Environ. Sci. Technol. 1988, 22, 664-672
Deposition and Evaporation of Polychlorobiphenyl Congeners to and from Siskiwit Lake, Isle Royale, Lake Superior Deborah L. Swackhamer,+ Bruce D. McVeety,t and Ronald A. Hites" School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The relative importance of polychlorobiphenyl (PCB) transport into and out of lakes was studied by constructing a mass balance of PCB congeners in Siskiwit Lake, a remote lake in the Isle Royale National Park in Lake Superior. Measurements of winter and summer air, rain, snow, water, and sediments were obtained over several months and used to determine PCB fluxes to and from the lake, assuming steady-state conditions over 1 year. By solving the mass balance equation under selected conditions, we estimated the deposition velocity and the overall liquid water-to-air mass transfer coefficient for PCBs to be 0.16 cm/s and 0.1 m/day, respectively. Wet deposition was generally 3 times as great as dry deposition and was dominated by particle washout. Rain was a more important removal process than snow. Removal from the lake by volatilization was more important than sedimentation for most congeners. Introduction Hydrophobic organic compounds have long residence times in the atmosphere and, thus, can be transported for long distances. These materials leave the atmosphere by dry and wet processes. The former include the simple fallout of particles and the sorption of vapor-phase materials. The wet processes include rainout and snowfall of both particulate and vapor phases. Transport to the atmosphere from water surfaces can occur from evaporation of the dissolved aqueous phase. The relative importance of these processes is in debate. In addition, several of the parameters needed to model these processes cannot be measured directly and must be estimated; these include dry deposition velocities and air-water mass transfer coefficients. Although the wet deposition of hydrophobic organics can be measured directly with event-only collectors, dry deposition measurements are not easily obtained (1, 2). Methods that have been used in the past (such as glycerol-coated plates or pans, Teflon sheets, filter paper, or water in shallow pans) are not reliable and give data that are difficult to relate to the actual environment ( I , 2). Modeling of dry deposition processes cannot substitute for this lack of real-world information. For example, dry deposition velocities for particles of different sizes have been modeled on the basis of wind-tunnel studies (3-6), and dry deposition velocities for contaminants can be estimated from these models for a given wind speed and for a given density and size distribution of the contaminantassociated particles. However, these parameters are difficult to determine in the field. The difficulties of measuring the gaseous transfer of contaminants across the air-water interface are also substantial (7-9). In this paper, the deposition of PCBs to water surfaces will be addressed. Our approach has been to balance the wet and dry depositional inputs against the outputs of a 'Present address: Environmental and Occupational Health, School of Public Health, Box 197 Mayo, 420 Delaware St. S.E., University of Minnesota, Minneapolis, MN 55455. *Presentaddress: Battelle Northwest Laboratories,Richland, WA 99352. 884
Environ. Scl. Technol., Vol. 22, No. 6, 1988
lake that can only receive atmospheric inputs. This mass balance approach has been used before for total PCBs in DDT in Lake Michigan (15),and the Great Lakes (10-14), polycyclic aromatic hydrocarbons (PAH) in Siskiwit Lake (16). However, with the exception of our sister study (16), all of these mass balance studies rely on estimates of unknown parameters in the mass balance equation and draw on data from a variety of sources. In this paper, a mass balance of PCB congeners is presented for Siskiwit Lake located on Isle Royale National Park in Lake Superior. This mass balance is unique in that it has been constructed with a self-consistentdata set. We obtained all of the field data required by the mass balance, including air, water, rain, snow, and sediment measurements, and these data were used to calculate parameters that could not be measured directly. The behavior of 17 selected PCB congeners that cover a wide range of physical and chemical parameters was modeled. These congeners serve as model compounds for the behavior of similar hydrophobic compounds. This is the first time PCBs have been treated as individual compounds rather than as a mixture in a mass balance. By using self-consistent data for single PCB congeners, some of the uncertainty posed by drawing from a number of data bases is eliminated, and thus, a clearer picture of PCB behavior in the aquatic environment is provided. Siskiwit Lake, our study site, is a lake on an island within a larger lake (Lake Superior). Its remote location and lack of point sources made it an ideal "laboratory" for studying atmospheric deposition. Furthermore, its island location is relevant for understanding transport processes in the broader region of the Great Lakes. The mass balance approach assumed a steady-state one-box model, and it equated total sources and sinks of each compound. Thus Frain
+ Fsnow + Fdry,aerosol + Fdry,vapor
--
+ Fevaporation where F refers to the flux of compound for each process. Other potential losses such as photodegradation, microbial degradation, or metabolism by aquatic animals are considered negligible. Photodegradation in the atmosphere or surface waters may occur, but it is thought to be a minor process (17,18).Microbial degradation in sediments has been documented in the environment (19,20),but it is a long-term process with half-lives of decades. Because our mass balance included data from recent sediments, this process was considered insignificant. The mass of PCB associated with higher levels of the food chain is small compared to the mass in the water or sediment compartments, so that losses due to uptake or metabolism were ignored. Groundwater inflow was considered minor, and outflow through Siskiwit Falls was measured and found to be insignificant (16). The mass balance equation can be expanded and rewritten as (lo3 L/m3)(C,J, + CJ,) + [3.16 X lo5 m.s/(cm year)]CaVd = (io4cm2/m2)Cs,dpd/@ + (365 day/year)Ko1[C$2'/H - (lo3 L/m3)CJ (1)
0013-936X/88/0922-0664$01.50/0
Fsedirnentation
0 1988 American Chemical Society
Fwre 1. Map of Lake S
e showing I& Royale and Siskiwii Lake.
