 18.0 86.4 61.2 27.8 12.2 31.1 12.3 8.9 12.0 11.4 9.6 6.5 5.1
Organic (‘i/O)
matter
126.
(/‘g hg) for the Norwegian
Sites referred to are marked in Fig. 1. a Tri-CB is sum of congeners 30, 18 and 28. b Tetra-CB is sum of congeners 40, 52, 61, 66, 74 and 77. “Penta-CB is sum of congeners 82, 101, 104. 105, 110, 118, 119 and d Hexa-CB is sum of congeners 138. 149, 151, 153 and 156. ‘Hepta-CB is sum of congeners 170. 180. 183, 185, 187 and 188. ’ Octa-CB is sum of congeners 194, 198, 201 and 202. s ZPCB is sum of all above congeners. ’ N.D. implies below detection limit.
Sample Sites Podzol Podzol Podzol Podzol Podzol Pocizol Podzol Podzol Podzol Podzol Podzol Podzol
Loam
Nmtqim I1 12 13 14 I5 16 17 18 19 20 21 22
-...
loam
2
I k .Smlp/c~ SiteA i Sandy
Soil type
labif I Sample hitc information
1.2 0.89 0.98 1.7 0.87 1.4 1.0 0.20 1.8 0.97 1.8 0.72
37 0.22 0.36 0.22 48 0.83 120 0.19 1.3 1.5 170 0.098 940 0.25 330 0.17 870 0.37 490 0.018
sample
0.73 0.46 0.67 1.5 0.46 1.6 1.3 0.62 2.8 0.71 2.3 3.0
19 0.23 0.22 0.088 27 0.43 65 0.27 1.6 1.5 92 0.048 510 0.22 320 0.13 470 0.18 330 0.063
Terra-CBh
1.8 1.2 1.6 4.1 2.0 3.0 5.1 2.3 7.9 2.5 7.6 6.3
3.7 1.4 1.3 0.26 7.5 1.2 19 2.2 2.0 2.2 16 0.14 84 0.68 78 0.4 69 1.5 34 0.81
Penta-CB’
sites and ten of the UK
sample
0.96 3.2 5.1 3.6 1.9 3.0 6.7 2.9 7.6 2.4 8.0 14
1.8 0.86 0.52 0.23 3.1 0.29 11 0.48 0.1 2.3 8 0.044 23 0.87 22 0.4 17 0.34 7.3 0.15
Hexa-CB”
sites
0.57 0.48 1.1 1.8 0.85 0.96 1.9 0.65 2.6 3.7 4.2 4.8
0.67 0.56 0.95 0.27 1.1 0.14 3.0 0.15 0.066 0.96 1.3 N.D. 6.2 0.73 7.1 0.37 3.4 0.24 1.4 0.13
Hepta-CB’
N.D. 0.32 0.33 0.32 0.08 0.33 0.34 0.17 0.79 0.28 0.93 0.73
0.019 0.037 0.25 N.D.h 0.24 0.01 I 0.18 0.058 0.086 0.22 0.42 N.D. 1.1 0.4 1.4 0.15 0.68 0.023 0.4 0.015
Octa-CB’
5.3 6.6 16 13 6.2 10 16 6.8 23 11 25 30
62 3.3 3.6 1.1 88 2.9 210 3.4 5.1 8.7 280 0.33 1600 3.1 760 1.6 1400 2.6 860 1.2
XPCB”
WA.
Lead
et al. II The Scirncr
of. the Total
possible to fully ascertain, however, whether the difference is truly a spatial one or whether it is due to the fact that all of the samples from Norway are woodland soils. Those from the UK are predominantly pasture and grassland samples. Previous studies have shown the concentration of semi-volatile organic compounds to be higher in forest soils than in nearby grassland/arable soils (Hagenmaier and Krauss, 1993; Rotard et al., 1994). However, there were four woodland soils which were analysed from the UK, and these samples were found to be evenly distributed in the PCA plot of the UK samples (Fig. 3). This suggests that the compositional difference in the UK and Norwegian data is a spatial not a landuse difference, but without either more woodland samples from the UK or some samples of pasture or grassland soils from Norway, this statement can not be fully supported. Importantly, it should also be noted that if concentrations were expressed on a per unit area basis, then the difference between the UK and Norwegian samples would be significantly reduced, since grassland soils generally have a much higher bulk density than woodland soils. Detailed bulk density data are not available to allow all the results to be expressed in this way. However, for illustrative purposes, if we assume all PCBs are in the top 5 cm and that the bulk density of the UK grassland samples is 1000 kg/m3, and that of the Norwegian forest soils is 250 kg/m3, then the mean concentration of CPCB equates to approximately 200 Llg/ mz for the UK soils; ca. 220 Llg/rn’ for the soils from southern Norway and ca. 120 pgg/m’ for the soils from the north of Norway. There is some support in the Norwegian data set for the global fractionation hypothesis, in that: (1) there is a slightly higher tri-CB:hexa-CB ratio in the northern samples than in the southern most Norwegian soils; and (2) there is a relative enrichment of the mid-molecular weight PCBs compared with the UK soils. These observations would be consistent with a latitudinal fractionation influenced by generally lower summer temperatures, particularly in the north of Norway, than in the UK. For the low molecular weight components, the temperatures seem to be high enough, even in the north of Norway, for volatili-
Emironmrnt
192 (1997)
229-3.36
233
sation to occur, i.e. it would seem that the less chlorinated PCBs are re-volatilising from temperate and circum-polar regions and are being transported to still colder areas. These findings are consistent with those reported for Norwegian mosses (Lead et al., 1996).
