Chemosphere 91 (2013) 320–327

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Chemosphere

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Significance of submarine groundwater discharge in the coastal fluxesof mercury in Hampyeong Bay, Yellow Sea

0045-6535/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2012.11.052

⇑ Corresponding author. Tel.: +82 62 715 2438; fax: 82 62 715 2434.E-mail address: shan@gist.ac.kr (S. Han).

MD. Moklesur Rahman a, Yong-gu Lee a, Guebuem Kim b, Kitack Lee c, Seunghee Han a,⇑a School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Republic of Koreab School of Earth and Environmental Sciences, Research Institute of Oceanography, Seoul National University, Seoul 151-742, Republic of Koreac School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea

h i g h l i g h t s

" We estimated mercury mass fluxes in the Hampyeong Bay, Yellow Sea." Submarine groundwater discharge was the major source of mercury." The coastal mercury budget estimation needs to include groundwater discharge.

a r t i c l e i n f o

Article history:Received 28 May 2012Received in revised form 23 November 2012Accepted 24 November 2012Available online 28 December 2012

Keywords:MercurySubmarineGroundwaterFluxCoastalInput

a b s t r a c t

Submarine groundwater discharge (SGD) and various solutes released with SGD have received particularattention recently; however, understanding of the impact of SGD on trace metal fluxes in the coastalocean is limited. To understand the contribution of SGD to the coastal Hg input, the Hg mass fluxes asso-ciated with SGD were estimated from Hampyeong Bay, a coastal embayment in the Yellow Sea. Hg con-centrations in filtered groundwater and seawater ranged from 1.3 to 4.4 pM and from 0.83 to 2.0 pM,respectively, and Hg concentrations in unfiltered seawater ranged from 1.7 to 4.6 pM. The Hg flux esti-mation showed that SGD was the prime input source of Hg in the bay (18 ± 12 mol yr�1), contributing65% of the total input. Atmospheric deposition was the second dominant source of Hg (8.5 ± 2.7 mol yr�1),contributing 31% to the total input. The results of the current study suggest that SGD can be a significantsource of Hg in estuarine/coastal systems; therefore, estimating the coastal mass budgets of Hg mustinclude SGD as a prime source of Hg.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Subterranean estuaries are coastal aquifer zones where land-derived freshwater is combined with intruding saltwater. Directdischarge of groundwater from subterranean estuaries into coastaloceans appears to be a common global phenomenon (Moore, 2006,2010). Based on recent estimates, the average annual fluxes of sub-marine groundwater discharge (SGD) all along the continentalshelf of the South Atlantic Bight were 3 times larger than the riverfluxes (Moore, 2010). Local groundwater fluxes contribute a signif-icant portion of the river discharge in many estuaries (Ferrarinet al., 2008; Santos et al., 2008; Peterson et al., 2010). For example,SGD contributed 10–80% of the river discharge in the Indian RiverLagoon, Florida (Peterson et al., 2010), and the SGD was 5–30 timesthe river discharge in the Venice Lagoon, Italy (Ferrarin et al.,2008). Submarine groundwater can enrich coastal waters with

nutrients; coastal eutrophication and algal blooms have been re-lated to SGD (Basterretxea et al., 2010; Hwang et al., 2010; Kneeet al., 2010).

Large amounts of land-derived pollutants, such as toxic tracemetals from agricultural and industrial sites, and atmosphericdeposition in the groundwater recharge area can be transportedto coastal waters along the groundwater flow path (Dowlinget al., 2003; Beck et al., 2009; Ganguli et al., 2012). Submarinegroundwater fluxes of Sr and Ba to the ocean were equal to the riv-er fluxes in the Bengal basin (Dowling et al., 2003). In Jamaica Bay,New York, the SGD is an important source of Fe, Zn, Co, and Ni forcoastal waters (Beck et al., 2010). The non-conservative mixingbehavior of metals has been found in subterranean estuaries, indi-cating that water–sediment reactions in subterranean estuaries arecritical for the mass flux of metals associated with SGD (Slomp andVan Cappellen, 2004; Charette and Sholkovitz, 2006; Beck et al.,2010).

