Mass Balance of Total Mercury and Monomethylmercury in Coastal

ARTICLE pubs.acs.org/est

Mass Balance of Total Mercury and Monomethylmercury in Coastal Embayments of a Volcanic Island: Significance of Submarine Groundwater Discharge Yong-gu Lee,† MD. Moklesur Rahman,† Guebuem Kim,‡ and Seunghee Han*,† † ‡

School of Environment Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea School of Earth and Environmental Sciences, Research Institute of Oceanography, Seoul National University, Seoul 151-742, Korea

bS Supporting Information ABSTRACT: To understand the contribution of submarine groundwater discharge (SGD) to the coastal mass budgets of Hg and monomethylmercury (MMHg), preliminary mass balance estimates were made for Hwasun and Bangdu Bays on Jeju Island, known to have large SGD due to the high permeability of the volcanic rocks. The mass balance results indicate that SGD is a main source of Hg in Hwasun Bay (23 ( 14  10
2 mol yr
1, 34%) and Bangdu Bay (23 ( 20  10
2 mol yr
1, 67%), although the contribution from atmospheric deposition was considerable (25% for Hwasun and 23% for Bangdu). MMHg was also discharged primarily from submarine groundwater at Hwasun (0.30 ( 0.17  10
2 mol yr
1, 55%) and Bangdu (0.65 ( 0.49  10
2 mol yr
1, 64%), which was higher than atmospheric deposition (6% for Hwasun and 2% for Bangdu) and sediment diffusion flux (5% for Hwasun and 3% for Bangdu). The overall mass balance results suggest that, although there are large spatial variations in SGD rates throughout the region, the coastal mass budgets of Hg and MMHg need to include SGD as well as atmospheric deposition and sediment diffusion as primary sources of Hg and MMHg.

1. INTRODUCTION Discharge of groundwater from subterranean estuaries into coastal oceans has drawn increasing attention.1,2 A number of studies have suggested that annual fluxes of submarine groundwater discharge (SGD) into local estuaries or coastal zones are equally important as, or often more important than, annual river fluxes.3
5 Submarine groundwater discharge (SGD) from subterranean estuaries significantly contributes to carbon and nutrient loading in coastal waters.2,6,7 Nutrient inputs derived from SGD were similar to, or higher than, those from ambient rivers in the northeastern Gulf of Mexico5 and the southern coast of Korea.8,9 Such large nutrient inputs via SGD had significant ecological effects on coastal ecosystems, for example, harmful algal blooms10,11 and thriving phytobenthos.12 A few studies have demonstrated that direct groundwater flow into the ocean is also important for trace metal budgets in coastal waters.13
15 For instance, SGD was an important source of dissolved Fe, Zn, Co, and Ni in Jamaica Bay, NY.14 A nonconservative mixing behavior of metals in subterranean estuaries was found, suggesting that biogeochemical reactions, occurring in the water
sediment interface, are associated with the source or sink functions of subterranean processes.14,15 r 2011 American Chemical Society

The presence of monomethylmercury (MMHg) in the coastal environment has received particular attention due to the compound’s high neurotoxicity to humans. Consumption of fish and shellfish is the major route of human exposure to MMHg.16 In microorganisms, MMHg is readily bioaccumulated from ambient water and then biomagnified through aquatic food chains.17,18 Hg loading in coastal waters may be linked to an elevated MMHg level in fish through microbial methylation of inorganic Hg(II) from near-shore sediment.19,20 In support of this hypothesis, a positive correlation between the atmospheric deposition of Hg and MMHg body burdens was reported for freshwater biota.21,22 The relative importance of various coastal Hg sources has been described using mass balance studies.23
26 These studies commonly report that the total Hg input into the coastal marine system is dominated by river and atmospheric input, while internal production and river discharge are important sources of MMHg.23
25 Received: June 20, 2011 Accepted: October 2, 2011 Revised: September 8, 2011 Published: October 05, 2011 9891

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Figure 1. Sampling locations in Hwasun Bay and Bangdu Bay on Jeju Island, Korea.

Recently, several studies have suggested that SGD from subterranean estuaries might contribute to the Hg budgets of local coastal zones.27
29 For instance, the amount of Hg introduced to the bay through SGD was similar to the atmospheric input and far greater than the riverine input in Waquoit Bay, Massachusetts.28 Hg and MMHg budgets in Seine Bay were also influenced by SGD, causing an increased Hg concentration in mussels from the SGD region, at least twice as high as the concentration observed in mussels from an adjacent area.27 Additionally, Black et al.29 have found fluxes of Hg and MMHg from SGD along the Central California coast. The Hg flux via SGD was greater than the net atmospheric input, and the flux of MMHg via SGD was similar to benthic fluxes.29 Despite these examples, studies addressing the influence of groundwater inputs on Hg and MMHg mass budgets in coastal waters are limited, in contrast to studies of lakes and rivers.30,31 In the present study, we constructed Hg and MMHg mass budgets for coastal embayments of a volcanic island based on the measurements of Hg and MMHg for diverse sources and sinks. The study was carried out at Hwasun Bay and Bangdu Bay on Jeju Island, which is known to have a high groundwater seepage rate and no considerable river water discharge. The overall goal was to understand the significance of SGD in the coastal mass budgets of Hg and MMHg in Hwasun and Bangdu Bays.

