Environ. Sci. Technol. 1988, 22, 931-941
Historical Atmospheric Inputs of High Molecular Weight Chlorinated Hydrocarbons to Eastern North America Robert A. Rapaportt and Steven J. Elsenreich” Department of Civil and Mineral Engineering, Environmental Engineering Sciences, University of Minnesota, Minneapolis, Minnesota 55455
rn The accumulation rates of polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB) in ombrotrophic peat cores taken in the midlatitudes of eastern North America are used to reconstruct historical input functions. These profiles are unique in that they represent an unequivocal signal of atmospheric deposition. These data and those for the hexachlorocyclohexanes (HCHs) are combined with previously reported accumulation rates and integrated burdens for DDT and toxaphene to assess historical and spatial deposition patterns. The input (source functions) derived from peat profiles is consistent with production and use information in the U.S.The onset of compound accumulation rates in peat and the similarity in shape to production data demonstrate a rapid response time of the atmosphere and ecosystems to changes in input. Peat core profiles of high molecular weight chlorinated compounds appear to be true measures of the atmospheric, time-variant signal. Introduction Atmospheric transport of toxic organic compounds from source regions and subsequent deposition to receptors (land, water) is the most important pathway for distributing anthropogenic organic compounds globally (1-3). Methods used to determine spatial distributions of atmospheric contaminants include measurement of their concentrations in precipitation and the atmosphere, analysis of dated lake sediment cores, and application of the paradigm of mass balances to remote or large lakes. In recent years, significant advances in the methodologies and measurement of organic contaminants in rain (4-8) and the atmosphere (9-11) have dramatically improved our ability to assess deposition. Hites and co-workers (12-14) have utilized data cores from the Great Lakes and small, remote lakes to assess the atmospheric signal. Mass balance constructs have been applied to remote lakes for Pb and 210Pb(15,16)and polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) (I 7-19) to back out the atmospheric portion of total deposition. Although these techniques offer valuable information on atmospheric deposition, they are not able to assess both spatial and historical deposition of an unequivocal atmospheric signal. Our approach to assess the temporal and spatial trends in the atmospheric deposition of high molecular weight chlorinated organic chemicals involves the collection and analysis of dated Sphagnum peat cores from bogs across the midlatitudes of eastern North America. Peat bogs represent a nearly ideal setting for such an investigation since they are ombrotrophic, deriving their nutrients from the atmosphere, are isolated from groundwater and surface water flow (20),and have an organic matrix (>go% organic carbon dry weight) (21,22). The latter property enhances the sorption of hydrophobic organics to the organic matrix +Present address: Environmental Safety Department, Procter and Gamble Co., Cincinnati, OH 45217. 0013-938X/88/0922-0931$01.50/0
minimizing postdepositional mobility (23,24). Lake sediment cores have also been used to determine the historical record of contaminant input, but interpretations are complicated by direct input from nonatmospheric sources and dynamic lake processes including bioturbation, sediment focusing, and resuspension (25-27). A number of studies have focused on mosses and lichens as atmospheric monitors of anthropogenic metal pollution (28-31). Selected organic contaminants monitored in epiphytic moss (e.g., Hypnum) and other plant species were correlated with land use and industrial emissions (32). In our paper, accumulation rates for polychlorinated biphenyls (PCBs), hexachlorobenzene (HCB), and hexachlorocyclohexanes (HCHs) in peat are presented. Atmospheric input or source functions derived from accumulation rates are described for PCBs and HCB and compared to previously published data on DDT (33) and toxaphene (34). Lastly, the integrated areal loadings of DDT, toxaphene, PCBs, HCB, HCHs, Zn, and Pb in peat cores across the midlatitudes of eastern North America are discussed in relation to land use, urban and industrial centers, and dominant weather patterns. Chemical Deposition to a n d Diagenesis in Peat. Hydrophobic organic compounds such as PCBs are delivered to peatlands by wet and dry atmospheric deposition. Once deposited, they are subjected to various diagenetic processes such as active uptake by growing moss and advective and diffusive transport. A brief discussion of these phenomena focusing on peat is necessary to acquaint the reader with this new area of investigation. Atmospheric Deposition. Bogs receive inputs of chemicals only from the atmosphere. In theory, the processes involved in the delivery of atmospheric chemicals to peat include