PCB and PAH fluxes to a dated UK peat core

ELSEVIER 0 2 6 9 - 7 4 9 1 ( 9 4 ) 0 0 0 4 8 - 4

Environmental Pollution 89 (1995) 17 25 © 1995 Elsevier Science Limited

Printed in Great Britain. 0269-7491/95/$09.50

PCB AND PAH FLUXES TO A DATED UK PEAT CORE

Gordon Sanders,* Kevin C. Jones,+ John Hamilton-Taylor Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, UK, LA1 4 YQ

&

Helmut Dorr § Institute of Earth Sciences, University of Heidelberg, Im Neuenheimer Feld 366, D-6900 Heidelberg, Germany

(Received 3 September 1993; accepted 18 June 1994)

Abstract Concentrations of PCBs and PAils have been determined from the individual sections of peat cores obtained from an ombrotrophic bog in rural north-west England. Chronological intervals throughout the core were determined from both radiometric (21°pb, IJ7Cs, 24tAm) and independent, non-chemical characteristics (pollen, mag- neticsJ information. Net fluxes of 25 individual PCB congeners and 14 PAH compounds to the bog were then derived. PCB inputs were apparent from the late-1930s/ early-1940s, with maximum sub-surface fluxes (~1300 pg cm : ),ear t) observed at a depth corresponding to 1964. Loadings decreased by ~65% over the following 15 year period before showing a surface enrichment. Initial increases in PA H loadings appear to coincide with the beginning of the bldastrial Revolution, with fluxes peaking in the early-1930s (305 ng cm 2 year-l). Introduction of emission controls and the decline of heavy industry has led to an 80 % reduction in the net flux of PAHs to the bog over the last three to four decades. Potential effects of post- depositional diagenesis are considered, with particular reference to alteration of contaminant chronologies.

Keywords: Polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), air concentrations, historical time trends, peat.

INTRODUCTION

Global contamination has resulted from large scale, heavy industrial manufacturing in the northern hemi- sphere since the Industrial Revolution. Until the late 1970s/early 1980s, however, assessments of long-term changes in environmental loading of contaminants have been limited, even though such information is necessary to monitor the success of measures to reduce emissions. Environmental samples laid down over time

Present Addresses: *Battelle-Europe, Centres de Rechereche de Geneve, 7 Route de Drize, CH 1227 Carouge, Geneva, Switzerland. ~Trischler und Partner GmbH, Berliner Allee 6, 6100 Darmstadt, Germany.

+Author to whom correspondence should be addressed.

in identifiable layers, or stored samples collected at some time in the past can be used to study the changing levels of contaminants in the environment retrospectively. Undisturbed marine and lacustrine sediments (MARC, 1985; Eisenreich et al., 1989; Sanders et al., 1992, 1993), Arctic and Antarctic ice cores (MARC, 1985), and archived samples (Jones et al., 1989, 1992) have all been used to construct contaminant input time-trend profiles. In this study, cores of ombrotrophic peat have been utilised to assess the historical atmospheric loadings of two important groups of hydrophobic organic contaminants (HOCs), the polychlorinated biphenyls (PCBs) and the polynuclear aromatic hydrocarbons (PAHs). Peats are an excellent medium for studies on HOCs (Rapaport et al., 1985; Rapaport & Eisenreich, 1988); their high organic carbon content binds non-polar compounds, minimising mobility or diagenetic alteration following deposition. In addition, the majority of inputs to ombrotrophic bogs are derived from the atmosphere; contaminant profiles down a core should be proportional to atmospheric loadings through time.

This paper reports data on PCBs and PAHs in an undisturbed ombrotrophic peat core obtained from a bog in rural Cheshire, north-west England, to enable inferences to be made about changing atmospheric loadings of these compounds.

17

MATERIALS AND METHODS

Sampling A specially constructed peat sampler for high resolution sampling (Clymo, 1988), was used to obtain three adjacent cores in October 1990 from a hollow on Flaxmere bog, Cheshire, north-west England (location 53°15'N, 2°40'W). Situated on level relief in a sparsely populated area 37 km south of Liverpool and 48 km south-west of Manchester, with prevailing wind direc- tion dominated by south westerlies, the site is considered to represent a semi-rural/rural location. Flaxmere occupies a small glacial sand basin in the Norley Parish of the Delamere Forest. At the time of sampling the bog could be described as being in a relatively 'dry' state; the water table position located at some 10-15 cm below the surface.

