Environ. Sci. Technol. 1997, 31, 2937-2943
Transfers of Airborne PCDD/Fs to Bulk Deposition Collectors and Herbage KEVIN C. JONES* AND RAQUEL DUARTE-DAVIDSON† Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K.
PCDD/F air concentrations, deposition fluxes, and grass concentrations/offtakes (pg m-2 day-1) were measured concurrently in Bolsover, Derbyshire, England over 1 year during 1992-1993. The data are used here to discuss spatial and temporal differences in atmospheric PCDD/F concentrations and how the airborne PCDD/Fs transferred to the bulk deposition collectors and grass. Three sample sites are discussed, namely, a regional background site (RBS), an urban site (US), and a site close to an industrial complex (IS) that has been a suspected source of selected PCDD/F homologs. Generally, the mixture of PCDD/F 2,3,7,8substituted congeners and homologs was the same in the air, in the bulk deposition, and in the grass over a given sampling period and site. This suggests that PCDD/Fs of different levels of chlorination were transferring with similar efficiencies from the air to the collectors and grass, despite different gas-particle partitioning in the air. Deposition or transfer velocities for individual congeners/ homologs to the bulk collectors throughout the whole study ranged between 0.034 and 0.82 cm s-1 at the sites, averaging 0.27 cm s-1 for PCDD/Fs at the RBS. An experiment conducted to assess potential volatilization of soilborne PCDD/Fs from contaminated areas near the IS showed that all the compounds had a strong affinity for the soil and that volatilization is not likely to significantly contaminate herbage growing in the contaminated soil or the overlying atmosphere near this site. Herbage PCDD/ Fs were therefore derived almost exclusively from the atmosphere in this study. The similarities in the PCDD/F mixture in air and grass suggest that the different PCDD/ Fs are scavenged by the grass sward from the atmosphere with similar efficiencies; the implications of this are discussed.
Introduction Emission to the atmosphere, atmospheric transport, and deposition results in the movement of semivolatile organic contaminants (SOCs) from source areas to rural and remote locations; subsequent deposition onto vegetation and soils cleanses the air (1) and provides the first step by which SOCs (including PCDD/Fs) enter agroecosystems via the pathway: deposition, vegetation/crops/soil, grazing livestock, milk/ meat/dairy products, human diet, human exposure. This pathway is of major importance in controlling the general (background) exposure of the human population to PCDD/ Fs and other SOCs in industrialized countries (2-5). In the study area described here, cow’s milk collected from farms * Author for correspondence; e-mail:
[email protected]. † Present address: MRC Institute for Environment and Health, 94 Regent Road, Leicester, LE1 7DD, U.K.
S0013-936X(97)00133-8 CCC: $14.00
1997 American Chemical Society
near an industrial complex in Bolsover, Derbyshire, U.K. between 1990 and 1995 contained elevated concentrations of PCDD/Fs, in some cases exceeding the maximum tolerable concentration (MTC) of 0.7 ng of ∑TEQ/kg for U.K. cow’s milk (6,7). This study (8) is one of several to establish the potential source(s) of the PCDD/Fs to the local area (9) and to monitor the levels in key environmental compartments (6,7,10). Despite the significance of SOCs deposition, there are still uncertainties over the processes of air-plant and air-soil exchange, the choice of appropriate techniques to measure deposition fluxes, and the significance of atmospheric and soil inputs of SOCs to crop plants (1-5). The congener/ homolog-specific data arising from the monitoring program are therefore presented here to contribute to our general understanding to these issues and processes. Specifically, we present and discuss: (a) spatial and seasonal variability of PCDD/Fs in air; (b) bulk deposition fluxes, obtained using Teflon-coated frisbees at the air monitoring sites; (c) deposition or mass transfer velocities (Vd, cm s-1) derived from the air concentrations and bulk deposition flux data; (d) a study to assess the potential significance of soil-borne PCDD/Fs, arising from historical contamination inputs in Bolsover soils (9, 10), as a source of volatilized PCDD/Fs back into the local atmosphere and hence overlying vegetation; (e) PCDD/Fs in grass sampled concurrently with air and bulk deposition; (f) an evaluation of the air-grass transfer processes, using the data obtained under (e). It should be noted that the study was originally conceived and funded as a monitoring program rather than one specifically designed to rigorously study these processes; nonetheless, it has yielded information whichsif interpreted with caresmakes a useful contribution to this area of scientific enquiry.
