Environ. Sci. Technol. 1999, 33, 7-13
Speciation and Cycling of Mercury in Lavaca Bay, Texas, Sediments NICOLAS S BLOOM* Frontier Geosciences Inc., 414 Pontius North, Suite B, Seattle, Washington 98109 GARY A. GILL Department of Oceanography, Texas A&M University at Galveston, 5007 Avenue U, Galveston, Texas 77551 STEVEN CAPPELLINO Parametrix Inc., 10540 Rockley Road, Suite 300, Houston, Texas 77099 CHARLES DOBBS AND LARRY MCSHEA Aluminum Company of America, State Highway 35, Point Comfort, Texas 77978-0101 CHARLES DRISCOLL Department of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse, New York 13244 ROBERT MASON Chesapeake Bay Laboratory, University of Maryland, P.O. Box 38, Solomons, Maryland 20688 JOHN RUDD Department of Fisheries and Oceans, Freshwater Institute, 01 University Crescent, Winnipeg, Manitoba, R3T 2N6 Canada
Sediment depth profiles of total mercury (THg) and monomethylmercury (MMHg) were collected at 15 sites in an anthropogenically contaminated estuarine system (Lavaca Bay, TX). THg in the solid phase increased with depth to a maximum located at 10-30 cm, which corresponds to historic industrial discharges to the bay. MMHg in the solid phase was highest in the upper 0-3 cm of the cores, decreasing rapidly with depth. The MMHg content of the surface sediment was a narrowly constrained fraction of the total (0.65 ( 0.34%) over a range of sediment types, while making up only 0.01-0.05% of THg at depth. Porewater concentrations exhibited trends similar to but more exaggerated than in the solid phase. The distribution coefficients (log Kd) for inorganic Hg (IHg ) THg - MMHg) were similar in most samples, averaging 4.89 ( 0.43. The log Kd for MMHg averaged 2.70 ( 0.78 over all sites and depths but exhibited a subsurface minimum of 2.29 ( 0.67 at the point of maximum dissolved Fe. A time series showed a maximum in both solid phase and porewater MMHg during the early spring, followed by a decrease throughout the remainder of the year.
Introduction As a result of contamination-free sample handling protocols (1) and ultra-trace speciation methods (2-5), several inves* Corresponding author phone: (206)622-6960; fax: (206)622-6870; e-mail:
[email protected]. 10.1021/es980379d CCC: $18.00 Published on Web 11/12/1998
1998 American Chemical Society
tigations of Hg cycling at the lacustrine sediment/water interface have recently been reported (6, 7). There remains, a paucity of data on the diagenesis of Hg in estuarine and marine sediments however. This is surprising, since the primary source of MMHg in the environment is believed to be sulfate reducing bacteria (8), and these organisms are very active in the sediments of estuarine systems (9-11). Before the development of current methods, few studies of Hg mobilization in estuarine sediments had been reported, with the exception of an extensive investigation in Bellingham Bay, Washington (12, 13). This work documented the decline in sediment THg concentrations after stringent pollution controls had been applied to a chlor-alkali plant discharge. Without methods for Hg speciation or for accurate collection and analysis of sediment porewaters, the authors based their inferences largely on a series of sediment cores that showed rapid decreases in the peak maximum over several years following the reduction of THg inputs. From this and from work with flux chambers, the authors concluded that Hg was being mobilized out of the sediment to the overlying water column by diffusion, although the bulk of the decrease was attributed to sediment redistribution in the shallow bay. Gagnon (14) reported on more detailed studies of THg and MMHg in the sediments of Saguenay Fjord, Quebec. This site was contaminated earlier by chlor-alkali plant discharges, and the THg maximum had since been buried by approximately 10 cm of sediment. MMHg concentrations of up to 10 ng/L (30% of the dissolved THg) were reported in the porewaters of the anoxic subsurface sediments. Dissolved MMHg and Fe showed similar profiles, with undetectable levels at the surface, a broad maximum over 10-20 cm, and a gradual decline thereafter. The author concluded that this pattern was consistent with the microbial production of MMHg by sulfate-reducing bacteria near the redoxcline. MMHg was apparently translocated by diffusion within the subsurface sediments, but very little escaped to the overlying water column due to the oxic barrier. In this study, MMHg and THg were investigated on broad areal, depth, and temporal scales. Mercury speciation data were accompanied in several intensive investigations by a variety of ancillary measures including porewater DOC, free sulfide, Fe, and Mn concentrations. The data presented illustrate the dynamic nature of the Hg cycle in the upper few centimeters of the estuarine sediment column as well as an intimate linkage to the Fe and Mn redox cycles.