Mercury is a well-documented neurotoxicant that poses a haz-ard to the environment and public health (Castoldi et al., 2008).

MD. Moklesur Rahman et al. / Chemosphere 91 (2013) 320–327 321

Human exposure to Hg occurs primarily through dietary uptake ofHg-contaminated fish (Campbell et al., 2008; Mahaffey et al.,2008). Once Hg is introduced into a coastal zone, Hg(II)-methylat-ing bacteria (e.g., sulfate-reducing and iron-reducing bacteria) con-vert inorganic Hg(II) to monomethyl Hg (MMHg) in anoxic andsuboxic compartments of the coastal zone (Warner et al., 2003).In general, the MMHg found in the coastal food web is regardedas arising from near-shore sediment (Fitzgerald et al., 2007). Con-sequently, Hg loading in coastal systems is associated with an ele-vated MMHg level in fish (King et al., 2002; Warner et al., 2003). Infact, atmospheric deposition of Hg showed a positive correlationwith the MMHg concentration in fish in lakes (Hammerschmidtand Fitzgerald, 2006).

Although many studies have reported coastal mass balances ofHg, the studies considered river discharge and atmospheric depo-sition as the primary input sources of Hg (Mason et al., 1999;Faganeli et al., 2003; Balcom et al., 2004). Recently, the limitedliterature has suggested that SGD may have a great influence onthe coastal mass budget of Hg (Bone et al., 2007; Laurier et al.,2007; Black et al., 2009; Lee et al., 2011; Ganguli et al., 2012).For instance, the total dissolved Hg input through SGD was higherthan the inputs received through atmospheric deposition and riv-erine input on the East Coast of the United States (Bone et al.,2007) and on the central and southern California coasts (Blacket al., 2009; Ganguli et al., 2012). Mussels collected near thegroundwater discharge zone of the French coast exhibited signifi-cantly higher Hg values than those found at the control site, sug-gesting a considerable amount of bioavailable Hg is dischargedfrom submarine groundwater (Laurier et al., 2007). In general,studies regarding the influence of groundwater discharge on thecoastal Hg budget are limited; thus, the role of SGD in Hg transportto the ocean is largely unknown.

In the present study, we aimed to estimate the significance ofHg input flux through SGD in Hampyeong Bay, a semi-enclosedembayment in the eastern Yellow Sea. We estimated the percentcontributions of Hg input through SGD as well as atmosphericdeposition and benthic diffusion, based on the measurements ofHg from these sources.

2. Methods and materials

2.1. Study site

Hampyeong Bay, on the southwest of Korean peninsula, is elon-gated northwest–southeast with a maximum width of 8.5 km anda length of 17 km (Fig. 1). The bay is situated in a pristine area de-void of large cities and industrial areas. Land use is dominated byfisheries, aquaculture, and agriculture (Waska and Kim, 2010).The bay has a total water area of 85 km2 and a mean water depthof 4 m (Waska and Kim, 2011). The surroundings of the northeastside of the bay are characterized by low mountains 100–400 mhigh, whereas the gently sloping Imsu Peninsula restricts thesouthwest side of the bay (Ryu, 2003). The mouth of the bay isdeep and narrow, with a maximum water depth of about 23 mand a width of 1.5 km (Ryu, 2003). A large area of tidal flats hasdeveloped along the side of the bay, characterized by small sandbars and cheniers (Ryu, 2003). Tides in this region are semidiurnal,and the mean tidal range is 3.46 m, with mean neap and spring ti-dal ranges of 2.37 m and 4.55 m, respectively (Waska and Kim,2011). No rivers flow into the bay, and the groundwater dischargerunoff in the intertidal area is large enough to form small streams,which causes thriving microphytobenthos (Waska and Kim, 2010).The tidal ranges on the sampling dates in 2010 were 5.09 m onJanuary 18 and 4.97 m on January 19, 5.32 m on May 1 and4.26 m on May 20, and 2.51 m on August 5 (Korea Hydrographic