18 G. Sanders et al.

The sampling technique consisted of insertion of a 20 cm diameter × 75 cm long sinuous toothed stainless steel cutting cylinder into the sample matrix until almost completely submerged. The cutter was withdrawn and a 60 cm long PVC core sleeve of the same diameter inserted into the template cavity until the surface of the living vegetation stood proud of the upper edge of the core tube. PVC lids were fixed onto the top of the cores at this stage. The in-situ sample cores were extracted from the bog hollow using a circular sharp-edged steel plate of 21 cm diameter mounted on aluminium rods. End-caps were affixed to the core-tube following extraction from the mire. Samples were transported in the upright position and sectioned immediately on returning to Lancaster. The surface growth was used as the initial datum point for sectioning. Sections of 2 cm were extruded and sliced down to a depth of 30 cm (FMP1-FMP15), and thereafter 3 cm divi- sions down to a maximum depth of 45 cm (FMP16- FMP20). Samples were air-dried and manually homogenised.

Dating Sample sections were radioisotopically dated by measur- ing 21°pb, bomb- and ChernobylJ37Cs and bomb- 241Am by gamma spectroscopy using a planar, high-purity germanium Canberra Packard LEGE detector. Energy resolution, in the presence of matrix, was 0.8 keV at 46 keV (21°Pb), and 1.4 keV at 662 keV (137Cs), giving a typical efficiency of 12 and 1.8% at respective energy levels. Atmospheric 21°Pb (21°pb~c) was corrected for in-situ derived 21°Pb. Employing the constant rate of supply (CRS) model (Appleby & Oldfield, 1978) on the unsupported 2~°pb profile, an unconstrained chronology has been calculated based on 2~°pb only, and a constrained time-frame derived by forcing the 2~°pb dates through the 241Am maximum (assumed to correspond to 1963/1964). Existing pollen and magnetics data on the bog has also been adapted and utilised.

saved, and the spent acid portions back extracted with a further aliquot of hexane to remove any residual PCBs. Both hexane extracts were combined, and reduced to ~ 1 ml under a gentle stream of oxygen free nitrogen. The final sample cleanup was carried out on a 6 g column of 1.25% deactivated Florisil (i.d. 15 mm), with elution of all PCBs achieved in 40 ml of pesticide grade hexane. The eluent was then spiked with a known quantity of congeners 30 and 209 to act as retention time correction reference standards. Samples were finally reduced to 0.5 ml prior to analysis. The sample extracts were analysed for PCBs on a Hewlett Packard HP5890 Series II gas chromatograph equipped with a 63Ni electron capture detector, as described previously (Sanders et al., 1992). Identification and quantification of PCBs was achieved by overlaying the chromatogram of a standard mix containing 51 congeners onto the sample chromatogram and matching and naming peaks by their retention times. This step was carried out automatically using a VG Minichrom data processing package. Individual chromatograms were checked and any baseline or identification alter- ations carried out manually. The standard mix contained 51 congeners, namely the following IUPAC congeners (given in elution order): 3, 10, 6, 8, 14, 30, 18, 15, 54, 28, 52, 104, 44, 37, 40, 61/74, 66, 155, 101, 99, 119, 77/110, 82/151, 149, 118, 188, 153, 105, 138, 187, 126, 183, 128, 185, 202, 156, 204, 180, 169, 170, 198, 201, 189, 208, 194/205, 206, 209. Of the 51 screened, only 25 were quantified in all the samples and are reported here. Congeners with large variability in peak areas between replicates, or peaks below the limit of detection were discarded and not considered. PCB recoveries averaged 83%, ranging from 72% to 101%. The maximum standard deviation between individual congeners in 4 spiked recovery samples was +21%, and as low as +4%. Congener replication between duplicate samples gave less than 15% deviation generally, but was slightly higher for the more volatile species.

Sample extraction and analysis Duplicate 10 g samples were Soxhlet extracted for 18 h with double re-distilled HPLC grade dichloromethane (DCM) on a Buchi 810 fat extraction system. Copper turnings were incorporated during extraction to remove any elemental sulphur which may be present. Follow- ing extraction a 60:40 split was performed on the sample extract for PCB and PAH analysis, respectively.

PCB purification and analysis The fraction for PCB analysis was solvent exchanged to hexane, then subjected to a pre-cleanup stage with concentrated sulphuric acid to remove a broad range of matrix constituents. Sample extracts were shaken in a 1 : 1 v/v ratio with 18 M Analar-grade H2SO4. After sep- aration of the two phases had taken place, the bottom, acid phase was run off and collected. This process was repeated two more times, or until the acid phase remained almost transparent. The hexane fraction was

P A H purification and analysis The PAH extract sample volume was reduced to ~ 1 ml and applied to a 3 g column of activated Florisil (i.d. 10 mm), and eluted with 30 ml of DCM. The eluent was reduced to 1 ml, and analysed for 14 unsubstituted PAH compounds by high performance liquid chroma- tography with fluorescence detection, as described previously (Sanders et al., 1993). Naphthalene, acenaphthene/ fluorene (co-eluters), phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene/chrysene, benzo[b] fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene and coronene were quantified by multiple regression equations derived from analysing and peak processing a range of composite PAH calibration standards. Recoveries of PAHs over the molecular weight distribution considered, averaged 103% for compounds of greater and equal to 3 rings in size and considerably less for the more volatile naphthalene (42%) and acenaphthene/fluorene (84%).

