Spatial Variations and Chronologies of Aliphatic Hydrocarbons in

PAUL V. DOSKEY*. Environmental Research Division, Argonne National. Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439. Four sediment cores ...
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Research Spatial Variations and Chronologies of Aliphatic Hydrocarbons in Lake Michigan Sediments PAUL V. DOSKEY* Environmental Research Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439

Four sediment cores were collected in fine-grained depositional areas of the southern basin of Lake Michigan. Spatial variations of aliphatic hydrocarbons in surficial sediments were consistent with a lakeward movement of riverine sediments in a series of resuspension-settling cycles in which an unresolved complex mixture (UCM) of hydrocarbons associated with dense sediments is deposited in nearshore areas, fine-grained sediments of terrestrial origin accumulate in the deep basin, and planktonic hydrocarbons are depleted by microbial degradation during transport to the deep basin. The rate of accumulation of the UCM (a marker of petroleum residues) in deep basin sediments has increased by more than an order of magnitude since 1880, from 60 µg m-2‚a-1 to approximately 960 µg m-2‚a-1 in 1980. Crude estimates of the atmospheric loading of the UCM (1100 µg m-2‚a-1) indicate that accumulations in deep-basin sediments might be supported by atmospheric deposition. Agreement was poor between the atmospheric flux of the terrestrial n-alkanes (∑C25, C27, C29, C31) to the deep basin (3200 µg m-2‚a-1) and the sediment accumulation rate (660 µg m-2‚a-1). Understanding of atmospheric fluxes, estimated from the very few available data, would be improved by more frequent measurement of the levels of aliphatic hydrocarbons in air and precipitation and a better knowledge of the particle deposition velocities and precipitation scavenging coefficients.

Introduction Atmospheric deposition is now recognized as the primary source of several classes of semivolatile organic compounds (SVOCs) in the Great Lakes. Although estimating atmospheric fluxes of the SVOCs from levels in air and the water column remains problematic, atmospheric loadings of polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs) in Lake Michigan have been inferred from accumulation rates in bottom sediments (1, 2). The semivolatile n-alkanes (C11-C32) are aliphatic hydrocarbons that have proven useful in investigations of air-water exchange (3) and in studies of the organic geochemistry of atmospheric particles (e.g., 4), settling sediments (e.g., 5), and bottom sediments of freshwater lakes (e.g., 6); however, data are insufficient to examine the spatial variation and chronology of this class of hydrocarbons in bottom sediments of Lake Michigan. * Corresponding author phone: (630) 252-7662; fax: (630) 2525498; e-mail: [email protected]. 10.1021/es001365m CCC: $20.00 Published on Web 12/01/2000

 2001 American Chemical Society

FIGURE 1. Locations of Lake Michigan sampling stations. The water column depth at each coring location is listed in parentheses. Aliphatic hydrocarbons in freshwater lake sediments have both biogenic and anthropogenic origins. Terrestrial plants and plankton are the prevalent biogenic sources. As for the PAHs, anthropogenic sources of aliphatic hydrocarbons include emissions from the transportation, storage, processing, and combustion of fossil fuels. Hydrocarbons from biogenic and anthropogenic sources have unique chemical signatures. For example, the predominant n-alkanes in nonsiliceous species of plankton are C15, C17, and C19 (7-10), whereas terrestrial plants contain mainly C21-C35 n-alkanes (11). Sediments containing petroleum residues exhibit an unresolved complex mixture (UCM) of branched and cyclic aliphatic hydrocarbons (12) with n-alkanes having a carbon preference index (CPI; 13) of 1 (14). This investigation was conducted to answer questions concerning the accumulation of aliphatic hydrocarbons in sediments of the southern basin of Lake Michigan: (a) Are spatial variations of hydrocarbon distributions in surficial sediments consistent with a lakeward movement of riverine sediments to the deep basin? (b) Are chronologies of anthropogenic aliphatic hydrocarbons in the deep basin consistent with the accumulations of other classes of organics at this location? (c) What fraction of the aliphatic hydrocarbons that accumulate in sediments of the deep basin is contributed by atmospheric deposition?

