Accumulation of polychlorinated biphenyls and polycyclic aromatic hydrocarbons in the snowpack of minnesota and lake superior

J Great Lakes Res 26(2):220–234 Internat Assoc Great Lakes Res., 2000 Accumulation of Polychlorinated Biphenyls and Polycyclic Aromatic Hydrocarbons in the Snowpack of Minnesota and Lake Superior Thomas P Franz† and Steven J Eisenreich* Department of Environmental Sciences Rutgers University 14 College Farm Rd New Brunswick, New Jersey 08901 ABSTRACT The winter snowpack is a significant reservoir of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), and may be utilized as a surrogate receptor for assessing net atmospheric deposition Seasonal snow cores were collected in late winter before snowmelt in northern and central Minnesota and at Eagle Harbor, Michigan on Lake Superior between 1982 and 1992 Snowpack concentrations of Σ-PCBs ranged from to 14 ng/L with no significant decrease in concentrations from 1986 through 1992 Σ21-PAH concentrations in 1989 and 1992 ranged from 35 to 3280 ng/L with significantly higher concentrations nearer urban areas Similarities between chemical accumulations in the snowpack and collection of integrated snowfall at Eagle Harbor support the hypothesis that dry deposition to accumulated snow is negligible at these remote locations Tributary discharges from spring snowmelt to Lake Superior in 1992 contributed to 11 kg of Σ-PCBs and 220 to 350 kg of Σ21-PAHs INDEX WORDS: PCBs, PAHs, snow, Lake Superior INTRODUCTION Atmospheric transport and deposition distributes chemical emissions from source regions to remote environments causing toxicological concern for the health of their biotic communities (Norstrom et al 1988; Muir et al 1988, 1990; Bidleman et al 1989; Hargrave et al 1992; Wania and Mackay 1993) Methods for assessing atmospheric loadings include mass balance modeling, the use of surrogate receptors, such as lake sediment, peat and snow cores, as well as direct measurements of precipitation and dry deposition Snow is an excellent tool for assessing atmospheric deposition Snowpacks in northern temperate and polar regions are a reservoir of accumulated chemicals that have been deposited by wet and dry processes over the winter Snow can account for to 40% of annual precipitation within the Great Lakes region (NCDC 1992) and about 75% of Arctic precipitation (Gregor 1990) Research on organic chemicals in snow has confirmed the long range transport of semivolatile organic compounds (SOCs) to polar regions (Peel 1975, Risebrough et al 1976, Tanabe et al 1983, McNeely and Gummer 1984, Hargrave et al 1988, Gregor and Gummer 1989, Patton et al 1989, Gregor 1990) However, few studies have determined concentrations of SOCs in snow from the upper Great Lakes region (Murphy and Rzeszutko 1977, Swain 1978, Strachan and Huneault 1979, Murphy and Schinsky 1983, Rapaport et al 1985, Boom and Marsalek 1988) and its relative contribution compared to other inputs Snow has been considered in mass balance models for PAHs and PCBs at Siskiwit Lake, Isle Royale, in Lake Superior (McVeety and Hites 1988, Swackhamer et al 1988) In Green Bay, the annual wet flux (snow + rain) of PCBs was included in atmospheric input calculations (Franz and Eisenreich 1993, Bierman et al 1993) However, in other mass balance efforts, annual wet deposition fluxes are based on rain concentrations (Strachan and Eisenreich 1988, Eisenreich and Strachan 1992) Omission of snow as a separate input pathway to the Great Lakes is because of a lack of information on SOC concentrations in snow within the region Snowpacks integrate various transport, scaveng- *Corresponding author: E-mail: eisenreich@envsci.rutgers.edu †Present address: Metropolitan Council Environmental Services, Research and Development, 2400 Childs Rd., St Paul, MN 55105 220 PCBs and PAHs in the Snowpack of Minnesota FIG Map illustrating location of sampling sites ing, and deposition phenomena in addition to various post-depositional diagenetic processes The concentrations observed are the net result of precipitation, dry particle deposition, gas exchange, and percolation Thus, the snowpack concentration CSnow is given by: Csnow = 221 Mass Wet + Mass dry + Mass Adsorption − Mass Volatilization − Mass Percolation Volume Precipitation − Volume Evaporation − Volume Percolation (1) Wet and dry (gas + particle) depositional processes, gaseous volatilization, and water percolation are the principal pathways whereby SOCs become first entrained and potentially lost in the snowpack A complete description of the important diagenetic procceses influencing accumulation is given in Franz et al (1997) This study was initially conducted to assess the similarities between atmospheric deposition and accumulations of PCBs and chlorinated pesticides in rural/remote peat bogs of North America (Rapaport et al 1985, Rapaport and Eisenreich 1988) Snow cores were collected from 1982 to 1985 in northern Minnesota during this phase of the study In 1986 and 1989, samples were taken to continue the chronological record and to compare PCB concentrations in snow to those in rain (Franz et al 1991, Franz and Eisenreich 1993) Field investigations in 1992 evaluated diagenetic processes