Rigorous statistical analyses, including student t-tests, analysis of variances (ANOVA) and PCA, have been performed on the contemporary UK data, but no correlations were found between PCB concentration and, for example. land use, organic matter content, soil type or sample region. This could be because in the UK there has been thorough environmental mixing of PCBs, so that differences can no longer be seen. Alternatively. it could be that there were insufficient samples in this study from, for example, each region for such rigorous tests to be carried out. A previous study by Alcock et al. (1993) reported contemporary UK CPCB soil concentrations as being 20-30 pg./kg. This study has found it to be approximately a factor of 5 lower (mean ZPCB concentration of 4 @g/kg). Soils in the earlier study were all air-dried, which probably led to some contamination from the atmosphere (Alcock et al., 1994). To minimise such contamination in this study. all contemporary samples were extracted wet. Tests were therefore carried out to ensure that this was a genuine difference in reported concentrations and was not due to differences in extraction efficiencies from wet and dry soil. No notable differences were found. The data reported by Alcock et al. (1993) have been used by others attempting to model fate processes of PCBs. For example, the data have been used by Harner et al. (1995) to model the long term exchange of PCBs between the air and soil and by Harrad et al. (1994) to estimate the contemporary PCB burden of UK soils. Obviously the results reported here will also have implications in such studies, e.g. using the same method as Harrad et al. (1994) we have reassessed the UK soil budget for congeners 28, 52, 101, 138, 153 and 180 as well as for CPCB (Table 2). However, it should be stressed that there are
234
W.A. Lead
Table 2 PCB concentration UK soils Congener -~-28 52 101 138 153 180 ZPCB Figure
Mean kg)
and
estimated
concentration
et al. 1 The Science
burden
(pgi
0.28 (1.7) 0.13 (1.1) 0.15 (1.8) 0.28 (1.0) 0.34 (1.5) 0.13 (0.69) 4.0 (30) in brackets
refers
UK
in
of the Total
of Harrad
193 (1997)
229-236
contemporary
soil burden
(t)
3.4 (21) 1.6 (14) 1.9 (22) 3.4 (12) 4.2 (19) 1.6 (8.6) 49 (370) to that
Environment
et al. (1994).
major uncertainties over this method for estimating soil budgets, primarily because it assumes that all of the PC& are present in the top 5 cm of soil and that the concentration is uniform throughout this depth. 3.3. Temporal trends in UK soils
Results for the XPCB concentration for all UK samples in relation to year of collection are shown in Fig. 4. It can be seen that there was an increase in the CPCB concentration up to the late 196Os/ early 1970s and there has since been a dramatic decrease in the XPCB content of UK soils. The date of the exact maximum cannot be inferred, due to the small number of archived samples from after 1970. The same trend is seen for all homologue groups; but the tri-chlorinated congeners decreased by a factor of approximately 1000 between 1968 and 1993 and the heavier octachlori?O”O -,
Fig. 4. Change rn UK soil samples over time. (1993 point represents all 46 sample sites. Concentration of ZPCB ranges from - 0.5 to 20 /cg/kg. with a mean value of 4 /‘g/kg.)
Fig. 5. Relative contribution of each homologue group to CPCB concentration for (a) a representative archived soil and (b) the matching contemporary sample.
nated biphenyls by a factor of approximately 5 over the same time period. The relative importance of the heavier PCBs has therefore increased over time (Fig. 5). These trends are comparable with those reported by Alcock et al. (1993) for soils collected from the ‘classical’ experimental plots at Rothamsted, an experimental research station in the south east of the UK. It is hypothesised that the dramatic decline in the PCB content of UK soils is largely due to volatilisation, with the air-soil system approaching a thermodynamic equilibrium. Following volatilisation, PCBs will be available for LRT and subsequent global distillation. It has been suggested that the lighter congeners could move further northwards than the higher chlorinated homologue groups resulting in a latitudinal fractionation (Wania and Mackay, 1993). This is supported by the fact that it is the lighter congeners which are being lost most rapidly from temperate soils. It must be stressed, however, that the archived soils used in this study were all air-dried prior to storage, albeit with the air-soil exposure time being kept to a minimum. This will undoubtedly have led to some alteration in the composition of these samples (Alcock et al., 1994; Lead et al., 1996). Results where samples have been air-dried should therefore be viewed with caution. In this study the PCB concentrations found in the archived samples are unlikely to be the true values at the time the soils were collected from the field; the observed changes in concentration are likely to have been exaggerated by the contamination artifacts and the scale of the decline in the PCB
W.A.
Lead et al. ! The Science
of’ the Total
content of UK soils since the early 1970s will probably have been less than has been reported here. Having said this, however, Lead et al. (1996) report that the contamination which occurs through air-drying of samples in their laboratory mainly arises from lower chlorinated congeners, particularly those of tri- and tetra-substitution. Therefore, in this study, the observed decline of a factor of 5 between the late 1960s and the early 1990s in the soil concentration of the octachlorinated congeners may be realisitic. Jones et al. (1995) reported that at two rural UK sites EPCB concentrations in outdoor air decreased by a factor of approximately 4.6 between 1972-1976 and 1987-1992. If we assume that this factor can also be applied to indoor air concentrations, and therefore to contamination fluxes onto drying soil samples, then based on a contemporary flux of 5 pg ZPCB/m2 per day (Alcock et al., 1994), fluxes in the 1970s may have been in the region of 23 pg ZPCB/m2 per day. In order to try to estimate the degree of contamination which may have occurred during the air-drying of the archived samples, if we assume a worse case scenario where in the early 1970s 1 kg of soil may have been spread over an area of 0.25 m2 to dry for 7 days, then using the above flux onto the soil, concentrations in the samples may have increased by 40 ,ug/kg. This compares with a measured concentration of CPCB in 1970 soil of around 1000 pg/kg (see Fig. 4). Using these estimates, it seems unlikely that contamination alone can account for the scale of the observed decrease in soil PCB concentrations over the last 2.5 decades.