2. MATERIALS AND METHODS 2.1. Study Sites. Jeju Island, a volcanic island with an area of 1830 km2, is located approximately 140 km off the southern coast of the Korean peninsula (Figure 1). Halla Mountain is at the center of the island (height 1950 m), sloping gently from the peak to the east and the west.32 Highly permeable basalt rocks, which form the principal aquifer, cover most of the island’s

surface area, and consolidated or semiconsolidated sedimentary rocks formed by hydrovolcanic activity underlie them. These consolidated sedimentary rocks, the Seoguipo Formation, are found only on the southern and western coasts. On the eastern coast, the basalt rocks, with the absence of the Seoguipo Formation, directly contact the sand and silt layer.32 On the western coast there are many artesian flows and freshwater wells, mostly originating from rainy season precipitation. On the contrary, the groundwater a few meters below is composed almost entirely of recirculated seawater on the eastern coast.32
34 There is little substantial streamflow on the island despite heavy rainfall (1946 ( 354 mm yr
1, 19-year average, Korea Meteorological Administration, KMA35). As a result of the high permeability of the basalt rocks, 44% of the total annual precipitation is recharged into the aquifer.33 The mountainous areas are composed of natural forests and grasslands, and the coastal land is agricultural.32 The study areas, Hwasun Bay and Bangdu Bay, are semienclosed bays located on the southwest and east coasts of Jeju, respectively (Figure 1). Bangdu Bay, with an area of approximately 0.80 km2 and a mean depth of 3 m, is relatively small and shallow compared to Hwasun Bay, which has an area of approximately 1.7 km2 and a mean depth of 6.5 m. Area normalized groundwater discharge is about three times greater at Bangdu (120
180 m3 m
2 yr
1)12 than at Hwasun (40
50 m3 m
2 yr
1),36 perhaps due to the absence of the Seoguipo Formation on the east coast of the island. Because Korea is in the Asian Monsoon Climate region, the period from late autumn to spring is a dry season, and summer to early autumn is a wet season. 2.2. Water and Sediment Sampling. Seawater and coastal groundwater samples were collected February 17
20 (winter), May 5
7 (summer), and June 2
4 (summer) 2010 from Hwasun and Bangdu Bays using ultraclean handling techniques.37 9892

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Table 1. Characteristics of Coastal Seawater and Groundwater in Hwasun Bay and Bangdu Bay (Data are Mean ( SD of n Samples) pH

salinity (ppt)

DOC (μM)

SPM (mg L‑1)

Hwasun Bay groundwater

winter (n = 4)

8.1 ( 0.31

27 ( 5.3

163 ( 17

inner bay seawater

summer (n = 4) winter (n = 6)

8.3 ( 0.085 8.2 ( 0.025

23 ( 8.9 31 ( 0.46

119 ( 19 129 ( 32

0.83 ( 0.30

summer (n = 3)

8.3 ( 0.039

30 ( 0.74

88 ( 9.7

0.60 ( 0.15

winter (n = 3)

8.1 ( 0.025

31 ( 0.44

111 ( 2.0

0.76 ( 0.36

8.3

30

77

0.44

outer bay seawater

summer (n = 1)

Bangdu Bay groundwater

winter (n = 4)

7.8 ( 0.19

28 ( 3.3

168 ( 12

summer (n = 4)

8.0 ( 0.13

17 ( 2.4

130 ( 52 166 ( 7.2

0.44 ( 0.17

88 ( 1.5 164

0.74 ( 0.074 0.48

85

0.65

inner bay seawater

winter (n = 5)

8.2 ( 0.027

31 ( 0.089

outer bay seawater

summer (n = 3) winter (n = 2)

8.3 ( 0.082 8.3

29 ( 1.8 30

summer (n = 2)