See glossary for defhition of t e r n . The air-to-water vapor flux and evaporation flux (last term on right) have been combined and expressed as the net vapor flux, where the direction of flux is dictated by the overall sign of the term. This is based on the thin film, two-resistance model (21-23). Recent evidence indicates that volatilization is a major process (11,14, 23, 24). All of the concentration terms were measured directly from field samples collected over several months. The p , d, @, and T terms were also measured. H constants for 15 "C (25) and J (16)were taken from the literature. The were estimated hy solving remaining two terms, V, and KO,, the mass balance equation under specific conditions. Methods Study Site. Siskiwit Lake is located on the south shore of Isle Royale National Park in northern Lake Superior (see Figure 1). Isle Royale is an archepegalo lying approximately 52 km southeast of Thunder Bay, Ontario. The island, as well as surrounding reefs and islets, was established as a national park in 1940, and thus, it has had m i n i anthropogenic activity. The area surrounding the
lake has never been mined or logged (26). The maximum length and width of the island are 70.8 and 14.5 km, and it rises 242 m above Lake Superior (26). Siskiwit Lake (see Figure 2) is a recharge lake with one outlet to Lake Superior through Siskiwit Falls. It is a dimictic, oligotrophic lake, approximately 11 km long having a 1.4 km mean width. The lake bed is part of the Precambrian Middle Keweenawan flows of the Portage Lakes Volcanics, and although it lies only 0.6 km from Lake Superior, it is 17 m higher. The mean depth and maximum depth are 21 m and 46 m, and its surface area is 16.8 km2. The hydraulic residence time is estimated to he 25 years, based on outflow measurements (16). The lake's watershed is 35-40 km2, lying in the same rock formation, and it consists largely of swamps and beaver ponds. The contribution to contaminant inputs from surface runoff is assumed to he negligible. Sample Collection. Sample collection took place from June through September 1983, and in January 1984. Space for sample handling was provided by the National Park Service on Matt Island. Air samples (1oo(t1400 ma) were collected every week during the summer sampling period by a battery-driven, high-volume collection system (Sierra-Misco, Berkeley, CA), modified with a polyurethane foam vapor collection system as described by Lewis and Jackson (27). Summer air samples were collected from a small island located approximately 750 m from the south shore, midway in the length of the lake (see Figure 2). Winter samples were collected from Fort Wilkens State Park in Copper Harlmr, MI, at the site of a weather reporting station of the National Oceanic and Atmospheric Administration. Seven winter air samples were collected over a 3-week period. Minimum and maximum temperature values were recorded continually during each sampling event. Vapor and particulate phases were collected separately, with phase separation operationally defined by glass fiber filtration (20 X 25 cm, Gelman type A/E). Rated filter retention is 99.9% for >0.3 pm and 90% for >0.1 pm particles. Vapor phase was collected by 10 X 10 em cylBB.15W
,",,,e
io""d."Ol ,Ir..,
,..I
8815OW
Flgure 2. Map of Siskiwit Lake on Isle Royale. The island sampling locatlon for rain and air collectors Is indicated. Numbers indicate sediment core sites. Water samples were collected at site 1.