Sample Collection Samples were collected from Lavaca Bay, a large (190 km2), shallow (average depth of 1.5 m) embayment on the southeastern Texas coast. Lavaca Bay was the site of an historic discharge of Hg from a chlor-alkali facility between 1966 and 1970, resulting in these samples being collected as part of an on-going remedial investigation. Most samples were collected as part of a 1-month synoptic survey of the bay conducted in April, 1996 (the “recon” study). Additionally, at one site (GF-2), samples continued to be collected on a monthly basis to allow investigation of the temporal variation of the chemistry. Collection of sediment cores, water column, and biota were made at 15 sites (Figure 1) chosen to be representative of five ecosystem types [intertidal mud flats (IM), grass flats (GF), open water (OW), oyster reefs (OR), and ship channel (SC)]. Samples were collected using established ultraclean handling protocols (1, 15). VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Detailed map of Lavaca Bay, TX, showing the locations of the 15 ‘recon’ study sampling sites. The historic chlor-alkali plant discharge was near site IM-2. Site GF-3 (inset) is in the relatively pristine northern portion of the bay.
Analytical Methods and QC Low-level mercury speciation was determined using cold vapor atomic fluorescence spectroscopy (CVAFS) and established extraction protocols, which are summarized below. After exhaustive prestudy methods evaluation, porewater extraction was conducted under N2 by centrifugation, followed by 0.4 µm filtration (15). THg was determined after acidic oxidation of the samples, by SnCl2 reduction to Hg0, purging onto gold traps, thermal desorption, and CVAFS detection (2, 16). The estimated method detection limit (3σ of all blanks over a 1-month period) obtained for water samples was approximately 0.15 ng/L (n ) 40), while for sediments, the MDL was 0.4 ng/g (n ) 40). Precision, as indicated by the relative percent difference (RPD) of duplicate digestions, was found to average 6.6% for THg in water (n ) 15 pairs) and 8.5% for total Hg in solids (n ) 17 pairs). Accuracy, as determined by spike recoveries and standard reference materials, averaged 99.0 ( 8.0% (n ) 94 recoveries). IHg was determined as the difference between THg and MMHg. 8
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MMHg was determined after acidic/chloride distillation to liberate it from the matrix (3). The distillates were analyzed using aqueous phase ethylation, trapping on Carbotrap, isothermal GC separation, and CVAFS detection (2, 4). The estimated MDL obtained for water samples was approximately 0.021 ng/L (n ) 72), while for the sediments the MDL was 0.005 ng/g (n ) 50). Precision, as indicated by the relative percent difference (RPD) of duplicate digestions, was found to average 8.5% for MMHg in water (n ) 21 pairs) and 8.2% for MMHg in solids (n ) 12 pairs). Accuracy, as determined by spike recoveries and standard reference materials, averaged 98.5 ( 12.6% (n ) 147 recoveries). Because of the potential for artifact formation during distillation (5), 12 sediment MMHg results were duplicated using a new low-artifact KBr/CH2Cl2 extraction procedure. In this study, the potential by which the distilled samples were biased was found to be between 0% and 15% with a mean of 3.7%. Because the effect was small, all distilled results are reported as observed. MMHg values for the long cores were determined using the newer extraction procedure;
TABLE 1. Variation in Hg Speciation of the Upper 1 cm of Sediment (n ) 3-4 Cores Per Site, Spring 1996)a sediment, ng/g (dry basis) inorganic Hg
porewater, ng/L
methylmercury
inorganic Hg
methylmercury
site
distb (km)
mean
SD
mean
SD
mean
SD
mean
SD
GF-1 GF-2 GF-3 IM-1 IM-2 IM-3 OR-1 OR-2 OR-3 OW-1 OW-2 OW-3 SC-1 SC-2 SC-3
0.96 1.28 9.60 0.42 0.35 2.11 2.68 1.18 1.42 3.64 1.33 2.06 3.13 1.24 0.28
783 618 5.1 723 200 19 226 164 458 287 352 337 214 264 710
82 183 0.3 76 71 10 36 21 122 74 68 23 29 37 254