Study Area Bolsover is a small town that lies between Sheffield and Chesterfield in an industrialized part of Derbyshire, northeast England. The area has a history of coal mining and heavy industry, although in recent years it has been affected by industrial recession. Coalite Products Ltd. operate two adjacent sites (hereafter called the industrial complex) in the Bolsover area. One produces smokeless fuel; the other processes chemical byproducts from the fuel plant, including the production of creosote, chlorinated oils, pitch, and phenols. Phenols are then chlorinated to produce materials used in antiseptics and chemical intermediates. An incinerator for the destruction of organic liquid residues was operated at the latter site, but this closed in November 1991 after the discovery of elevated PCDD/F concentrations in cow’s milk in the area (6). Her Majesty’s Inspectorate of Pollution (now the Environment Agency) commissioned a study to investigate whether operations at the industrial site were contributing to elevated levels of PCDD/Fs in the local environment. Some of these compounds were detected in flue gases emitted from the chemicals waste incinerator together with much lower emissions to the atmosphere from the stack serving the carbonization process and a smoke abatement stack at the smokeless fuels works (9). This was prior to the current monitoring program, which ran for 12 months beginning October 1992.
Materials and Methods Air Sampling. Seven air sampling sites were initially established for monitoring purposes (full details in ref 8), but only data from three sites are discussed here. These are the regional background site (RBS) about 6 km upwind of the industrial complex, an urban site (US) in the Bolsover town
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center about 2 km from the complex, and the industrial site (IS) closest to the complex (ca. 100 m from the main gate of the Chemicals Works) and located on a farm; milk samples from this farm had the most elevated concentrations of PCDD/ Fs (6-8). Each site was equipped with a high-volume pesticide air sampler (General Metal Works Inc., USA), comprising a Whatman glass microfiber filter (GF/A) backed by a polyurethane foam plus (PUF). Weekly samples of typically ca. 300-500 m3 of air were taken: the filters and PUFs were changed every week and stored at 4 °C, prior to bulking four or six weekly samples to obtain an integrated sample of ca. 2000-3000 m3 of air. A 2-µL mixed standard containing 333 pg/µL C-13 labeled 1,2,3,4-TCDD, 1,2,3,7,8,9HxCDD, and 1,2,3,4,7,8,9-HpCDF was applied as the sampling spike on the filters before they were placed in the field. Air sampling commenced on October 14, 1992, and finished October 26, 1993. Initially the weekly samples were bulked every four weeks, but from mid-February 1993 onward, they were bulked over periods of six weeks to comply with contractual requirements. Ten samples were analyzed from each site (8). Bulk Deposition and Grass Sampling. The design of the bulk deposition collectors used in Bolsover was chosen to match those used on the U.K. Toxic Organic Micro-Pollutants (TOMPS) program so that the fluxes measured in the Bolsover area could be placed in the context of those elsewhere in the U.K. (11). These ‘upturned frisbee’ collectors have been widely used for heavy metals studies (12). They have been tested for their collection efficiency for dry particulate deposition in wind tunnel experiments (13). Collection efficiency varies with wind speed, direction, and particle size and drops to 50 µm above wind speeds of ∼4 m s-1. The collectors used for trace organics studies have been modified by coating the surface with Teflon to better retain particulates (12). However, there may be a potential problem in using bulk deposition collectors of this design for SOCs where the organic compounds of interest have a significant vapor-phase component. The collectors may not mimic deposition fluxes to real environmental surfaces because (a) they are smooth and therefore less efficient at entraining fine particles; (b) PCDD/Fs, particularly the TCDDs, PeCDDs, and HxCDDs, have a measurable vaporphase component (14, 15); (c) PCDD/Fs in wet deposition can be adsorbed onto the sampler surface. It is uncertain how fluxes measured using these samplers are underestimated due to these three factors. The collector consisted of a 5-L glass jar in a lagged plastic container with a thermostat to prevent freezing. Inserted into the neck of the jar was a 2-m Teflon tube, onto which a Teflon-coated inverted Al frisbee collector was fitted. The whole sampler was rinsed with dichloromethane (DCM) and MilliQ water prior to its transport to the field; 25 mL of 5% CuSO4 solution was added to the jar as an algicide. A 2-µL sample of the mixed sampling spike was applied to a filter inside the glass jar before it was placed in the field. Six bulk deposition samples were collected at each site as follows (the number of corresponding bulked air samples is given in parentheses): October 28, 1992-December 8, 1992 (2); December 18, 1992-January 5, 1993 (1); January 5, 1993March 30, 1993 (2); March 30, 1993-June 22, 1993 (2); June 22, 1993-September 14, 1993 (2); September 14, 1993October 30, 1993 (1). Grass was also sampled at each site, and the yield was recorded. An area of grassland was fenced off in February 1993 to ensure that no animals grazed on the area; it was not cut initially until March 30, 1993, but thereafter 3 samples (from 1 m2) were collected from each of the sites, coinciding with the sampling dates given above. Sample Preparation. Samples were spiked with a mixture of labeled PCDD/F internal standards to correct for analytical losses and ensure quality control (namely, 3-6 ng of C-13