Experimental Procedures Sediments were collected in September 1980 at four sites in the areas of fine-grained deposition in the southern basin of Lake Michigan (Figure 1). The composition of sediments in these areas was described by Cahill (15) as silty clay. Undisturbed sediment cores (250 cm2 × 20 cm) were collected VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Concentrations (on the basis of dry sediment) of n-C17, pristane, and n-C29 vs. depth in four Lake Michigan sediment cores. with a box coring device. The surface sections of all four cores were light brown and very flocculent, but deeper sections were grayish black and consolidated. Immediately after collection the box cores were sectioned at 1- or 2-cm intervals, and the sections were transferred to solvent-rinsed glass jars and frozen. Details of the analytical methodology have been described elsewhere (16-18). Briefly, sediments were freeze-dried with a Virtis (model 10-100) freeze-drier. Dried samples were pulverized with a mortar and pestle and then Soxhlet extracted with methylene chloride for 36 h. Concentrated extracts were eluted through silica gel to isolate the aliphatic hydrocarbon fraction. Analysis was performed on a HewlettPackard 5840 gas chromatograph equipped with a 25 m × 0.2 mm SE-30 fused-silica capillary column (Scientific Glass Engineering, Austin, TX) and a flame ionization detector. The n-alkanes, pristane, and phytane were quantified by using authentic standards (Alltech Associates, Deerfield, IL). The UCM was quantified by measuring the unresolved area with a graphics tablet and by using the average response factor of the n-alkanes eluting above the UCM to convert the unresolved area to a UCM concentration. Total carbon and inorganic carbon were measured in dried samples by using a Perkin-Elmer model 203B elemental analyzer. Organic carbon was calculated by difference. Precision for the individual n-alkanes was (8%. Procedural recoveries increased with carbon chain length and reached a maximum at n-C20. The mean recovery for n-C18 to n-C32 was 78.8%, and the mean for n-C11 to n-C17 was 62.6%. To check possible evaporative losses of n-alkanes during freeze-drying, the technique, which included extraction of freeze-dried sediment with methylene chloride, was compared to a method in which wet sediment was extracted with methanol. The results of the two techniques were not significantly different, indicating that evaporative losses of n-alkanes during freeze-drying are insignificant. Precision for the organic carbon analysis was (1.5%.

Results and Discussion Spatial Variations. The carbon preference index (CPI), which is simply defined as the ratio of the mole fractions of the odd- to even-numbered homologues in a hydrocarbon distribution (13), is one of several characteristics used to 248

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derive information on the origin of aliphatic hydrocarbons. The CPI14-20 in all sediment core sections varied from 1.3 to 1.7. The prominence of n-C15, n-C17, and n-C19 in Lake Michigan sediments resulted in a CPI14-20 value greater than 1, which is typical of sediment hydrocarbons having a nonsiliceous planktonic origin (7-10). Concentrations of the planktonic n-alkane observed in greatest abundance in surficial sediments, n-C17, at sediment core locations 12 and 24 were approximately twice the levels at locations 6 and 18 (Figure 2). The CPI20-32 of n-alkanes in all sediment core sections was 2.0-3.6. The slight predominance of odd carbon numbers over the range C20-C32 indicates that a fraction of the hydrocarbons have a terrestrial origin. The waxy coatings of terrestrial plants contain n-alkanes in the range C25-C33 (11, 19-21) with a CPI of 4-10 (22). The sediment samples exhibit CPIs lower than those of purely terrestrial plant waxes, suggesting additional contributions from other sources of aliphatic hydrocarbons. Concentrations of n-C29, the terrestrial plant wax n-alkane observed in greatest abundance in surficial sediments, were greatest at sediment core location 18 and were similar in magnitude at the other locations (Figure 2). The most prominent feature of the aliphatic hydrocarbon distribution in surficial sediments at all coring locations was a UCM. Concentrations of the UCM were greatest at locations 12 and 24 (Figure 3). Levels of the UCM were similar to the total n-alkane concentrations at locations 6, 18, and 24 and exceeded the n-alkane level at location 12 (Figure 3). The UCM has been observed in sediments from diverse environments and has been used as an indicator of petroleum pollution (14, 23). The CPI of n-alkanes in petroleum is approximately one (24). Consequently, the CPI of a hydrocarbon assemblage having a planktonic and terrestrial plant wax origin should be decreased by additions of petroleum residues. Because the CPIs of the n-alkanes in sediments from the southern basin of Lake Michigan are > 1, petroleum residues cannot be the sole source of aliphatic hydrocarbons in these sediments. The n-C29/n-C17 ratio has been used to indicate relative strengths of allochthonous and autochthonous inputs of n-alkanes to aquatic systems (25). Because n-C17 is prevalent in nonsiliceous species of plankton, it can be used as an