and Oceanographic Administration, KHOA, http://www.khoa.go.kr/). The seasonal precipitation pattern is characterized byheavy rainfall in summer, as Korea is situated in the Asian mon-soon climate region. The monthly precipitation amounts in 2010were 37 mm in January, 99 mm in May, and 338 mm in August(Korea Meteorological Administration, KMA, http://www.kma.go.kr/). Hampyeong Bay has a considerable intertidal zone (>50%of the total bay area), and the SGD magnitudes, calculated basedon a mass balance model of 226Ra, were 0.14–0.35 m3 m�2 d�1 inthe literature (Waska and Kim, 2011).

2.2. Sampling methods

Groundwater samples from shallow wells of different depths,ranging from 30 cm to 1 m, were collected from Woldu, Dolmori,and Songseokri on January 18 and 19, May 1 and 20, and August5, 2010. All the groundwater samples were collected in active seep-age runoff areas in the intertidal zone. Zero salinity groundwatersamples after heavy rain events were collected from Songseokrion May 20, 2010. At low tide, a well was excavated to a depth of30–100 cm. After the dirty water was discarded, clear groundwatersamples were collected using a peristaltic pump along with Teflontubing connected to a Millipore groundwater filter capsule(0.45 lm pore size) in 500 mL pre-acid-cleaned Teflon bottles.Using recorded GPS points, pH, salinity, and temperature weremeasured in situ using the Orion 5 Star™ Series Meters.

Surface seawater samples were collected on May 17 and August3, 2010. Samples were collected in a similar way as the groundwa-ter, using the peristaltic pump and the same filter attached to theTeflon tubing at a depth of approximately 20 cm from the surfacewater. Filtered and unfiltered samples were collected in 500 mLTeflon bottles that had previously been treated by soaking themin an HCl solution (30% v/v) to make them Hg free. Returning tothe laboratory, the groundwater samples were acidified withapproximately 0.4% 12 N HCl (trace metal grade), while the seawa-ter samples were acidified with 0.2% 9 N H2SO4 (trace metal grade),and kept at 4 �C in a dark environment (Lee et al., 2011). To mea-sure dissolved gaseous mercury (DGM), filtered seawater samplesfrom the same depth (20–30 cm from the surface) were collectedin 1 L glass bottles and 500 mL Teflon bottles on January 2 andFebruary 13, 2012. The glass and Teflon bottles provided almostthe same result for the DGM. No free space was allowed at thetop of the sampling bottle to prevent even a subtle loss of DGMthrough evasion to the headspace before analysis.

Seawater samples for ancillary measurements of dissolvedorganic carbon (DOC), particulate organic carbon (POC), suspendedparticulate matter (SPM), and chlorophyll-a (Chl-a) content werecollected along with Hg samples. DOC and POC samples werecollected in precombusted (450 �C for 10 h) 40 mL glass vials and500 mL acid-cleaned, low-density polyethylene (LDPE) bottles,respectively. Unfiltered seawater samples for SPM and Chl-a anal-ysis were collected in 2 L LDPE bottles.

Sediment samples were collected in acrylic cores 30 cm deepwith a 12 cm diameter on September 9, 2010, and February 14,2011. The total Hg concentrations in the sediments and porewaters were determined with concomitant measurements of or-ganic carbon content and sediment density. Sediment cores forchemical analysis were sectioned within 5 h of collection. Coreswere placed inside a N2-filled glove box before the overlying waterwas removed, after which the cores were sectioned up to 4 cmdown the core. Pore waters were extracted from the sedimentswith centrifugation, and the supernatant was filtered through0.45 lm syringe filters (Mason et al., 1998). The filtered porewaters were acidified to about 0.4% final concentration with 12 NHCl (trace metal grade) and stored at 4 �C in a dark environmentuntil analysis. After being removed from the glove box, the

Fig. 1. Location of the groundwater (GW) and seawater (SW) sampling sites in Hampyeong Bay, Korea.