8.3

Surface seawater was pumped from a depth of approximately 50 cm into Teflon bottles, through Teflon tubing connected to a peristaltic pump. Filtered seawater samples were collected by the same method using a 0.45-μm pore size polyethersulfone cartridge filter.38 Unfiltered seawater samples were collected in polyethylene bottles for measurement of suspended particulate matter (SPM), and filtered seawater samples were collected in precleaned glass bottles for measurement of dissolved organic carbon (DOC). Salinity, pH, and temperature were measured in the field. The detailed sampling methods for groundwater, sediment core, and rainwater are provided in Supporting Information S1. 2.3. Analysis of Hg Species and Ancillary Parameters. Concentrations of Hg and MMHg in seawater, groundwater, sediment digests, and pore water were analyzed following EPA method 163139 and Bloom et al.40 The analytical methods for Hg species and ancillary parameters, and statistical methods are shown in Supporting Information S2 and S3. 2.4. Flux Calculation. 2.4.1. Wet and Dry Deposition. For the wet deposition flux of water, we multiplied the mean annual precipitation (1946 ( 354 mm yr
1, KMA) by the bay area. Then, the wet deposition flux of water was multiplied by the mean Hg concentration in the rainwater. For the dry deposition of Hg, we used the literature value measured from nine different areas in Japan, 0.040 ( 0.013 mol m
2 yr
1,41 since no data were currently available for Korea. 2.4.2. Sediment Diffusion. The diffusive fluxes of Hg and MMHg from sediment were estimated by Fick’s first law:42   dC J ¼
ϕDs ð1Þ dz
2


1

where J (ng m day ) is the flux of a solute with concentration gradient dC/dz (ng m
4), and ϕ is sediment porosity, ϕ = (Mw/Fw)/[(Ms/Fs) + (Mw/Fw)]43 where Mw is the weight of water lost, Ms is dry sediment weight, Fw is water density, and Fs is dry sediment density. The diffusion coefficient (Ds) defined in Ullmann and Aller44 is Ds ¼ θ2 D0

ð2Þ √ where the θ is a tortuosity term that can be calculated by (1
ln(ϕ2)).45 We calculated 0.35 and 0.46 for sediment porosity

30

(dimensionless) and 1.8 and 1.6 for tortuosity (dimensionless) in Hwasun and Bangdu Bays, respectively. The molecular diffusion coefficient (D0) in seawater was 5.0  10
6 cm2 s
1 for Hg46,47 and 1.2  10
5 cm2 s
1 for MMHg.48 2.4.3. Volatilization. At the air
water boundary, the evasion flux (F) of Hg0 can be estimated from the Hg0 difference between air and water, Δcair
water = (cair/H)
cwater, and the gas-exchange transfer velocity k(u).49 The Henry’s constant (H) for Hg0 at the corresponding water temperature was 0.28.50 F ¼ kðuÞ  Δcair
water

ð3Þ 49

From the above equation, k(u) is determined as follows: kðuÞ ¼ ð0:365u2 þ 0:46uÞ  ðScHg =660Þ
0:5

ð4Þ

In eq 4, the mean wind speeds (u) at Hwasun and Bangdu Bays were 4.3 m s
1 and 6.1 m s
1, respectively (Korea Hydrographic and Oceanographic Administration, KHOA, 51), and the Schmidt number, ScHg, was 689.52 2.4.4. Photodemethylation. MMHg loss by photodemethylation was determined using the eq 5 constructed for the photodecomposition rate (Kdecomp) of MMHg in arctic lakes.53 Kdecomp ¼ k  CMMHg  PAR

ð5Þ

We used the annual mean flux of photochemically active radiation (PAR) of 25 E m
1 y
1,25 a PAR attenuation coefficient of 2 m
1 determined from Jeju coastal water,54 and k of 2.60  10
3 m2 E
1 from the literature.53

3. RESULTS AND DISCUSSION 3.1. DOC in Coastal Groundwater and Seawater. DOC concentrations were significantly higher for coastal groundwater than for seawater (p = 0.039 in winter and p = 0.0054 in summer, Table 1), suggesting that recirculation of seawater in the subterranean estuary increases DOC loads. Considerable transport of DOC into coastal water through the subterranean process has been reported in Rhode Island and the Gulf of Mexico in the United States.55,56 Koh et al.32 have suggested that surfacederived contaminants (e.g., nitrate) could be easily transported to the aquifer due to the high permeability and high recharge rate 9893

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Figure 2. Concentrations of HgD, HgT, and HgP (mean ( standard error) in the groundwater and seawater of Hwasun (HS) and Bangdu (BD) Bays.