4. General comments and conclusions
In summary, this study has highlighted a difference in the patterns of PCBs between contemporary soil samples from the UK and from Norway. It is thought that this is due to a spatial difference and not to a land-use difference. The concentrations for the contemporary UK soil samples was a factor of approximately five lower than previously reported for the UK. A change has been seen in the UK soil PCB concentration over time which is
Enrironment
193 (1997)
229%236
23s
comparable with that found by Alcock et al. (1993). There has also been an increase in the relative contribution of the heavier molecular weight congeners since the late 1960s. It is emphasised that, because of the possibility for post-collection changes in sample composition, we believe the archived samples only allow general trends and indications of concentrations from the past to be derived. The spatial difference and the results for the decreasing concentration in UK samples lead us to suggest that PCBs are being lost from the soils of temperate areas, largely as a result of volatilisation. This implies that between the late 1960s and the present day, temperate soils have been acting as a source of these compounds to the atmosphere. Once volatilised, the compounds will have been available for LRT and subsequent global distillation. McLachlan (1996) has suggested that soil-borne PCBs in rural parts of Germany are now approaching a steady state with the air, so that the air-soil fugacity quotient is nearing 1. It may be, therefore, that in the summer, when temperatures are higher, contemporary soils are acting as a source, while in the winter as a sink. This may be a factor in explaining the seasonality of PCB concentrations in air (Halsall et al., 1995). Finally, it should be pointed out that although the decline in soil PCB concentrations here has mainly been attributed to volatilisation, biodegradation of these compounds, particularly of lower chlorinated compounds, will also have occurred. This process will therefore also have played a part in the temporal change in both concentration and congener profile. In addition, it has been suggested that persistent organic pollutants in soils may become more strongly bound over time, thereby becoming harder to extract (Alexander. 1995; Pignatello and Xing, 1996). This may result in an effective decrease in concentrations of PCBs extractable in hexane. However, this process could occur in both the archived samples during storage and also in the soil in the field. A recent study on sediments suggests that this process may be relatively unimportant for PCBs (McGroddy et al., 1996). Research is continuing in our laboratory to establish the relative importance of soil out-gassing and other processes in contributing to
‘36
M’.A.
Leud et al. : T/w Scierrw
of r/w Tutu/
longterm changes in soil concentrations, However, we currently believe that air-soil exchange is of particular importance in governing the concentrations of PCBs in soils.
Acknowledgements Collection of archived and contemporary UK soil samples was funded by the Natural Environment Research Council (NERC), and analysis of these samples by the UK Ministry of Agriculture, Fisheries and Food (MAFF). Thanks are also due to Peter Loveland and Dick Thompson of the Soil Survey of England and Wales for their help in choosing the archived soil collection. W.A.L. is funded by a studentship provided by the Environmental Science Department at Lancaster University.
References Alcock, R.E.. Johnston, A.E.. McGrath, S.P., Berrow, M.L.. Jones, KC. (1993) Long-term changes in the polychlorinated biphenyl content of United Kingdom soils. Environ. Sci. Technol. 27, 19181923. Alcock. R.E., Halsall, Cl., Harris, C.A., Johnston, A.E., Lead, W.A., Sanders, G., Jones, KC. (1994) Contamination of environmental samples prepared for PCB analysis. Environ. Sci. Technol. 28, 1838- 1842. Alexander. M. (1995) How toxic are chemicals in soil? Environ. Sci. Technol. 29. 27132717. Calamari, D., Bacci, E., Focardi, S., Gaggi, C., Morosini, M., Vighi, M. (1991) Role of plant biomass in the global environmental partitioning of chlorinated hydrocarbons. Environ. Sci. Technol. 25, l489- 1495.