19

1 0

Pb-210 Activity (eq/s) 0.00 0£5 0.10 0.15 0,21) 0.25 0.30

0 ! " ' "

10

40'

20

(a)

Am-241 Activity (mBqt 8)

0 1 2 3 41 S 0 I ! I I I ~ m

I I I

I I

30

O.(3O O ~ 0 | " i .

P C B and P A H f luxes to a dated UK peat core

(b)

Cs-la7 Activity (Bq/8)

Ol)t 0,06 Oat 0.10 0.12

10'

20 ̧

30 ̧

40

0.14

(c)

Fig. 1. Radioisotope activity/depth profiles in the Flaxmere peat core: (a) 2J°pb, (b) 241Am, (c) 137Cs.

R E S U L T S A N D D I S C U S S I O N

Dating the peat Applying chronologies to peat profiles can be a difficult task due to potential in-situ diagenetic processes which may affect the distribution of the radioisotopes which are commonly employed in standard dating techniques ( 2 ] ° p b , 1 3 7 C s ) (Urban et al., 1990). It is therefore important to corroborate the chronology derived by one method with that of another. This section presents the three radioisotope activity/depth distributions (21°pb, 2 4 J A m , 1 3 7 C s - - Fig. l) and considers their implications. Complementary pollen and remnant magnetisation data are also reported for independent cores from the same bog.

21°pb ' 241Am ' lSZCs

Figure l(a) and Table 1 give the activity/depth relation- ships for 21°pb. The 2t°pb profile for Flaxmere peat does not demonstrate a log-linear activity decrease with depth (Fig. l(a)). However, even undisturbed 21°pb activity/depth profiles in peat may not show monotonic log-linear distributions. Aerobic decomposition is tak- ing place in the upper part of the core (the acrotelm), which reduces the original mass of peat produced at the surface to ~10-20% of its original weight. As well as affecting the apparent radionuclide activity/time-depth distributions this process will also have similar implications for pollutant time-trend profiles. Cation mobility and diagenesis within organic-rich bog waters may also result in an alteration of the original chronology. Urban et aL (1990) undertook a comprehensive study of inputs and outputs of Pb and 2~°Pb to an ombrotrophic bog in northern Minnesota. Their work suggested that Pb and 2]°pb inventories from adjacent peat hummocks and hollows were consistently higher for the hummocks. The apparent depletion of Pb from (primarily) the surface layers of peat in hollows is believed to be a consequence of water table level and the acidity of ombrotrophic bogs. Because the greatest amounts of 2~°Pb are being deposited onto the surface material, and the water table in a hollow will consistently lie closer to this point compared to that in an adjacent hummock, then loss by lateral flow of water is likely to be much more considerable. Losses are probably also facilitated by the elevated dissolved organic matter (DOM) fraction typically found in bog waters. As well as the height of the water table assisting lateral removal of Pb and 2~°Pb, it may also aid the sub-surface enhancement of the latter, as seen in the Flaxmere core (Fig. l(a)). Fluctuations in the position of the water table will affect dissolution and transport of particulate bound Pb. This may explain the sub-surface activity of 2~°pb, where the position of the maxima may correlate to the average height of the water table.

Figure l(b) illustrates the activity/depth profile for 24~Am in the core, and Table 1 gives actual activities within the peat. Greatest activity is seen at 19 cm below the surface (4.5 + 0.7 mBq g-l), in a relatively tight- band. This probably corresponds to the atomic bomb


Table 1. Activities of 21°Pb, 241Am and 137Cs in Flaxmere peat core. Corresponding dates derived from the CRS model (Appleby & Oidfield, 1978) are given for both unconstrained and constrained 21°pb and the mean of both

Depth Dry wt Cumm. Unsupp. 21°pb 241Am 137Cs Dates (cm) density, dry mass activity activity activity