(over 40% of total precipitation), and relatively short flow path of the basalt aquifer on Jeju Island. Indeed, nitrate contamination was noticeable in coastal areas due to agricultural land use,32 and the fluxes of dissolved inorganic nitrogen (DIN) from SGD contributed approximately 90% of total DIN flux in the coastal water.12 The mean DOC concentrations in coastal groundwater and seawater were commonly higher in winter than in summer (p = 0.0093 for groundwater and p < 0.001 for seawater). This is the opposite of the Gulf of Mexico, where DOC flux was higher in summer than winter due to enhanced remineralization in the summer aquifer.56 The higher mean DOC concentrations in winter versus summer suggest that a large volumetric input of groundwater in the summer, attributable to large precipitation, may dilute DOC supplied from the land surface and/or from the remineralization processes in the aquifer. 3.2. Total Hg in Coastal Groundwater and Seawater. The dissolved Hg (HgD) concentration found in the Hwasun coastal groundwater ranged from 3.4 to 4.2 pM in winter and from 1.6 to 3.2 pM in summer (Figure 2a). In Bangdu, it ranged from 0.84 to 2.0 pM in winter and from 1.1 to 1.6 pM in summer. For Hwasun Bay, the mean HgD levels were higher for groundwater than for seawater. Like DOC, recirculation of seawater in the aquifer appeared to increase HgD loads. However, the same trend was not found in Bangdu Bay. The mean Hg concentration in the Hwasun sediment (10 ( 0.92 ng g
1, n = 4), collected from the intertidal region, was seven times the concentration in the Bangdu sediment (1.5 ( 0.11 ng g
1, n = 4), which might contribute to the increased HgD in the groundwater of Hwasun Bay compared to seawater. The unfiltered Hg (HgT) concentration found in Hwasun seawater ranged from 2.5 to 3.2 in winter and from 1.2 to 1.7 pM in summer (Figure 2b). In Bangdu, it ranged from 1.6 to 2.0 pM in winter and from 1.2 to 1.4 pM in summer. Among four data sets, substantially increased HgT was found for Hwasun in winter, which follows the trend of HgD in coastal groundwater and

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Figure 3. Concentrations of MMHgD, MMHgT, and %MMHgT (mean ( standard error) in groundwater and seawater of Hwasun (HS) and Bangdu (BD) Bays.

seawater. Regarding particulate Hg (HgP), significantly (p < 0.05) low particulate HgP levels for summer versus winter were found in Bangdu Bay. The depletion of Hg in Bangdu summer particles might be attributable to the nature of suspended particles associated with large primary production. Because Hg tends to be enriched in mature organic particles, phytoplankton blooms that bring fresh organic matter to the SPM often cause a dilution effect in HgP.57,58 The large nutrient input via SGD, as well as the consequent benthic primary production, is wellknown in Bangdu Bay.12 3.3. MMHg in Coastal Groundwater and Seawater. The dissolved MMHg (MMHgD) concentration found in Hwasun coastal groundwater ranged from 0.023 to 0.041 pM in winter and from 0.033 to 0.043 pM in summer (Figure 3a). In Bangdu, it ranged from 0.038 to 0.060 pM in winter and from 0.022 to 0.047 pM in summer. The trends of unfiltered MMHg (MMHgT) followed that of MMHgD in seawater (Figure 3b). The fraction of MMHgT over HgT (%MMHgT) was higher in Bangdu than in Hwasun, and it was higher in summer than in winter. The larger %MMHgT in Bangdu seawater versus Hwasun seawater is consistent with the trend of sediment %MMHg, which is in agreement with the idea that MMHg production in near-shore systems mainly arises from sediment.59 The MMHg contents in dry sediment collected during the summer were 3.3 ( 1.1 pg g
1 (n = 4) for Hwasun and 7.1 ( 0.96 pg g
1 (n = 4) for Bangdu, and the percentage of MMHg in dry sediment was 0.032% for Hwasun and 0.47% for Bangdu. Organic matter content could be a limiting factor for MMHg production in sediments, since both bay deposits are comprised largely of low organic sandy sediment. Large benthic primary production in Bangdu Bay,12 which may increase the availability of metabolizable organic matter, appears to cause greater MMHg production in Bangdu versus Hwasun.36,60 The higher percentage of MMHg 9894

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Figure 4. Simplified Hg mass balance for Hwasun and Bangdu Bays on Jeju Island. Units are 10
2 mol yr
1.

in summer, which is typical in near-shore systems, may be associated with enhanced microbial activity at higher temperatures.61,62 3.4. Sources of Total Hg and MMHg. The annual wet atmospheric deposition flux of Hg was estimated to be (10 ( 5.0)  10
2 mol yr
1 and (5.0 ( 2.4)  10
2 mol yr
1 for Hwasun and Bangdu, respectively (Figure 4), applying the mean HgT concentration in rainwater collected from Gwangju and Jeju (31 ( 14 pM, n = 6). The wet atmospheric deposition flux of MMHg was estimated to be (0.033 ( 0.021)  10
2 mol yr
1 and (0.016 ( 0.010)  10
2 mol yr
1 for Hwasun and Bangdu, respectively (Figure 5), applying the mean MMHgT concentration in rainwater (0.10 ( 0.063 pM, n = 5). The uncertainty term for wet atmospheric deposition was calculated by a propagation of errors based on the variability associated with the mean annual precipitation and the variability associated with the HgT or MMHgT in rainwater. The estimated dry deposition flux of Hg for Hwasun and Bangdu was (6.8 ( 2.3)  10
2 mol yr
1 and (3.2 ( 1.1)  10
2 mol yr
1, respectively (Figure 4). The rainwater HgT concentration, scavenging reactive gaseous Hg and particulate Hg in the atmosphere, of Jeju and Gwangju was relatively low, and similar to that of Long Island Sound (LIS, 20
30 pM24) but lower than that of Chesapeake Bay (CB, 60
100 pM63). At LIS, the overall wet and dry deposition flux of 41 nmol m
2 yr
1 (Table 2) is similar to those of Midwest and Northeast U.S. points, indicating that long-range transport is a major source of atmospheric Hg.24 The wet and dry depositions