Environment
193 (1997)
229-236
Hagenmaier, H., Krauss, P. (1993) Attempts to balance transport and fate of polychlorinated dibenzo-p-dioxins and dibenzofurans for Baden-Wiirttemberg. Organohalogen Compounds 12, 81-84. Halsall, C.J., Lee, R.G.M.. Coleman, P.J., Burnett, V., Harding-Jones, P., Jones, K.C. (1995) PCBs in UK urban air. Environ. Sci. Technol. 29. 2368-2376. Harner, T., Mackay. D.. Jones, KC. (1995) Model of the long-term exchange of PCBs between the soil and the atmosphere in the southern UK. Environ. Sci. Technol. 29, 1200- 1209. Harrad, S.J., Sewart, A.P., Alcock, R., Boumphrey, R., Burnett, V., Duarte-Davidson. R., Halsall. C., Sanders, G., Waterhouse. K., Wild, S.R., Jones, K.C. (1994) Polychlorinated biphenyls (PCBs) in the British environment: sinks, sources and temporal trends. Environ. Pollut. 85, I31 -146. Jones, K.C., Duarte-Davidson, R., Cawse, P.A. (1995) Changes in the PCB concentration of United Kingdom air between 1972 and 1992. Environ. Sci. Technol. 28, 272275. Lead, W.A.. Steinnes, E., Jones, K.C. (1996) Atmospheric deposition of PCBs to moss (Hylocomium splendens) in Norway between 1977 and 1990. Environ. Sci. Technol. 30, 524-530. McGroddy, S.E., Farrington, J.W., Gschwend, P.M. (1996) Comparison of the in-situ and desorption sediment-water partitioning of polycyclic aromatic hydrocarbons and polychlorinated biphenyls. Environ. Sci. Technol. 30, 1722177. McLachlan, M. (1996) Bioaccumulation of hydrophobic chemicals in agricultural food chains. Environ. Sci. Technol. 30. 252-259. Pignatello, J.J., Xing, B. (1996) Mechanisms of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 30, 1 - 11. Rotard, W., Christmann, W., Knoth, W. (1994) Background levels of PCDD/F in soils in Germany. Chemosphere 29, 2193-2200. Wania, F., Mackay, D. (1993) Global fractionation and cold condensation of low volatility organochlorine compounds in polar regions. Ambio 22, 10-18.
The Scienceof the Total Environment 193 (1997) 229-236
Polychlorinated
biphenyls in UK and Norwegian soils: spatial and temporal trends
Wendy A. Lead”.*, Eiliv Steinnesb, Jeffrey R. Bacon”, Kevin C. Jones” “Institute
of Environmental and Biological Sciences, Lancaster University, Lancaster, LA 1 4YQ, bDepartmenf of Chemistry, University of Trondheim. N7055 Dragvoll, Norma) ‘MLURI, Craigiebuckler, Aberdeen, AB15 SQH. UK
UK
Received26 July 1996; accepted4 October 1996
Abstract Contemporary soil samples from 46 sites across the UK and 12 sites in Norway have been analysed for a range of PCB congeners. Results show spatial differences, in terms of concentration and congener profile. The difference is partly caused by an increased proportion of the mid-molecular weight congeners in the samples from Norway. The soils from southern Norway and the UK contained similar amounts of PCBs per unit area: those from northern Norway contained lesser amounts. The possible influence of long-term air-soil exchange, latitudinal fractionation processes and differences in land management practices on the observed patterns is discussed. Archived soils (1951- 1974) from the UK sites have also been analysed and the results show increasing concentrations of these compounds up to the late 1960s/early 197Os, after which there has been a substantial decline. This temporal trend is in accordance with that reported in previous studies. However, it is possible that some of the archived samples were contaminated in the process of air-drying. Due to this contamination artifact, it is not possible to ascertain whether the scale of the observed temporal differences truly reflect changes in the environment. The data are discussed in terms of possible spatial/temporal trends and the potential for air-soil exchange of these compounds. Copyright 0 1997 Elsevier Science B.V. Keywords:
Global fractionation; Polychlorinated biphenyls; Soils; Spatial and temporal trends
1. Introduction The ubiquity of polychlorinated biphenyls (PCBs) in the modern environment is well docu--.__ * Corresponding author.
mented, but fate processes of these compounds are not yet fully understood. It has been hypothesised by various groups that semi-volatile compounds, such as PCBs, will volatilise from terrestrial and aquatic environmental compartments in warm and temperate regions, will un-
0048-9697/97/$17.00Copyright 0 1997Elsevier ScienceB.V. All rights reserved PII SOO48-9697(96)05345-4
230
W’.A. Lead et (11. :I The Science
of the Total
dergo long-range atmospheric transport (LRT) and will then recondense when they reach colder circum-polar zones (Calamari et al., 1991; Wania and Mackay, 1993). In order to test this theory, it is necessary either to obtain spatial and temporal data for changes in concentrations of such compounds in samples of the same environmental matrix or to assemble data on the rates of change in concentrations at different locations over time. In an attempt to further our understanding of the transport and fate processes of semi-volatile organic compounds we have analysed a collection of contemporary surface soil samples from across the UK and Norway for a range of PCB congeners. The UK samples were all collected from sites where soil profiles had previously (1951- 1974) been collected. These archived samples have also been analysed for PCBs with the hope that any observed changes in congener concentrations and/ or profiles would give evidence to support/disprove the global fractionation hypothesis.
2. Materials
Environment
193 (1997)
229-236
for surveys of soil type. The archived samples dated from between 1951 and 1974. To reduce the risk of post-collection changes in the PCB concentration of the archived soils, they needed to have been sealed as soon as possible after collection with minimal air-drying exposure time and stored unopened and undisturbed. Sample sites were also chosen where land management would have been kept to a minimum, to help ensure that changes in contaminant composition could be attributed to changes in the field (such as air-soil exchange, biodegradation and other processes) rather than to agrochemical or sewage sludge amendments, In the summer of 1990, samples of surface soil from podzol profiles from 12 remote woodland areas in Norway were collected (Fig. 1). The samples were taken from the upper 5 cm of the humus layer after removal of litter, and in most cases represented the F-horizon. Each sample was a composite of several sub-samples taken within a 5 x 5 m area. The predominantly organic soil (typically around 90% organic matter) was sealed in an aluminium box and stored frozen.