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labeled 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 1,2,3,6,7,8-HxCDD, 1,2,3,4,6,7,8-HpCDD, OCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDF, 1,2,3,4,7,8-HxCDF, 1,2,3,4,6,7,8-HpCDF, and OCDF). The deposition jars (i.e., rainwater, particulate matter, etc.) were filtered through a Whatman No. 1 filter paper, and the particulate fraction was dried under ambient conditions. The aqueous filtrate was then extracted with three 50-mL portions of DCM, evaporated to incipient dryness, and transferred to a Soxhlet thimble containing the particulate fraction. In brief, samples were Soxhlet extracted with toluene for a minimum of 16 h, evaporated to incipient dryness, dissolved in hexane, and washed with concentrated sulfuric acid and 20% potassium hydroxide (1:1). Further purification was carried out by passing the sample extract through a multilayered column containing sulfuric acid-impregnated silica, potassium hydroxide, and anhydrous sodium sulfate interfaced to an activated Florisil column. After elution of the interfaced column with 100 mL of hexane, the Florisil column alone was eluted with 50 mL of dichloromethane (DCM) to yield a PCDD/F-containing fraction. After solvent exchange and concentration to incipient dryness, the sample extract was transferred to a 200-µL sample vial for analysis. Vegetation samples were ground and Soxhlet extracted as just described. Blank values were also monitored. The ‘sampling’ blanks consisted of 100 mL of n-hexane and 200 mL of Milli-Q water for deposition samples and a Soxhlet thimble for the vegetation and soil samples. Extraction and cleanup procedures were carried out at Scientific Analysis Laboratories (SAL) Ltd., Manchester, on samples supplied ‘blind’ from Lancaster. Analysis. All the 2,3,7,8-substituted PCDD/Fs and the total homolog groups were quantified. Analysis was carried out using isotope dilution high-resolution gas chromatography/ mass spectrometry (GC/HRMS) on a VG 7070H mass spectrometer operated at a resolution of 5000 and interfaced with a HP 5790A gas chromatograph fitted with a J&W DB5 column (60 m × 0.25 mm i.d., 0.25 µm film thickness, and a head pressure of 30 psi). The data system was a VG 11250/J and samples were injected with a Shimadzu ADC 14 autoinjector. Full QA/QC details are available elsewhere (8). Detection limits for the various 2,3,7,8-substituted PCDD/Fs varied between ∼0.4 and 2.5 pg on column. This was equivalent to ∼1 fg m-3 for air, ∼0.1 ng kg-1 dry weight for vegetation, and ∼0.5 pg m-2 day-1 for deposition samples. Field blanks were always well below those detected in the corresponding batch of samples. Typical recoveries for sampling spikes were in the 70-95% range with internal standards varying between ∼50 and 80% for air and deposition and between ∼80 and 98% for vegetation (see ref 8 for full details). Experiment on Potential Volatilization of Soil-Borne PCDD/Fs. Surface soil (∼0-5 cm) was collected in 1994 from a grassland field at the industrial site. This site was selected because the soils here were shown by previous surveys (10) to have the highest elevated concentrations of PCDD/Fs in the Bolsover region. The soil (loam, 5% organic matter) was partially air-dried to enable it to be passed through a 2-mm mesh sieve in readiness for the experiment. The experimental design consisted of closed circulating systems with a chamber for the soil (Omnifit glass chromatography column with Teflon end pieces, length 250 mm, i.d. 50 mm), connected to a glassfiber on-line filter (to remove fine soil particulates), and a Teflon chamber containing precleaned solvent-extracted polyurethane foam (PUF) to trap vapor-phase PCDD/Fs. Air was circulated around the chamber by a Teflon-headed pump, and gas meters and flow meters were also installed. Teflon tubing was used throughout to connect the apparatus, and Teflon tape was used to seal connections. Different treatments were run simultaneously in replicate chambers. In one treatment (treatment 1), the soil was maintained relatively dry (∼24% moisture) at 20 ( 5 °C to simulate summer conditions; in another (treatment 2), the soil was maintained
TABLE 1. ∑PCDD/F and ∑TEQ Concentrations (pg m-3) in Air at Regional Background Site and Site Closest to Industrial Complex background site
industrial site
meana
range
meana
range
2,3,7,8-TCDD 1,2,3,7,8-PeCDD 1,2,3,6,7,8-HxCDD 1,2,3,4,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,7,8-HpCDD OCDD
0.0072 (4.2) 0.026 (9.2) 0.085 (4.6) 0.033 (1.8) 0.11 (5.9) 0.88 (4.9) 2.71 (1.5)