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Figure 5. Simplified MMHg mass balance for Hwasun and Bangdu Bays on Jeju Island. Units are 10
4 mol yr
1.

of Hg at Tokyo Bay (TB) were larger than those of Hwasun and Bangdu (64; Table 2). This might be attributed to relatively large local Hg emission in Tokyo, considering similar wet precipitation between Jeju and Tokyo (1530 mm yr
1 for Tokyo and 1946 mm yr
1 for Jeju). In Jeju, long-range transport appears be a major source of Hg, instead of local deposition. Regarding MMHg, the atmospheric deposition flux of Hwasun and Bangdu was smaller than the one in the New York/New Jersey Harbor Estuary (NY/NJ) and LIS, and similar to that in CB and Bay of Fundy (BF, Table 2). The ratio of MMHg/Hg was about 3% in NY/NJ and LIS.24,25 In contrast, it was 0.32% in Jeju, more or less similar to CB (0.22
0.37%, 63). Jeju and CB may have no local sources of MMHg from the atmosphere. The sediment diffusive fluxes of Hg were estimated to be (5.4 ( 2.2)  10
2 mol yr
1 in Hwasun and (0.36 ( 0.12)  10
2 mol yr
1 in Bangdu (Figure 4). For this estimate, we applied the mean pore water and seawater Hg concentrations of 38 ( 4.3 pM (n = 4) and 1.9 ( 0.78 pM (n = 10), respectively, in Hwasun, and 5.7 ( 0.64 pM (n = 4) and 0.99 ( 0.31 pM (n = 7), respectively, in Bangdu. The MMHg diffusive fluxes were estimated to be (0.028 ( 0.0075)  10
2 mol yr
1 in Hwasun and (0.033 ( 0.010)  10
2 mol yr
1 in Bangdu (Figure 5). The pore water MMHg concentrations in Hwasun and Bangdu were 0.10 ( 0.010 pM (n = 4) and 0.22 ( 0.022 pM (n = 4), respectively. The seawater MMHg concentrations in Hwasun and Bangdu were 9895

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Table 2. Mass Balance Estimates of Total Hg and MMHg (in Parentheses) for Different Coastal Embayments (Units are nmol m
2 yr
1) term atmospheric wet/dry deposition river/watershed water treatment facilities

a

Hwasun Bay

Bangdu Bay

a

a

60/40 (0.24 ) n/ac

Tokyo Bay

62/40 (0.21 ) n/a

94/99 363

New York/New

Long

Jersey Harbor Estuary

Island Sound

a

b

Chesapeake

a

a

60 (1.8 ) 4540 (42)

41 (1.1 ) 303 (7.0)

Bay b

Bay of Fundy a

108 (0.54 ) 177 (2.3)

55a (0.49a) 674 (43)

n/a

n/a

n/a

280 (6)

19 (0.47)

n/a

n/a

sediment diffusion

32 (0.16)

4.5 (0.41)

n/a

n/a (16)

n/a (17)

n/a (5.3)

27 (2.2)

SGD

135 (1.8)

289 (8.1)

n/a

n/a

n/a

28 (0.85)

n/a

total input

267 (2.2)

396 (8.7)

556

4880 (66)

363 (26)

313 (9.0)

756 (46)

volatilization

163 (n/a)

154 (n/a)

254

120 (n/a)

125 (n/a)

48 (n/a)

122 (n/a)

ocean export

106 (1.9)

258 (11)

68

3120 (28)

25 (0.47)

90 (3.2)

318 (35)

photodemethylation bioaccumulation

n/a (1.1) n/a (0.14)

n/a (1.8) n/a (0.29)

n/a n/a

n/a (4) n/a (26)

n/a (1.6) n/a (16)

n/a 4.2 (4.2)

n/a (3.9) n/a

burial

130 (0.043)

19 (0.092)

2571

1640 (8)

213 (4.7)

158 (0.8)

133 (1.0)

total output

399 (3.2)

431 (13.2)

2893

4880 (66)

363 (23)

300 (8.2)

573 (40)

reference

this study

this study

64

25

24

23

68

Wet deposition. b Wet + dry deposition. c n/a = not available.