Environ. Sci. Technol.. Vo1. 22, No. 6. 1988
665
indrical, polyurethane foam (PUF) plugs (27,28). Filters were cleaned before use by rinsing in dichloromethane and by ashing at 400 "C for 24 h. They were transported to the field in cleaned foil envelopes. The PUF plugs were washed with Alconox, rinsed with distilled water followed by acetone, and Soxhlet extracted for 12 h each with acetone, dichloromethane, and petroleum ether. Plugs were dried in a heated vacuum desiccator and transported to the field in cleaned tin cans. After sample collection, PUF plugs were returned to the cans, and filters were placed in clean glass Soxhlet thimbles and sealed in glass jars. Filters and PUF plugs were stored at -18 "C within 36 h of collection until analysis. Precipitation samples included all individual rain events during the summer months and a snow core obtained in midwinter. Rain was collected from the island sampling site by a battery-driven wet-only collector built by Aerochem Metrics (Miami, FL) to our specifications. The stainless steel collection funnel was 1m2and was covered by an automated stainless steel roof, which isolated the funnel during dry periods with a Teflon-coated foam seal. The rain sensor was a grid-plate conductor design. Rain was collected in a 45-L glass carboy that was precleaned with acid and solvent. The contents were collected after each event and transported weekly to Park Service Headquarters for extraction. A 1-m2snow core was obtained from the same site the following winter using an aluminum shovel and a Teflon collection bag. The sample was returned to Indiana University, thawed, and transferred to clean glass carboys for extraction. Lake water (18.6 L) was collected weekly from site 1(see Figure 2) from the epilimnion (3 m), metalimnion (8-10 m), and hypolimnion (25 m). Temperature profiles (2-m intervals) and Secchi depths were also recorded. Water was collected by an 8.2-L nickel-lined kemmerer bottle (Wildlife Supply Co., Saginaw, MI) and transported to Mott Island for extraction. Sediments were analyzed from locations 3 and 5 (see Figure 2) and were taken by a 0.1-m2Eckman brass box corer (Wildlife Supply Co., Saginaw, MI). Cores were taken in such a way as to obtain undisturbed surficial sediments. Four subcores were taken by polycarbonate tubes (8 X 30 cm), extruded, and sectioned in 0.5-cm intervals for the first 3 cm and 1-cm intervals down to 1 2 cm. The extruder and sectioning device was modified after Kemp et al. (29) and Fast and Wetzel (30). Surface sections that had high water content were removed by a large glass turkey baster. Corresponding intervals from the four subcores were combined and stored in clean glass jars at -18 "C until analysis. Prior to freezing, samples were homogenized and subsampled for in situ density measurements. Field and laboratory blanks were included with each sample matrix. Field blanks for air samples consisted of clean filters and PUF plugs carried to the field, installed in the sampler, removed immediately, and returned to the laboratory for analysis. Laboratory blanks were clean, unused filters and plugs. Field blanks for rain and water samples consisted of solvent taken to the field, opened, and returned. Laboratory bottle blanks were prepared by rinsing unused clean carboys with solvent and carrying the solvent through the analytical procedure. Solvent blanks were included with every nine sediment analyses as laboratory blanks. Sample Analysis. Air filters were Soxhlet extracted with 300 mL of dichloromethane for 24 h. Air PUF plugs were Soxhlet extracted with 1.5 L of petroleum ether for 24 h. Rain, thawed snow, and lake water samples were 666
Environ. Scl. Technol., Vol. 22,
No. 6, 1988
liquid-liquid extracted with 1 L of dichloromethane for every 18.6 L of sample. Samples were stirred rapidly for 24 h, and the solvent layer was removed by a Teflon syphon. Wet sediments (10-15 g) were Soxhlet extracted with sequential, 24-h extractions of 300 mL each of isopropyl alcohol and dichloromethane. All extracts were reduced in volume to approximately 1 mL by rotoevaporation and solvent exchanged to dichloromethane, if necessary. All extracts had interferences removed by silica gel and alumina column chromatography. Silica gel was preextracted with dichloromethane, activated at 180 "C for 24 h, and deactivated with 1% (w/w) water. Alumina was activated at 450 "C for 4 h and deactivated with 10% (w/w) water. Extracts were first passed over microcolumns (Pasteur pipets) containing anhydrous sodium sulfate over 1.2 g of silica gel and eluted with hexane (2 mL), followed by 1:l hexane-dichloromethane (2 mL), followed by dichloromethane (3 mL). Fractions were sufficiently clean to be combined and were exchanged to hexane. Sediment extracts required a larger column (1.5 X 18 cm) containing 18 g of silica gel and successive elution volumes of 25, 25, and 40 mL of the above solvents. All extracts were reduced in volume to approximately 4 mL and then passed over microcolumns of alumina and eluted with 6 mL of hexane. Elemental sulfur was removed from sediment extracts by passing them over a column containing 2-3 g of acid-activated copper grains. All extracts were reduced to 3-4 mL and stored at -18 "C until analysis. Immediately prior to analysis, extracts were reduced to 100-200 pL by a gentle N2 stream. Sample Quantitation. Extracts were analyzed for 17 PCB congeners by capillary column gas chromatography with a 63Nielectron capture detector. These 17 congeners were chosen to span a wide range of chlorination, molecular weight, solubility, and vapor pressure and because they were prominent in the samples and contribute significantly to the mass of commercial Aroclors. Congeners are indicated in Table I. The