6.40 4.69 0.028 10.34 1.60 0.13 0.76 2.11 3.60 1.24 1.36 1.69 0.80 0.78 1.96
1.61 0.61 0.008 6.48 1.14 0.05 0.20 0.94 1.20 0.27 0.74 0.55 0.11 0.13 0.25
11.35 13.57 7.06 (20)c 11.39 7.77 3.22 3.39 5.63 1.61 4.21 2.69 2.38 2.76 11.01
4.92 8.68 1.51 (63)c 9.04 7.62 0.81 1.29 2.94 0.61 0.52 1.17 2.91 0.52 6.29
45.23 39.55 0.11 242.8 11.60 0.77 0.13 7.93 6.35 1.28 2.72 1.82 0.08 0.23 0.74
7.99 21.90 0.02 56.3 8.53 0.73 0.23 5.06 2.66 0.97 3.34 2.85 0.02 0.11 0.54
a Distance is kilometers from the site to the point of Hg discharge. North Bay. c Includes two negative difference values.
b
Straight line distance. IM-3 is about 4 km distant by water, in the less impacted
however, because the very high THg levels at depth coupled with very low MMHg concentrations could result in errors of up to +50% for some samples. For these cores, an intercomparison of the two extraction procedures both above and below the subsurface THg maxima gave similar results. Dimethylmercury (DMHg) was determined on a small subset of oxic and anoxic sediments after digestion in 25% KOH/methanol (2). Aliquots of the digest were analyzed using purge-and-trap on Carbotrap, isothermal GC separation, and CVAFS detection. The estimated MDL was 0.0002 ng/g. Since DMHg was not detected in any sample, no further analysis was made for this species. Fe and Mn were determined in porewaters after 10- or 100-fold dilution by stabilized platform Zeeman corrected graphite furnace atomic absorption spectrometry (SPZGFAAS). Matrix modification using 50 ppm Pd was employed along with the instrument manufacturer’s recommended settings.
Results Field Variability. When looking at changes in Hg speciation with depth and time, consideration must be given to the fact that “replicate” cores were collected at different locations within a defined sampling site. This introduces a natural level of variability, which limits the resolution of differences that can be quantified. Mason et al. (15) discuss in detail the within-site and analytical variability for sediment and porewater Hg speciation. For cores simultaneously collected within a 0.5-m radius, sediment concentrations varied by about (5-20% (similar to the analytically induced variability), while porewater concentrations varied by (10-40%. When samples were collected from within a 25-m radius, sediment variability increased to (20-50%, while variations in porewater levels of (50-90% were not uncommon. All core profiles presented here are of the single long core collected at each site, while when surface sediments are discussed, the results are the mean of 3-4 surface sections (0-1 cm) of cores collected within 25 m at each site. Between Site Variability. Surface sediment THg concentrations were found to vary dramatically between sites, with a general trend of higher values closer to the point of historic mercury discharge (Table 1). MMHg in both the sediments and the porewater followed the same pattern, with additional variability introduced due to factors such as grain size and organic carbon content. Although MMHg was found to be a small fraction of THg in the surficial sediments (0.64
FIGURE 2. Relationship between IHg (open circles) and MMHg (closed diamonds) log Kd, total organic carbon (TOC), and percent clay (O < 4.0) in surface sediments of Lavaca Bay. ( 0.34%), much of the Hg found in porewater (41 ( 33%) was in the methylated form. This results in the calculation of sediment/water distribution coefficients (unitless Kd ) [Msed]/ [MPW]) for MMHg, which is 2 orders of magnitude lower than those for IHg (Figure 2). Although the speciation of mercury at these sites was seen to be dynamic on a time frame of hours (17) to days, the use of the static Kd is still an appropriate comparative measure, as spiking experiments showed that equilibration between the solid and liquid phases occurs over a time period of seconds to minutes. The observed Kd increases for both Hg species with the average sediment organic carbon and percent