Change of Dissolved Gaseous Mercury Concentrations in a Southern

A 12-month field study was conducted consecutively from June 2003 to May 2004 to quantify temporal variations of dissolved gaseous mercury (DGM) conce...
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Environ. Sci. Technol. 2006, 40, 2114-2119

Change of Dissolved Gaseous Mercury Concentrations in a Southern Reservoir Lake (Tennessee) Following Seasonal Variation of Solar Radiation HONG ZHANG,* CHRISTOPHER DILL, TODD KUIKEN, MELISSA ENSOR,† AND WILLIAM CHAD CROCKER Department of Chemistry, Box 5055, Tennessee Technological University, Cookeville, Tennessee 38505

A 12-month field study was conducted consecutively from June 2003 to May 2004 to quantify temporal variations of dissolved gaseous mercury (DGM) concentrations in Cane Creek Lake, a southern reservoir lake (Cookeville, TN). Diurnal changes of DGM concentrations in two periods (morning increase vs afternoon decrease with an aroundnoon peak) were observed, and the changes closely followed daily solar radiation variation trends. The diurnal patterns prevailed in the late spring and summer, but became vague in the late fall and winter. The monthly mean DGM concentrations peaked at 40.8 pg L-1 in July and reached the lowest at 14.2 pg L-1 in December and 21.9 pg L-1 in January; this DGM concentration change closely followed the monthly mean solar radiation variation trend. The increase of the lake DGM concentration from January to July and its decrease from July to December mirror the typical daily rhythm of DGM concentration variations in the two periods. This finding supports the following hypothesis: The natural phenomenon of daily oscillation of freshwater DGM concentrations that follows diurnal solar radiation variation would manifest on a seasonal scale. High DGM concentrations were found in the spring and summer and low in the fall and winter (seasonal mean: 34.2, 37.5, 20.0, 24.4 pg L-1, respectively). This seems to suggest an annual occurrence of two periods of the seasonal DGM level fluctuation (spring and summer high vs fall and winter low DGM levels). Linear relationships of the monthly mean DGM concentrations were found with the monthly mean global solar radiation (R2 ) 0.82, P < 0.05) and UVA radiation (R2 ) 0.84, P < 0.05). Linear relationships of the seasonal mean DGM concentrations were also found with the seasonal mean global solar radiation (R2 ) 0.85, P ) 0.08) and UVA radiation (R2 ) 0.93, P < 0.05).

Introduction The threats of mercury (Hg) to human and ecological health and related environmental social debate and concern are leading it to emerge as the toxic and environmental twin of lead. The past decade or more witnessed wide research on environmental Hg aimed at understanding its global bio* Corresponding author phone: (931)372-6325; fax: (931)372-3434; e-mail: [email protected]. † Current address: The University of the South at Sewanee. 2114