322 MD. M. Rahman et al. / Chemosphere 91 (2013) 320–327

sediment samples were freeze-dried and stored frozen. The bulkorganic content of the sediments was measured gravimetricallyas the loss-on-ignition (LOI) of lyophilized material heated to550 �C for at least 1 h (Heiri et al., 2001).

Six rainwater samples were collected between February andOctober 2010 at Gwangju, 35 km east of Hampyeong Bay. The rain-water was collected using acid-cleaned glass funnels (15 cm diam-eter) connected to a 2 L Teflon bottle. The rainwater samples wereimmediately preserved after collection by adding 12 N HCl (0.4% v/v). Before each sampling, the funnels, Teflon bottles, and glass fun-nels were carefully cleaned with an HCl solution and Milli-Q water.

2.3. Sample analysis

DOC and POC concentrations were measured with ShimadzuTOC-5000 and LecoCHNS-932 analyzers, respectively. POC sampleswere prepared filtering 100–300 mL samples through GF/F (0.7 lmpore size) filter paper and stored frozen until processed. In the caseof DOC, the filtered samples were acidified and sparged with ultra-high pure N2 gas to remove the inorganic carbon. Before the anal-ysis, the blank with carbon-free distilled water and the calibrationstandard were tested, and eventually the samples were analyzedusing a Shimadzu TOC-5000 analyzer. The SPM concentrationwas determined by the mass difference of the membrane filters(polycarbonate membrane with 0.40 lm pores) before and afterseawater filtration.

For Hg analysis in water, 500 lL of BrCl was added to the100 mL pre-acidified samples at least 12 h before analysis, andthen 200 lL of NH2OHHCl was added (Lee et al., 2011). The totalmercury in all samples was then quantified by using cold-vapor

atomic fluorescence spectroscopy (CVAFS) (Bloom and Fitzgerald,1988). Acid-cleaned Teflon bottles were filled with Milli-Q waterduring the May and August sampling campaigns, and the methodblank was performed concomitantly following the same procedureas the sample analysis. DGM was analyzed immediately afterreturning to the laboratory by decanting a 500 mL sample withgreat care into a 1 L borosilicate bubbler purging ultra-high pureN2 gas at a flow rate of 40 mL min�1 for 30 min. Elemental Hgwas then collected in the gold trap connected to the outlet of thebubbler and quantified by using gold-amalgamation CVAFS (Bloomand Fitzgerald, 1988).

For Hg analysis of the sediment pore water, the pre-acidifiedpore water samples were digested with BrCl similar to the dis-solved seawater samples and quantified using CVAFS. For Hg anal-ysis in sediment, approximately 0.5–1.0 g of freeze-dried sedimentwas oxidized in 125 mL Teflon bottles with 8 ml of 12 N HCl and2 mL of 14 N HNO3 (Choe et al., 2004). Sediment digests were di-luted on the following day with approximately 100 ml of Milli-Qwater. After 2–4 h, the Hg concentration was measured from sed-iment digests using CVAFS (Choe et al., 2004).

Recovery of certified reference materials for Hg in water (BCR�-579, coastal seawater, 1.9 ± 0.5 ng kg�1) averaged 107 ± 3% (n = 10)and for Hg in sediment (ERM-CC580, Estuarine Sediment,132 ± 3 mg kg�1) averaged 108 ± 9% (n = 3). Recovery of the matrixspike for Hg for water averaged 103 ± 6% (n = 10) and for sedimentaveraged 105 ± 12% (n = 5). Analytical precision for the filteredwater samples, estimated as the relative percent difference (RPD)of duplicate analysis, averaged 20 ± 19% (n = 41) for water and7 ± 5% (n = 12) for sediment. The value for the method blank ran-ged from 0 to 0.67 with an average of 0.13 pM (n = 5).

