Environ. Sci. Technol. 1998, 32, 936-942
Air-Pasture Transfer of PCBs GARETH THOMAS,† ANDREW J. SWEETMAN,† WENDY A. OCKENDEN,† DONALD MACKAY,‡ AND K E V I N C . J O N E S * ,† Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and Environmental and Resource Studies, Trent University, Peterborough, Ontario, Canada, K9J 7B8
A field experiment was conducted to study the air to pasture transfer of PCBs at a rural site in northwest England. Strong positive linear correlations were obtained between the log plant-air partition coefficients (m3 of air g-1 of plant dry weightsdefined here as the scavenging coefficient) and log octanol-air (Koa) partition coefficients. Pasture typically retained amounts of PCB per g dry weight equivalent to that in ∼7 m3 of air for congener 18 and ranging up to ∼64 m3 for congener 170, regardless of whether the pasture growth (exposure) time had been 2, 6, or 12 weeks. This indicates that airborne PCBs partition onto freshly grown pasture and approach plant surfaceair gas-phase equilibrium rather rapidly at this site, i.e., within 2 weeks of exposure. In late April-June, when grassland production is at a maximum, sequestering rates could approach 1.2 ng of PCB-18, 0.17 ng of PCB-170, and 12 ng of ∑PCB m-2 day-1. With 7 million ha of managed and rough grassland in the U.K., fresh pasture production in the spring and summer is estimated to remove an average of ∼0.8 kg of ∑PCB day-1 from the air during these times (∼80 kg of ∑PCB per growing season). Some buffering influence may be exerted on surface air concentrations during the most active periods of plant biomass production, while the incorporation of PCBs into pasture following air-pasture transfer processes controls the supply of PCBs to grazing animals and the human food chain.
Introduction There is growing interest in and awareness of the important role vegetation plays in the global cycling and food chain transfer of persistent, semivolatile organic contaminants (SOCs) (1). Because of the propensity for vegetation to retain gas-phase SOCs and to scavenge particle-phase SOCs, it can play a role in ‘cleansing’ the atmosphere (2, 3) and introducing SOCs, many of which can bioaccumulate, to natural and agricultural terrestrial food chains (4-8). Vegetation should be viewed as an important, dynamic, and active environmental compartment that influences the atmospheric transport, global turnover, and cycling of many SOCs or persistent organic pollutants (POPs). However, at the present time many uncertainties remain in our knowledge of the processes of air-vegetation exchanges of SOCs and in our ability to model these transfer processes (4-7). This largely arises * Corresponding author e-mail:
[email protected]. † Lancaster University. ‡ Trent University. 936
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 7, 1998
from a lack of detailed, mechanistic, field-based studies on the kinetics and fugacity relationships of air-vegetation transfer. To date, the focus has been on acute exposure studies in the laboratory and studies in controlled growth chambers that may not adequately mimic the processes of deposition in the field (8-10). The few field studies that have been conducted are typically limited to simple observations of the relationship between SOC concentrations in air and vegetation (e.g., refs 11 and 12); such investigations ignore potentially important environmental and vegetation factors that will influence the transfer processes. These shortcomings place uncertainties, at least in the short term, on the extent to which vegetation can be used as a biomonitor of spatial and temporal variations in atmospheric SOC concentrations (1, 13, 14). This paper reports the results from a detailed field study conducted to investigate and quantify the transfer of one well-characterized class of SOCs, the polychlorinated biphenyls (PCBs), from the atmosphere to an established sward of upland grazing pasture at a site in northwest England. Our interest was both in the propensity of the vegetation to interact with atmospheric PCBs, which have been well studied at this field site (15-17), and the supply of atmospherically derived PCBs to grazing livestock (4, 6, 7).
