Environ. Sci. Technol. 2004, 38, 715-723
Observations on Historical, Contemporary, and Natural PCDD/Fs N I C H O L A S J . L . G R E E N , †,‡ ASHRAF HASSANIN,† A. E. JOHNSTON,§ AND K E V I N C . J O N E S * ,† Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, U.K., and Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 2JQ, U.K.
PCDD/Fs were determined in samples of archived surface soils collected from different locations around the world in the early 1880s, in contemporary surface soils from around the world, in archived subsurface soils collected at Rothamsted Experimental Station in the 1870/1880s, and in sections of peat core deposited between 5000 BP and the present. PCDD/Fs were detected in most of the samples. The potential sources and implications of the levels and mixtures of PCDD/Fs present in the samples are discussed. The homologue and isomer patterns observed in most of the contemporary European surface soils are commonly observed for European air samples and soil samples. The homologue pattern in the Rothamsted surface soils collected in the 1800s was similar, suggesting that similar sources of atmospheric emissions of PCDD/ Fs were operating in the UK in the 1800s as currently. Very different patterns, dominated by OCDD and with contributions of HpCDD and HxCDD, were found in some other samples. It is proposed that the PCDD/Fs present in the subsurface Rothamsted soils, archived (1880s) surface soils from Illinois and the Congo, clay beneath the peat bog (deposited ∼5000 BP), and possibly surface soil samples from Thailand and Australia are of a natural origin. The most abundant TeCDD/F congeners measured in the peat samples here were also those observed by previous workers who studied a Canadian peat bog and are consistent with the microbially mediated oxidative coupling of chlorophenols. The study provides evidence for the widespread occurrence of PCDD/Fs in the environment prior to 1900 and for a complex array of sources (including natural) and environmental transformation processes.
Introduction The twentieth century saw a major rise and fall in the emissions of polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) to the environment. Typically, atmospheric emission fluxes into the environment increased in the early to mid-1900s, peaked in the 1960s to early 1970s, and have since declined (1, 2). The increases have been variously linked to many different human activitiessnotably the systematic * Corresponding author e-mail:
[email protected]; phone: +44-1524-593-972; fax: +44-1524-593-985. † Lancaster University. ‡ Present address: Department of Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden. § Rothamsted Experimental Station. 10.1021/es034599p CCC: $27.50 Published on Web 12/30/2003
2004 American Chemical Society
incineration of wastes, burning of fossil fuels, production of chlorinated aromatic chemicals, and metal smelting activities. However, the relative contributions of these anthropogenic sources are complex and have changed over time, and most are not unique to the twentieth century. Anthropogenic combustion of fossil fuels and metal smelting have been widespread human activities throughout Europe for centuries, for example. Whereas burning of fuel and waste may have increased markedly since the 1900s, large-scale combustion processes have become more efficient over time and may have had considerably higher PCDD/F emission factors (amount generated per unit fuel/waste consumed) in the 1800s than now. It is therefore not surprising that PCDD/Fs were present in European soils and sediments well before the 1900s (3-7). What is less clear, however, is information about the environmental burdens and sources of PCDD/Fs going further back in time and the role of natural PCDD/F formation. The purpose of this paper is to (i) expand the database of PCDD/Fs in surface soils sampled prior to 1900 (and archived since then) and from the present day; (ii) consider the potential sources of PCDD/Fs that were prevalent in the UK during the 1800s; (iii) present further evidence for naturally formed PCDD/Fs from subsurface soils and in a peat core going back over 5000 yr; and (iv) briefly discuss possible natural formation mechanisms of PCDD/Fs.