within the snowpack (Franz 1994) and determined snow scavenging of atmospheric SOCs (Franz and Eisenreich 1998) The objective of this paper is to summarize the 1982 to 1992 snow data and to report the concentrations and regional variability of PCBs and PAHs in annual snowpacks Precipitation data from the Integrated Atmospheric Deposition Network (IADN) site at Eagle Harbor (Gatz et al 1994, Hoff et al 1996) are compared to snowpack concentrations to evaluate the importance of dry deposition And finally, snowmelt contributions to tributary loadings to Lake Superior during the spring snowmelt are estimated EXPERIMENTAL Site Description Snow was collected at four sites in Minnesota and at Eagle Harbor, Michigan (Fig 1) near the end of winter before snowmelt Table lists the loca- 222 TABLE Location Franz and Eisenreich Location of sampling sites and snow core characteristics Number # Cores Marcell State Forest, MN (Lat 47° 32′ N, Long 93° 28′ W) Year Water Equivalent Surface Area Snow Depth Depth (m2) (cm) (cm) Snow Density (g/cm3) Number of Accumulation Days until Sampled Precipitation during Accumulation (cm) Percent of Precipitation Sampled NA 112 12.3 92 5.35 NA 11.9 NA 5.55 NA 14.0 NA 16.7 ± 0.8 0.25 ± 0.02 11.3 ± 0.7 0.24 ± 0.01 76 86 67 ? 135 131 114 5.5 12.2 6.5 14.4 18.6 10.8 98 97 85 97 90 ± 105 ± 1981–82 0.5 NA 2 2 1982–83 1983–84 1984–85 1985–86 1988–89 1991–92 1.38 0.62 1.55 0.63 0.75 1.05 NA NA NA NA 66.5 47.5 1985–86 1.25 41 13.6 ± 0.3 0.34 ± 0.2 114 13.1 104 ± 1988–89 1.25 14.5–23.5a 6.8 ± 1.4 0.36 ± 0.01 102 10.5 109 ± 30 1988–89 0.5 67 10.5 ± 1.0 0.16 ± 0.01 57 11.8 89 ± GFBI, MN (Lat 44° 57′ N, Long 93° 39′ W) 1991–92 1.75 18.6 6.35 ± 1.5 0.34 ± 0.1 91 9.6 67 ± Eagle Harbor, MI (Lat 47° 28′ N, Long 87° 52′ W) 1991–92 (1/7/92) (3/21/92) 1.38 32.9 10.8 0.33 ± 0.1 46 12.1 89 0.6 1.5 30 17 11.0 5.4 0.37 0.32 74 46 120 12.8 12.1 24.9 86 45 66 Cedar Creek Natural History Area, MN (Lat 45° 19′ N, Long 93° 17′ W) Lake Itasca State Forest, MN (Lat 47° 13′ N, Long 95° 12′ W) Topb Bottomc Total 11.3 aSamples taken in meadow with small hillocks and depressions, some drifting snow Depths highly irregular and listed for each sample section of snowpack represents snowfall accumulation from Jan to 21 March 92 cBottom section is replicate sample of January snowpack that accumulated snow 23 Nov 91 to Jan 92 bTop tions of the sites and characteristics of the snow cores Within the Marcell Experimental Forest in northern Minnesota, snow was collected in an open meadow of 0.4 hectare surrounded by forest and maintained as a National Atmospheric Deposition Network (NADP) site At the Lake Itasca State Forest, sampling occurred in a meadow located near the shore of Lake Itasca, headwaters of the Mississippi River The Cedar Creek Natural History Area is located about 50 km NW of Minneapolis/St Paul in an agricultural region Sampling occurred in a grassy field used as an atmospheric monitoring site The Gray Freshwater Biological Institute (GFBI) is located approximately 35 km west of Minneapolis/St Paul in a suburban setting The IADN Eagle Harbor, Michigan site is near the northwest tip of the Keweenaw Peninsula Samples were taken within 50 m of Lake Superior Sampling Protocol All equipment was washed with Alconox and rinsed with tap water, Milli-Q® water (Millipore), acetone and hexane, or methanol and dichloromethane and wrapped in aluminum foil prior to transport to the field In the autumn of each year (1982 to 1986 samples), 1-m2 sheets of mil plastic were secured on the ground at each sampling location In subsequent years, no plastic sheeting was used because it was deemed unnecessary In late winter, a one-square-meter area was inscribed on PCBs and PAHs in the Snowpack of Minnesota the snow surface, quartered, and the snow on two sides removed to ground level to allow access to the entire core Duplicate snow cores were collected in 0.25 m quadrants in 110 L anodized aluminum cans and covered with an aluminum foil-lined lid Samples for dissolved organic carbon (DOC) and suspended particulate matter (SPM) were collected using a 6.5 cm i.d plexiglass tube and kept frozen in plastic bags Monthly IADN wet-only precipitation samples were taken at Eagle Harbor as described by Sweet et al (1993), Hoff et al (1996), and Hillery et al (1998) In the laboratory, the snow containers were weighed to determine water volumes Between 1982 and 1989, the snow was allowed to melt for to days within a walk-in refrigerator at 4°C Melted snow was then passed through an XAD-2 resin (Sigma Chemical Co.) column (glass cartridge 2.5 cm i.d × 20 cm) using a peristaltic pump at flow rates of 100 to 200 mL/min Particulate matter was trapped on glass wool plugs holding the resin within the column The empty snow container was rinsed with either acetone (1982 to 1986), or methanol and dichloromethane (1989) to collect adhered particles and compounds sorbed to the container walls These rinses were later added to the extract In 1992, the snowmelt was maintained at 4°C and filtered using a submersible pump, a stainless steel filter head, and precleaned 293 mm diameter glass fiber filters (GFFs) (Schleicher and Schuell No 25) The filtrate, collected in precleaned 65 L