MD. Moklesur Rahman et al. / Chemosphere 91 (2013) 320–327 323

3. Results and discussion

3.1. Variation of Hg in coastal groundwater and seawater

In such a large tidal region as Hampyeong Bay, the seawater cir-culation process in and out of sediment depends on the tidal cycle(Robinson et al., 2007; De Sieyes et al., 2008; Waska and Kim,2010). Lower salinities were observed in coastal groundwater inAugust when the tidal range was lower and wet precipitationwas higher than in May (Fig. 2a). The dissolved Hg (HgD) measuredin the coastal groundwater from Woldu, Dolmori, and Songseokriranged from 1.2 to 4.1 pM (Table 1 and Fig. 2b). This range is sim-ilar to the low ranges of HgD found in the groundwater off thenorthern shore of the Seine estuary (4.9 ± 0.75 pM at Etretat and2.2 ± 0.85 pM at Yport; Laurier et al., 2007) and off the central Cal-ifornia coast (1.2–28.3 pM; Black et al., 2009). The Songseokri datacommonly showed lower salinities and higher HgD levels than theWoldu and Dolmori data. This may be associated with the highpermeability of the Songseokri aquifer, which allows a relativelylarge penetration of freshwater through the aquifer sediment. In-deed, fine-grained sediment (mud) dominates the tidal flats onthe eastern and western sides of the bay, while coarse sediment(muddy sandy gravel and gravelly muddy sand) covers the bay

Woldu Dolmori Songseokri Woldu Dolmori Songseokri

0

10

20

30

salin

ity

(a) May August

Woldu Dolmori Songseokri Woldu Dolmori Songseokri0

1

2

3

4

5

HgD

(pM

)

(b) May August

Fig. 2. Temporal and spatial variations of salinity (a) and dissolved Hg (HgD) (b) atthe sampling sites in May and August 2010. Box plots show 25th, 50th (median) and75th percentiles, and error bars represent 1.5 times the interquartile ranges.

mouth and the tidal flats on the southern side of the bay (Ryu,2003).

The relationship between HgD and salinity in the subterraneanestuary is disputable; for instance, thermodynamic studies sug-gested that Hg could desorb off metal (hydr)oxides with increasingchloride by forming soluble Hg-chloride complexes (Bone et al.,2007). However, a plot of Hg versus salinity for the Waquoit Baygroundwater profile showed that the mid-salinity region has ele-vated levels of HgD, suggesting that Hg mobilization is drivennot only by chlorinity (Bone et al., 2007). The same authors sug-gested that solid phase Hg loadings, DOC, and dissolved Fe mightalso contribute to the amount of HgD released from aquifer sedi-ments. We found a nearly linear correlation between salinity andHgD for the coastal groundwater and seawater of HampyeongBay (Fig. 3), suggesting that HgD conservatively behaves duringsubterranean water mixing. High levels of groundwater HgD foundin August at salinities close to 0 might be attributable to enhancedHg release processes such as microbial Fe(III) dissolution(Peretyazhko et al., 2006; Harris-Hellal et al., 2011).