Materials and Methods Samples. Archived Surface and Subsurface Soil Samples from the Rothamsted Collection. We have previously demonstrated that appropriately sealed archived soil and vegetation samples from the Classical Agricultural Experiments at Rothamsted Experimental Station in the UK enable PCDD/F time trends to be ascertained (3-6). For this study, further soil samples from the collection were examined. Surface soil samples (023 cm) were collected from farms in the 1880s from four different parts of the world (Illinois, USA; Brandon, Canada; Winnipeg, Canada; former Belgian Congo, Africa) and a fifth location (India) sampled in 1914 and returned to Rothamsted for soil description and nutrient analysis. These have been stored in the archive. At one of the locations (Winnipeg), a subsurface soil sample had also been kept in the archive, and this was also analyzed. Soil cores were collected at Rothamsted Experimental Station itself during the 1800s, from the long-term “Classical (agricultural) Experiments” by hammering a square iron frame to a depth of 23 cm (6). The earth around the frame was dug away, and the soil within the frame was collected. The frame was then hammered a further 23 cm deep and the process repeated to collect up to 12 depths of soil, each one of 23 cm. These soil samples were dried and sieved before storage in glass jars with cork stoppers. The cork stoppers were sealed with wax and then capped with lead. Samples selected for PCDD/F analysis were removed from the archive and transported to Lancaster. The cap, seal, and cork of each jar were carefully removed in the laboratory, and a subsample of the soil was taken from below the sample surface. Approximately 100 g of dried soil was used for PCDD/F analysis. Samples were selected from depth series taken from three long-term experimental fields of the Rothamsted farm, Hoosfield, Agdell, and Barnfield to compare with data previously obtained at two other locations (6). Contemporary Surface Soils from Around the World. We have recently reported on the occurrence of PCBs and other organochlorines in surface (0-5 cm) soil samples, collected from ∼200 global background locations (8). Twelve samples VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. PCDD/F and PCB Homologue Concentrations (pg/g Dry Weight) in Archived Global Soils sample
Sylhet, India
Illinois, USA 0-23 cm
Illinois, USA
Brandon, Canada
Brandon, Canada
Congo, Africa
Congo, Africa
0-23 cm
duplicate
Winnipeg, Canada
Winnipeg, Canada
blank
depth
0-23 cm
duplicate
0-23 cm duplicate
monofurans difurans trifurans tetrafurans pentafurans hexafurans heptafurans OCDF
7.6 9.9 2.7 1.2 0.32 0.43 0.08
39 15 2.7 0.84 0.28 0.18 0.05 0.15
44 14 3.0 0.79 0.33 0.33 0.14 0.22
1.9 0.53 0.19 0.18 0.05 0.02 0.02 0.01
2.3 0.50 0.20 0.15 0.01 0.00 0.00 0.00
11 3.3 1.1 1.0 0.83 1.4 0.43 0.77
17 3.9 1.5 1.3 0.85 1.5 0.32 0.76
monodioxins didioxins tridioxins tetradioxins pentadioxins hexadioxins heptadioxins OCDD
0.03 0.08 0.08 0.23 0.11 1.1 0.42 4.6
0.43 0.48 0.34 0.76 1.5 8.1 21 380
0.65 0.57 0.32 0.47 1.3 7.1 19 370
0.03 0.13 0.18 0.43 0.37 0.61 0.08 0.56
0.44 0.16 0.21 0.36 0.29 0.55 0.16 1.6
0.17 0.29 0.82 2.6 1.4 9.6 58 1200
∑P(4-8)CDD/F 9 ∑P(1-8)CDD/F 29
410 470
400 460
2 5
3 7
1300 1300
∑-TrCB ∑-TeCB ∑-PeCB ∑-HxCB ∑-HpCB ∑-OcCB
n/a 180 23 8.9 2.2 0.1
190 54 11 9.5 2.6 0.2
440 160 22 5.8 1.1 0.1
32 16 4.1 3.4 1.1 0.1
∑-PCB
220
270
630
57
were selected from the global set for PCDD/F analysis, which had very low concentrations of PCBs, together with samples selected from western European background sites (9) for comparison. Peat Core. The Archaeological Unit of Lancaster University has conducted a detailed characterization of a 6 m core from a lowland peat site at Fenton Cottage, Over Wyre, Lancashire, UK (10). This site forms a record of peat accumulation spanning the second half of the Holocene (i.e., over the last 5000 yr), with human influence on landscape development evident since the mire began growing. A simplified description of the stratigraphy is given in Supporting Information Figure S1. Broadly, the stratigraphy is as follows: marine clays at the base, dating from the