stainless steel tanks, was passed through a XAD-2 resin column as described The snow cans were rinsed with L of Milli-Q water and filtered with the remaining sample No solvent rinse of the cans was performed This method allowed the determination of both dissolved (XAD-2) and particulate (GFF) fractions within the snowmelt Subsamples for DOC and SPM were transfered to L glass beakers and allowed to thaw at room temperature while covered with aluminum foil Approximately 250 to 750 mL of the melt water was filtered through a 0.4 µm Nuclepore filter for suspended particulate matter (SPM) analysis The remainder was filtered through 47 mm GFFs with the filtrate collected in polyethylene bottles for DOC analysis Analytical Procedure Although sampling and analysis occurred over a decade, similar analytical procedures were em- 223 ployed with minor variations Basically, the procedure consisted of 24 hr sequential Soxhlet extractions of the XAD resin and GFFs using acetone and hexane, or methanol and dichloromethane Surrogate standards of mirex (1982 through 1986), or PCB congener #166 (2,3,4,4′,5,6-hexachlorobiphenyl) and d12 -chrysene (1989 and 1992 samples) were added to the resin in the Soxhlet prior to extraction to evaluate analytical recoveries The extracts and rinses were back-extracted with Milli-Q water to remove water soluble solvents, concentrated in a Kuderna-Danish apparatus with a solvent switch to hexane, cleaned and fractionated using a Florisil or alumina/silica column, concentrated in a Kuderna-Danish apparatus, and reduced with N gas to final volume Internal quantification standards (2,4,6-trichlorobiphenyl, IUPAC #30 and 2,2′3,4,4′,5,6,6′-octachlorobiphenyl, IUPAC #204; and deuterated PAHs d 10 anthracene, d 12 benzo(a)anthracene, d 12 benzo(a)pyrene and d 12 benzo(g,h,i)perylene) were added prior to final volume reduction in 1989 and 1992 samples The concentrated extracts were analyzed on either an Hewlett-Packard (HP) 5840A or HP-5890 GC with 63Ni electron capture detector (PCBs) or HP-5890 GC with an HP-5970 mass selective detector (PAHs) Selective ion monitoring and retention times were used to identify the PAH compounds using a 30m DB-5 (J & W Scientific), 0.32 mm i.d., 0.25 µm film thick glass capillary column Helium was the carrier gas with a linear velocity of about 33 cm/sec Injection was splitless with an initial column temperature of 50°C held for minute, then ramped at 25°C/min to 125°C and then at 10°C/min to 290°C and held for 10 Injection port and GC-MS interface temperatures were 290°C and 300°C, respectively Electron multiplier voltage was either 1,800 or 2,000 emv Compounds were quantified using either external (1982 to 1986) or internal standards (1989 and 1992) Details of analytical methods and GC-ECD instrumental conditions for PCBs are described in Rapaport et al (1985); Rapaport and Eisenreich (1988) (1982 through 1985 samples); Franz et al (1991) (1986 samples); and Franz and Eisenreich (1993) (1989 and 1992 samples) Nuclepore filters (SPM) were dried overnight at 50°C and placed in a dessicator prior to weighing on a Perkin Elmer Model AD-2 microbalance Dissolved organic carbon (DOC) was measured by IR following either persulfate-enhanced UV digestion in a Dohrmann DC-80 Carbon Analyzer or combus- 224 TABLE Franz and Eisenreich Summary of total PCB concentrations (ng/L) in snow Location Marcell, MN Cedar Creek, MN Lake Itasca, MN GFBI, MN Eagle Harbor, MI Number of Cores 2 2 3 2 1/7/92 (2) 3/21/92 Top (1) 3/21/92 Bottom (1) VWM (c) VWM (d) Total PCB Concentration Year 1981–82 1982–83 1983–84 1984–85 1985–86 1988–89 1991–92 1985–86 1988–89 1988–89 1991–92 1991–92 1.4 6.8 ± 2.7 8.5 ± 6.8 13.6 ± 5.0 1.9 ± 0.8 0.76 ± 0.44 1.3 ± 0.2 1.3 ± 1.0 1.4 ± 0.7 2.8 ± 0.8 2.3 ± 0.3 1.8 ± 0.5 2.02 1.45 1.84 1.70 SPM (a) (mg/L) DOC (b) (mg C/L) 6.1 ± 0.3 3.0 ± 0.5 0.96 ± 0.03 0.9 ± 0.2 18.6 ± 5.4 5.6 ± 1.4 2.1 ± 0.9 3.3 ± 0.1 8.8 ± 0.6 1.5 ± 0.3 2.7 ± 0.5 51 ± 14 (a) Suspended particulate matter in snow (b) Dissolved organic carbon in snow (c) Volume weighted mean of January and March top snow cores (d) Volume weighted mean of March top and bottom snow cores tion at 750°C in an Ionics Model 555 Total Organic Carbon Analyzer Quality Control/Quality Assurance Quality control and assurance (QA/QC) details are described elsewhere (Rapaport et al 1985, Rapaport 1985, Rapaport and Eisenreich 1988, Franz et al 1991, Franz and Eisenreich 1993, Franz 1994) Briefly, instrument detection limits (defined as 3x signal:noise ratio) ranged from 0.001 to 0.2 ng for PCB congeners (0.7 to 10 ng for Σ-PCBs) and from 0.01 to 0.1 ng for individual PAHs Matrix blanks accounted for ~10 to 20% of the sample mass for PCBs and ~5 to 10% for PAHs Breakthrough of dissolved SOCs was evaluated by two XAD columns in series The primary column recovered an average of 82 ± 12% (n = 5) of Σ-PCBs and 97 ± 3% of individual PAHs Annual average surrogate recoveries ranged from 71 to 108% for mirex or PCB congener #166 and from 74 to 89% for the PAH surrogate d12-chrysene Data for 1982 to 1985 are not corrected for surrogate recoveries or blanks Samples in 1986 were corrected for the recovery of mirex and the average mass from XAD Blanks Similarly, all