fines (φ < 4.0). There were no strong correlations between parameters as a function of habitat type, although the near-shore grass flats and intertidal marshes were found to have approximately half again as much CH3Hg (0.84 ( 0.31%, n ) 6) as the more open water sites (0.52 ( 0.32%, n ) 9) despite the lower TOC at these sandy locations. Long Core Profiles. Long core profiles collected for this study (Figure 3) show a pronounced subsurface maximum VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Long core profiles for THg (open circles) and MMHg (closed diamonds) at sites near the historic discharge point. Core OW-3 was dated using 210Pb and 137Cs, allowing the assignment of the date 1963 to the point at the middle of the THg maximum. in THg, typically at 20-30 cm below the surface, despite the use of a relatively coarse (10 cm) sampling interval below the initial 12 cm. High-resolution cores collected as part of a later investigation (18) showed that the subsurface peak was in fact very sharp, covering a depth of only a few centimeters, thus indicating a shallow (4-7 cm) mixed layer depth. Core profiles were age-dated using 210Pb and 137Cs, showing that the subsurface Hg profiles are consistent with the historical discharges from the chlor-alkali plant. Subsurface maxima are greatest (up to 12 000 ng/g) near the historic plant discharge area, becoming attenuated to a few nanograms per gram at the remote GF-3 site. Although THg is seen to increase more than 50-fold from the surface down to the maximum, the MMHg increases very little over this range. The fraction MMHg in the highest subsurface sediments is only approximately 0.01-0.02% of THg, more similar to uncontaminated subsurface sediments (0.02-0.1% MMHg) than to the biologically active upper few centimeters of the cores, where MMHg is found to be 0.3-1.4% THg. Short Core Profiles (Detailed Mercury Chemistry). Depth profiles from the 12 cm short core at a typical shore site (IM-1) and open water site (OW-3) are presented in Figure 4. At most of the sites studied, core profiles exhibited two clear characteristics illustrated by these examples. First, MMHg was almost always at a maximum near the sediment surface, with a rapid decrease over the first few centimeters 10
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of depth in the core. Second, a subsurface minimum in Kd for MMHg usually was observed in the 1-3 cm segment, coinciding with or adjacent to the maximum in dissolved porewater Fe. Cores not following this pattern were those from the ship channel, which showed the Kd minimum and dissolved MMHg and Fe maxima much deeper (>10 cm). These cores were unusual because they consisted exclusively of very flocculent sediments that are repeatedly remixed by ship traffic and periodic maintenance dredging in the channel. The log Kd for IHg was consistent throughout the entire bay, averaging 4.89 ( 0.43 (n ) 70) over all core depths at 14 of the 15 sites investigated. The one site excluded from the data set used to derive this mean was GF-3, a sandy background site with low THg (mean 5.5 ng/g) that had log Kd values of about 3.7 for IHg and 1.7 for MMHg. At all sites and depths, MMHg was less strongly bound to the solid phase, exhibiting a mean log Kd of 2.70 ( 0.78 (n ) 69). If the MMHg log Kd is calculated at the depth of the dissolved Fe maximum in each core, a mean value of 2.29 ( 0.67 (n ) 14) is obtained, illustrating a 3-fold increase in MMHg solubility in that layer. Sediment O2 profiles with millimeter scale resolution showed the redoxcline to be located within several millimeters of the sediment surface for all cores (the visible boundary between brown and black sediment was usually seen at about 1 cm). This means that the upper 0-1 cm layer represents
FIGURE 4. Detail of upper sediment chemistry at intertidal marsh site IM-1 (closed squares) and open water site OW-3 (open circles). Each point is a single determination from the 12 cm long core taken at each site.