gas chromatograph was a Hewlett-Packard 5890 equipped with an HP 3392A integrator, an HP7673 autosampler, a 30m X 0.2 mm i.d. fused silica, DB-5 bonded phase column (J&W,Rancho Cordova, CA), splitless injection, hydrogen carrier gas (1.2 mL/min), 95% / 5 % aergon-in-methane makeup gas (20 mL/min), and an IBM X T personal computer. All raw data were visually examined prior to computer reduction. Column conditions were as follows: initial temperature, 100 "C; initial hold, 1min; ramp to 160 "C at 30 deg/min; ramp to 260 "C at 2 deg/min; and ramp to 280 "C at 10 deg/min. The injection port and detector were held at 225 and 325 "C, respectively. Individual PCB congeners were identified on the basis of retention times relative to a mixed PCB congener standard solution. A standard of the 17 congeners was run with each day's analyses. The presence of PCBs was confirmed in selected samples by gas chromatographic mass spectrometry (GC-MS) with a Hewlett-Packard 5985B. The GC contained the same column as above, with splitless injection and helium carrier gas. The column was programmed from 100 to 280 "C at 4 deg/min with an initial 1-min hold. The injection port and transfer lines were at 285 "C. The MS was operated in the electron capture, negative ion mode with methane enhancement gas and an ion source temperature and pressure of 100 "C and 0.35 Torr. PCBs were confirmed from full mass spectra of peaks found in the total ion chromatogram. Quantitation was done by the internal standard method. The total mass of each PCB in the sample extract was
Table I. Average Concentrations of Selected PCB Congeners in Air, Precipitation, Water, and Sediments of Siskiwit Lake’
IUPAC PCB congener no. 2,2‘,4 2,2’,5 2’,3,4 2,2‘,4,5’ 2,2’,5,5’ 2,2’,3’,4,5 2,2’,4,5,5‘ 2,3’,4,4’,5 2,2‘,3,4,4‘,5 2,2’,3,4,4’,5’ 2,2’,3,4’,5’,6 2,2’,3,3’,4,5,6’ 2,2’,3,4,4’,5,5’ 2,2’,3,4’,5,5’,6 2,2’,3,3’,4,4’,5,5’ 2,2‘,3,3’,4,4’,5,6 2,2’,3,3’,4’,5,5’,6 total PCB
17 18 33 49 52 97 101 118 137 138 149 174 180 187 194 195 201
summer air concn, ng/m3 vapor, particulate, n = 13 n = 13 0.15 (40) 0.27 (33) 0.15 (38) 0.053 (37) 0.062 (38) 0.010 (96) 0.049 (40) 0.011 (28) nd 0.0081 (63) 0.013 (45) nd nd 0.0033 (17) nd nd nd 2.8
winter air concn, ng/m3 vapor, particulate, n=7 n=7
0.00067 (78) 0.010 (62) 0.0033 (66) 0.048 (60) 0.0026 (58) 0.026 (62) 0.0020 (76) 0.012 (61) 0.0017 (81) ndc nd 10.0038 (77) 0.00097 (55) 0.014 (67) 0.00030 (57) 0.01 0.00035 (47) nd 0.00014 (97) 0.0036 (51) 0.00095 0.0068 (91) 0.000 36 (15) nd 0.000 16 (138) nd 0.00051 (70) 0.0021 0.00040 (180) nd 0.00094 (63) nd 0.00033 (141) nd 0.06 0.59
0.0036 0.00082 (82) 0.001 5 (57) 0.001 1 (97) nd 0.00040 (40) 0.0023 (100) nd 0.000 64 0.00067 (50) 0.00035 (66) nd 0.00082 (102) 0.00026 (7) nd nd nd 0.025
rain, ng/L, n = 12
snow, ng/L, n=1
water, ng/L, n = 11
0.38 (88) 1.1(89) 0.42 (81) 0.20 (85) 0.44 (111) 0.31 (154) 0.84 (79) 0.63 (89) 0.22 (74) 0.25 (87) 0.18 (102) nd 0.23 (234) 0.19 (76) nd nd nd 13
0.51 1.5 0.76 0.97 0.75 0.074 0.39 0.15 0.12 0.23 0.15 nd nd 0.085 nd nd nd 17
0.11 (128) 0.14 (159) 0.076 (201) 0.079 (143) 0.070 (83) 0.011 (81) 0.20 (123) 0.20 (144) 0.035 (57) 0.058 (128) 0.063 (103) 0.024 0.051 (65) 0.041 (106) nd nd nd 2.3
sediment, ng/g, solvedb n = 4 % dis-
25 25 24 18 18 14 14 14 11 11
10 7.7 7.8 7.4 6.0 5.9 5.7
1.5 (27) 3.4 (36) 1.9 (24) 2.2 (15) 1.9 (32) 0.34 (19) 1.1 (46) 0.87 (13) 0.60 (25) 1.0 (24) 0.50 (28) 0.15 (1) 0.37 (42) 0.24 (19) 0.15 (17) 0.070 (26) 0.30 (32) 48
Concentrations are geometric averages of data; coefficients of variation (in W )are shown in parentheses. bThe percent dissolved phase = not detected. was estimated by model. See text for explanation.
calculated from the ratios of the known amounts and measured GC peak areas of internal standard in the sample and standard solution, the amount and GC peak area of the congener of interest in a standard solution, and the GC peak area of the congener in the sample. The limits of detection for each sample matrix (expressed as final concentrations) were 0.014 ng/L, 1.5 X ng/m3, 7.8 X ng/m3, 0.15 ng/g, and 0.013 ng/L for rain, air vapor, air particulates, sediment, and water, respectively. The use of 4,4’-dibromobiphenyl, originally added to each sample prior to initial extraction, later proved unsuccessful as an internal standard. Instead, a PCB congener not present in commercial Aroclors or as an environmental contaminant (2,2’,3,4,4’,5,6,6’-octachlorobiphenyl) was used as the quantitation internal standard, and it was added prior to the alumina column chromatography in each case. We were able to monitor extraction recoveries of samples up to this step by quantitating the recovery of the dibromobiphenyl with the octachlorobiphenyl internal standard. Recoveries of the dibromobiphenyl averaged 81% with a standard deviation of 13% Spike recovery experiments with the alumina chromatography and Nz volume reduction steps indicted negligible losses resulted from these procedures. Reported concentrations are not corrected for procedural losses. Two sets of triplicate water samples were obtained and analyzed to evaluate our sampling and analytical reproducibility. The average coefficient of variation from these studies was 30%. Sediment Dating. Measurements of weight fractions of water and dry solids densities were used to calculate in situ densities (mass of dry sediment per unit volume sediment and pore water). One of the two composited cores (no. 3) was dated by 13Ts (31,32), pollen counting (33,34), and charcoal analysis (35,361. Core 5 was dated by 137Cs and 210Pb(32). The 13’Cs analyses were done by the Indiana University Cyclotron, and the 210Pbanalyses were done by S. Norton a t the University of Maine, Orono. Pollen counting and charcoal analyses were done in this laboratory.