PCB results in 1989 and 1992 were blank corrected after being adjusted for the recovery of surrogate PCB congener #166 The PAH results (1989 and 1992) are blank corrected but not adjusted for surrogate recovery RESULTS AND DISCUSSION Table lists the location of the snow cores and their characteristics Snow events at Cedar Creek and Itasca in 1989 and at Marcell and GFBI in 1992, had densities of 0.12 to 0.18 g/cm3 (Franz 1994) Seasonal snow cores exhibited densities ranging from 0.16 to 0.37 g/cm3 Cores from northern Minnesota, which experience few days with above-freezing temperatures during winter, had densities of 0.16 to 0.25 g/cm3, compared to central Minnesota cores with densities of 0.34 to 0.36 g/cm3 which experienced some melting These densities are similar to the 0.38 ± 0.03 g/cm3 density in cores from Canada (Strachan and Huneault 1979); 0.3 to 0.4 g/cm in Canadian Arctic snow cores (McNeely and Gummer 1984) and the 0.25 to 0.41 g/cm in cores from Sault Ste Marie, Ontario (Boom and Marsalek 1988) The Eagle Harbor cores exhibited densities of 0.32 to 0.37 g/cm3, similar to central Minnesota Based on daily precipitation records at nearby PCBs and PAHs in the Snowpack of Minnesota National Weather Service sites (NCDC 1992), the water retention efficiency of the snowpack relative to the amount of precipitation that occurred during the accumulation period was calculated The percent of precipitation sampled in the snow core (Table 1) is defined as the water equivalent snow depth relative to the amount of recorded precipitation Deviations from unity are attributable to sublimation, percolation of water to the ground surface, snow drifting, and snowfall variability between the snowpack sampling site and the snowfall recording site Water loss by sublimation was not significant during the winter Snow cores retained an average of 91 ± 10% of the precipitation that occurred during snowpack accumulation Obvious exceptions occurred in suburban MN (GFBI) and Eagle Harbor The seasonal snow core at GFBI exhibited low water recovery (67 ± 2%) that was attributed to significant snow melt Melting may not significantly increase the density of a snowpack if some water percolates out of the core The measured density then reflects the packing density of the remaining snow cover Cores at Eagle Harbor were obtained on January and 21 March 1992 to examine temporal changes in the snowpack Low water recovery (45%) was noted in the bottom section of the March core, a replicate sample of the January core This section had the same density as in January (0.33 ± 0.1 g/cm3), but half the water content The January and March cores were taken within m of each other and were visually similar with no obvious indication of melting PCB Snow Concentrations The concentration of total PCBs (Σ-PCBs) in seasonal snow cores from 1982 to 1992 ranged from 0.8 to 14 ng/L (Table 2) The coefficient of variation (RSD) amongst several sets of replicate cores averaged 41 ± 22% With the exception of Σ-PCB concentrations of ~10 ng/L in 1983 to 1985, concentrations were about to ng/L, similar to the values in Great Lakes rain (Hoff et al 1996) Winter deposition in terms of concentrations of atmospheric PCBs has not diminished significantly since 1986, a behavior reminiscent of atmospheric PCBs (Hillery et al 1997) It is now known that atmospheric PCBs measured at some IADN sites are decreasing with a half-life of about to years (Hillery et al 1997; Simcik et al 1999) Interestingly, IADN Lake Superior data at Eagle Harbor 225 not show any statistical decrease This agrees with measurements of atmospheric PCBs over and near Lake Superior which have not decreased appreciably (Baker and Eisenreich 1990, Hornbuckle et al 1994, Hillery et al 1997) Also, with the exception of Marcell in 1983 to 1985, there is no clear spatial variation among the sites suggesting a well-mixed atmospheric source signal The mean Σ-PCB concentrations are equivalent among the sites during any one year (p < 0.05) Samples collected within 50 km of the Minneapolis/St Paul metropolitan area at Cedar Creek and at suburban GFBI have approximately the same concentrations as those from remote northern Minnesota (Marcell and Lake Itasca) and at Eagle Harbor on Lake Superior The range of PCB concentrations in snow are similar to other values within the Great Lakes region (Table 3) and are similar to rain concentrations (Strachan 1990, Franz and Eisenreich 1993, Gatz et al 1994, Hoff et al 1996, Hillery et al 1998) The volumeweighted mean (VWM) concentration of Σ-PCBs in snowpack at Eagle Harbor in March was 1.7 ng/L The wet-only VWM Σ-PCB concentration from December through mid-March in IADN precipitation samples was 2.0 ng/L (Gatz et al 1994) In the 1992 snowpack, 47 to 80% of Σ-PCBs were in the particulate phase The di- and tri-chlorinated congeners were primarily in the dissolved phase (< 50% particulate), while the higher chlorinated congeners were predominantly in the particle phase PCB Snow Accumulations The mean concentration of PCBs in the snowpack and the water equivalent depth were used to calculate the winter accumulation (Fig 2) Winter accumulation of Σ-PCBs ranged from 0.13 to 1.0 µg/m2 No significant