FIGURE 3. Percent organic carbon and concentrations (on the basis of dry sediment) of UCM and total n-alkanes (C11-C32) vs depth in four Lake Michigan sediment cores.

FIGURE 4. Values of the n-C29/n-C17 and n-C17/pristane ratios in four Lake Michigan sediment cores. indicator of authochthonous inputs of organic matter, whereas n-C29 is abundant in terrestrial plant waxes and therefore can be used to represent allochthonous inputs. Spatial variations of the ratio are apparent (Figure 4). Locations 6 and 18 exhibit a larger contribution from allochthonous material. Another method of apportioning sources of aliphatic hydrocarbons in sediments is to separate the n-alkanes into CPI ) 1 and CPI ) 10 members via a mass balance (17, 26). The total aliphatic hydrocarbon concentration is then defined as the sum of the concentrations of CPI ) 1 n-alkanes, CPI ) 10 n-alkanes, and the UCM. These groups represent contributions from plankton and petroleum, terrestrial plants, and petroleum, respectively. Aliphatic hydrocarbons in surficial sediments from coring locations 12 and 24 exhibit the highest contribution by the UCM and the lowest contribution by the CPI ) 10 n-alkanes (Figure 5). Contributions by the CPI ) 10 and CPI ) 1 n-alkanes at these locations are nearly equal. Terrestrial n-alkanes represent a greater proportion of the aliphatic hydrocarbons in sediments at

stations 6 and 18, in agreement with trends indicated by the n-C29/n-C17 ratios. Spatial variations of chemical concentrations in surficial sediments are influenced by changes in material inputs and also by physical, chemical, and biological processes. The rivers of the southern basin of Lake Michigan deposit a substantial amount of sediment particles into the lake. These particles are sorted and deposited by a succession of resuspension and resettling cycles as they are transported lakeward (27). We observed levels of organic carbon in nearshore surficial sediments to be less than those at the offshore sites (Figure 3), in agreement with the observation of Eadie et al. (28) that surficial sediment concentrations of organic carbon (a) increased with distance from the mouth of the Grand River and (b) correlated with the fine-grain-size material of surface sediments. Carbon isotopic analysis of settling sediments in Lake Michigan also suggests that organic matter moves laterally downslope from coastal regions of higher productivity to the deep basin (5). VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Contributions of the UCM, CPI ) 10 n-alkanes, and CPI ) 1 n-alkanes to the aliphatic hydrocarbons in the surface layers of four Lake Michigan sediment cores. Hydrodynamics might alter hydrocarbon distributions of surficial sediments via differential particle dispersal (29), because particles with different sizes and densities have unique hydrocarbon signatures (30-32). Thompson and Eglinton (30) found that the planktonic n-alkanes and the UCM in the bottom sediments of a small lake in England were concentrated in the silt/clay fraction ( 125 µm) containing terrestrial plant detritus. Contrary to the results of Thompson and Eglinton (30), analyses of a Washington coastal sediment (32) and bottom sediments from Naragansett Bay (31) indicated that plant wax n-alkanes were concentrated in the silt/clay fraction, suggesting that the source of the bulk sediment (lacustrine, estuarine, etc.) and the sediment’s location relative to the point of introduction to the aquatic system influence hydrocarbon associations with various particle sizes. Prahl and Carpenter (32) found that alkylated PAHs characteristic of an unburned fossil organic source (e.g., petroleum, oil shales, and their refined products) were concentrated in a dense (> 1.9 g cm-3), lithic phase of bottom sediments, suggesting that these substances would be deposited close to their source. The origin of the UCM is believed to be petroleum residues, and thus the UCM might also be incorporated in a similar sediment phase; however, Wade and Quinn (31) speculated that the UCM is associated with fine particles because of their large surface area, in agreement with the data of Thompson and Eglinton (30). If the terrestrial and planktonic n-alkanes and the UCM in Lake Michigan sediments are all associated with fine-grained sediments, the spatial distribution of terrestrial n-alkane accumulations in surficial sediments supports these findings; however, the planktonic n-alkanes and UCM accumulations do not. The UCM, which is prevalent in urban runoff (33, 34), might enter the southern basin of Lake Michigan via riverine discharges. Helfrich and Armstrong (35) found high levels of alkylated naphthalenes in surficial sediments collected offshore from the mouths of the Grand and St. Joseph Rivers in Lake Michigan, in close proximity to the sites where we observed the highest concentrations of the UCM. The alkylated naphthalenes are typical of sewage effluents containing petroleum hydrocarbons from industrial sources (36). Tolosa 250