TABLE 2. Comparison of Mean (n ) 3-4 Cores per Site) Sediment Chemistry (Surface 1 cm) at Two Sites in Spring 1996 and Winter 1997 GF-2
FIGURE 5. Time course study of mercury speciation at site GF-2 (March 15, 1996-April 2, 1997), showing the percent of 0-1 cm sediment THg as MMHg (open circles) and the log Kd for MMHg (closed circles). THg (0-1 cm) at GF-2 averaged 597 ( 147 ng/g (n ) 14 events), with no temporal pattern. a mix of oxic and anoxic sediments and that all lower core sections were clearly anoxic. As illustrated by the example in Figure 4, porewater Fe concentrations were generally low in the 0-1 cm (oxic) section, highest in the following section, and then decreased with depth in the core. Dissolved Mn was often found to be highest in the surface section, tapering to lower, relatively uniform levels at depth.
GF-1
parameter
spring
winter
spring
winter
sediment Hg(II) (ng/g) porewater Hg(II) (ng/L) Hg(II) log Kd sediment MMHg (ng/g) porewater MMHg (ng/L) MMHg log Kd
723 10.23 4.85 6.45 31.02 2.32
511 18.83 4.44 1.40 1.21 3.06
789 11.35 4.84 6.39 45.23 2.15
448 16.43 4.44 1.93 1.97 2.99
Seasonal Hg Speciation Cycle. At site GF-2, a series of sediment and porewater collections was made over a 1-year period between March 1996 and April 1997. In the early spring, corresponding to the intensive synoptic sampling, the sediments displayed a dramatic increase in MMHg activity (Figure 5). The fraction of sediment Hg found in the methylated form peaked at 1.59% in mid-April and then declined steadily to a value of 0.27% in late January. The Kd for MMHg at the sediment surface showed an inverse pattern, with the minimum value at the MMHg concentration maximum. These two effects combined to yield an annual porewater MMHg cycle that varied in concentration by a factor of 38 from peak to trough. The fraction of porewater THg in the methylated form was greater than 95% of the VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Sediment profiles for (a) total Hg and (b) methylmercury at GF-2, comparing the spring of 1996 (open circles) with the winter of 1997 (closed squares). total during the spring activity peak and then decreased to only 6% by mid-winter. To help further elucidate the seasonal effect and to gauge whether it was a general phenomenon in Lavaca Bay, two sites (GF-1 and GF-2) were sampled in January 1997. A comparison of key sediment parameters between spring and winter (Table 2) shows that these two sites track each other extremely well in terms of absolute MMHg levels, Kd, and
porewater profile shape. During the spring, very high MMHg concentrations in porewater and sediments were seen in the surface layer, while by mid-winter, the peak was below the surface and an order of magnitude lower. Presented in Figure 6 is a comparison of detailed sediment profiles taken in the spring and winter at GF-2, while Figure 7 shows the porewater constituents. The sediment showed an increase in total mass of MMHg during the spring peak, while in the porewater large amounts of Fe, Mn, and MMHg were released to the dissolved phase. Inorganic Hg(II), on the other hand, showed a solubility maximum in the winter and a minimum during the period of greatest methylation activity. In the detailed winter core, significant levels of dissolved sulfide were observed in the porewater below 4 cm. Increasing dissolved sulfide corresponded with the very high levels of dissolved Hg(II) seen at this site, perhaps indicating complexation and solubilization of Hg(II) as polysulfide species (19). Dissolved organic carbon (DOC) also increased with depth, although over a narrower range than for Hg(II) and sulfide. Using the same detailed winter 1997 core, we also estimated the speciation of sediment phase inorganic Hg using selective leaching techniques. Labile Hg(II) was determined using 1 M HCl, and HgS was determined using saturated Na2S solution after pre-extraction of non-HgS with 12 M HNO3 (20). These tests indicated that approximately 1% of the total Hg in the sediment was recoverable as labile
FIGURE 7. Porewater profiles for (a) Hg(II), (b) methyl mercury, (c) dissolved organic carbon (DOC), (d) sulfide, (e) iron, and (f) manganese at GF-2, comparing the spring of 1996 (open circles) with the winter of 1997 (closed squares). 12
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Hg(II) and 5-10% was recoverable as HgS. No patterns were observed related to depth, MMHg levels, or porewater sulfide, Fe, or Mn concentrations. Sediments in a core collected from the same site in April 1997 were leached with an acidic NH2OH procedure expected to solubilize amorphous iron and manganese oxides. Less than 1% of the inorganic Hg was released at any depth, while 30-60% of the MMHg was solubilized. These tests, while qualitative, suggest that MMHg is bound to amorphous metal oxides, while Hg(II) is strongly bound with organic carbon and sulfides.