(g cm-') (g cm 2) (Bq g ') (mBq g ~) (Bq g-L) 21°Pb (Uncon.) 21°pb (Con.) Mean

0-2 0.045 0.045 0.178 + 0.007 <1 0-037 + 0.003 1989 1989 1989 2-4 0.049 0.140 0.103 + 0.005 <1 0.044 + 0.004 1988 1988 1988 4-6 0.061 0.250 0.170 + 0.009 1.7 + 0.7 0.113 + 0.007 1986 1987 1986.5 6-8 0.068 0.379 0-261 + 0.010 1.9 + 0.7 0.049 + 0.005 1983 1984 1983.5 10-12 0.069 0.638 0.199 + 0-009 1.7 + 0.7 0-029 + 0.004 1974 1977 1975.5 12-14 0.072 0-779 0.193 + 0.008 2.0 + 0.7 0.056 + 0.004 1969 1973 1971 16-18 0.054 1.024 0.134 + 0.007 2.6 + 0-7 0.078 + 0.006 1960 1967 1963.5 18-20 0.063 1.141 0.247 + 0.011 4.5 + 0.7 0.094 + 0.007 1954 1963 1958.5 22-24 0.083 1.436 0.127 + 0.006 1-8 + 0.5 0.108 + 0.005 1930 1935" 1932.5 26-28 0.077 1.752 0.079 + 0.005 0.6 5- 0.5 0.043 + 0.003 1895 1910 a 1902.5 28-30 0.079 1.907 0.026 + 0.002 <1 0.038 + 0.002 1876 1897 a 1886.5 33-36 0.087 2.359 0.010 + 0.003 <1 0.014 + 0.001 1813 187V 1842 42-45 0.103 3.217 <0.002 <1 0.003 + 0.001 1718 1832" 1775

aData points extrapolated from unconstrained 2t°Pb.

testing of ~1963/64. Detectable 241Am activity is first seen in appreciable quantities at between 23 to 25 cm down the core, and is likely to be representative of the initiation of atomic weapons testing (between 1945 and 1950). 24~Am is generally considered to be less mobile within peat profiles and subject to greater adsorption onto organic rich material than 2~°pb and ~37Cs (Appleby et al., 1991).

Caesium-137 can be mobile in bog water and subject to uptake into the live, growing surface moss (Oldfield et al., 1979; Urban et al., 1990), especially within acidic environments. This obviously can cause difficulties in using this radioisotope as a chronological marker when dating peat cores. Figure l(c) illustrates the activity/depth distribution of ~37Cs, and activities are also given in Table 1. The profile demonstrates a double sub-surface maxima at depths of 5 and 23 cm, probably indicative of Chernobyl- and bomb-derived 137Cs, respectively. The bomb-related 137Cs maximum occurs 4 cm lower down the core than the 241Am peak input. Since both isotopes are derived from a common source, at least one has been subject to some down-core diffusion. In addition, the relative broad- ness of the bomb-~STCs input peak (between 13 and 23 cm) may reflect its up-core advection into surface mosses and lichens (Oldfield et al., 1979; Urban et al., 1990).

In summary, classic radioisotopic dating techniques are not always consistent and reliable enough to either use alone, or in combination with other methods, for the dating of peat. However, despite the problems associated with 21°pb dating in peat cores, the existing activity and cumulative weight data have been applied to the constant rate of supply (CRS) model (Appleby & Oldfield, 1978), under both unconstrained and constrained cir- cumstances. An unconstrained chronology based solely upon 21°pb activity using a constant rate of supply model (i.e. constant atmospheric flux assumptions) yields the dates given in Table 1. This method dates the 24tAm peak at ~1954 and the bombJa7Cs, occurring 4

cm deeper, dates at 1930. Therefore, this chronology either demonstrates the effect of diffusion and advection of 241Am and 137Cs, or is overestimating the age of the core sections. It is interesting to note the spike of 137Cs at 5 cm which dates to 1986 under this regime. It is therefore possible that whilst the 137Cs associated with weapons testing has migrated, the Chernobyl material may, for the present, be sufficiently in place to be a reliable chronological marker. Extrapolation of the unconstrained 2~°pb data gives a date for the base of the core of ~ 1718, with peat accumulation rates ranging from 0.051-0.088 g cm -2 year -~ at the surface, ~0.030 g cm 2 year ~ mid-core, and ~0.010 g cm -2 year -~ at the base of the core.

Table 1 also shows the dates derived for 2~°Pb when applied to the CRS model constrained to the 241Am maximum, i.e. the dates have been forced through 19 cm = 1963/1964. Theoretically this approach can be used only for allocating a chronology to that part of the core corresponding to post-24tAm deposition. How- ever, using the accumulation rate derived by unconstrained 2t°pb at a depth of 23 cm (i.e. 0.012 g cm -2 year-l), which corresponds to the visually determined onset of complete humification, constrained dates have been extrapolated down-core from 19 cm to the bottom of the core. This approach dates the bomb 137Cs maximum at ~1935, and the base of the core to ~1832, rep- resenting significantly younger material than calculated via the unconstrained method. In general, the agreement between chronologies for constrained and unconstrained instances is good in the top 20 cm of the core, and demonstrates an increasing divergence with depth (Table 1). The mean year derived from the two procedures is also given in Table 1.

Independent dating techniques Knowledge of the local and regional agriculture and vegetation can be used to date peat cores, by inferring changes in pollen numbers and species. The pollen profile of a peat core obtained from Flaxmere in May


Table 2. Table relating major features in pollen diagram and magnetic analysis. (Adapted from Ainsworth (1990))

21

Depth Magnetic Pollen

Regional Local

17-1~

24

27

42-44

50

1950s: Main increase in SIRM (magnetic concentrations) possibly related to post-war industrialisation and increased burning of fossil fuels in power generation plants.