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et al. (37) found higher levels of alkylated PAHs and the UCM in river delta sediments than in deep basin sediments of the northwestern Mediterranean, suggesting that these petroleum derived substances are deposited in close proximity to their source. Thus, if hydrodynamics controls spatial distributions of the UCM, this complex mixture of aliphatic hydrocarbons must be associated with coarse or dense sediment particles because its spatial distribution is not typical of the distribution of contaminants associated with fine-grained sediments. Spatial distributions of the planktonic n-alkane concentrations could not be explained by an association with finegrained sediments, because the highest concentrations were observed in nearshore sediments. Instead, microbial decomposition appears to have influenced the accumulation of planktonic hydrocarbons in bottom sediments. Several investigations have indicated that n-alkanes in plankton are more susceptible to decomposition than those in terrestrial plant wax (5, 18, 38-42). Furthermore, isoprenoid hydrocarbons such as pristane (Pr) and phytane (Ph) are not as readily degraded as straight-chain hydrocarbons (24). The UCM, comprising a complex mixture of branched and cyclic hydrocarbons, tends to persist in sediments. Biodegradation of hydrocarbons in aerobic sediments is also more rapid than microbial decomposition of hydrocarbons in anaerobic sediments (43). Thus, n-alkane distributions in sediments having slow accumulation rates or deep mixing zones would exhibit a lower planktonic n-alkane contribution, because sediments within the mixed layer spend a longer time in the aerated zone. Sedimentation rates at the offshore locations in Lake Michigan are lower than those in the nearshore areas (44). Therefore, the planktonic n-alkane accumulations at sites 12 and 24 might be greater than the accumulations at sites 6 and 18 because (a) the primary productivity in nearshore areas is greater than that in the offshore areas and (b) the autochthonous organic matter is better preserved in the rapidly depositing sediments of the nearshore areas. Also, as a fraction of the sediments that are easily resuspended move laterally from nearshore areas to the deep basin in a series of settling-resuspension cycles, bacterial decomposition of the most labile fraction of the organic matter, the planktonic n-alkanes, would deplete these hydrocarbons. A

FIGURE 6. Distributions of n-alkanes in settling sediments (5) and surface sediment at coring location 18 (contributions normalized to the n-alkane homologue in greatest abundance). comparison of the n-alkane distributions of settling sediments collected at a depth of 140 m in the zone of resuspension near the location of core 18 (5) with the n-alkane distribution of the surface sediment of core 18 supports this hypothesis (Figure 6). The resuspended sediments contain a higher proportion of planktonic n-alkanes than do the surficial sediments. Longer-chain n-alkanes, which can originate from diatoms, are also present in greater abundance in settling sediments collected at a depth of 140 m. Clark and Blumer (7) reported a predominance of C29 in a fairly uniform n-alkane homologue distribution over the range C25-C31 for the diatom species Skeletonema costatum. Diatoms are one of the major plankton populations in Lake Michigan. Microbial decomposition of the most labile fraction of the organic matter is apparently altering the aliphatic hydrocarbon distribution during the resuspension of the finegrained sediments at this location. The ratios of n-C17/Pr and n-C18/Ph are greater than one in the settling sediments