atmospheric vapor exchange and wet and dry deposition of vapor and particles. Wet deposition of chlorinated hydrocarbons (CHs) may occur by the scavenging of aerosol by or the vapor partitioning into precipitation. The relative importance of these processes is a function of the aerosol/vapor distribution, Henry’s law constant, particle size distribution, and intensity of precipitation (18). Dry deposition of particles is proportional to the deposition velocity (ud) and the chemical concentration in the particle phase. The deposition velocity is expected to be 0.1-1.0 cm/s for particles of mass median diameter of 0.1-1.0 pm. Particle sizes of less than 1 pm appear to dominate particulate-phase CHs in the atmosphere (35, 36). Deposition of particulate matter to tree canopies can be 2-16 times higher than those measured in open terrain (37). CH fluxes derived from snow cores collected in open and canopied sites in north central Minnesota showed no consistent or clear difference. For example, CDDT (DDT, DDD, DDE) fluxes for each winter from 1981-1982 through 1984-1985 were 0.33 f 0.24 pg/m2 per year (n = 7) for the clear cut area and 0.40 f 0.29 for the canopied site (38). On the basis of field measurements over the Great Lakes (3,18)and on CH concentrations measured at the Marcel1 site, the dominant input pathway to peat occurs via sca-
@ 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 8 , 1988 931
Table I. Locations of Peat Coring Sites in Eastern North America
date
latitude (north)
longitude (west)
Marcell, MN
1982, 1983; summer, fall
47'32'
93'28'
Diamond, ON
1982, summer
48'50'
80'38'
Alfred, ON
1981, summer
46'
76'
Lac St. Jean, QU
1982, summer
48'54'
71'54'
Croatan, NC Big Heath, ME Forchu, NS
1984, spring 1981, summer 1981, summer
44' 15' 45'12'
68' 15' 60'15'
site
mass accum rate, g/m2 per year
water table depth; cm
annual precipitation, cm
mean temp, 'C
605" 115b 630" 117b 390" 8gb 1215" 215b 94" 260" 1037" 350b
39
76
4
40
87
-0.5
33
104
35
99
1.2
5 50 37
127 122 128
17.9 7.4
Mass accumulation rate at core surface. Mass accumulation rate over entire core. From the surface of the bog hummock to the top of the water table.
venging of CHs on fine particles (3, 18, 39). Vapor scavenging by rain and particle fallout appear to be less important. However, atmospheric vapor partitioning into live surficial peat may be an important transfer process. Actively growing peat in hummocks (-2-5 cm deep) represents a relatively small exposure area and mass in comparison to the water-unsaturated hummock peat below; thus, total fluxes of CH to the bog are probably dominated by wet deposition. Diagenetic Pathways. Once CHs are added to peat, they undergo several forms of diagenesis. All materials deposited on the bog surface move downward as a result of the continual accumulation of fresh peat at the surface (300-1000 g/m2 per year; Table I). Depending on the sorptive nature of a chemical, it can be displaced downward by advection and/or diffusion. The sum of these processes is referred to as postdepositional mobility. Advection occurs in rain events where particles containing CHs may be washed downward by the water flux (40). Precipitation low in CH content could desorb CH from the overlying peat matrix and displace it downward. There is no biological advective mechanism operating in bogs with a parallel to lake sediments (25, 27). Calculation of advective fluxes on the basis of field data is complicated by a changing atmospheric signal, the rainfall intensity and frequency over time, and the growth rate of the peat. The rate of chemical diffusion in bogs is inversely related to the partition coefficient describing the sorption by the organic matrix, the porosity of the medium, and the chemical's diffusivity in water. The peat matrix is >90% organic matter by dry weight; partitioning is expected to be strong. Wet peat is about 90% water by weight, but an estimated 25-40% of the total pore space is unavailable (41). The high dead volume likely contributes to reduced diffusivity. In contrast to 13'Cs diffusivities in peat reported by Schell (31),peat matrix CH diffusivities as high as loF8cm2/s account for less than 10% of the CH mobility in cores where postdepositional mobility is evident (38). The diffusivity for CH is calculated considering a bulk water diffusivity of cm2/s (42) and a partition coefficient of 100. At least a partially advective mechanism is needed to explain postdepositional mobility in peat. Postdepositional mobility is evident in only a small fraction of the cores analyzed. Two other mechanisms may explain CH diagenesis in peat. The first involves the transfer of CHs from waterunsaturated peat to the atmosphere as surficial peat is buried by new growth. Under aerobic conditions, decomposition of the organic matrix results in the loss of peat mass (43-45). As evidence of decomposition, measure932
Environ. Sci. Technol., Vol. 22, No. 8, 1988
ments of Sphagnum peat accumulation rates in this study decreased from 2-4-fold in young surficial peat (