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geochemical cycle and unveiling its fate in the environment (1, 2). The number of studies has been rapidly rising on aquatic biogeochemistry of Hg in natural lakes (e.g., for dissolved gaseous mercury (DGM) in lakes, see refs 3-17). Yet, few studies have been reported on the photochemical behavior of Hg in reservoir lakes, especially the southern ones. It is currently considered that Hg(0) and Hg(II) dominate the stable aquatic Hg redox species, and the two are coupled by the superficially simple redox cycle between each other. DGM predominantly consists of Hg(0) (>99%) (15) and is the main source of Hg evasion from surface water to the atmosphere. The fate of the Hg(II) species includes, among others, two important aspects: transformation to methylated Hg (CH3Hg+ and (CH3)2Hg, the most toxic environmental Hg species) and reduction to Hg(0). Hg(0), or DGM, is highly important in the aquatic biogeochemical cycle of Hg because of its two fates, i.e., evasion to the atmosphere (14, 16, 18), or oxidation to Hg(II) followed by methylation to become bioavailable in the food chain (1, 2, 7, 9, 19). Modern advancements in the analytical chemistry of environmental Hg have made DGM concentration a parameter that can be determined conveniently with reproducibility and accuracy in quantification of trace levels of aquatic Hg species (8, 14, 15, 20, 21). DGM has thus emerged as a key to understanding the biogeochemical cycle of aquatic Hg. There have been a growing number of studies on the behavior of freshwater DGM. These were conducted mostly for the northern lakes (e.g., for some Canadian lakes, see refs 3, 10-12; some Nordic lakes, see refs 16, 17; the Great Lakes, see refs 6, 7, 22-24) and several for some southeastern costal wetlands (e.g., the Florida Everglades, see refs 25, 26). These studies revealed the diurnal patterns of DGM level changes that closely followed daily solar radiation variation rhythms (e.g., 3, 10, 22, 25, 26). It was observed that usually, low in the early morning, the DGM levels rose continuously to peak around noontime, then decreased in the afternoon, and became low again in the night, exhibiting typical diurnal rhythms. The studies on temporal variations of freshwater DGM levels, however, commonly featured field observations only in short terms such as several days or weeks or months (e.g., 3, 6, 21, 25, 26) as a result of various research designs as well as logistic constraints. The changes of freshwater DGM levels over the entire seasonal spectrum (i.e., every month through a year) remained to be thoroughly characterized. Hypothetically, if the DGM level oscillates following diurnal variation of solar radiation as a result of the earth’s rotation, then, intuitively, the DGM level would also change closely following the seasonal solar radiation variation rhythm through a year imposed upon the earth by its revolution. Although the hypothesis appears plausible, to our best knowledge, there was a lack of long-term systematic field research aimed at its verification. Observation of the rhythmic phenomenon regarding long-term variations of DGM levels would help to fully reveal and firmly establish the essential relationship between freshwater DGM regimes and solar radiation. A comparison between diurnal and seasonal variations of DGM levels will provide insights into the mechanism of the aquatic photochemical cycle of freshwater Hg. Unlike the northern lakes, the southern ones seldom freeze in wintertime, which makes the aquatic photochemical cycle of Hg proceed in liquid phase coupled with the Hg air/water exchange throughout the year. Tennessee (TN) abounds with reservoir lakes and artificial impoundments (∼44 or more reservoir lakes, mainly operated by the Tennessee Valley 10.1021/es0513990 CCC: $33.50

 2006 American Chemical Society Published on Web 03/03/2006

FIGURE 1. Cane Creek Lake (Cookeville, Putnam County, TN), a reservoir lake located in the Caney Fork Watershed of the Cumberland River Basin, one of the three major basins in TN. Authority, the U.S. Corps of Engineers, and the Tennessee Wildlife Resources Agency). Moreover, Tennessee has four distinct seasons. The needs and local opportunities for investigation into long-term temporal variations of freshwater DGM levels are thus readily recognizable. Here we report a 12-month field study on DGM concentration variations in a southern reservoir lake of Tennessee. The primary purposes of this study were to quantify long-term temporal variations of DGM concentrations in the lake and to characterize the seasonal pattern of the variations. An overarching goal was to observe the hypothetical close match between the temporal trends of the DGM concentration variation and the solar radiation variation on seasonal scales; of special interest is the comparison between diurnal and seasonal trends of DGM level variations.

Site and Methods Site Description. This research was conducted in a southern reservoir lake, Cane Creek Lake of Cookeville (Putnam County, TN), located in the Caney Fork Watershed of the Cumberland River Basin of Tennessee (Figure 1). The lake was created in 1981 through the joint efforts of the USDA, the U.S. EPA, and the City of Cookeville. Table 1 summarizes its basic characteristics (for details, see refs 27, 28). Little aquatic stratification was present throughout the year in this rather small, shallow, and still lake. Thus, it may resemble a simplified one-box reactor as the first approximation. Our field study indicated that the lake was generally well-mixed without significantly large spatial variations of DGM concentrations (27). Sampling and Analyses. The field study was carried out each month consecutively from June 2003 to May 2004 for a total of 34 sampling days. Lake water sampling was conducted approximately following a biweekly sampling schedule based on the modified random sampling method

TABLE 1. Basic Characteristics of Cane Creek Lake (Cookeville, TN) lake characteristics location (km2)

area depth (m) shoreline development Secchi disk reading (m) pHa DO (mg L-1)a total Hg (ng L-1)b DOC (mg L-1)b TOC (mg L-1)b Fe (mg L-1)b Mn (mg L-1)b

value 36°, 09.73′ N; 85°, 32.64′ W 0.18 1.5-3.0 1.9 >3 8.1 7.4 e0.2 3.6 3.6 0.14 0.07

a Mean of all data from May of 2003 to January of 2004. the data of summer of 2003.