at 140 m and nearly one in the surface sediment, another indicator of microbial decomposition (because the n-alkanes are more easily decomposed than Pr and Ph). However, organic carbon concentrations in surficial sediments were greatest at the offshore locations. If the hypothesis about the planktonic hydrocarbons is correct, this carbon is probably refractory and most likely of terrestrial origin, a theory supported by the terrestrial signature of the offshore surficial sediments. Chronologies. The UCM concentrations in southern basin sediments approached background levels (1 µg g-1) at different depths in each core (Figure 3). Because the UCM has been attributed to anthropogenic inputs, this feature of the UCM profiles indicates that sedimentation rates at coring locations 12 and 24 are greater than those at locations 6 and 18, in agreement with extensive measurements of sedimentation rates in the southern basin (44). The UCM profiles for cores 12 and 24 both contained subsurface maxima (Figure 3). Maxima in cores 6 and 18 could have been obscured by wide sectioning intervals, because sedimentation rates at these locations are lower than those at the other coring sites. A mass sedimentation rate of 0.016 g cm-2 a-1 was determined for a duplicate core of 18 (45), in good agreement with the value of 0.012 g cm-2 a-1 reported by Robbins and Edgington (46) for this location. However, mass sedimentation rates of 0.039 g cm-2 a-1 (2) and 0.033 g cm-2 a-1 (44) have been reported for locations in close proximity to site 18, indicating the spatial heterogeneity of mass sedimentation in the deep basin. Applying a value of 0.016 g cm-2 a-1 to UCM concentrations reported here indicates that the UCM first appeared in about 1880 at coring location 18, which is consistent with the onset of PAH accumulations near this location (2). The appearance of the UCM in Lake Michigan sediments is also consistent with observations in other aquatic sediments (37, 47, 48). However, Bourbonniere and Meyers (49) found that the UCM first appeared in Lake Ontario sediments in approximately 1910. The pre-1900 flux of the UCM at coring location 18 is approximately 60 µg m-2 a-1, and the 1980 flux is about 960 µg m-2 a-1. In comparison, the flux of PAHs at this location, which is attributed to atmospheric deposition of aerosols originating in the urban/ industrial complex of Chicago, Illinois, and Gary, Indiana, is about 700 µg m-2 a-1. The input of petroleum residues to the southern basin of Lake Michigan has apparently increased by more than an order of magnitude since about 1880, similar to the increase observed in continental-shelf sediments of the northwestern Mediterranean (37). All four cores exhibited a decrease of n-C17 relative to pristane within the top few centimeters of sediment accumulation (Figure 2). Profiles of n-C29 are similar to those of n-C17 in the upper 3 cm of each core except 18 (Figure 2). Subsurface maxima are present in three of the four cores. The n-C29 maximum in core 24 coincides with the subsurface maximum exhibited by the UCM. Vertical profiles of the hydrocarbons are affected by several processes. Subsurface maxima observed in profiles of energyrelated substances have been attributed to changes in material inputs. Goldberg et al. (50) found a subsurface maximum in the profile for charcoal particles > 38 µm in diameter in a core taken from a depositional region in the southern basin of Lake Michigan. Charcoals in this size range are products of coal combustion. The maximum occurred in sediments deposited in about 1955, the same period in which subsurface maxima in PAH profiles have been hypothesized to reflect decreases in inputs caused by a change in home heating fuels from coal to oil and natural gas (51, 52). Helfrich and Armstrong (35) and Simcik et al. (2) observed subsurface maxima in PAH profiles in sediments from the southern basin of Lake Michigan. Like the PAHs, the UCM has been detected in unburned coal and vehicle exhaust (53, 54). The UCM has VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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also been found in aerosols and precipitation collected in the Midwest region (17, 55). Consequently, subsurface maxima in UCM profiles might reflect decreases in material inputs. Biological activity can also produce subsurface maxima in profiles. Oligochaetes in sediments selectively feed on fine particles containing high concentrations of organic matter (56). Feeding by oligochaetes can also alter profiles of hydrocarbons in sediments by providing a means of advecting substances from depths of 2-10 cm in sediments to the surface (57-59). Because the UCM is associated with organiccarbon-rich, fine particles (30), its vertical profile can also be altered by this mechanism. Planktonic n-alkanes are concentrated in the same size fraction as the UCM (30) and might be expected to exhibit a similar vertical profile; however, the dissimilarity between profiles of the planktonic n-alkanes and the UCM is probably caused by recalcitrance of the UCM to biodegradation. Microbial decomposition of the planktonic n-alkanes is illustrated by the decrease of n-C17 relative to pristane within the top few centimeters of each core (Figure 2). Differences in n-C17 and pristane concentrations in deeper sections of the cores become constant, indicating that n-C17 is being preserved within the anoxic layers of the sediment. Atmospheric Loadings. The settling sediment fluxes of trace contaminants have been used to estimate atmospheric loadings and accumulation rates of chemicals in bottom sediments of lakes (3, 18, 28, 60). The distribution of n-alkanes in settling sediments collected at a depth of 80 m within the hypolimnion of Lake Michigan during the period of summer stratification near the location of site 18 (5) is shown in Figure 6. Settling sediments collected at greater depths during summer stratification and also during winter (a period of intense mixing) contain a substantial amount of resuspended bottom sediments (27). Thus, a settling sediment flux measured below the thermocline during summer stratification might accurately reflect the annual flux of autochthonous hydrocarbons to the bottom sediments but might underpredict the annual flux of externally derived hydrocarbons because atmospheric and riverine inputs continue during the winter. The settling sediment fluxes of the planktonic (∑C15, C17, C19) and the terrestrial (∑C25, C27, C29, C31) n-alkanes at a depth of 80 m during the period of summer stratification near the location of site 18 are 540 µg m-2 and 3100 µg m-2, respectively (5). Sediment accumulation rates at this location of 70 µg m-2 a-1 and 660 µg m-2 a-1 for the planktonic and terrestrial n-alkanes, respectively, are considerably less than the settling sediment fluxes. These accumulation rates were calculated by using a mass sedimentation rate of 0.016 g cm2 a-1. The accumulation rate of terrestrial n-alkanes is less (by approximately a factor of 2) than the settling sediment flux when the mass sedimentation rate determined by Simcik et al. (2) for this location is used in the calculation. Fluxes of planktonic n-alkanes by settling sediments are typically greater than their accumulation rates in bottom sediments and indicate extensive microbial decomposition during settling (5, 18, 38-42). However, values of the accumulation rate and settling sediment flux of the terrestrial n-alkanes are expected to be similar because the terrestrial n-alkanes are contained in a plant wax matrix and thus are protected from microbial degradation. A fraction of the n-alkanes in settling and bottom sediments that we have defined as being derived from terrestrial plant waxes could in fact originate from diatoms (7). If the diatom-associated n-alkanes were remineralized during dissolution of the diatom frustules, settling sediment fluxes would be greater than accumulation rates in bottom sediments. Any estimate of the atmospheric loading of aliphatic hydrocarbons for Lake Michigan must be considered specu252