0.024 ( 0.006 pM (n = 10) and 0.038 ( 0.011 pM (n = 7), respectively. Because Hwasun pore waters were not available, the pore water Hg and MMHg values for Hwasun were estimated from the Bangdu values, applying the sediment Hg and MMHg ratios between Hwasun and Bangdu Bays. The uncertainty term for sediment diffusion was calculated based on the standard deviations associated with the mean HgD or MMHgD in pore water and seawater. Benthic MMHg fluxes normalized to the surface areas of Hwasun and Bangdu were considerably lower than those of U.S. coasts (Table 2). This is mostly attributable to low MMHg levels in the sand pore waters of Hwasun and Bangdu. For example, the pore water MMHg range was 10
30 pM in LIS.65 The Hg and MMHg flux from a subterranean estuary was estimated by the measurements of Hg and MMHg in coastal groundwater and the SGD rate in the literature. The uncertainty term for the SGD flux was estimated based on the standard deviations associated with the mean HgD or MMHgD in coastal groundwater, and those associated with the SGD rate. In the literature, SGD fluxes in Hwasun and Bangdu were calculated on the basis of the mass balance models of 222Rn, 224Ra, and 226Ra, and the literature variation (standard deviation) of the SGD flux was 20% and 35% for Hwasun and Bangdu, respectively.12,36 For a conservative estimate, we applied a 50% variation for SGD uncertainty, which results in 75 ( 37 m3 yr
1 for Hwasun and 127 ( 63 m3 yr
1 for Bangdu. The coastal groundwater HgD concentrations in Hwasun and Bangdu were 3.0 ( 1.0 pM (n = 10) and 1.8 ( 1.3 pM (n = 9), respectively, and MMHgD concentrations in Hwasun and Bangdu were 0.040 ( 0.011 pM (n = 10) and 0.051 ( 0.029 pM (n = 9), respectively. The estimated fluxes of Hg for Hwasun and Bangdu were (23 ( 14)  10
2 mol yr
1 and (23 ( 20)  10
2 mol yr
1, respectively, with those of MMHg being (0.30 ( 0.17)  10
2 mol yr
1 and (0.65 ( 0.49)  10
2 mol yr
1, respectively (Figures 4 and 5). The Hg fluxes normalized to the surface area are 135 and 289 nmol m
2 yr
1 for Hwasun and Bangdu, respectively (Table 2), which are comparable to the SGD flux of Hg in Waquoit Bay: 172
694 nmol m
2 yr
1.28 In Waquoit Bay, the mean Hg concentration in groundwater was higher (50 pM), but the overall groundwater discharge rate was lower (37 000 m3 day
1) than in Hwasun and Bangdu.

3.5. Sinks of Total Hg and MMHg. The overall evasion fluxes of Hg0 were estimated to be (28 ( 20)  10
2 mol yr
1 for Hwasun and (12 ( 6.1)  10
2 mol yr
1 for Bangdu Bay (Figure 4), using the literature value of annual mean gaseous Hg concentration in Jeju Island (3.85 ( 1.67 ng m
3)66 and seawater dissolved gaseous Hg measured in the current study: 0.08 ( 0.04 pM (n = 3) for Hwasun and 0.05 ( 0.01 pM (n = 3) for Bangdu. Normalized to the surface area, the estimated evasion fluxes of Hg0 were 163 ( 118 nmol m
2 yr
1 for Hwasun and 154 ( 76 nmol m
2 yr
1 for Bangdu. The Hg evasion flux relies on wind speed, Hg(II) deposition, biological activity, and UV radiation.67 Although there may be a large variability of Hg0 evasion flux by time and space, estimates of Hg0 volatilization of Hwasun and Bangdu were similar to those for temperate estuaries, such as NY/NJ, LIS, and BF (Table 2). Similar evasion flux between Jeju and LIS is validated by the comparable mean gaseous Hg concentrations in the atmosphere (3.0 ng m
3) and seawater (0.10
0.42 pM), as well as the mean wind speed (3
6 m s
1) of LIS.67 Hg0 evasion at the seawater interface was greater than the Hg input from wet and dry atmospheric deposition (Table 2), suggesting that Hwasun and Bangdu Bays are a net source of Hg to the atmosphere, as shown in NW/NJ, LIS, and BF.24,25,68 On the basis of the water balance, the export flux of water from the bay to the ocean is equivalent to the net water discharge into the bay: (78 ( 37)  106 m3 yr
1 for Hwasun and (129 ( 63)  106 m3 yr
1 for Bangdu. The export amount of Hg was then estimated by multiplying the net water discharge by the mean HgT in Hwasun seawater (2.2 ( 0.73 pM, n = 10) and that in Bangdu seawater (1.6 ( 0.24 pM, n = 10). The estimated export fluxes of Hg were (17 ( 9.9)  10
2 mol yr
1 for Hwasun and (21 ( 11)  10
2 mol yr
1 for Bangdu (Figure 4). The mean MMHgT in Hwasun seawater was 0.041 ( 0.0071 pM, (n = 10) and that in Bangdu seawater was 0.065 ( 0.014 pM, (n = 7). The estimated export fluxes of MMHg were (0.32 ( 0.16)  10
2 mol yr
1 for Hwasun and (0.84 ( 0.45)  10
2 mol yr
1 for Bangdu (Figure 5). Here, the uncertainty term was calculated by the error propagation based on the standard deviations associated with the mean water discharge into the ocean and the mean HgT or MMHgT in bay water. The estimated export fluxes 9896