.
Results Average concentrations and the coefficients of variation of PCB congeners in each environmental matrix are shown
in Table I. All concentrations have been blank corrected. Although there was temporal variability in the data, a yearly average was needed for the purpose of our model. Thus, measurements were geometrically averaged to approximate the seasonal or yearly average (see below). Vapor concentrations ranged from 0.3 to 0.002 ng/m3. (All concentration ranges are reported for PCB congeners arranged by decreasing vapor pressure and increasing chlorination.) Winter air vapor concentrations were lower than summer values. Less chlorinated congeners were enriched in the vapor phase, and several of the higher chlorinated congeners were not detected in the vapor phase, which is expected due to the decrease in vapor pressure with increasing chlorination (37). The decrease in vapor concentrations in the winter is also expected due to the temperature dependence of vapor pressure. Winter and summer particulate air concentrations were comparable to one another and ranged from 4 X 10” to loW4 ng/m3. While the overall distribution favored the less chlorinated congeners, higher chlorinated congeners occurred in summer particulate samples. For congeners present in both phases, an average of 96% and 88% occurred in the summer and winter vapor phases, respectively. These findings support previous studies that have reported that the vapor phase accounted for approximately 90% of the total PCB and that the vapor phase is enriched in the less chlorinated congeners (7, 11, 38, 39). Concentrations of PCBs were similar in rain and snow samples with a few exceptions. Values ranged from 0.07 to 1.5 ng/L. The congener distribution showed enrichment of the less chlorinated congeners, similar to air samples. The congener distribution in the air and precipitation samples may be due to the long residence times of vapor-phase PCBs and the distance from sources to Siskiwit Lake (40). The more chlorinated congeners, which are more prevalent in the particulate phase, may be depleted by the time the air mass reaches Isle Royale. Total washout ratios, defined as the concentration of a congener in rain divided by its concentration in air (vapor and particulate) multiplied by 1000 L/m3, ranged from 2.5 X lo3 to 1.5 X lo6 (2.2 X lo4 geometric average). These washout ratios are much higher than can be explained by air-water partitioning and Henry’s law constants. If rain concentrations are due solely to vapor scavenging, then the Envlron. Sci. Technol., Vol. 22,
No. 6, 1988 667
Concentration. ng/L
Concentration. ng/t
t
lo
E
0.1
:
0.01
1
Rain
O'l 0.01 1
1eo
160
I40
1
200
220
240
260
150
200
175
225
250
nqim3
0.5 1
0.1
0.01
0.01
:
1
Summer Air O.Ool
'
1o;
1BO
200
220
W i n t e r Air 240
Julion Day
'
260
0001'
15
20
25
Julion Day
Flgure 3, Temporal variations in concentration of three representative PCB congeners in Slskiwit Lake rain, water, and summer and winter air. The solkl, dashed, and dotted lines refer to congeners 18, 52, and 101, respectively.