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lative because of the paucity of data on the levels of aliphatic hydrocarbons in the atmosphere of the Lake Michigan region. Meyers and Hites (55) reported an average concentration of terrestrial n-alkanes in precipitation of 3.2 µg L-1 over an 8-month period for events sampled in Bloomington, Indiana, an urban area in an agricultural region located south of Lake Michigan. Using a precipitation rate of 76 cm a-1 for the Lake Michigan region yields an estimate of 2400 µg m2 a-1 for the wet flux of terrestrial n-alkanes. Doskey (3) evaluated precipitation scavenging coefficients and particle size distributions of the n-alkanes from a number of investigations and estimated values of 850-1200 g air (g precipitation)-1 and 0.60-0.78 cm sec-1 for the scavenging coefficients and deposition velocities of the terrestrial n-alkanes, respectively. These parameters were used in an air-water exchange model to estimate the atmospheric loading of n-alkanes in a precipitation-dominated seepage lake in a semiremote, forested area of north central Wisconsin, approximately 190 km west of the northern basin of Lake Michigan. Using the midpoint of the range of the estimated precipitation scavenging coefficients and the precipitation data of Meyers and Hites (55) to calculate the concentration of terrestrial n-alkanes in air yields a value of 3.7 ng m-3. This value is smaller than the level of 5.2 ng m-3 measured in north central Wisconsin (17) but is nevertheless reasonable, given that the density of vegetation is considerably less in an agricultural area than in a forested region. Using the estimated particle deposition velocities to calculate the dry particle flux of the terrestrial n-alkanes for the Lake Michigan region gives a value of 820 µg m2 a-1; the total atmospheric flux (wet plus dry) is then approximately 3200 µg m2 a-1, in excellent agreement with the settling sediment flux (3100 µg m-2) but much greater than the accumulation rate in bottom sediments (660 µg m-2 a-1). The settling sediment flux represents the period of summer stratification and is probably an underestimate of the annual flux of terrestrial n-alkanes. In addition, whether a portion of the terrestrial n-alkanes is contributed by diatoms is unclear. Thus, we cannot determine whether the atmospheric flux is over- or underestimated. Crude estimates of the atmospheric flux of the UCM can also be made from the available data. The total n-alkane concentration in the air of Bloomington, Indiana, was estimated from the precipitation data and scavenging coefficients to be 4.6 ng m-3. Brorstro¨m-Lunde´n and Lo¨vblad (61) reported that the level of the UCM at a rural location in Sweden was greater, by a factor of about 3, than the total n-alkane concentration. Applying this factor to the Indiana data indicates that the concentration of the UCM might be 14 ng m-3. If atmospheric deposition is an important source of the UCM for Lake Michigan, this value probably underestimates the UCM concentration, because (a) levels of the UCM are greater in urban areas (62) and (b) urban areas in the southern basin of Lake Michigan are major sources of various particle-associated organic compounds (2). The UCM is associated with particles of micron and submicron dimensions (63). A dry particle flux of 550 µg m-2 a-1 is calculated for the UCM by assuming that the UCM is associated with particles that are 1 µm in diameter and by using the Sehmel and Hodgson (64) model to estimate particle deposition velocities for Lake Michigan. Poster and Baker (65) reported scavenging coefficients of 12-120 g air (g precipitation)-1 for PAHs associated with submicron particles in precipitation events in the Chesapeake Bay region. Applying a value of 66 (the midpoint of the range of measured scavenging coefficients) to the estimated concentration of the UCM in air and using a precipitation rate of 76 cm a-1 for the Lake Michigan region yields a value of 590 µg m-2 a-1 for the wet flux of the UCM. The total atmospheric flux is approximately 1100 µg m-2 a-1, in good agreement with the accumulation rate in bottom sediments (960 µg m-2 a-1).