Discussion MMHg in sediments was found to be highest in the upper several centimeters of the cores, decreasing exponentially with depth, while THg exhibited a pronounced subsurface maximum at 10-30 cm, corresponding to historic discharges from the chlor-alkali plant. At the sediment surface, the fraction of MMHg was narrowly constrained to the range of 0.3-1.6% (at the spring maximum) of the THg concentration, resulting in the spatial distribution of MMHg being similar to that of THg. MMHg showed much greater variation with depth in the cores, ranging from a maximum of 1.6% of the total in the surface layer of near-shore sediments to less than 0.02% at depth in the long cores. MMHg in the contaminated deep core layers was higher than the noncontaminated layers below it but lower than at the sediment surface. MMHg concentrations were highest in the surface 3 cm of all cores (excepting the flocculent material found in the ship channels), despite THg concentrations up to 50 times higher at depth. When expressed as a fraction of THg, the lowest MMHg ratios were found in these deep Hg contaminated layers, indicating that the historic levels of MMHg expected to be present during the period of active Hg releases to the bay are not recorded in the sediment history. Whether MMHg is lost from the surface sediments during diagenesis by demethylation or diffusion cannot be assessed from these data, although the low Kd for MMHg in these sediments supports a hypothesis that diffusion could be significant (14, 17). In contrast to an earlier study (21), we observed no dimethylmercury (DMHg < 0.0002 ng/g) in any sediment sample, irrespective of its location along the redoxcline. Thus, for Lavaca Bay at least, the conversion of MMHg to DMHg, which may rapidly diffuse out, does not appear to be a significant geochemical pathway. Temporal studies showed the bay sediments to have a strong seasonal methylation cycle with a short, active period in early spring, followed by a slow year-long decrease. The minimum in MMHg in all compartments occurs just prior to the sudden spring “bloom.” Since all synoptic measurements in this study were made during the spring bloom period, caution must be taken in extrapolating directly from those concentrations to long-term integrated measures, such as fish tissue levels. However, based on the close parallels between sites GF-1 and GF-2 in spring and winter, it appears as though the seasonal cycle might be extrapolated, if not to the whole bay, then at least to the more active shoreline areas. Detailed core porewater profiles taken at the maximum and minimum of sediment activity suggest that, after the spring bloom, MMHg is slowly degraded back to Hg(II), a significant amount of which remains in a soluble (reactive) form until the next year’s onset of intense methylation. Perhaps the high dissolved Hg(II) found at the end of winter serves as the substrate for the following spring methylation bloom. Dissolved Fe and Mn during the spring bloom show patterns similar to those measured in other estuarine systems (10) as well as modeled predictions (9). These indicate a shallow subsurface maximum in dissolved Fe that occurs just below the redoxcline, at the point of maximum sulfate reducing bacterial activity. Mn is predicted to be maximal
just above the Fe maximum, as dissolved Fe(II) may reduce Mn(IV) to Mn(II) while itself being oxidized to insoluble Fe(III). The distribution coefficient for MMHg is lowest at the point of the Fe maximum, whereas the dissolved Hg(II) concentrations appear to be unrelated to changes in dissolved Fe and Mn. In our only core where all ancillary data were collected, dissolved Hg(II) does appear to be directly related to both DOC and dissolved sulfide (Figure 5c,d) These results indicate that MMHg mobility is be tied to the Fe redox cycle, with maximal availability to overlying water occurring when the Fe reducing zone is near the surface. Mobility of Hg(II) is clearly not controlled by metal oxide solubilization but may instead be controlled by the formation of soluble polysulfide forms or soluble organic complexes.