washout ratio is given by RTIH. Since the measured washout ratios are much greater than RT/H, the main source of PCBs to rain is due to washout of the particulate phase rather than vapor scavenging (1). Washout ratios of less than lo5 in rain indicate that the aerosol is not efficiently attached to cloud water and is likely in the 0.1-1.0 pm size range (41). Congeners with lower rain washout ratios may be attached to different size particles or, more likely, undergo below cloud vapor scavenging to a greater extent due to their higher solubilities. Previous field measurements of total PCB washout ratios (rain) vary from lo3 to lo5 (42-44). Water concentrations ranged from 0.14 to 0.01 ng/L. Concentrations of nine of the same congeners measured in this study were also measured in Lake Superior waters taken in 1983 (45). The concentrations in Siskiwit Lake reported here averaged 2.5 times those in Lake Superior, probably due to the smaller lake volume and differences in particle populations, dynamics, and removal rates. Swain (26) also found that aqueous concentrations of total PCB were higher in Siskiwit Lake than in Lake Superior, although the values he reported for Siskiwit Lake were overestimated due to the interference of toxaphene in his quantitation. Higher total PCB levels were found in the Isle Royale region of Lake Superior compared to other lake regions by Cape1 and Eisenreich (46),who suggerst that prevailing winds from Thunder Bay, Ontario, may be a significant source to this region. The sediment concentrations given in Table I are the geometric average of the top two sections (top 1cm) from the two cores after correcting for sediment focusing. A sediment focusing factor was estimated for each core by comparing the total, integrated 13'Cs in the core to the amount expected for the Great Lakes region based on the historical input function (25.2 dpm/cm2, 47). Focusing factors of 2.1 and 2.3 were estimated for cores 3 and 5. 668
Environ. Sci. Technol., Vol. 22, No. 6, 1988
Concentrations ranged from 0.03 to 1.6 ng/g. Concentrations were more evenly distributed between lower and higher chlorinated congeners than in the other matrices. This suggests that processes within the water column have altered the congener distribution between source (atmosphere) and sink (sediment). The estimated total PCB concentrations are shown at the bottom of Table I. These estimates are included to allow comparison of these data with previously published data; they were calculated from the percent molar contribution of these congeners to Aroclor standards (48). The total PCB values for each of the compartments is in agreement with recent data for other remote sites (7,11, 14, 49, 50). The coefficients of variation for the air samples averaged 70%. There was greater variation in the rain and water samples (106%). Sediments showed the least variation (30%). Because sediments would not be influenced by temporal fluctuations, variations should be lowest for these samples. The changes in concentration with time are shown in Figure 3 for winter and summer air, water, and rain for three representative congeners. There are more temporal changes in rain and water samples than in air samples. Concentrations in rain are dependent on the length, time, and severity of the event in addition to air concentrations. Water concentrations are dependent on many physically, chemically, and biologically mediated processes over short time periods (hours-weeks); this contributes to greater sample variation.
Discussion Fluxes. The annual precipitation flux for each congener is calculated directly from C,, C,, J,,and J, as shown previously in the expanded mass balance equation. On the basis of historical records, the annual precipitation rate for this region is estimated to be 75 cmlyear with ap-
Table 111. Deposition Velocities for PCBs
Table 11. Fluxes of PCB Congeners into and out of Siskiwit Lake, ng/(m2 year)
vd,
congeneru 17 18 33 49 52 97 101 118 137 138 149 174 180 187 194 195 201
totalPCB
dry aerosolb vaporC
rain
snow
sediment
210 630 230 110 240 180 470 360 130 140 100 0 130 110 0 0 0
100 280 140 180 140 14 72 28 23 43 28 0 0 16 0 0 0
170 390 220 250 220 39 130 100 69 120 58 17 43 27 17 8 35
200 190 190 140 120 19 150 14 47 38 61 17 46 36 19 44 16
340 710 340 180 280 170 560 300 130 100 130
7300
3200
5600
3600
8500
Od
130 140 2d 36d -lgd
proximately 75% falling as rain (16). Thus, J, and J, are 56 and 19 cm/year. The rain and snow fluxes for each congener are shown in Table 11. Rain fluxes averaged 3.5 times as great as snow fluxes. Although snow fluxes are based on a single core, these values agree well with fluxes calculated from snow cores collected in north central Minnesota from 1981 to 1985 (51); thus, annual input estimates of hydrophobic organic compounds on the basis of extrapolation of only rain data may be overestimated. Sediment fluxes, shown in Table 11, were calculated directly from Csed, p , d, and @. Sedimentation rates for cores 3 and 5 were estimated to be 0.19 cm/year on the basis of the 210Pb,137Cs,pollen, and charcoal dating. In situ densities were 0.13 g/cm3 for the top two sections of core 3 and 0.13 and 0.14 g/cm3 for the top two sections of core 5. The calculation of dry particulate flux requires a deposition velocity for PCB-associated aerosols. This parameter is very difficult to measure directly because of the uncertainties in extrapolating the efficiencies of experimental dry collector surfaces to natural water surfaces. The deposition velocity is a function of particle size distribution, wind speed, and near-surface turbulence (1). While attempts have been made to measure v d for PCBs with surrogate surfaces, there is still considerable uncertainty in the estimated values. We were fortunate to have a means of estimating v d from our data: We simply solved the mass balance equation for v d for those congeners that were not detected in the vapor phase. When a congener is not in the vapor phase, the mass balance equation reduces to + Frain + Fsnow = Fsedimentation
method
ref
0.91" 0.18O 0.53 0.13' 0.06" 0.23" 0.5 0.75 0.16
mineral oil/Teflon plates glycerin-water/ aluminum pans theoretical model glycerin-water/glass plates glycerin-water/aluminum pans (Aroclor 1016 only) glycerin-water/aluminum pans (Aroclor 1254 only) estimated estimated mass balance (individual congeners)
56 57 2 42 43 43 8 49
b
" Particle and vapor phases not distinguished. This study.
aFor identification, see Table I. bThe summer and winter C, values were given equal weight. The flux was calculated with V , = 0.3 cm/s; see text. cCalculated by difference assuming inputs equals outputs. Summer concentration data used. Positive value indicates flux is from water to air. dCongeners174, 194, 195, and 201 were not detected in vapor-phase samples. Because an average Vd was used to calculate the dry aerosol flux, a vapor flux results from the balancing of the other flux terms.