1850: Metal magnetic take-off and increased loadings of trace metals.

1970s: Ulmus decline due to Dutch Elm disease.

1948-1956: Pine plantations of 1923-26 mature. 1939-1946: Peak in cereal pollen and high percentages of arable and pastoral weeds. Intensification of agriculture during the Second World War.

1820: High pine percentages. Plantation of pine in 1795 matures.

1980s: Birch declines due to felling of trees.

1970s: Birch pollen increase due to maturity of trees..

1989 has been studied and reported by Ainsworth (1990). Significant changes in pollen types and loadings were found to be present within the core, and are reported in Table 2, along with corresponding time periods associated with the individual events. Once more, agreement with 21°Pb dating is good in the upper reaches of the core (0--20 cm) but becomes increas- ingly poor with depth. Using the same core, Muir (1990) conducted investigations into the remnant magnetism associated with the organic rich material. Iron impurities produced during fossil fuel combustion, iron and steel manufacture, and metal smelting, are con- verted into ferro- and antiferro-magnetic iron oxides (Hunt et al., 1984), which give distinctive magnetic properties associated with industrial and urban particulate emissions. Peat samples from Cumbria subjected to saturation isothermal remnance magnetism (SIRM) (Oldfield et al., 1979) have shown a sub-surface escala- tion in values, believed to be indicative of the onset of the Industrial Revolution. This boundary was found at 42 to 44 cm, and corresponds well with the increase in trace metal concentrations, both indicative of the mid- 1800s. Pollen constitutes a crude chronological method. It should also be noted that both procedures are likely to be subject to post-depositional movement, upward and downward, especially within the uncompacted zone (Clymo & Mackay, 1987). Local and regional vegetation changes in pollen spores and significant magnetics information are compared with key chronological features of the present core in Table 2.

PCB time trends EPCB refers to the sum of 25 individual congeners: 18, 28, 52, 44, 66, 101, 99, 110, 82/151 (coelute), 149, 118, 188, 153, 105, 138, 183, 128, 185, 156, 180, 170, 201, 194/205. The EPCB concentration and flux found in each section, along with the flux of eight abundant individual congeners (18, 28, 66, 110, 138, 149, 153, 180) throughout the profile given are in Table 3. Mea- surable concentrations of PCBs were detected in all sections of the core, including slices dated as much older than the date of first commercial production of PCBs in 1929. This important feature of the profile will be discussed later.

The onset of increases in PCB loadings is observed at 20 to 24 cm, dated at between 1932 and 1945. Net fluxes at this time were 100 to 300 pg cm -2 year ~l. Thereafter, the net flux increased rapidly to a sub- surface maximum of 1300 pg cm -2 year I at 17 cm, dated at ~1964. Within the next 6 cm, net fluxes declined sharply to 450 pg cm -2 year l in ~1976, repre- senting an estimated decrease in atmospheric loadings of 65%. In the top three slices of the core (0--6 cm) there has been an apparent substantial increase in PCB net fluxes (Table 3), although this may be an artefact produced by the living vegetation at the surface, or by upward migration/outgassing of PCBs from deeper in the core. Other temporal trend data for the UK does not support a recent increase in fluxes (Jones et al., 1992; Alcock et al., 1993). Rapaport and Eisenreich (1988) also noted a surface maximum in their study of


Table 3. Fluxes of 8 major congeners, and YPCB concentrations and fluxes for 25 congeners with relation to depth

Section Depth Approximate 18 28 66 110 138 149 153 180 EPCB EPCB Flux (cm) year (/zg k g I) (pg cm -2 year -I)

FMP 1 0--2 1989 140 105 58 35 88 152 111 76 23.5 1371 FMP 2 2-4 1988 165 146 39 19 48 116 68 58 11.3 1096 FMP 3 4-6 1986 179 145 56 22 62 117 78 56 20.0 1119 FMP 4 6-8 1984 83 80 23 10 33 43 33 33 16.2 539 FMP 5 8-10 1979 44 47 10 7 41 41 37 34 12.6 426 FMP 6 10-12 1976 27 34 24 51 84 31 31 41 13.1 449 FMP 7 12-14 1971 62 40 34 93 65 52 52 62 23.5 725 FMP 8 14-16 1967 60 53 60 129 89 66 69 73 29.5 976 FMP 9 16-18 1964 85 64 78 173 109 106 92 92 36.8 1299 FMP 10 18-20 1958 26 18 23 67 41 35 33 39 27.7 456 FMP 11 20-22 1945 45 25 15 38 21 15 16 20 24.2 304 FMP 12 22-24 1932 32 20 14 4 2 1 2 5 8.5 100 FMP 13 24-26 1918 29 17 1 1 1 ND ND 3 6.4 67 FMP 14 26-28 1902 27 15 ND ND ND ND ND 1 7.0 64 FMP 15 28-30 1886 37 25 2 4 1 4 ND ND 6.9 77 FMP 16 30-33 1864 37 30 8 1 ND 1 ND ND 8.5 81 FMP 17 33-36 1842 35 27 8 2 2 3 2 ND 12.0 94 FMP 18 36-39 1820 20 17 3 1 ND ND 1 ND 7.3 57 FMP 19 39-42 1798 18 12 ND 1 ND ND ND ND 4.6 36 FMP 20 42-45 1775 13 11 2 2 ND 1 ND 1 5.4 42