differences (p < 0.05) were found among the sampling sites in the snowpack deposition in 1982 and 1983 and from 1986 through 1992 Thus, no temporal or spatial differences in the regional deposition of PCBs is evident even at suburban sites (Cedar Creek and GFBI) within 50 km of Minneapolis/St Paul This suggests a nearly uniform atmospheric source signal throughout the region in winter with the accumulation of PCBs ranging from 0.2 to 0.4 µg/m2 since 1986 The apparent deposition of Σ-PCB reflected in snow accumulations are generally less than other snow deposition estimates from the Great Lakes region—range: ~0.4 to 3.5 µg/m (Murphy and Schinsky 1983, Swackhamer et al 1988, Franz and Eisenreich 1993) Snow accumulation in 1992 is 226 TABLE Franz and Eisenreich Concentrations of PCBs in snow in Great Lakes region PCB Concentration Year Location mean (range), ng/L Type of Snow Sample Reference 1974–76 Duluth, MN 50 Snow Events Swain (1978) ″ Isle Royale, Lake Superior 230 Snow Events ″ 1975–76 Ontario, Canada 18–43a Snowpack Strachan and Huneault (1979) 1975–76 Chicago, IL 212 ± 97 Snow Events Murphy and Rzeszutko (1977) 1982–83 Isle Royale, Lake Superior 17 Snowpack Swackhamer et al (1988) 1982–85 Marcell, MN 7.6 ± 4.4 (1.4–13.6) Snowpack This Study 1986 Madison, WI 12.4b Snow Events Murray and Andren (1992) 1985–86 Minnesota 1.6 ± 0.3 (1.3–1.9) Snowpack This Study 1988–89 Minnesota 1.7 ± 0.8 (0.8–2.8) Snowpack This Study 1988–89 Minnesota 2.0 – 6.5 Snow Events Franz 1994 1989–90 Green Bay region, WI (1.4 – 5.1) Integrated Snow Events Franz and Eisenreich (1993) 1991–92 Eagle Harbor, MI 2.0 (1.3 – 2.6)c Integrated Snow Events Gatz et al 1994 1991–92 Minnesota & Michigan 1.8 ± 0.4 (1.3–2.3) Snowpacks This Study 1991–92 Minnesota 0.7 – 7.9 Snow Events Franz 1994 aRange of means within various regions bEvent began as rain, then turned to snow, half of the precipitation amount in each form cVolume-weighted mean and range of wet-only precipitation between 12/3/91 to 3/17/92 for total PCBs for same congeners as analyzed in this study FIG Mean ⌺-PCB accumulation (µg/m2 ± one standard deviation) in snowpack from winter 1981 to 1993 The values given are the averages of two snow cores each similar to the upper range for Arctic winter accumulation of 0.01 to 0.3 µg/m2 (Gregor 1991) A direct comparison between rain and snow loadings can be made from precipitation studies at Cedar Creek in 1986 (Franz et al 1991) and Green Bay in 1989 and 1990 (Franz and Eisenreich 1993) The PCB flux from rain at Cedar Creek was 1.4 ± 0.3 µg/m2/yr Snowfall during the 1985–86 winter accounted for about 20% of annual precipitation at this site while contributing 0.18 ± 0.14 µg/m2/yr of PCBs Thus, snow contributed ~12% of the annual PCB flux At three sites near Green Bay, Lake Michigan, the mean Σ-PCB flux ranged from 1.0 to 2.0 µg/m2/yr for rain and 0.36 to 0.54 µg/m2/yr for snow (Franz and Eisenreich 1993) Thus snow was responsible for 22 to 27% of annual PCB loadings PCBs and PAHs in the Snowpack of Minnesota from wet deposition to Green Bay while accounting for 20 to 30% of annual precipitation in the region Dry deposition of PCBs to the snowpack was evaluated by comparing cumulative snowfall deposition versus accumulation in the snowpack at Eagle Harbor Cumulative wet deposition was calculated from the measured monthly IADN Σ-PCB concentrations and snowfall (water equivalent) during the 1991-92 winter (Gatz et al 1994) Cumulative snowpack accumulation is the sum of measured Σ-PCB deposition in the Eagle Harbor snow core taken in January and the top section of the March core that integrated atmospheric inputs from 23 November 1991 to January 1992 and from January to 21 March 1992, respectively Cumulative snow deposition is the sum of the integrated snowfall measurements at the IADN site collected monthly from December 1991 through 19 March 1992 No significant difference (p < 0.05) was observed between the cumulative snow deposition of PCBs (0.32 ± 0.05 µg/m2) and accumulation within the snowpack (0.40 ± 0.11 µg/m2) This suggests that falling snowfall is the dominant source of PCBs in snowpacks Thus if gaseous PCBs are sorbed to ice/snow crystals, it likely happens during snowfall Atmospheric concentrations of S-PCBs seldom exceeeds 60 pg/m3 in the cold of winter Comparison of Σ-PCB annual snow accumulation of ~ 0.4 µg/m2 to other fluxes in Lake Superior is informative The surface sediment accumulation rate of Σ-PCBs is ~1 to µg/m /y (Jeremiason et al 1994), and the atmospheric loading is ~1 µg/m2/y from wet and dry particle deposition and ~5 µg/m2/y if PCB gas absorption is included (Hoff et al 1996) Σ-PCB fluxes on settling particles in Lake Superior for this time period were about 18 µg/m2/y (Jeremiason et al 1998) However benthic recycling ratios of ~20 lead to observed sediment accumulation rates (Baker et al 1991, Jeremiason et al 1998) Thus Σ-PCB snow accumulation rates are comparable to assessed atmospheric deposition and surficial sediment accumulation rates PAH Snow Concentrations Total PAHs (Σ21-PAHs) as the sum of 21 individual PAHs ranged from 35 to 3,300 ng/L among 1989 and 1992 seasonal snow cores (Table 4) Replicate variability of all individual