b

Mean of

(i.e., random selection of one sampling day during each twoweek period, see ref 29). This schedule represents a reasonable DGM sampling coverage over a complete seasonal spectrum considering the constraints in resources and logistics for the field study of such a scale. In addition to the regular sampling, intensified field campaigns were conducted in June (9 days) and July (6 days) of 2003, the two months normally with the strongest solar radiation. Sunny days were preferentially selected for field sampling in order to study the link between DGM levels and solar radiation. Deviations from the designed sampling schedule occurred occasionally because of the sampling condition requirement as well as logistic constraints. To evaluate the field sampling, the monthly mean global solar radiation values obtained from field measurements were compared with the calculated values of the monthly mean extraterrestrial global horizontal solar irraVOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Diurnal variations of dissolved gaseous mercury (DGM) concentrations in Cane Creek Lake (Cookeville, TN) for each month from June 2003 to May 2004 generally under sunny conditions, shown in (a) summer, (b) fall, (c) winter, and (d) spring. diance for the study period of June 2003-May 2004. The measured and theoretical solar radiation values were found to share the same variation trend (r ) 0.88, p < 0.01 for the measured vs the calculated). This indicates that the actual sampling for this study was reasonably representative. The water sampling proceeded usually from early morning until late evening at a frequency as high as possible (approximately one sample per hour at most). Ideally, sampling at regular intervals should be pursued; however, this did not prove feasible in practice. The actual sampling interval had to vary daily according to weather and other conditions. Nevertheless, the water was sampled generally at daily frequencies of 1 sample per ∼1-1.5 h. About 90% of the samples (ntotal ) 238) were taken at the intervals of 1.0-1.5 h (∼7% at intervals >2 h). The DGM data obtained generally cover the period of ∼08:00 to ∼19:30, except for wintertime (usually ∼09:00 to ∼17:30). All the data interpretations and statistics were thus made based on the DGM data sets obtained within the time frames studied. Fresh water samples were taken at a master site near shore on the south side of the lake by surface grab using a Chemglass glass bottle (1 L, with a glass stopper; acid-cleaned and preheated at 500 °C for ∼4 h). Upon each sampling, in situ measurements were also made of water and air temperatures, global solar radiation (Rg) using a LiCor pyranometer, and UVA radiation using a Fischer Scientific UVA photometer. Each fresh water sample (normally 1 L) was immediately transported in a cooler to our laboratory; upon the sample’s arrival, it was immediately purged (usually ∼30-40 min between field sampling and purging) with Hg-free N2 gas at 1.0 L min-1 for 40 min using a cleaned, preheated Chemglass glass purge insert (with a frit tip) fitted with the sample bottle. The DGM collected on a pre-blanked Tekran gold-sand trap through purging was then analyzed for Hg(0) by means of cold vapor atomic fluorescence spectroscopy using a Tekran 2600 mercury analyzer modified by the manufacturer for 2116

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gaseous elemental Hg analysis. This was calibrated each sampling day by an external standard Hg vapor source kept at 17.9 °C in a refrigerated water bath. The blank of the purge and collection system was routinely inspected at least twice a day (usually at the beginning and the end of each sampling day) and corrected appropriately. Clean technique and appropriate quality management measures were implemented throughout the study. The total Hg of the lake water was analyzed following a slightly modified version of the EPA 1630 method (27, 28). The metals were analyzed using an atomic absorption spectrometer. The organic carbon (TOC/DOC) was determined using a Shimadzu total organic carbon analyzer within 24 h after a water sample was taken. The dissolved oxygen (DO) was measured in situ with a YSI 85 DO/EC/T meter and the pH with a YSI 100 pH meter. Detailed procedures for the field operations and the laboratory analyses are documented elsewhere (27, 28).