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Environmental Science & Technology of Hg and MMHg normalized to the surface area were similar to those of temperate estuaries, with the exception of NY/NJ (Table 2). Relatively large ocean exports of Hg and MMHg in NY/NJ appear to be associated with extremely large freshwater discharge from the Hudson River (1.4  1010 m3 yr
1).25 The burial flux of Hg and MMHg to the bay sediment was estimated by multiplying the sedimentation rate (0.10 cm yr
1) reported for the coastal area around Seoguipo Harbor69 by the surface area of the bay and the Hg and MMHg sediment concentrations. Hg and MMHg sediment concentrations were 50 ( 4.6 and 0.016 ( 0.0055 pmol g
1 (n = 4), respectively, in Hwasun and 7.5 ( 0.55 and 0.035 ( 0.0048 pmol g
1 (n = 4), respectively, in Bangdu. The variability associated with sedimentation was obtained from the standard deviations of Hg or MMHg sediment concentrations. The annual burial fluxes of Hg were (22 ( 2.0)  10
2 mol yr
1 for Hwasun and (1.6 ( 0.11)  10
2 mol yr
1 for Bangdu (Figure 4). The annual burial fluxes of MMHg were (0.71 ( 0.24)  10
4 mol yr
1 for Hwasun and (0.74 ( 0.10)  10
4 mol yr
1 for Bangdu (Figure 5). The burial Hg fluxes normalized to the surface areas of Hwasun and Bangdu were similar to those of LIS, CB, and BF, but one to two orders lower than those of TB and NY/NJ (Table 2). The high burial flux of TB and NY/NJ seems to be associated with high sediment Hg concentrations (2.1 nmol g
1 in Tokyo bay;64 4.8 nmol g
1 in NY/NJ25), despite the comparable sedimentation rates (0.16 cm yr
1 in Tokyo bay;64 0.08 cm yr
1 in NY/NJ25). The high burial flux of MMHg in NY/NJ and LIS is expected, given that the MMHg sediment concentrations of NY/NJ and LIS are higher than our results (e.g., 17
32 pmol g
1 in NY/NJ)70. Overall, MMHg transport from sediment to overlying water by diffusion processes exceeds the removal of Hg from the water column by settling particles (Table 2). This indicates that sediment is a net source of MMHg into the water column, providing MMHg for pelagic food webs. The bioaccumulation flux of MMHg was estimated using primary production reported for the eastern coast of Jeju (95 g C m
2 yr
1)71 and the particulate MMHg normalized to the phytoplankton carbon concentration (1.5 ( 0.85 pmol g
1 C for Hwasun and 3.1 ( 0.91 pmol g
1 C for Bangdu). Here, phytoplankton carbon concentrations were estimated from chlorophyll-a measurements using the carbon to chlorophyll-a ratio in the literature.58 Overall, annual bioaccumulation fluxes were estimated to be (0.024 ( 0.014)  10
2 mol yr
1 for Hwasun and (0.026 ( 0.0069)  10
2 mol yr
1 for Bangdu (Figure 5). Relatively low area-normalized bioaccumulation fluxes of MMHg as compared to that in other coastal systems (Table 2) could be associated with the oligotrophic nature of Jeju coastal water as well as low MMHg content in primary producers. In fact, primary production in LIS was 200
400 g C m
2 yr
1 and mean MMHg concentration in primary producers was 4.5 pmol g
1 wet weight or about 90 pmol g
1 C dry weight.24 The estimated loss of MMHg by photodemethylation, integrated over a 2-m water depth, was (0.19 ( 0.048)  10
2 mol yr
1 for Hwasun and (0.14 ( 0.041)  10
2 mol yr
1 for Bangdu (Figure 5). Photodemethylation fluxes normalized to the surface area were similar to those of NY/NJ, LIS, and BF located in a temperate zone (Table 2). 3.6. Mass Balance of Total Hg and MMHg. Finally, the estimated annual input and output of Hg to Hwasun Bay were 45  10
2 and 67  10
2 mol, respectively, while those of Bangdu Bay were 32  10
2 and 35  10
2 mol, respectively (Figure 4). SGD was a main source of Hg for Hwasun Bay (34%) and