Fdry,aeroeol
cm/s
(2)
and hence (3)
As discussed above, all of these terms are known. It was
Table IV. Estimated KO1for PCBs K0i9m/day
comment
ref
0.096 1.08 0.92 0.2 0.17-0.21 0.24 0.28-1.0 0.1 0.43" 0.16b 0.24 0.2 0.11
estimate from field data wave tank studies theoretical model model calibrated to field data theoretical model wave tank studies model model theoretical model theoretical model estimate from field data model mass balance
42 57 57 13 58 59 60 14 53 53 61 62 C
"Using wind speed of 4.1 m/s (16) and gas- and liquid-phase Schmidt numbers of 2 and 1000 (49). *Same as footnote a with average summer wind speed of 2.2 m/s (16). CThisstudy.
assumed that volatilization of congeners 174,194,195, and 201 was negligble. Solving the equation for these congeners yielded an average vd of 0.16 f 0.13 cm/s. This number is compared to other estimates in Table 111. Because of the uncertainty in this estimate, the geometric average of all estimates was used and yielded a v d of 0.3 cm/s. This value is reasonable for particles with a mass mean diameter of 0.1-1.0 pm at wind speeds of 1-15 mJs (2). The dry deposition fluxes, calculated with this v d , are shown in Table 11. Winter and summer air data (mean values; see Table I) were averaged. With four of the five terms known, the mass balance equation was solved for the net vapor flux. The net direction of flux was found to be from water to air for most congeners and is shown in Table 11. The overall liquidphase mass transfer coefficient, Kol, is another parameter that is not easily measured and was an unknown in our mass balance equation. We were able to solve the vapor flux equation for KO,in this manner: The dissolved-phase concentration of each congener, C1, was estimated from the three-phase models of Gschwend and Wu (52) and Baker et al. (45). The suspended particulate matter and dissolved organic carbon concentrations were estimated to be 1 mg/L, and the particulate organic carbon fraction was estimated to be 0.20 on the basis of similar oligotrophic lakes (50). Water-particle partition coefficients for each congener were taken from Swackhamer (50) and H values for 15 "C were from Burkhard et al. (25). The expected percent dissolved phase of each congener was calculated and applied to the water concentrations (see Table I). The predicted range of percent dissolved PCB is similar to that for Lake Superior (49). The resulting KO, value is shown in Table IV along Environ. Scl. Technol., Vol. 22, No. 6, 1988 669
MASS BALANCE FLUX. n o / m 2 v r
TOTAL FLUXES I N DRY PARTICULATE FLUX n S N O W FLUX
600 300 n
TOTAL FLUXES OUT FLUX. n /m2yr 120%
CONGENER
Flgure 4. Total fluxes In and out of Siskiwlt Lake for 17 selected PCB congeners. The congeners are ordered from left to right by decreasing vapor pressure.
with other estimates from the literature. Our value is well within the range estimated or modeled previously. One from this study was estimust keep in mind that the KO, mated by an annual gas-phase flux and summer water and air concentrations. Because ice covers Siskiwit Lake 5-6 months of the year, the annual gas flux applies to only half a year. Thus, the time-averaged Kolonly applies to lakes having ice cover for similar periods of time. Note that this would match the estimated KO, from Mackay and Yeuns (53)equations and the average summer wind speed on Isle Royale of 2.2 m/s (16) (see Table IV).In winter, Cl is likely to be much lower because of decreasing inputs from ice cover and depletion of PCBs in the water column by particle removal processes. Some of this may be balanced by particle resuspension due to seasonal mixing. The strong temperature dependence of H will give an overall increase in C$T/H even though winter C, values are lower than summer values. Thus, the concentration gradient may decrease or reverse direction in the winter. Mass Balance. The mass balance is completed by assembling all of the fluxes given in Table 11, and it is depicted in Figure 4. The precipitation flux is of greater importance than the dry particulate flux, and rain was more important than snow. The wet/dry flux ratio ranged from 27 to 1.5, with a geometric average of 3.3. Early measurements by Eisenreich et al. (11) indicated wet/dry ratios were 0.4. Slinn and Slinn ( 2 )estimated that wet and dry deposition of anthropogenic organic pollutants would be equal. Andren (8) estimated that the wet/dry ratio for PCBs was 9 on the basis of the Sehmel and Hodgson (3) model calibrated to zloPbfield deposition data (54). More recent estimates seem to agree that wet deposition is more important than dry (24,49),and a mass balance model for PCBs in the Great Lakes calculates an average wet/dry ratio of about 1.3 for the five lakes (55). Our field measurements indicate that wet removal processes are approximately 3 times more important than dry deposition of PCBs. The mass balance presented here indicates that removal of PCBs from Siskiwit Lake by volatilization is of equal or greater importance than sedimentation. The average loss to the atmosphere was 65% of total losses. This supports a similar finding for several remote Wisconsin 670