and methods
2.1. Sampling
The contemporary UK soil samples were taken from 46 sites where soils had previously been collected (Fig. 1). Obviously, sampling depth is critical when examining contaminants introduced to the soil from the atmosphere. Therefore, a small hand held auger was used to collect the soils, with sample depth varying between O-2.5 and O---25 cm (depth was identical to that used in the collection of the archived soils). Samples were taken to represent an area of 100 rn’. Therefore, approximately 20 cores were taken from across the area, and the cores bulked together to give the sample. Samples were frozen immediately on return to the laboratory until required for extraction and analysis. The archived soil samples from the UK were obtained from collections held at the Soil Survey of England and Wales, Silsoe, and the Macaulay Land Use and Research Institute (MLURI), Aberdeen. They were originally collected as profiles
Fig. 1. Location of the 46 British and 12 Norwegian sites. Numbered sites referred to in Table 1.
sample
W.A. Lead
et al. / The Science
of‘ the Total
Environment
193 (1997)
231
229-236
congener basis are available from the authors on request. 3.1. Spatial differences
Fig. 2. Simplified PCB concentration
2.2. Extruction
conceptual diagram of the change on exposure to air.
in sample
and unalysis
The extraction, analysis and quality assurance methods used were identical to those given by Alcock et al. (1993), in order to ensure that results would be comparable with the earlier study, except that all contemporary samples were extracted wet, i.e. they were mixed with sodium sulphate prior to extraction to remove water. This is because recent studies (Alcock et al., 1994; Lead et al., 1996) have shown that exposure of samples to air can cause either contamination by or loss of PCBs depending on the equilibrium position of the sample with the air (Fig. 2). In this study, contamination or loss was minimised for the contemporary samples by avoiding air drying. For the archived soils, the artifact was minimised by selecting soils which were stored sealed and unopened. Although analysis of archived material is not ideal for the inference of temporal trends of volatile/semi-volatile compounds, it is one of the few techniques available (Lead et al., 1996).
in contemporary
soils
It was found that soil PCB concentrations in the south of Norway were significantly higher (99% confidence limit) than those in samples collected from the north of the country. It was also found that the PCB content of the Norwegian samples was higher than that of the UK samples. For example, the mean total PCB concentration (CPCB, see Table 1 for definition) in the UK soils was 4 pgg/kg; in samples from the south of Norway, it was 17.5 ,ug/kg and from the north of Norway, it was 9.5 ,ugg/kg. Principal components analyses (PCA) were used to further investigate this spatial difference. These analyses revealed differences between the UK and Norwegian samples when all factors having Eigen values greater than 1 were plotted against each other (e.g. Fig. 3). It is found that the clustering observed in Fig. 3 is due to mid-molecular weight PCBs, particularly congeners 101, 105, 110, 138, 149 and 187. It can therefore be concluded that it is these midmolecular weight congeners which are having the greatest influence on the difference in the patterns between UK and Norwegian samples. It is not
3. Results and discussion
Due to the quantity of data obtained (104 samples plus replicates: 37 PCB congeners) it is necessary to be selective in the presentation of results. Information on all the Norwegian sites and ten representative UK sites (archived and contemporary samples), together with concentrations of tri- to octachlorinated PCB congeners, are shown in Table 1. The location of these sites is highlighted in Fig. 1. Results on an individual
I
-41
-3
-2
I 0
-1
I
I
0 FACTOR
1
I
I
2
3
(1)
Fig. 3. PCA plot of factor 2 vs. factor 1 for contemporary UK (U) and Norwegian (N) soil samples. Woodland UK samples are highlighted (0). Factors 1 and 2 together explain approximately 25% of the variance of the data.
Clay
Clayloam
Peat
Clay
Loam
Sandyloam
Loam
Loam
3
4
5
6
I
8
9
10
Pasture
Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland Woodland
Grassland
Pasture
Grassland
Pasture
Pasture
Woodland
Grassland
Scrub
use
.~
Pasture
-. --~
Land
1990 1990 1990 1990 1990 1990 1990 1990 1990 1990 1990 1990
1951 1993 1953 1993 1955 1993 1956 1993 1959 1993 1961 1993 1963 1993 1965 1993 1966 1993 1968 1993
Sample
,md PCB concentration data
95.2 91.6 94.2 92.8 82.9 94.9 94.2 46.5 94.5 71.4 93.6 95.0
7.2 16.2 9.1 7.9 13.0 9.0 11.7 18.0 86.4 61.2 27.8 12.2 31.1 12.3 8.9 12.0 11.4 9.6 6.5 5.1
Organic (‘i/O)
matter
126.
(/‘g hg) for the Norwegian
Sites referred to are marked in Fig. 1. a Tri-CB is sum of congeners 30, 18 and 28. b Tetra-CB is sum of congeners 40, 52, 61, 66, 74 and 77. “Penta-CB is sum of congeners 82, 101, 104. 105, 110, 118, 119 and d Hexa-CB is sum of congeners 138. 149, 151, 153 and 156. ‘Hepta-CB is sum of congeners 170. 180. 183, 185, 187 and 188. ’ Octa-CB is sum of congeners 194, 198, 201 and 202. s ZPCB is sum of all above congeners. ’ N.D. implies below detection limit.