EPCB is the sum of 25 congeners: 18, 28, 52, 44, 66, 101, 99, 110, 82/151, 149, 118, 188, 153, 105, 138, 183, 128, 185, 156, 180, 170, 201, 194/205. ND: Compound not detected. All concentrations are expressed in/xg k g 1.

PCBs in peat cores from mid-latitudes of the north- eastern United States. In their 8 cores, they generally noted increased PCB accumulation from ~1930 and maximum inputs between 1967 and 1970, averaging 25 /~g EPCB kg -~. Their surface maxima ranged in concentration between 3 to 15 /xg EPCB k g I (up to 930 pg cm 2 year~), within the range found at Flaxmere (Table 3). These trends are also in good agreement with those derived for a dated rural lacustrine sediment core obtained from Esthwaite Water in the English Lake District, about 80 km from Flaxmere (Sanders et al., 1992). However, the period of maximum input to the Flaxmere peat was for a smaller time interval than for the Esthwaite Water sediment core; this is probably an artefact of the physical mixing processes which re-work the deposited lake sediments. Sorption of HOCs onto the organic-rich peat may restrict post-depositional contaminant movement.

The PCB trends generally follow the pattern of U K production. Commercial production commenced in 1954 and peaked in the late 1960s, with production between 1965 and 1969 accounting for 21 000 out of the estimated total of 66 748 tonnes of PCBs produced in the UK (Harrad et al., 1994). Manufacture remained high (~- 18 000 tonnes) between 1970 and 1974, prior to the discontinuation of UK sales in 1977. In the peat core this chain of events is recognisable in the flux/depth associations between the depth of 11 and 19 cm. If UK production figures accurately coincide with fluxes to Flaxmere, then a broader sub-surface maximum would be expected. However, atmospherically derived burdens entering the natural environment are more likely to reflect the particular PCB usage pattern, rather than total production, i.e. 'open' versus 'closed'

system usage. In addition, long range atmospheric transport will import PCBs from elsewhere to the UK atmosphere. This is presumably why PCB fluxes to Flaxmere increased prior to U K production (i.e. between the 1930s and 1954) (Sanders et al., 1992).

PCB composition There are three main changes in PCB composition over the depth of the core. At the bottom of the profile (below 21 cm; pre-1945), the mixture is almost entirely dominated by 3- and 4-C1 congeners. There are several possible explanations to account for this. These congeners are more volatile and more water soluble. For example, congener 18 is approximately 30 and 400 times more soluble in water than the hexa-CBs 153 and 138, respectively. These compounds may therefore have been subject to post-depositional transportation down the core, either by chemical diffusion, rainfall dissolution or through water table fluctuations, to contribute to the detectable concentrations present in the deeper slices. Diagenesis of higher chlorinated congeners in situ by microbially mediated anaerobic de-chlorination is not thought to be of any significance, since the low pH of the bog environment restricts bacterial prolifera- tion and the higher chlorinated congeners are not very susceptible to microbial breakdown.

Above 23 cm there is a change in congener mixture favouring those PCBs of >5-C1. This continues up to and including a sub-surface depth of 9 cm (i.e. during the rise and fall of U K PCB manufacture), Within these seven samples, congeners of _>5-C1 averaged 72 + 6% of the EPCB content. This feature is similar to that seen in the Esthwaite Water dated lacustrine sediment core (Sanders et al., 1992). It is worth noting that


Table 4. Individual and ~PAH fluxes, and ~PAH concentrations in Flaxmere peat with relation to depth

23

Depth Flux (ng cm -2 year -t) ]~PAH ]~PAH Flux (cm) (mg k g l) (ng cm -2 year -1)