PAHs averaged 17 ± 13% Relatively low concentrations of Σ 21 -PAHs (35 to 120 ng/L) were found at the rural/remote sites Higher concentrations (Σ21-PAHs 227 230 to 3,280 ng/L) were found nearer the urban areas at Cedar Creek and GFBI Table compares the concentrations of PAHs in winter precipitation at a number of remote and urban locations Snowpack concentrations in Sault Ste Marie, Ontario at the eastern shore of Lake Superior (Boom and Marsalek 1988) are significantly higher than observed elsewhere and are attributable to nearby steel manufacturing In Portland, Oregon (Ligocki et al 1985a,b), PAH concentrations of the lower molecular weight species (< Pyr) in winter rain are higher than Lake Superior snow concentrations, while the higher molecular weight PAHs are in close agreement All other samples taken in the Lake Superior region are similar, although high concentrations of low molecular weight PAHs (< Pyr) were observed on Isle Royale (McVeety and Hites 1988) Filtration of 1992 snow samples determined that 28 to 100% of PAHs were associated with particulate matter Only the low MW PAHs acenaphthylene (Acy), acenaphthene (Ace), and fluorene (Flr) were found primarily in the snow filtrate The dominance of the particulate fraction of medium and high molecular weight PAHs in winter snowpack suggests that snow scavenging of soot particles is likely the primary atmospheric removal mechanism However, Schmitt (1982) suggested that the importance of particle scavenging diminishes with distance from urban sources as particulate emissions are efficiently washed out close to the source The data in Table supports this observation such that the proportion of particulate PAHs in rural snowpacks (~80%) is somewhat less than found in suburban snow (~98%) PAH Snow Accumulations The winter accumulation of Σ 21 -PAHs ranged from 4.7 to 13 µg/m2 at the remote sites in 1989 and 1992 and from 20 to 210 µg/m at the suburban sites Urban sources contribute significantly to the suburban snowpack Deposition is much less than estimated annual emissions of PAHs in the Great Lakes region which range from approximately 400 to 6,400 µg/m2/yr (Johnson et al 1992) At Eagle Harbor, the deposition of PAHs in the January 1992 snowpack was similar to that calculated from the VWM concentration of the top and bottom sections of the March snowpack This suggests that losses from meltwater percolation during this period equaled gains from subsequent snowfalls A comparison of the cumulative IADN snowfall 228 TABLE Concentrations (ng/L) of PAHs in seasonal snow cores Marcell, MN 1988–89 PAH Symbol Mean SD Lake Itasca, MN 1988–89 Mean SD Cedar Creek, MN 1988–89 Mean SD Marcell, MN 1991–92 % Mean SD Particle Eagle Harbor, MIa 1991–92 % Mean SDb GFBI, MN 1991–92 % Particle Mean SD Particle Acy 0.9 0.1 0.5 0.1 0.4 0.1 0.3 0.1 31 0.4 0.1 61 1.6 0.1 89 Acenaphthene Ace 0.5 0.2 0.8 0.2 1.5 0.4 0.7 0.1 32 0.9 0.4 16 19 0.7 90 Fluorene Flr 1.5 0.2 1.4 0.4 2.4 0.6 0.9 0.01 35 1.1 0.6 24 33 1.5 91 1-Methyl Fluorene 1-mF 0.9 0.1 0.6 0.2 0.7 0.1 1.2 0.2 28 0.5 0.2 44 7.8 0.3 86 Phenanthrene Phen 6.2 1.3 13.3 0.9 31.4 7.7 6.6 1.2 51 6.2 3.0 52 449 18.4 93 Anthracene Anth 0.3 0.1 0.6 0.0 1.8 0.5 0.4 0.1 85 0.3 0.1 77 23 1.3 97 2-Methyl Phenanthrene 2-mP 1.3 0.3 2.1 0.2 3.9 0.6 1.2 0.2 64 1.3 0.4 63 61 2.8 94 Methylene mP 0.7 0.2 1.6 0.1 3.7 0.9 0.5 0.1 61 0.5 0.2 71 54 1.1 97 1-Methyl Phenanthrene 1-mP 0.6 0.2 1.2 0.2 1.8 0.3 0.6 0.1 63 0.6 0.2 62 36 1.9 94 Fluoranthene Fln 3.2 0.9 18.7 2.1 26.5 4.7 4.7 0.6 75 4.5 1.0 80 608 5.7 97 Pyrene Pyr 2.2 0.7 13.4 1.7 18.7 3.4 3.0 0.3 82 3.3 0.8 86 462 16.1 98 Retene Ret 1.0 0.4 3.4 1.7 5.9 0.6 1.5 0.2 96 0.6 0.1 95 4.0 0.1 99 Benzo(a)anthracene BaA 0.7 0.2 3.7 0.2 6.4 1.3 0.8 0.01 95 0.7 0.1 99 118 45.6 100 Chrysene Chr 1.5 0.4 8.8 1.2 9.3 0.6 1.9 0.1 92 2.1 0.2 94 231 8.8 99 Benzo(b+k)fluoranthene BFlns 3.4 1.3 18.3 0.7 45.3 14.1 3.4 0.1 99 3.4 0.3 98 478 17.4 100 Benzo(e)pyrene BeP 2.1 0.8 7.8 0.3 20.4 6.4 1.4 0.2 97 1.8 0.1 98 204 10.1 100 Benzo(a)pyrene BaP 6.6 0.6 11.8 1.5 20.8 3.5 9.4 0.2 100 3.4 0.4 99 150 4.4 100 Indeno(c,d)pyrene IDP 1.2 0.5 6.6 0.1 12.6 2.7 1.3 0.1 100 1.4 0.1 99 165 0.4 100 Dibenzoanthracene DBA 0.2 0.1 0.8 0.1 1.9 0.3 0.3 0.01 100 0.2 0.03 100 36 2.0 100 Benzo(g,h,i,)perylene BghiP 1.3 0.5 6.1 0.1 11.5 2.5 1.4 0.02 100 1.7 0.2 100 141 0.3 100 36 121 12 227 51 41 80 35 78 3280 139 98 Total PAHs (a) Volume-weighted mean concentration of top and bottom core on 3/21/92 (b) Standard deviation calculated from replicate coefficient of variation of January snow cores Franz and Eisenreich Acenaphthylene TABLE PAHs a b c d e Portland, ORa 1984 Rain Events Mean ± sd Isle Royale, LSb 1983–84 Snowpack Mean ± sd 37 ± 13 5.4 ± 2.0 14.4 ± 4.3 Sault Ste Marie, ONT c 1986–87 Snowpack Range Arcticd Narragansett Bay, RIe Eagle Harbor, MIf Rural/Remoteg 1988 1992–93 1991–92 1991–92 Snow Event Rain Integrated Snow Events Snowpack Particle Only Range VWM Range of Means < 50–150 < 50–98 < 50–237 0.05 0.7 1–21 0.2–3.7 1.5– 18 0.99 0.84 1.5 9– 95 0.6– 24 7.1 1.3 94 ± 29 5.1 ± 2.0 33 ± 10h 26 ± 3.0 ± 0.3 < 50–3,560 2.6 0.5 52 ± 20 43 ± 16 17.5 ± 1.4 10.4 ± 0.8 < 50–7,020 < 50–3,750 4.8 ± 2.4 11.5 ± 6.2 11 ± 12i 3.4 ± 3.7 3.0 ± 3.1 2.6 ± 0.3 