Sample Sites Podzol Podzol Podzol Podzol Podzol Pocizol Podzol Podzol Podzol Podzol Podzol Podzol
Loam
Nmtqim I1 12 13 14 I5 16 17 18 19 20 21 22
-...
loam
2
I k .Smlp/c~ SiteA i Sandy
Soil type
labif I Sample hitc information
1.2 0.89 0.98 1.7 0.87 1.4 1.0 0.20 1.8 0.97 1.8 0.72
37 0.22 0.36 0.22 48 0.83 120 0.19 1.3 1.5 170 0.098 940 0.25 330 0.17 870 0.37 490 0.018
sample
0.73 0.46 0.67 1.5 0.46 1.6 1.3 0.62 2.8 0.71 2.3 3.0
19 0.23 0.22 0.088 27 0.43 65 0.27 1.6 1.5 92 0.048 510 0.22 320 0.13 470 0.18 330 0.063
Terra-CBh
1.8 1.2 1.6 4.1 2.0 3.0 5.1 2.3 7.9 2.5 7.6 6.3
3.7 1.4 1.3 0.26 7.5 1.2 19 2.2 2.0 2.2 16 0.14 84 0.68 78 0.4 69 1.5 34 0.81
Penta-CB’
sites and ten of the UK
sample
0.96 3.2 5.1 3.6 1.9 3.0 6.7 2.9 7.6 2.4 8.0 14
1.8 0.86 0.52 0.23 3.1 0.29 11 0.48 0.1 2.3 8 0.044 23 0.87 22 0.4 17 0.34 7.3 0.15
Hexa-CB”
sites
0.57 0.48 1.1 1.8 0.85 0.96 1.9 0.65 2.6 3.7 4.2 4.8
0.67 0.56 0.95 0.27 1.1 0.14 3.0 0.15 0.066 0.96 1.3 N.D. 6.2 0.73 7.1 0.37 3.4 0.24 1.4 0.13
Hepta-CB’
N.D. 0.32 0.33 0.32 0.08 0.33 0.34 0.17 0.79 0.28 0.93 0.73
0.019 0.037 0.25 N.D.h 0.24 0.01 I 0.18 0.058 0.086 0.22 0.42 N.D. 1.1 0.4 1.4 0.15 0.68 0.023 0.4 0.015
Octa-CB’
5.3 6.6 16 13 6.2 10 16 6.8 23 11 25 30
62 3.3 3.6 1.1 88 2.9 210 3.4 5.1 8.7 280 0.33 1600 3.1 760 1.6 1400 2.6 860 1.2
XPCB”
WA.
Lead
et al. II The Scirncr
of. the Total
possible to fully ascertain, however, whether the difference is truly a spatial one or whether it is due to the fact that all of the samples from Norway are woodland soils. Those from the UK are predominantly pasture and grassland samples. Previous studies have shown the concentration of semi-volatile organic compounds to be higher in forest soils than in nearby grassland/arable soils (Hagenmaier and Krauss, 1993; Rotard et al., 1994). However, there were four woodland soils which were analysed from the UK, and these samples were found to be evenly distributed in the PCA plot of the UK samples (Fig. 3). This suggests that the compositional difference in the UK and Norwegian data is a spatial not a landuse difference, but without either more woodland samples from the UK or some samples of pasture or grassland soils from Norway, this statement can not be fully supported. Importantly, it should also be noted that if concentrations were expressed on a per unit area basis, then the difference between the UK and Norwegian samples would be significantly reduced, since grassland soils generally have a much higher bulk density than woodland soils. Detailed bulk density data are not available to allow all the results to be expressed in this way. However, for illustrative purposes, if we assume all PCBs are in the top 5 cm and that the bulk density of the UK grassland samples is 1000 kg/m3, and that of the Norwegian forest soils is 250 kg/m3, then the mean concentration of CPCB equates to approximately 200 Llg/ mz for the UK soils; ca. 220 Llg/rn’ for the soils from southern Norway and ca. 120 pgg/m’ for the soils from the north of Norway. There is some support in the Norwegian data set for the global fractionation hypothesis, in that: (1) there is a slightly higher tri-CB:hexa-CB ratio in the northern samples than in the southern most Norwegian soils; and (2) there is a relative enrichment of the mid-molecular weight PCBs compared with the UK soils. These observations would be consistent with a latitudinal fractionation influenced by generally lower summer temperatures, particularly in the north of Norway, than in the UK. For the low molecular weight components, the temperatures seem to be high enough, even in the north of Norway, for volatili-
Emironmrnt
192 (1997)
229-3.36
233
sation to occur, i.e. it would seem that the less chlorinated PCBs are re-volatilising from temperate and circum-polar regions and are being transported to still colder areas. These findings are consistent with those reported for Norwegian mosses (Lead et al., 1996).
Rigorous statistical analyses, including student t-tests, analysis of variances (ANOVA) and PCA, have been performed on the contemporary UK data, but no correlations were found between PCB concentration and, for example. land use, organic matter content, soil type or sample region. This could be because in the UK there has been thorough environmental mixing of PCBs, so that differences can no longer be seen. Alternatively. it could be that there were insufficient samples in this study from, for example, each region for such rigorous tests to be carried out. A previous study by Alcock et al. (1993) reported contemporary UK CPCB soil concentrations as being 20-30 pg./kg. This study has found it to be approximately a factor of 5 lower (mean ZPCB concentration of 4 @g/kg). Soils in the earlier study were all air-dried, which probably led to some contamination from the atmosphere (Alcock et al., 1994). To minimise such contamination in this study. all contemporary samples were extracted wet. Tests were therefore carried out to ensure that this was a genuine difference in reported concentrations and was not due to differences in extraction efficiencies from wet and dry soil. No notable differences were found. The data reported by Alcock et al. (1993) have been used by others attempting to model fate processes of PCBs. For example, the data have been used by Harner et al. (1995) to model the long term exchange of PCBs between the air and soil and by Harrad et al. (1994) to estimate the contemporary PCB burden of UK soils. Obviously the results reported here will also have implications in such studies, e.g. using the same method as Harrad et al. (1994) we have reassessed the UK soil budget for congeners 28, 52, 101, 138, 153 and 180 as well as for CPCB (Table 2). However, it should be stressed that there are
234
W.A. Lead
Table 2 PCB concentration UK soils Congener -~-28 52 101 138 153 180 ZPCB Figure
Mean kg)
and
estimated
concentration
et al. 1 The Science
burden
(pgi
0.28 (1.7) 0.13 (1.1) 0.15 (1.8) 0.28 (1.0) 0.34 (1.5) 0.13 (0.69) 4.0 (30) in brackets
refers
UK
in
of the Total
of Harrad
193 (1997)
229-236
contemporary
soil burden
(t)
3.4 (21) 1.6 (14) 1.9 (22) 3.4 (12) 4.2 (19) 1.6 (8.6) 49 (370) to that
Environment
et al. (1994).