Neolithic and including abundant remains of Phragmites australis in its upper stratum. This underlies a wood peat layer, which is succeeded by a band of Scheuchzeria palustris/Eriophorum peat. Eriophorum-dominated peats lie above this with Sphagnum imbricatum peats at the top. Above these five strata, the Molinia-dominated top section is indicative of “modern” cereal agriculture from ca. 1500 A.D. on. A new core was obtained for this study. The surface 10 cm were dug away with a spade to expose the peat proper, as opposed to the current-growth grasses. Below this ambiguous layer the peat was well defined. Two cores were taken within 30 cm of each other using a Russian corer (11) with a barrel of 50 cm depth and 5 cm diameter. The process of removing each 50 cm section of peat destroys the 10 cm immediately below it. To counter this, 50 cm sections were taken alternately from the two cores. The outside of the collected material was cut away and discarded to counter the possibility of contamination by smearing of material as the corer was inserted. The exposed inner material was placed in precleaned glass jars and sealed with aluminum foil capped screw-threaded lids. Samples were frozen on the day of sampling and kept at -20 °C until required for analysis. Twenty-four samples were taken representing different eras of bog growth and varying environmental conditions (Figure S1). The core penetrated into the underlying marine silty clay, which lies ca. 3 m above 716
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0-23 cm 23-46 cm 14 4.4 0.99 0.35 0.26 0.21 0.12 *
11 3.3 0.80 0.97 0.19 0.24 0.20 0.13
0.07 0.09 0.02 0.01 0.00 0.00 0.00 0.00
0.35 0.36 1.3 2.8 2.1 12 67 1300
1.6 4.3 5.9 21 13 13 1.2 3.0
1.7 3.9 5.0 15 12 13 1.3 1.4
0.04 0.03 0.01 0.01 0.01 0.00 0.00 0.11
1400 1400
52 83
44 70
0.1 0.4
current sea level. Key archaeobotanical horizons of the current core were compared to the record generated for the previously studied core to confirm the archaeological progression and to provide estimates of dates. Analytical Methods. PCDD/Fs were analyzed according to the method described previously (6, 7). Briefly, for archived soils, the seal of the storage jar was broken carefully and a subsample (ca. 100 g) was taken from below the surface of the archived material. The subsample was spiked with 13C12labeled PCDD/F surrogates and Soxhlet extracted in toluene. The organic carbon content of the soils was determined by the Walkley-Black method. As such, carbonates did not contribute to the carbon data. Peat and contemporary soil samples had been stored frozen. They were thawed, subsampled (typically ∼10 g), spiked with 13C12-labeled PCDD/ Fs and PCBs, and extracted with DCM (soil) or acetone (peat) for 16 h then DCM for 8 h. The combined extracts were washed with water to remove acetone and dried over sodium sulfate, and the DCM was reduced in volume under reduced pressure. The DCM was passed through silica gel and then solvent transferred to hexane without letting the sample go to dryness at any time. The extract was cleaned up by gel permeation chromatography and adsorption chromatography, as described previously for sediment samples (7). Quantification of PCBs and PCDD/Fs was by HRGC/HRMS (6).
Results and Discussion Archived Surface Soils from Around the World. PCDD/Fs were detected in all the archived surface soil samples at levels between 20 and 10 000 times that found in the laboratory blank. In the soils analyzed for PCBs, lighter congeners were measured up to 10 times the levels found in the laboratory blank, with ∑PCB values between 240 and 650 pg/g dry weight. PCDD/F and PCB results are presented in Table 1. Three soils were analyzed in duplicate, and the results of these are also presented in Table 1; they show good repeatability. The homologue profiles of the sample with
FIGURE 1. PCDD/F homologue patterns in archived global soils.
TABLE 2. PCDD/F Homologue Concentrations in a Range of Contemporary Surface Soil Samples (pg/g Dry Weight) sample
Canada1
Canada2
Canada3
Brazil
Borneo
Australia
Bolivia
Thailand
Alaska
monofurans difurans trifurans tetrafurans pentafurans hexafurans heptafurans OCDF