7.6 ± 0.5 1.2 1.5 < 0.05 0.4 0.9 0.1 5.8 ± 0.3 3.2 ± 0.2 5.2 ± 0.6 < 100–560 < 100–500 6.0 ± 6.3 4.6 ± 0.6 < 100–470 Ligocki et al 1985 a,b McVeety and Hites 1988 Boom and Marsalek 1988 Welch et al 1991 Latimer (1994) < 100–1,640 f g h i 0.01 0.02 0.5 0.1 0.7– 8–57 5–50 0.6–5.7 2–13 2–27 1– 16 0.5–14 0.2–4.7 < 0.5–2.2 < 1–15 8.7 5.8 1.2 2.0 6.8 11.1 4.0 4.9 6.4 4.2 5.1 0.3– 0.9 0.5– 0.9 0.9–1.5 0.5–1.2 6.2–13.3 0.3–0.55 1.2–2.1 0.5–1.6 0.6–1.2 3.2–18.7 2.2–13.4 0.6–3.4 0.7–3.7 1.5–8.8 3.4–18.3 1.4–7.8 3.4–11.8 1.2– 6.6 0.2– 0.8 1.3–6.1 Suburban, MNg 1991–92 Snowpack Range of Means 0.4–1.6 1.5–19 2.4–33 0.7–7.8 31–450 1.8–23 3.9–61 3.7–54 1.8–36 27–610 19–460 4.0–5.9 6.4–120 9.3–230 45–480 20–200 21–150 13–165 1.9–36 12–140 PCBs and PAHs in the Snowpack of Minnesota Acy Ace Flr 1-mF Phen Ant 2-mP mP 1-mP Fln Pyr Ret BaA Chr Bb,kF BeP BaP IDP DBA BghiP PAH concentrations (ng/L) in winter precipitation at various locations IADN Data (Gatz et al 1994) This study Sum of methylphenanthrene isomers Sum of benzo(b+j+k)fluoranthene isomers 229 230 Franz and Eisenreich FIG Mean PAH accumulation (µg/m2 ± one standard deviation) in 1991–92 snowpack at rural sites (a) Marcell, MN, (b) Eagle Harbor, MI, and (c) the suburban Minneapolis, MN Gray Freshwater Biological Institute (GFBI) deposition at Eagle Harbor (Gatz et al 1994) of Σ17-parent PAHs with snowpack accumulation in this study yielded no significant difference (p < 0.05): deposition 12.7 ± 1.1 µg/m2; accumulation within the snowpack 9.7 ± 2.2 µg/m The agreement in the cumulative estimates between snowfall deposition and snowpack accumulation demonstrates that snow deposition dominates winter inputs to the snow cover in remote areas and that contributions by dry deposition are within the sampling variability of ±20% This is consistent with the low dry particle deposition of both PCBs and PAHs to exposed plates occurring in the winter of 1994–95 in the northern Lake Michigan basin (Franz et al 1998), and was largely attributed to low particle emisisons combined with minimum resuspension of soil particles in remote areas Figure compares the accumulation of PAHs in PCBs and PAHs in the Snowpack of Minnesota the 1991–92 snowpack at the rural sites of Marcell and Eagle Harbor and at the suburban GFBI location At both rural sites, phenanthrene and lower molecular weight species are more prevalent in the snowpack than at the suburban GFBI site emphasizing the importance of particulate deposition at the latter site The particulate fraction of phenanthrene and other low MW species is ≤ 0.5 in the snowpack at these sites In contrast, the strong particulate signal depicted in the GFBI snow core is > 85% of the mass for each PAH The proximity to urban particle sources and/or meltwater percolation out of the core of the more soluble PAHs may explain this pattern and the predominance of the particulate fraction The winter accumulation of PAHs estimated from the rural snowpack is similar to the winter deposition reported for Isle Royale (McVeety and Hites 1988) and the surficial sediment accumulation rates in Lake Superior (Gschwend and Hites 1981, Baker et al 1991), while the suburban snowpack accumulation brackets the annual wet deposition of PAHs in rural areas of Chesapeake Bay (Baker et al 1997) In the Arctic, the winter deposition of total PAHs is ~4.2 µg/m2 (Gregor 1991) similar to the remote values in this study (6.8 ± 3.6 µg/m2) Thus, the winter loading of PAHs in remote areas of Lake Superior is generally low and reflects continental background levels The similarity with Arctic snow accumulations suggests that the atmospheric signal transported to these regions may have similar sources or comparable source strengths Lake Superior Loadings Winter loadings of SOCs to Lake Superior occur by both direct deposition to the lake surface and indirectly by tributary discharge of snowmelt Assuming a winter deposition of 0.2 µg/m2 for PCBs from winter snowfall throughout the Lake Superior basin, ~16 kg directly entered Lake Superior over the winter of 1991–92, while ~26 kg accumulated within the snowpack of the watershed For Σ 21 PAHs, with a basin-wide deposition of 6.8 µg/m2, ~560 kg directly entered the lake and ~870 kg accumulated in the snowcover of the basin The concentration of chemicals in runoff during snowmelt is influenced by the rate of thaw, runoff/terrestrial interactions, and additional rain inputs (Cadle et al 1984a) Soluble compounds are released during early snowmelt periods, while particulate SOCs are released in final meltwaters (Schöndorf and Herrmann 1987, Quémerais et al 1994) Thus, there may be pulses of inputs to rivers 231 and lakes with the relative importance of the dissolved and particulate phases changing as snowmelt proceeds Biogeochemical interactions, partitioning to vegetative and soil surfaces or filtration of particles by ground litter, can reduce the concentration in runoff In addition, temporal and spatial variations in snowmelt and runoff over the drainage basin can disperse the concentration in runoff over longer periods (Hibberd 1984) To estimate riverine inputs of these chemicals during spring snowmelt, two tributaries were chosen for analysis The St Louis River