Results and Discussion Verification of Diurnal Trends of DGM Concentration Changes. The temporal changes of the DGM concentrations in Cane Creek Lake usually exhibited a clear diurnal pattern. This is characterized by a daily march of the DGM concentration from low values in the morning to around-noon peak (morning increase period) and then from the peak to low values in the afternoon through the evening (afternoon decrease period); moreover, the DGM concentration changes followed the trends of diurnal solar radiation variations under normal sunny conditions. Figure 2 provides representative examples of the diurnal variations of DGM concentrations in the lake generally under sunny conditions for each month during June 2003-May 2004. These observations for Cane Creek Lake generally resemble those obtained for the northern lakes, e.g., some Canadian lakes (3, 10, 12) and the

FIGURE 3. Variations of (a) monthly and (b) seasonal mean and maximum concentrations of dissolved gaseous mercury (DGM) in Cane Creek Lake (Cookeville, TN) from June 2003 to May 2004 in comparison with the corresponding mean global solar radiation and UVA radiation. Great Lakes (6, 22, 24), and for some southern coastal wetlands (25, 26). A detailed characterization of the daily temporal variations of DGM concentrations in the lake is available elsewhere (27, 28). Clearly, the summer months saw the greatest daily increases and decreases of DGM concentrations in the lake (e.g., Figure 2a), while the winter months witnessed the smallest temporal changes of DGM concentrations (e.g., Figure 2c); the fall and spring months appeared as transitional periods (e.g., Figure 2b and Figure 2d). In other words, typical diurnal DGM concentration changes occurred in the late spring and summer, while vague diurnal patterns were prevalent in the late fall and winter. Temporal Variation of DGM Concentrations through the Year. This can be presented on various temporal scales (Figure 3). Figure 3a provides a characterization of the temporal variation for Cane Creek Lake from June 2003 to May 2004 by means of the monthly mean DGM concentration (i.e., the average of all DGM data for a certain month) in comparison with the variations of the monthly mean global solar radiation and UVA radiation (defined similarly to the monthly mean DGM concentration). A demonstration of the temporal trend through the year in the conventional monthly sequence (January-May 2004 to June-December 2003) can be seen in SI-Figure 1 in the Supporting Information. Clearly recognizable is the oscillating trend of the DGM concentration change (Figure 3a). Following the conventional monthly sequence, the monthly mean DGM concentration was as low as 21.9 pg L-1 in January, then gradually rose and peaked at 40.8 pg L-1 in July (86% increase), and then continuously fell to the lowest at 14.2 pg L-1 in December (65% decrease) (Figure 3a and SI-Figure 1 in the Supporting Information). The variation of the monthly maximum DGM concentration (i.e., the maximum DGM concentration for a certain month) shares the similar characteristics in the temporal change (Figure 3a). The monthly maximum DGM