major uncertainties over this method for estimating soil budgets, primarily because it assumes that all of the PC& are present in the top 5 cm of soil and that the concentration is uniform throughout this depth. 3.3. Temporal trends in UK soils
Results for the XPCB concentration for all UK samples in relation to year of collection are shown in Fig. 4. It can be seen that there was an increase in the CPCB concentration up to the late 196Os/ early 1970s and there has since been a dramatic decrease in the XPCB content of UK soils. The date of the exact maximum cannot be inferred, due to the small number of archived samples from after 1970. The same trend is seen for all homologue groups; but the tri-chlorinated congeners decreased by a factor of approximately 1000 between 1968 and 1993 and the heavier octachlori?O”O -,
Fig. 4. Change rn UK soil samples over time. (1993 point represents all 46 sample sites. Concentration of ZPCB ranges from - 0.5 to 20 /cg/kg. with a mean value of 4 /‘g/kg.)
Fig. 5. Relative contribution of each homologue group to CPCB concentration for (a) a representative archived soil and (b) the matching contemporary sample.
nated biphenyls by a factor of approximately 5 over the same time period. The relative importance of the heavier PCBs has therefore increased over time (Fig. 5). These trends are comparable with those reported by Alcock et al. (1993) for soils collected from the ‘classical’ experimental plots at Rothamsted, an experimental research station in the south east of the UK. It is hypothesised that the dramatic decline in the PCB content of UK soils is largely due to volatilisation, with the air-soil system approaching a thermodynamic equilibrium. Following volatilisation, PCBs will be available for LRT and subsequent global distillation. It has been suggested that the lighter congeners could move further northwards than the higher chlorinated homologue groups resulting in a latitudinal fractionation (Wania and Mackay, 1993). This is supported by the fact that it is the lighter congeners which are being lost most rapidly from temperate soils. It must be stressed, however, that the archived soils used in this study were all air-dried prior to storage, albeit with the air-soil exposure time being kept to a minimum. This will undoubtedly have led to some alteration in the composition of these samples (Alcock et al., 1994; Lead et al., 1996). Results where samples have been air-dried should therefore be viewed with caution. In this study the PCB concentrations found in the archived samples are unlikely to be the true values at the time the soils were collected from the field; the observed changes in concentration are likely to have been exaggerated by the contamination artifacts and the scale of the decline in the PCB
W.A.
Lead et al. ! The Science
of’ the Total
content of UK soils since the early 1970s will probably have been less than has been reported here. Having said this, however, Lead et al. (1996) report that the contamination which occurs through air-drying of samples in their laboratory mainly arises from lower chlorinated congeners, particularly those of tri- and tetra-substitution. Therefore, in this study, the observed decline of a factor of 5 between the late 1960s and the early 1990s in the soil concentration of the octachlorinated congeners may be realisitic. Jones et al. (1995) reported that at two rural UK sites EPCB concentrations in outdoor air decreased by a factor of approximately 4.6 between 1972-1976 and 1987-1992. If we assume that this factor can also be applied to indoor air concentrations, and therefore to contamination fluxes onto drying soil samples, then based on a contemporary flux of 5 pg ZPCB/m2 per day (Alcock et al., 1994), fluxes in the 1970s may have been in the region of 23 pg ZPCB/m2 per day. In order to try to estimate the degree of contamination which may have occurred during the air-drying of the archived samples, if we assume a worse case scenario where in the early 1970s 1 kg of soil may have been spread over an area of 0.25 m2 to dry for 7 days, then using the above flux onto the soil, concentrations in the samples may have increased by 40 ,ug/kg. This compares with a measured concentration of CPCB in 1970 soil of around 1000 pg/kg (see Fig. 4). Using these estimates, it seems unlikely that contamination alone can account for the scale of the observed decrease in soil PCB concentrations over the last 2.5 decades.