Envlron. Scl. Technol., Vol. 22,
No. 6, 1986
lakes (14). While the net annual flux is estimated to be from water to air, the scenario likely consists of intense, short-term input during precipitation events and gradual, long-term volatilization driven by the water-air concentration gradient during favorable weather conditions (24). Uncertainties. The reader has probably noticed that, although this mass balance was derived from a self-consistent data set, there is considerable variability in the individual fluxes. The coefficients of variation in Table I and the uncertainties of additional parameters were used to propagate the uncertainties in the model. This analysis indicated that the precipitation and sedimentation mass balance terms vary by approximately f l O O and 40%, respectively. The dry atmospheric particulate flux uncertainty includes the concentration error (f75%) and the error of the estimated v d parameter (2~170%).The V, estimate was more sensitive to rain and snow concentration than other terms of the model. The uncertainty in the volatilization term is calculated to be approximately f100%. The error for KO] is approximately f220%. The assumptions that other loss mechanisms are neglible may add additional errors. Conclusions The mass balance presented here serves as a useful approximation of the magnitude of PCB congener fluxes in and out of Siskiwit Lake. The estimate of v d was obtained by a method that is different from those in the literature, and it verifies the order of magnitude of previous estimates. This study supports the idea that PCBs are associated with submicron particles in air. From the magnitude of the KOland volatilization terms, it is likely that the atmosphere has been a major sink of PCBs in recent years. This may have a marked effect on the global cycling of these contaminants. Because these compounds are being recycled through the atmosphere, the residence time of PCBs in the environment prior to permanent burial in the sediments may be much longer than previously expected. Acknowledgments We extend our sincerest appreciation to the National Park Service on Isle Royale for the use of their facilities and their cooperation throughout the study. The 'loPb dating of sediments was provided by S. Norton, University of Maine at Orono. 13'Cs dating was provided by the Indiana University Cyclotron. S. Safe kindly provided PCB congeners 149,174,187, and 201. We thank I. Basu for technical assistance and J. Dorset for construction of sampling equipment. Glossary
cr
vd Csed
concentration of contaminant in rain, ng/L annual precipitation rate of rain, m/year concentration of contaminant in snow, ng/L annual precipitation rate of snow, m/year surface level concentration of contaminant in aerosol, ng/m3 deposition velocity, cm/s concentration of contaminant in surficial sediment,
P
ng/g in situ density, g/cm3
Jr
c,
J, c a
d @
KO1
c,
sedimentation rate, cm/year sediment focusing factor, dimensionless overall liquid mass transfer coefficient, m/day surface level concentration of contaminant in gas phase, ng/m3
R T H C1
universal gas constant, 8.21 X m3.atm/(K.mol) temperature a t air-water interface, K Henry’s law constant, atmm3/mol concentration of contaminant in surface water dissolved phase, ng/L
Registry No. 2,2’,4-PCB, 37680-66-3; 2,2’,5-PCB, 37680-65-2; 2’,3,4-PCB, 38444-86-9;2,2’,4,5’-PCB, 41464-40-8; 2,2’,5,5’-PCB, 35693-99-3;2,2’,3’,4,5-PCB,41464-51-1; 2,2’,4,5,5’-PCB, 37680-73-2; 2,3’,4,4’,5-PCB, 31508-00-6; 2,2’,3,4,4’,5-PCB, 35694-06-5; 2,2’,3,4,4’,5’-PCB, 35065-28-2; 2,2’,3,4’,5’,6-PCB, 38380-04-0; 2,2‘,3,3’,4,5,6‘-PCB, 38411-25-5; 2,2’,3,4,4’,5,5‘-PCB, 35065-29-3; 2,2’,3,4’,5,5’,6-PCB, 52663-68-0; 2,2’,3,3’,4,4’,5,5’-PCB,35694-08-7; 2,2,3,3’,4,4’,5,6-PCB, 52663-78-2; 2,2,3,3’,4’,5,5’,6-PCB, 68194-17-2.
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terson, S., Eisenreich, S. J., Simmons, M. S., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; Chapter 21. Bopp, R. F. J . Geophys. Res. 1983,88, 2521-2529. Capel, P. M.S. Thesis, University of Minnesota, Minneapolis, MN, 1983. Astle, J. W.; Gobas, F. A. P. C.; Shiu, W. Y.; Mackay, D. In Sources and Fates of Aquatic Pollutants; Hites, R. A.,
Eisenreich, S. J., Eds.; Advances in Chemistry Series 216, American Chemical Society,Washington, DC, 1987;Chapter 111. Received for review February 26,1987. Accepted December 18, 1987. This project was funded in part by the US.Department of Energy (Grant 80EV10449).
Characterization of Aerosol Organics by Diffuse Reflectance Fourier Transform Infrared Spectroscopy Robert J. Gordon,” Nirupam J. Trlvedi, and Brij P. Singh Global Geochemistry Corporation, 69 19 Eton Avenue, Canoga Park, California 9 1303
Elizabeth C. Ellis Southern California Edison Company, 2244 Walnut Grove Avenue, Rosemead, California 9 1770
Complete analysis of urban aerosol samples has been difficult because of the complexity of the organic portion. Previous techniques have seldom accounted for more than a few percent of the total mass of the organic matter because they determine individual components, of which there are hundreds in urban aerosol. We have instead characterized fiie-particle (