levels of January, July, and December were 32.0, 79.5, and 23.5 pg L-1, respectively, showing an increase of 148% and a decrease of 70% relative to the July peak. Similar DGM concentration variations were also observed in the Florida Everglades where the peak DGM levels of 33, 22, 39, and 33 pg L-1 were reported for July and December of 1995 and March and June of 1996, respectively (26). Interestingly, the DGM concentration changes in terms of the monthly mean and maximum DGM concentrations for Cane Creek Lake also closely followed the solar radiation variation rhythms in terms of both Rg and UVA over the 12month period (Figure 3a); the monthly mean global solar radiation also was low at 148 W m-2 in January, had a peak of 627 W m-2 in July, and became lowest at 133 W m-2 in December. A detailed summary of the monthly DGM concentration regimes is available in the Supporting Information (SI-Table 1). The temporal change of DGM concentrations on a bimonthly scale also consistently demonstrates the same phenomenon as described above on a monthly scale (see Supporting Information, SI-Table 1). It thus emerges that a similar natural phenomenon manifests on two temporal scales. First, on the daily scale, the DGM concentration change closely follows the rhythm of diurnal solar radiation variation as a result of the earth’s rotation; second, through the 12-month period, the monthly mean DGM concentration change closely synchronizes the rhythm of the monthly mean solar radiation variation as a result of the earth’s revolution. The monthly phenomenon thus clearly mirrors the daily phenomenon. This finding agrees with the hypothesis described previously regarding the seasonal change of DGM concentrations in relation to solar radiation. Figure 3b provides a characterization of the temporal DGM concentration variation through the year in another perspective, i.e., by means of the seasonal mean DGM concentration (i.e., the average of all DGM data for a certain season defined by the solar position) in comparison with the variations of the seasonal mean global solar radiation and UVA radiation (defined similarly to the seasonal mean DGM concentration). A detailed summary of the seasonal DGM concentration regimes is available in the Supporting Information (SI-Table 1). Figure 3b shows a clear seasonal trend featuring high DGM concentrations in the spring (34.2 pg L-1) and summer (37.5 pg L-1) and low DGM concentrations in the fall (20.0 pg L-1) and winter (24.4 pg L-1). The same characteristics were also found for the seasonal maximum DGM concentrations (Figure 3b). On the seasonal scale, the temporal change of DGM concentrations in the lake also shared the same trend with the seasonal solar radiation variation (Figure 3b). The high seasonal mean DGM concentrations in the spring and summer are consistent with the high seasonal mean global solar radiation and UVA radiation (Figure 3b; also see SI-Table 1 in the Supporting Information). An overall comparison among the monthly, seasonal, and yearly mean DGM concentrations can be made. The yearly mean DGM concentration of Cane Creek Lake for the study period was found to be 32 pg L-1 (n ) 238). The monthly mean concentrations of DGM in the lake for the months of June-September 2003 and May 2004 and the seasonal mean DGM concentrations for the spring and summer were all above the yearly mean value. It appears that the overall yearly mean DGM concentration may serve as a boundary index distinguishing between the spring and summer high DGM concentration period and the fall and winter low DGM concentration period. This seems to suggest that there may exist two distinct periods of freshwater DGM regimes over a year, i.e., the strong photochemical period spanning spring and summer and the weak photochemical period spanning late fall and winter. The photochemistry of aquatic Hg could VOL. 40, NO. 7, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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differ to some degrees in the two periods. Further comparative studies should provide more knowledge about the characteristics and nature of each period and the biogeochemical significance of the phasing. Evaluation of the Relationship between DGM Concentration and Solar Radiation Variations during the Year. The essential control of solar radiation over daily DGM level variations in freshwaters has been increasingly observed (3, 10, 12, 22). This study demonstrated that the seasonal solar radiation variations and the seasonal changes of DGM concentrations in Cane Creek Lake also shared similar trends (Figure 3). To further characterize this phenomenon, the relationship between the DGM level and solar radiation was examined by linear regression analysis (see SI-Figure 2 in the Supporting Information). Linear relationships of the monthly mean DGM concentrations for Cane Creek Lake were found with the monthly mean global solar radiation (R2 ) 0.82, P < 0.05, DGM (pg L-1) ) 0.046Rg (W m-2) + 9.8) and the UVA radiation (R2 ) 0.84, P < 0.05, DGM (pg L-1) ) 0.026UVA (µW cm-2) + 12.9) (monthly maximum DGM concentrations vs monthly mean global solar radiation and UVA radiation: R2 ) 0.78, P < 0.05 and R2 ) 0.88, P < 0.05, respectively). Linear relationships of the bimonthly mean DGM concentrations were found with the bimonthly mean global solar radiation (R2 ) 0.85, P < 0.05) and UVA radiation (R2 ) 0.93, P < 0.05) (bimonthly maximum DGM concentrations vs bimonthly mean global solar radiation and UVA radiation: R2 ) 0.78, P < 0.05 and R2 ) 0.91, P < 0.05, respectively). Linear relationships of the seasonal mean DGM concentrations were found with the seasonal mean global solar radiation (R2 ) 0.85, P ) 0.08) and UVA radiation (R2 ) 0.93, P < 0.05) (seasonal maximum DGM concentrations vs seasonal mean global solar radiation and UVA radiation: R2 ) 0.90, P ) 0.05 and R2 ) 0.88, P ) 0.06, respectively). These consistent statistical evaluation results reinforce the notion that the temporal changes of DGM concentrations in the lake were mediated photochemically on the seasonal as well as daily scales. Supersaturation of DGM in Cane Creek Lake. Sunlightinduced production of DGM results in a wide occurrence of supersaturation of DGM in aquatic systems (14, 15, 24). Our study for Cane Creek Lake further shows that the supersaturation could be prevalent throughout a year (see SI-Figure 1 in the Supporting Information, a comparison of the monthly mean DGM concentrations in the lake with the equilibrium DGM concentrations at the corresponding monthly mean lake water temperatures). The comparison demonstrates that even in the fall and winter the monthly mean DGM concentrations for daytime were still significantly higher than the corresponding equilibrium DGM concentrations (Figure 3a; equilibrium concentrations of DGM for January, July, and December being 10.4, ∼6, and 10.3 pg L-1, respectively), indicating prevalent existence of DGM supersaturation in the lake. But, exceptions did occur in January ([DGM]min ) 6.1 < 10.4 pg L-1) and in December ([DGM]min ) 7.9 < 10.3 pg L-1). This suggests that when solar radiation was the lowest in a year, the photochemical DGM production was too weak to compensate for DGM removal; this would lead to an ultimate net DGM loss during nighttime, although in daytime the DGM levels would rise again above the saturation threshold. Some Potential Implications of This Study to Environmental Toxicology and Chemistry of Hg. The major goal of this study was to test the hypothesis that DGM concentrations in Cane Creek Lake would change following seasonal solar radiation variation. Our 12-month field observation clearly supports the hypothesis. This study further revealed the essential relationship between freshwater DGM chemodynamics and solar radiation and can provide the field data for modeling the photochemical redox transformation of 2118