drains 8,880 km2 of northeastern Minnesota entering Lake Superior at Duluth The Ontonagon River in the Upper Peninsula of Michigan enters the lake near Ontonagon and drains about 3,600 km2 The annual mean discharge of the St Louis River is ~64 m3/sec, representing about 8.8% of total tributary flow to Lake Superior The Ontonagon River represents ~3.6% of annual riverine water inputs to the lake with an annual mean discharge of approximately 26 m3/sec (D Dolan, International Joint Commission 1993, personal communication) Daily precipitation and snow records (NCDC 1992) from monitoring sites within each drainage basin were used to determine the mean snowpack depth and the snowmelt period Daily discharges for each river from the U.S Geological Survey (Have, M and Blumer, S., U.S Geological Survey, Water Resources Division, Minnesota and Michigan Districts, respectively 1993, personal communications) were used to calculate the volumetric discharge from each river during the 1992 spring thaw Total water equivalent accumulations (in cm) within the snowpack could then be compared to the discharged volume, normalized to the basin area (in cm), to estimate the percentage of water that entered the rivers during snowmelt relative to the amount that had accumulated within the watershed during the winter As a first approximation, runoff concentrations were assumed to be equal to the bulk snowpack concentrations and snowmelt occurred uniformly across the drainage basin Thus the total discharge from the onset of meltwater discharge to the first spring rain was summed A substantial amount of water was discharged with the onset of spring rains because of saturated soils, but this discharge was not included in this estimation In the St Louis River basin, snowmelt began in mid-March and lasted until the end of April, 1992 Peak discharge from snowmelt lasted about weeks from March to early May During this time ~470 × 106 m3 of water entered Lake Superior accounting 232 Franz and Eisenreich for 37% of the accumulated water in the snowpack If the concentration of PCBs in the discharge is similar to the snowpack at Marcell, MN (1.3 ng/L), then ~0.6 kg of PCBs entered the lake during this 6week period Assuming that the discharge from the St Louis River accounted for 8.8% of total riverine water inputs to Lake Superior, the total indirect loading of PCBs from snowmelt projected over the lake basin from all tributaries amounted to ~7 kg A similar estimate of Σ21-PAH loading from the St Louis River during snowmelt suggests that ~19 kg of PAHs were discharged from the river with ~220 kg projected from all tributaries in the lake basin Similar treatment of the Ontonagon River basin resulted in an estimated 224 × 106 m3 of water discharged during spring thaw or ~31% of the accumulated water in the snowpack Applying a PCB concentration of 1.7 ng/L from Eagle Harbor, ~0.4 kg entered the lake during the spring melt Assuming the Ontonagon River represents 3.6% of total tributary discharges, suggests that 11 kg of PCBs entered the lake with meltwaters For Σ-PAHs, kg entered the lake with Ontonagon River spring discharge and 220 kg from all tributaries Assuming that 30 to 40% of the snowpack water within the basin enters Lake Superior during a few spring weeks, the estimated Σ-PCB loading with meltwaters is to 10 kg, while total PAH loading ranges from 220 to 350 kg ACKNOWLEDGMENTS The authors acknowledge the technical support of C Sweet, I Basu, and K Harlin of the Illinois State Water Survey for their analysis of the IADN samples and M Auer at Michigan Technological University for his diligent operation of the Eagle Harbor site The authors wish to thank Dave Dolan of the International Joint Commission for discussions on Lake Superior tributaries and to Mark Have and Steve Blumer of the U.S Department of the Interior, Geological Survey, Water Resources Division from the Minnesota and Michigan Districts, respectively, for supplying daily discharge records for the St Louis and Ontonagon rivers The authors thank D VanRy for preparing the map This research was funded in part by: National Science Foundation, Grant No DEB 7922142; U.S Environmental Protection Agency, Great Lakes National Program Office, Grants Nos R00584001, R005038-01 and X-995786-01; the Great Lakes Protection Fund, Grant No FG6901029; and the NJ Agricultural Experiment Station of Rutgers University REFERENCES 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Canadian Arctic Environ Sci Technol 25:280–286 Submitted: May 1999 Accepted: 28 February 2000 Editorial handling: Paul V Doskey ... resuspension of soil particles in remote areas Figure compares the accumulation of PAHs in PCBs and PAHs in the Snowpack of Minnesota the 1991–92 snowpack at the rural sites of Marcell and Eagle Harbor and. .. tributaries in the lake basin Similar treatment of the Ontonagon River basin resulted in an estimated 224 × 106 m3 of water discharged during spring thaw or ~31% of the accumulated water in the snowpack. .. polychlorinated biphenyls to Green Bay, Lake Michigan Chemosphere 26:1767–1788 ——— , and Eisenreich, S.J 1998 Snow scavenging of polychlorinated biphenyls and polycyclic aromatic hydrocarbons in Minnesota