Although riverine inputs of the UCM to southern Lake Michigan are probably substantial, assuming that UCM levels in deep basin sediments are supplied by atmospheric deposition might be reasonable. A similar analysis of aliphatic and aromatic hydrocarbons in northwestern Mediterranean sediments led to the conclusion that riverine and atmospheric inputs to coastal and deep sea basin sediments, respectively, were responsible for the sediment inventories at those locations (37). The results of this investigation suggest that the aliphatic hydrocarbons might be useful in future studies of the airwater exchange of SVOCs (e.g., PAHs, PCBs, and other organochlorine compounds). The aliphatic hydrocarbons represent a wide range of physicochemical properties that would facilitate the evaluation of air-water transfer coefficients, particle deposition velocities, and precipitation scavenging coefficients. Estimates of atmospheric loadings would benefit from more frequent measurements of the levels in air and precipitation and from direct measurements of air-water exchange rates. In a typical analytic scheme, the aliphatic hydrocarbons are isolated in the same fraction as the PCBs; thus, the simultaneous investigation of several classes of SVOCs is not an unreasonable goal.

Acknowledgments We thank the captain and crew of the RV Roger Simons for their assistance with sampling and also Norman Draeger for laboratory assistance in the early phases of this work. Karen Haugen’s help with editing is greatly appreciated. This investigation was supported by the University of Wisconsin Sea Grant Institute through Federal Grant NA 800-AA-00086, Project R/MW-13, from the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce. This manuscript has been created by the University of Chicago as operator of Argonne National Laboratory under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research.

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Received for review June 12, 2000. Revised manuscript received October 16, 2000. Accepted October 24, 2000. ES001365M