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freshwater Hg. Moreover, it becomes necessary that a model for the aquatic photochemical redox chemistry of Hg needs to be able to describe and explain the trends of the changes of DGM concentrations with solar radiation variations on both daily and seasonal scales. Our study showed that the diurnal DGM concentration oscillation in Cane Creek Lake following solar radiation variation can be active nearly year-long, weak only in wintertime. The decrease of the DGM concentrations from the summer peaks to the low values in the winter period suggests that the removal of the DGM is occurring also on a seasonal scale. Aquatic photochemical oxidation of Hg(0) to Hg(II) has been implicated in a growing number of studies to be one of the major mechanisms responsible for removal of DGM in natural waters (e.g., 4, 7, 9, 10). The finding from this study thus may suggest that there could be a prolonged (nearly year-long in the south) regularly recurring supply of freshly produced inorganic Hg(II) to the lake by means of in situ aquatic photochemical oxidation of Hg(0). The pivotal role of fresh inorganic Hg(II) in aquatic Hg toxicity has been revealed recently by a whole-scale lakeshed study, the METAALICUS project (5). The results from this project suggest that the inorganic Hg(II) freshly introduced into the watershed may be more mobile and prone to methylation (5). The removal of DGM can also stem from its evasion. Our recent Hg air/water exchange study carried out for Cane Creek Lake using the dynamic flux chamber method showed that the general magnitude of the daytime Hg evasion flux in the lake averaged ∼1.2 ng m-2 h-1 for summertime and ∼0.5 ng m-2 h-1 for wintertime (for details, see ref 30). As a first-approximation, these values are equivalent to decreases of 6.0 and 2.5 ng m-3 or pg L-1, respectively, for a 5-h evasion from a water column of 1 m2 × 1 m. This estimation of the DGM loss from the evasion can account for only a portion of less than ∼25% of the large daily and seasonal decreases of DGM concentrations as observed in this study (also see refs 27, 28). Hence, sunlight-induced photochemical oxidation of DGM is likely to be responsible for the observed daily and seasonal DGM decreases to a greater degree. The sustained production of DGM during the year shown by this study suggests that photochemical reducible Hg(II) is available in the lake through the year, even though the total Hg of the lake water appeared to be low (∼0.2 ng L-1). In addition to atmospheric deposition (both wet and dry), photochemical oxidation of DGM may partially contribute to the available pool of reducible Hg(II). Further study to follow the total Hg and reducible Hg in the lake together with DGM concentrations and other relevant parameters on various temporal scales is desirable to advance the understanding of the aquatic photochemical cycle of Hg in the lake. Among the factors controlling aquatic photochemical reduction of Hg(II) are three essential parameters: the level of the reductants for Hg(II), the level of the reducible Hg(II) available, and the rate constant for the reduction reactions (3, 7, 10). Further study is desirable to reveal the sensitivity of each parameter to solar radiation on a seasonal scale. This will help to understand the observed seasonal variation of the DGM production peaks in the lake. An elaborate quantification of the rates of photochemical reduction and oxidation of Hg under both field and controlled conditions using authentic lake water samples as well as simulation systems is an imperative research need. This will shed light on why the DGM concentration variation manifests itself on the daily and seasonal scales in the similar ways observed.