4. General comments and conclusions
In summary, this study has highlighted a difference in the patterns of PCBs between contemporary soil samples from the UK and from Norway. It is thought that this is due to a spatial difference and not to a land-use difference. The concentrations for the contemporary UK soil samples was a factor of approximately five lower than previously reported for the UK. A change has been seen in the UK soil PCB concentration over time which is
Enrironment
193 (1997)
229%236
23s
comparable with that found by Alcock et al. (1993). There has also been an increase in the relative contribution of the heavier molecular weight congeners since the late 1960s. It is emphasised that, because of the possibility for post-collection changes in sample composition, we believe the archived samples only allow general trends and indications of concentrations from the past to be derived. The spatial difference and the results for the decreasing concentration in UK samples lead us to suggest that PCBs are being lost from the soils of temperate areas, largely as a result of volatilisation. This implies that between the late 1960s and the present day, temperate soils have been acting as a source of these compounds to the atmosphere. Once volatilised, the compounds will have been available for LRT and subsequent global distillation. McLachlan (1996) has suggested that soil-borne PCBs in rural parts of Germany are now approaching a steady state with the air, so that the air-soil fugacity quotient is nearing 1. It may be, therefore, that in the summer, when temperatures are higher, contemporary soils are acting as a source, while in the winter as a sink. This may be a factor in explaining the seasonality of PCB concentrations in air (Halsall et al., 1995). Finally, it should be pointed out that although the decline in soil PCB concentrations here has mainly been attributed to volatilisation, biodegradation of these compounds, particularly of lower chlorinated compounds, will also have occurred. This process will therefore also have played a part in the temporal change in both concentration and congener profile. In addition, it has been suggested that persistent organic pollutants in soils may become more strongly bound over time, thereby becoming harder to extract (Alexander. 1995; Pignatello and Xing, 1996). This may result in an effective decrease in concentrations of PCBs extractable in hexane. However, this process could occur in both the archived samples during storage and also in the soil in the field. A recent study on sediments suggests that this process may be relatively unimportant for PCBs (McGroddy et al., 1996). Research is continuing in our laboratory to establish the relative importance of soil out-gassing and other processes in contributing to
‘36
M’.A.
Leud et al. : T/w Scierrw
of r/w Tutu/
longterm changes in soil concentrations, However, we currently believe that air-soil exchange is of particular importance in governing the concentrations of PCBs in soils.
Acknowledgements Collection of archived and contemporary UK soil samples was funded by the Natural Environment Research Council (NERC), and analysis of these samples by the UK Ministry of Agriculture, Fisheries and Food (MAFF). Thanks are also due to Peter Loveland and Dick Thompson of the Soil Survey of England and Wales for their help in choosing the archived soil collection. W.A.L. is funded by a studentship provided by the Environmental Science Department at Lancaster University.
References Alcock, R.E.. Johnston, A.E.. McGrath, S.P., Berrow, M.L.. Jones, KC. (1993) Long-term changes in the polychlorinated biphenyl content of United Kingdom soils. Environ. Sci. Technol. 27, 19181923. Alcock. R.E., Halsall, Cl., Harris, C.A., Johnston, A.E., Lead, W.A., Sanders, G., Jones, KC. (1994) Contamination of environmental samples prepared for PCB analysis. Environ. Sci. Technol. 28, 1838- 1842. Alexander. M. (1995) How toxic are chemicals in soil? Environ. Sci. Technol. 29. 27132717. Calamari, D., Bacci, E., Focardi, S., Gaggi, C., Morosini, M., Vighi, M. (1991) Role of plant biomass in the global environmental partitioning of chlorinated hydrocarbons. Environ. Sci. Technol. 25, l489- 1495.
Environment
193 (1997)
229-236
Hagenmaier, H., Krauss, P. (1993) Attempts to balance transport and fate of polychlorinated dibenzo-p-dioxins and dibenzofurans for Baden-Wiirttemberg. Organohalogen Compounds 12, 81-84. Halsall, C.J., Lee, R.G.M.. Coleman, P.J., Burnett, V., Harding-Jones, P., Jones, K.C. (1995) PCBs in UK urban air. Environ. Sci. Technol. 29. 2368-2376. Harner, T., Mackay. D.. Jones, KC. (1995) Model of the long-term exchange of PCBs between the soil and the atmosphere in the southern UK. Environ. Sci. Technol. 29, 1200- 1209. Harrad, S.J., Sewart, A.P., Alcock, R., Boumphrey, R., Burnett, V., Duarte-Davidson. R., Halsall. C., Sanders, G., Waterhouse. K., Wild, S.R., Jones, K.C. (1994) Polychlorinated biphenyls (PCBs) in the British environment: sinks, sources and temporal trends. Environ. Pollut. 85, I31 -146. Jones, K.C., Duarte-Davidson, R., Cawse, P.A. (1995) Changes in the PCB concentration of United Kingdom air between 1972 and 1992. Environ. Sci. Technol. 28, 272275. Lead, W.A.. Steinnes, E., Jones, K.C. (1996) Atmospheric deposition of PCBs to moss (Hylocomium splendens) in Norway between 1977 and 1990. Environ. Sci. Technol. 30, 524-530. McGroddy, S.E., Farrington, J.W., Gschwend, P.M. (1996) Comparison of the in-situ and desorption sediment-water partitioning of polycyclic aromatic hydrocarbons and polychlorinated biphenyls. Environ. Sci. Technol. 30, 1722177. McLachlan, M. (1996) Bioaccumulation of hydrophobic chemicals in agricultural food chains. Environ. Sci. Technol. 30. 252-259. Pignatello, J.J., Xing, B. (1996) Mechanisms of slow sorption of organic chemicals to natural particles. Environ. Sci. Technol. 30, 1 - 11. Rotard, W., Christmann, W., Knoth, W. (1994) Background levels of PCDD/F in soils in Germany. Chemosphere 29, 2193-2200. Wania, F., Mackay, D. (1993) Global fractionation and cold condensation of low volatility organochlorine compounds in polar regions. Ambio 22, 10-18.