Acknowledgments We would like to thank the following colleagues, who spent almost unbearably long hours collecting, processing, and analyzing samples so that ongoing results could be used as a guide for future sampling efforts: Eric J. von der Geest, Beverly A. Heaphey, and Amara M. Fischer (analysts at Frontier) as well as Beth Kuhn and Daniella Paluzzi (field sampling crew at Parametrix). This research was supported by the Aluminum Company of America (ALCOA).
Literature Cited (1) Gill, G. A.; Fitzgerald, W. F. Deep Sea Res. 1985, 32, 287-297. (2) Bloom, N. S. Can. J. Fish. Aquat. Sci. 1989, 46, 1131-1140. (3) Horvat, M., Bloom, N. S.; Liang, L. Anal. Chim. Acta 1993, 282, 153-162. (4) Liang, L.; Horvat, M.; Bloom, N. S. Talanta 1994, 41, 371-379. (5) Bloom, N. S.; Coleman, J. A.; Barber, L. Fresenius J., Anal. Chem. 1997, 358, 3371-377. (6) Hurley, J. P.; Krabbenhoft, D. P.; Babiarz, C. L.; Andren, A. W. Environmental Chemistry of Lakes and Reservoirs; Baker, L. A., Ed.; American Chemical Society: Washington, DC, 1994; pp 425-448. (7) Bloom, N. S.; Effler, S. W. Water Air Soil Pollut. 1990, 53, 251. (8) Gilmour, C. C.; Henry, E. A.; Mitchell, R. Environ. Sci. Technol. 1992, 26, 2281-2287. (9) Van Capellen, P.; Wang, Y. Metal Contaminated Aquatic Sediments; Allen, H. E., Ed.; Ann Arbor Press: Chelsea, MI, 1995; pp 22-64. (10) Thamdrup, B.; Fossing, H.; Jorgensen, B. B. Geochim. Cosmichim. Acta 1994, 58, 5115-5129. (11) Hines, M. E.; Jones, G. E. Estuarine Coastal Shelf Sci. 1985, 29, 729-742. (12) Bothner, M. H. Mercury: Some Aspects of its Marine Geochemistry in Puget Sound. Doctoral Dissertation, University of Washington, Seattle, WA, 1973. (13) Bothner, M. H.; Jahnke, R. A.; Peterson, M. L.; Carpenter, R. Geochim. Cosmichim. Acta 1980, 44, 273-285. (14) Gagnon, C.; Pelletier, E.; Mucci, A.; Fitzgerald, W. F. Limnol Oceanogr. 1996, 41, 428-434. (15) Mason, R. P.; Bloom, N. S.; Cappellino, S.; Dobbs, C.; Driscol, C. T.; Gill, G. A.; Benoit, J. Environ. Sci. Technol. In press. (16) Bloom, N. S.; Crecelius, E. A. Mar. Chem. 1983, 14, 49-59. (17) Gill, G. A.; Bloom, N. S.; Cappellino, S.; Dobbs, C.; Driscoll, C. T.; Mason, R.; McShea, L.; Rudd, R. Environ. Sci. Technol. Submitted for publication. (18) Santchi, P. H.; Allison, M.; Asbill, S.; Eek, A. B.; Cappellino, S.; Dobbs, C.; McShea, L. Environ. Sci. Technol. Submitted for publication. (19) Paquette, K.; Helz, G. Water Air Soil Pollut. 1995, 80, 10531056. (20) Revis, N. W.; Osborne, T. R.; Sedgely, D.; King, A. Analyst 1989, 114, 823-825. (21) Bartlett, P. D.; Craig, P. J. Water Res. 1981, 15, 37-47.
Received for review April 15, 1998. Revised manuscript received September 30, 1998. Accepted October 5, 1998. ES980379D VOL. 33, NO. 1, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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