Acknowledgments This study was supported by a grant donation from Ray and Michelle Whitford, to whom we are greatly grateful. We thank the four anonymous reviewers for their comments, sugges-

tions, and insights, which led to a great improvement of this paper. We are thankful to Drs. Dale Ensor, Eugene Kline, and Hollings Andrews (TTU) for their advice and help. We thank Dr. Peter Li (TTU) for his help with preparation of the site map and Dr. Jerry Lin (Lamar University) for his help with calculation of the extraterrestrial global horizontal solar irradiance. We thank Dr. Steve Lindberg for his advice and help. We thank Ginger Ensor of the Center for the Management, Utilization, and Protection of Water Resources (the CMUPWR, TTU) for the editorial suggestions for the manuscript. We are grateful to the CMUPWR and the Biology Department (TTU) and the Leisure Service Department of the City of Cookeville for their support and assistance for this study.

Supporting Information Available The Supporting Information includes SI-Figures 1 and 2 and SI-Table 1. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Schroeder, W. H.; Munthe, J. Atmospheric mercury-an overview. Atmos. Environ. 1998, 32, 809-822. (2) Mason, R. P.; Fitzgerald, W. F.; Morel, F. M. M. The biogeochemical cycling of elemental mercury: Anthropogenic influences. Geochim. Cosmochim. Acta. 1994, 58, 3191-3198. (3) O’Driscoll, N. J.; Beauchamp, S.; Siciliano, S. D.; Rencz, A. N.; Lean, D. R. S. Continuous analysis of dissolved gaseous mercury (DGM) and mercury flux in two freshwater lakes in Kejimkujik park, Nova Scotia: Evaluating mercury flux models with quantitative data. Environ. Sci. Technol. 2003, 37, 2226-2236. (4) Siciliano, S. D.; O’Driscoll, N. J.; Lean, D. R. S. Microbial reduction and oxidation of mercury in freshwater lakes. Environ. Sci. Technol. 2002, 36, 3064-3068. (5) Hintelmann, H.; Harris, R.; Heyes, A.; Hurley, J. P.; Kelly, C. A.; Krabbenhoft, D. P.; Lindberg, S.; Rudd, J. W. M.; Scott, K. J.; St. Louis, V. L. Reactive and mobility of new and old mercury deposition in a boreal forest ecosystem during the first year of the METAALICUS study. Environ. Sci. Technol. 2002, 36, 50345040. (6) Zhang, H.; Lindberg, S. E. Trends in dissolved gaseous mercury in the Tahquamenon river watershed and nearshore waters of Whitefish bay in the Michigan Upper Pennisula. Water, Air, Soil Pollut. 2002, 133, 379-389. (7) Zhang, H.; Lindberg, S. Sunlight and Fe(III)-induced photochemical production of dissolved gaseous mercury in freshwater. Environ. Sci. Technol. 2001, 35, 928-935. (8) Amyot, M.; Auclair, J. C.; Poissant, L. In situ high temporal resolution analysis of elemental mercury in natural waters. Anal. Chim. Acta 2001, 447, 153-159. (9) Lalonde, J. D.; Amyot, M.; Kraepiel, A. M. L.; Morel, F. M. M. Photooxidation of Hg(0) in artificial and natural waters. Environ. Sci. Technol. 2001, 35, 1367-1362. (10) Amyot, M.; Mierle, G.; Lean, D. R. S.; McQueen, D. J. Effect of solar radiation on the formation of dissolved gaseous mercury in temperate lakes. Geochim. Cosmochim. Acta 1997, 61, 975987. (11) Amyot, M.; Lean, D. R. S.; Mierle, G. Photochemical formation of volatile mercury in high artic lakes. Environ. Toxicol. Chem. 1997, 16, 2054-2063. (12) Amyot, M.; Mierle, G.; Lean, D. R. S.; McQueen, D. J. Sunlightinduced formation of dissolved gaseous mercury in lake waters. Environ. Sci. Technol. 1994, 28, 2366-2371.