Phen Anth Fluor Pyr Ben/ChryB(b)F B(k)F B(a)P B(ghi)P Cor Nap Ac/F1

0-2 7-0 7.6 9.9 4.1 11.7 8.2 7.6 3.5 0.6 3.5 8.7 0.6 1.25 72.9 2-4 12-6 14.6 15-5 9.7 20-4 15.5 13.6 6.8 1.0 6.8 14-6 1.0 1.36 132.0 4-6 9.5 7.8 8.4 3.9 14.6 6-7 6-7 3.4 0.6 2.8 7.3 1.1 1.30 72.8 6-8 5-0 5-3 6.6 2.3 8.3 3.0 4.7 3.0 0.7 2-0 4.7 0-7 1.48 49.2 8-10 6.4 6.4 6.8 2.4 8.9 3.4 5.1 3-4 0.7 2-4 4.7 0-7 1.51 51.0 10-12 8.6 4-8 7.9 2-4 13-0 5.5 6.5 5.1 1.4 2-7 7.9 0.7 1.94 66.6 12-14 9.3 4.3 8.3 2.5 19.1 7.7 7.7 7.7 2.8 4.0 9.0 0.3 1-93 59.6 14-16 7.6 5.3 18.8 3.3 36.0 10.9 11-9 15.5 6.9 8.3 17.9 3.3 4.41 145.8 16-18 3.5 7.1 28.6 4.6 57.9 20.5 18.4 29.3 12.4 8.8 26.5 5.3 6.31 222.8 18-20 5.6 3.9 22.1 3.1 36.2 16.5 11.4 23.0 10.0 6.7 10.5 4.1 9.31 153.2 20-22 3.1 3.5 11.4 3.6 22.0 14-4 10.4 17.6 5.3 5.3 8-3 1-4 19.78 248.2 22-24 6.6 6.0 32.9 6.6 64.4 44.8 26.1 54.3 18-9 13.3 26.2 5.2 25.88 305.4 24-26 10.0 7.9 27.3 7.7 49.3 32.7 20.1 41-7 13.9 12-0 22.9 4.7 23.95 250.2 26-28 9.0 3.9 19.6 3.9 31.1 18.2 13.0 23.5 7.3 5.5 11.1 1.9 16-26 148.0 28-30 5.0 3.1 16.7 2.7 27.8 11.6 8.7 17.4 5.0 3.4 9.7 2.0 10.11 113.2 30-33 1.9 2.0 12.1 1.7 19-2 4-9 4.2 8.8 2.1 1.7 5-3 1.7 6.88 65-6 33-36 1.8 2.3 10.1 1.4 15.2 4.9 3-1 6.6 1.6 1.5 4.3 1.8 6.98 54.8 36-39 0-8 1.7 8.6 0.7 7.9 2.6 1.4 2-5 0.8 0.7 1.7 5.5 3.82 30.0 39-42 1-3 1.8 7.4 0-6 3.8 1.7 1.2 1.i I-0 0.5 0.5 ND 2-56 20.1 42-45 2.2 0.9 5.9 0.8 13.2 1.4 1-1 0.9 0.2 0.5 0.4 ND 2.23 17.5

Naphthalene (Nap), acenaphthene (Ac), fluorene (F1), phenanthrene (Phen), anthracene (Anth), fluoranthene (Fluor), pyrene (Pyr), benz[a]panthracene (Ben), chrysene (Chry), benzo[b]fluoranthene (B[b]F), benzo[k]fluoranthene (B[k]F), benzo[a]pyrene (B[a]P), benzo[ghi]perylene (B[ghi]P), coronene (Cot).

although the percentage contribution from 3- and 4-C1 congeners remained fairly stable at these depths, there is a slight increase in the proportion of <4-C1 congeners between 19 and 9 cm (21-32%). This may reflect the actual input distribution at the time, indicating increases in loadings of lighter congeners. Alternatively, this could result from interception by the near-surface peat of congeners which are being volatilised from sub- surface material (i.e. a 'wicking' effect).

The most recent changes in distribution are the least dramatic. From a depth of 7 cm ascending to 3 cm (1984-1988), there is an overall increase in burdens of 3- and 4-C1 congeners in relation to >5-C1, such that both ~roups account for approximately equal propor- tions c.f the EPCB content. Finally, there is a decline in the lower-C1 species in the top surface layer. This may reflect outgassing of the lighter congeners, which domi- nate the vapour phase mixture of PCBs in the contem- porary atmosphere (Alcock et al., 1993; Halsall et al., 1993). These trends may infer a two-phase accumulation pattern in the bog, with the majority of post-depositional diagenesis occurring in the more dynamic surface zone, whilst the congener distribution in buried, well decomposed material may generally reflect the recalcitrant fraction of PCBs originally deposited.

Occurrence of PCBs in the deep slices As mentioned above, PCBs were not commercially produced until 1930 and not manufactured in the U K until 1954. Despite this, quantifiable levels of these compounds were detected throughout the core. This is not the first report of such findings. In their comprehensive study Rapaport and Eisenreich (1988) recognised signi-

ficant quantities of PCBs at depths corresponding to pre-production periods (up to 30% of the total core inventory) and proposed downward percolation by rainfall to account for this observation. The occurrence of similar pre-1930 concentrations of PCBs is seen in the Flaxmere core, constituting 11% of the estimated total post-1930 burden. It is possible that these concentrations have arisen partly as a result of post-depositional mobility similar to that discussed by Rapaport and Eisenreich (1988). As discussed above, the deep peat congener mixture supports this argument, since it is dominated by the 3- and 4-C1 species.