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Bangdu Bay (67%), although the contribution from atmospheric deposition was considerable: 25% for Hwasun and 23% for Bangdu. Combined, SGD and atmospheric flux accounted for most transport of Hg into the bays. Volatilization, ocean export, and burial flux were equally important as a sink of Hg in Hwasun, and major Hg sink was an ocean export in Bangdu. The large difference in the burial flux of Hwasun versus the one of Bangdu was caused by the large variation in Hg sediment concentration. The missing input, estimated from the difference of total output and total input, was 33% for Hwasun and 9% for Bangdu, which could be attributable to sediment erosion of Hg. With respect to the MMHg loadings, the estimated annual input and output were 0.36  10
2 and 0.54  10
2 mol, respectively, for Hwasun, and 0.70  10
2 and 1.0  10
2 mol, respectively, for Bangdu (Figure 5). The prime source of MMHg was SGD: 55% of total input for Hwasun and 64% of total input for Bangdu. The SGD flux of MMHg largely exceeded the sediment flux of MMHg (5% for Hwasun and 3% for Bangdu), implying that most sediment MMHg might indeed be transported via groundwater discharge rather than molecular diffusion. Combined, the SGD flux and sediment diffusion flux account for most MMHg transport to the bay; thus, internal sediment production appears to be a prime source of MMHg in Hwasun and Bangdu, as shown in other coastal areas.23,24,65 The relative importance between sediment production and fluvial discharge as a source of MMHg in coastal water might be related to the geographical location of the bay. Significant river discharge of MMHg was found at BF68 and NY/NJ,25 which are located close to the mouth of a large river. Ocean export was a main sink of MMHg in both bays. The missing input, estimated by the difference of total output and total input, was 34% for Hwasun and 31% for Bangdu, which could be caused by sediment erosion or periphyton production of MMHg.72 Regarding caveats for the current study, there might be large uncertainties in the SGD estimation, because SGD varies temporally and spatially, and experimental determination of SGD flux often does not fully cover these fluctuations. Another large uncertainty of the mass flux estimation may occur from the fact that submarine groundwater was collected from shallow wells exposed to oxic conditions, which may cause underestimation of groundwater Hg and MMHg concentrations. It is likely that oxic exposure of groundwater enhances coprecipitation of Hg and MMHg with Fe(III)- and Mn(IV)-oxyhydroxide, thereby reducing dissolved Hg and MMHg levels from the actual groundwater concentrations.73,74 This could be part of the reason for the relatively large amount of missing input for Hg and MMHg. Nevertheless, submarine groundwater discharges into coastal water through the surface oxic layer of sediment; therefore, we argue that the underestimation of SGD flux involved in oxic collection would not be critical. 3.7. Environmental Implication. The SGD-driven HgD and MMHgD fluxes over the entire island were estimated by multiplying the mean HgD and MMHgD concentrations in the groundwater of Hwasun and Bangdu by an overall SGD rate for Jeju Island (1.65  1010 m3 yr
1).36,75 The overall inputs of HgD and MMHgD through SGD were 40 ( 20 and 0.70 ( 0.33 mol yr
1, respectively. This SGD flux of HgD is comparable to the overall HgD flux from major rivers in Korea that flow into the Yellow Sea (i.e., the Han River, 36 ( 16 mol yr
1; the Geum River, 13 ( 5.7 mol yr
1; the Yeongsan River, 3.0 ( 1.3 mol yr
1; Rahman et al., submitted). The groundwater 9897

dx.doi.org/10.1021/es202093z |Environ. Sci. Technol. 2011, 45, 9891–9900

Environmental Science & Technology transport of HgD and MMHgD from Jeju to the Yellow Sea appears to be as important as river transport from the Korean Peninsula. Direct atmospheric deposition and in situ production from sediment are often cited as important sources of Hg and MMHg, respectively, to estuaries.23,24 There were large discharges of Hg and MMHg from the submarine groundwater of Hwasun and Bangdu Bays, which are considerably higher than atmospheric deposition and sediment diffusion. We suggest that conventional coastal input estimations of Hg and MMHg, based on atmospheric deposition and diffusion calculation, may possibly underestimate overall Hg and MMHg fluxes to coastal waters. Although the mass balance results show that SGD is an important source of Hg and MMHg in the coastal water of Jeju, ambiguity remains as to large spatial and temporal variations in SGD flux, and Hg and MMHg levels in groundwater. Extensive mass balance studies therefore would be necessary for a complete understanding of the SGD contribution to the Hg and MMHg mass budgets of coastal systems.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text S1
S3 and References. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +82-62-715-2438; fax: +82-62-715-2434; e-mail: shan@ gist.ac.kr.

’ ACKNOWLEDGMENT We are grateful for the kind support of Hyunji Kim, Jiyi Jang, Seam Noh, Miji Kim, and Chunho Lee for sample collection, and Taehoon Kim for DOC measurement. This study was supported by a National Research Foundation of Korea grant, funded by the Korean government (2009-0085216) and by the “Innovative Technology of Ecological Restoration” project at GIST. ’ REFERENCES (1) Moore, W. S. The role of submarine groundwater discharge in coastal biogeochemistry. J. Geochem. Explor. 2006, 88 (1
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