(13) Watras, C. J.; Morrison, K. A.; Host, J. S.; Bloom, N. S. Concentrations of mercury species in relationship to other sitespecific factors in the surface waters of northern Wisconsin lakes. Limnol. Oceanogr. 1995, 40, 226-565. (14) Fitzgerald, W. F.; Mason, R. P.; Vandal, G. M. Atmospheric cycling and air-water exchange of mercury over mid-continental regions. Water, Air, Soil Pollut. 1991, 56, 745-767. (15) Vandal, G. M.; Mason, R. P.; Fitzgerald, W. F. Cycling of volatile mercury in temperate lakes. Water, Air, Soil Pollut. 1991, 56, 791-803. (16) Xiao, Z. F.; Munthe, J.; Schroeder, W. H.; Lindqvist, O. Vertical fluxes of volatile mercury over forest soil and lake surfaces in Sweden. Tellus 1991, 43B, 267-279. (17) Meili, M.; Iverfeldt, A.; Hakanson, L. Mercury in the surface water of Swedish forest lakes-Concentrations, speciation and controlling factors. Water, Air, Soil Pollut. 1991, 56, 439-453. (18) Lindberg, S. E.; Meyers, T. P.; Munthe, J. Evasion of mercury vapor from the surface of a recently limed acid forest lake in Sweden. Water, Air, Soil Pollut. 1996, 85, 2265-2270. (19) Nriagu, J. O. Mechanistic steps in the photoreduction of mercury in natural waters. Sci. Total Environ. 1994, 154, 1-8. (20) Lindberg, S. E.; Vette, A. F.; Miles, C.; Schaedlich, F. Mercury speciation in natural waters: Measurement of dissolved gaseous mercury with a field analysis. Biogeochemistry 2000, 48, 237259. (21) O’Driscoll, N. J.; Siciliano, S. D.; Lean, D. R. S. Continuous analysis of dissolved gaseous mercury in freshwater lakes. Sci. Total Environ. 2003, 304, 285-294. (22) Zhang, H.; Wang, X. Q.; Marsik, F. J.; Lehman, J.; Xu, X. H.; Nriagu, J.; Keeler, G.; Lindberg, S. E. Photochemodynamics of aquatic mercury in Saginaw Bay of Lake Huron (Michigan, USA). American Chemical Society 223rd Annual Meeting, Orlando, FL, April, 2002. (23) Sullivan, K. A.; Mason, R. P. The concentration and distribution of mercury in Lake Michigan. Sci. Total Environ. 1998, 213, 213-228. (24) Vette, A. F. Photochemical Influences on the Air-Water Exchange of Mercury. Ph.D. Thesis, University of Michigan, Ann Arbor, MI, 1998. (25) Zhang, H.; Lindberg, S. E. Air/water exchange of mercury in the Everglades I: The Behavior of dissolved gaseous mercury in the Everglades Nutrient Removal Project. Sci. Total Environ. 2000, 259, 123-133. (26) Krabbenhoft, D. P.; Hurley, J. P.; Olson, M. L.; Cleckner, L. B. Diel variability of mercury phase and species distributions in the Florida Everglades. Biogeochemistry 1998, 40, 311-325. (27) Dill, C. Aquatic photochemokinetics of mercury in Cane Creek Lake, Putnam County, TN. M.S. Thesis, Tennessee Technological University, Cookeville, TN, 2004. (28) Dill, C.; Kuiken, T.; Zhang, H.; Ensor, M. Diurnal variations of dissolved gaseous mercury (DGM) levels in a southern reservoir lake (Tennessee, USA) in relation to solar radiation. Sci. Total Environ. 2006, 357, 176-193. (29) Akland, G. G. Design of sampling schedules. J. Air Pollut. Control Assoc. 1972, 22, 264-266. (30) Crocker, W. C. Air/water exchange of aqueous gaseous mercury in a southern reservoir lake: Cane Creek Lake, Putnam County, TN. M.S. Thesis, Tennessee Technological University, Cookeville, TN, 2005.

Received for review July 18, 2005. Revised manuscript received January 17, 2006. Accepted January 23, 2006. ES0513990

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