However, another contributory factor is contamination of the samples during air-drying in the laboratory. Contamination of samples of pre-1929 peats and soils collected prior to 1929 and stored sealed has recently been investigated by exposing different samples to air for periods of a few hours to many days (Alcock et al., 1994). Unexposed samples of peat (i.e. samples of wet peat which was not air-dried) contained little or no PCBs. Detectable levels of the lower chlorinated PCBs were present in samples which had been air-dried for only a few hours (Alcock et al., in press). As mentioned above, the lower chlorinated species dominated the mixture in air (Halsall et al., 1993). Consequently, all future work in this laboratory will be performed on undried samples straight from the field. Rapaport and Eisenreich (1988) analysed their samples wet, with no air drying stage.

P A H time trends Net fluxes of the 14 individual compounds quantified and concentrations are given in Table 4. There are


striking differences in the PAH loadings throughout the core. The deepest section (42--45 cm), corresponding to ~1775, has a net ]~PAH flux of 18 ng cm -2 year I. The first significant increase in flux occurred between 36 and 30 cm (approximately 1842-1864). This was fol- lowed by a further doubling of the flux at a depth of 29 cm, corresponding to 1886 (113 ng cm 2 year-l). These increased fluxes are thought to mark the onset of the Industrial Revolution (Sanders et al., 1993) The greatest increases in net flux occurred between the 29 and 23 cm interval (1886-1932), rising to a maximum at 23 cm (305 ng c m 2 year-l). EPAH net fluxes to Flaxmere remained high until the early-1970s, since when substantial reductions started and have continued to the present (to about 20% of the maximum).

The recent declines presumably coincide with the decline of heavy industry and implementation of clean- air legislation. A decline in fluxes was also noted at 19 cm, although the reason for this is unknown. Since the early-1970s, inputs have remained fairly consistent until ~1986 (49-66 ng cm 2 year 1). There is a surface effect comparable to that noted for PCBs.

Surface fluxes are only 24% of the maximum EPAH input, reflecting a substantially greater decrease from peak inputs than the 59% reduction seen in the surface sediment of the Esthwaite Water core from north-west England (Sanders et al., 1993). No other studies report- ing historical trends for PAHs in peat could be found, so it is impossible to draw comparisons with other data. However, the profile is generally consistent with results from UK sediments (Sanders et al., 1993). Cran- well and Koul (1989) describe a drop of 70% in PAHs over the same time scale for another lake in the Lake District, near Esthwaite Water. Neither of the lake studies showed such a dramatic fall in levels from peak loadings as this peat study. This may reflect differences in the sampling media. These lakes are known to receive some additional catchment (runoff) sources and the historical record of the core may become 'smeared' by sediment mixing, whereas inputs to the bog surface are considered to be entirely atmospherically derived. In summary, however, all three studies show significant declines in PAH inputs over the past two decades. These trends give clues about the source input function. Fossil fuel combustion has been the major source of PAHs to the environment over the last 100-150 years. Broadly, the Flaxmere trends fit the pattern of coal consumption in the UK, which peaked in the 1950s (Jones et al., 1989). However, changes in the pattern of use, the balance between coal, oil, gas and petroleum combustion and the effect of combustion conditions will have influenced PAH emissions from source. A shift in the percentage of total coal consumption used domestically (i.e. many small combustion sources) and for electricity generation (few localised large scale, effi- cient plants), 'cleaner' residential heating methods (such as natural gas and electricity), declines in heavy industry and the growth in nuclear and oil-fired power generation have probably all contributed towards the reduction in PAH fluxes in recent decades.

CONCLUSIONS

To date there has been little use of peat profiles to re-construct temporal trends of HOCs. This is some- what surprising, since peat binds HOCs strongly and inputs to ombrotrophic bogs are almost exclusively derived from the atmosphere. The greatest problems arise over accurate dating, because of post-depositional mobility of the radioisotopes. PCB and PAH time trends derived here are generally in good agreement with those obtained by other historical monitoring techniques for UK sites. As a consequence, there is now good and consistent evidence that the UK atmospheric burden of both groups of compounds has declined in recent decades. Because of the problems associated with air-drying of samples (either potential loss by volatilisa- tion from contaminated samples, or contamination of previously clean samples), all future work in our laboratory will be conducted on wet samples.

ACKNOWLEDGEMENTS

The authors are grateful to the following for their assistance and advice during this work: Professor R. S. Clymo, Queen Mary College, London, for the loan of the coring equipment; Professor F. Oldfield and Dr. N. Richardson, University of Liverpool, for assistance during sampling, and application of the CRS model to the radiochemical data; the Nature Conservancy Coun- cil for granting permission to sample, and the Natural Environment Research Council for funding this work under grant number GT4/89/AAPS/28.

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PCB and PAH fluxes to a dated UK peat core