Experimental Section Field Site and Experiment. A field plot was isolated on an established sward of unimproved upland pasture at a semirural field station site owned by Lancaster University, in northwest England. The sward comprised Lolium perenne (∼30%), Holcus lanatus (∼30%), Agrostis capillaris (∼10%), Poa pratensis (∼10%), Cynosurus cristatus (∼10%), and a range of other grasses and forbs. A meteorological site and ambient PCB monitoring equipment have been based at the site for a number of years, and PCB air data have been reported previously (15-17). The field plot was established in early April 1996 and divided into 1 m2 subplots. Three management regimes were established on randomly selected subplots; ‘simulated grazing’ in which triplicate subplots were harvested every 2 weeks through the growing season between April and October 1996; ‘simulated silage production’ in which triplicate subplots were harvested at 6-week intervals (i.e., late May, late July and mid September); ‘long-term growth’ in which triplicate subplots were harvested every 3 months (i.e. in July, September, and January). Pasture yields (g m-2) were always recorded. Air samples were also taken over 1 or 2 week periods (∼600 m3 sampled, with a filter and polyurethane foam plug on a high volume air sampler; 15) and coincided with pasture harvesting. The glass fiber filters were spiked with PCB recovery standards before sampling commenced. Great care was taken to avoid contamination of the vegetation during sample handling, by avoiding an air-drying step and ensuring minimal air contact after sampling (18, 19). Samples were placed into bags in the field, immediately sealed, and frozen until required. The sample for analysis (30 g wet weight) was frozen with liquid nitrogen, ground with powdered sodium sulfate (50 g), spiked with labeled recovery standards ([13C] PCB-28, 52, 101, 153, 138, 180, 209), and Soxhlet extracted for 8 h with 250 mL of 4:1 hexane: acetone. The extract was first cleaned up on a silica/acidified silica double-layered column, eluted with hexane, and secondly on a Biobeads S-X3 GPC column, eluted with 1:1 DCM:hexane. Internal standards and keeper solvent (dodecane) were then added, and the final sample was taken down to 50 µL. Air filter and PUF samples were spiked with labeled S0013-936X(97)00761-X CCC: $15.00
1998 American Chemical Society Published on Web 02/24/1998
FIGURE 1. Congener concentrations averaged for all the two weekly air and pasture samples. recovery standards, extracted separately with hexane, and fractionated on a silica column. Quantification was achieved by GC-MS on a Fisons MD-800 in SIM mode with separation on a CPSil-8 50 m column. A total of 52 PCB congeners purchased as individual standards was screened in every sample. Pasture lipid contents were estimated by hexane extraction on the dry, ground samples. Recoveries of the 13C-labeled PCBs averaged 85%, ranging between 65 and 115%. Results were not corrected for recovery. An in-house reference material was run as every tenth sample. Duplicate analyses were performed on 10% of the samples; on average a 15% variation in ∑PCB was noted between duplicates. Method blanks formed 20% of the samples analyzed. Levels were found to be low, with trichlorinated and tetrachlorinated congeners being the only ones regularly detected. Trichlorinated congeners were present at a maximum of 20% of the levels found in real samples after blank subtraction.
Results and Discussion Air Concentrations. Air concentrations (sum of 52 congeners) varied between 54 and 288 pg m-3, with values in the warmer summer months (late May to late September) being generally higher, as reported previously for this site (16). The lighter congeners (e.g., 18, 31, 28, 33, 47, 52) were the most abundant; this is apparent in the averaged air pattern shown in Figure 1; virtually all of the atmospheric burden of the PCBs was associated with the PUF plugs (>93%) (i.e., was in the gas phase). This has been noticed previously, e.g., ref 14. Consequently, partitioning of the gas-phase PCBs to the plant surface is envisaged as being the dominant air-pasture transfer mechanism (7, 8). Pasture Concentrations after 2 Weeks Exposure. Pasture concentrations in the samples taken every 2 weeks varied between 620 and 3050 pg ∑PCB/g dry weight (equivalent to ∼10-180 ng of ∑PCB/g of plant lipid, given the measured lipid contents of 0.5-3.2%). On average, a 30% standard deviation was noted in the concentrations measured from different subplots sampled on the same day. Broadly, it is possible to define groups of congeners on the basis of abundances as follows: typically >100 pg/g DW, PCB-18, 31, 28, and 33; typically 10-100 pg/g, PCB-4, 37, 52, 49, 47, 44, 61/74, 66, 60, 101, 87, 110, 118, 105, 151, 149, 153,
138, 187, 183, 180, 170, and 194; typically 1-10 pg/g, PCB141, 128, 156, and 202; typically