The HgD and unfiltered Hg (HgT) concentrations in inner bayseawater ranged from 0.83 to 2.0 pM and 1.7 to 4.6 pM, respec-tively (Table 2), which is in accordance with the level of HgD andHgT in uncontaminated coastal waters (HgD of 0.3–1 pM andHgT of 0.8–6 pM; Cossa et al., 1997; Kotnik et al., 2007). The meanHgD and DOC values were significantly higher (p < 0.05) in Augustthan in May, while the mean HgT, particulate Hg (HgP), POC, andSPM concentrations in May and August were similar (p > 0.05).These results suggest that a large supply of coastal groundwaterin August, implied by the low salinities of August groundwaterand coastal water, may influence DOC and HgD levels in coastalseawater, which is supported by the Hg flux estimation shown inTable 3. In August, the mean HgT was higher in the inner bay thanin the outer bay, which seems to be caused by the higher SPM inthe inner bay. HgT showed a strong positive correlation withSPM for the inner (r = 0.84) and outer (r = 0.99) bays. SPM andHgT enhancements in the inner bay versus the outer bay suggestthat sediment resuspension from the tidal mud flats may resultin SPM and HgT increases in inner bay surface waters. Multi-annual sedimentological observation has reported that Hampy-eong Bay is subject to consistent and light erosion (Ryu, 2003). Incontrast to HgT, significantly (p < 0.05) higher HgP concentrationswere found in the outer bay, along with higher POC. High organicmatter content, including living phytoplankton, in the outer baySPM appears to cause Hg enrichment in particles.

3.2. Sources of Hg in Hampyeong Bay

3.2.1. Atmospheric sourceUsing our measurements and literature data, we assessed the

Hg fluxes for various sources, as summarized in Table 3. The wetatmospheric deposition flux of Hg was determined using the 18-year average rainfall in Korea between 1992 and 2010(1.95 ± 0.35 m yr�1, KMA) multiplied by the area of the bay water(85 km2) and the mean HgT concentration in rainwater(31 ± 14 pM) for 6 rainwater samples collected between Februaryand October 2010. The uncertainty term for wet atmospheric depo-sition was calculated with the propagation of errors based on thestandard deviations associated with the mean annual precipitationrate and the HgT in rainwater. We used the literature value of a dryHg deposition of 0.040 ± 0.013 lmol m�2 yr�1 determined from 9different areas in Japan (Sakata and Marumoto, 2005), since nodata were currently available on dry deposition for coastal areasin Korea. The total Hg input through wet and dry atmosphericdeposition was 8.5 ± 2.7 mol yr�1 or 100 ± 32 nmol m�2 yr�1.

Salinity 0 10 20 30 40

HgD

(pM

)

0

1

2

3

4

5

GW (May 2010)GW (Aug 2010)SW (May 2010)SW (Aug 2010)

Fig. 3. Dissolved Hg (HgD) versus salinity in the groundwater (GW) and surfaceseawater (SW).

Table 1Concentration of dissolved Hg (HgD), pH, and salinity in coastal groundwater of Hampyeong Bay, Korea.

Sampling month Sites pH HgD (pM) Salinity

January 2009 Woldu1 8.1 1.94 –Woldu2 8.1 2.11 –Woldu3 8.0 1.98 –Woldu4 8.0 3.01 –Dolmori1 8.1 2.95 –Dolmori2 7.6 2.46 –Dolmori3 7.3 1.23 –Dolmori4 7.9 2.61 –Songseokri1 7.4 2.47 –Songseokri2 7.3 2.69 –Songseokri3 7.3 2.32 –Songseokri4 7.6 2.33 –

May 2010 Woldu1 8.0 1.23 23Woldu2 8.0 1.38 27Dolmori1 8.0 1.46 23Dolmori2 8.0 1.50 26Songseokri1 7.6 2.22 15Songseokri2 7.5 2.71 18Songseokri3 6.7 3.22 0Songseokri4 7.5 3.18 0Songseokri5 7.2 2.68 0.2

August 2010 Woldu1 7.7 2.28 18Woldu2 7.7 1.47 19Woldu3 7.6 1.32 18Dolmori1 7.4 2.43 12Dolmori2 7.5 2.28 15Dolmori3 7.6 2.69 7.9Songseokri1 7.4 4.06 1.3Songseokri2 7.6 4.11 1.9Songseokri3 7.5 3.88 0.9

324 MD. M. Rahman et al. / Chemosphere 91 (2013) 320–327

3.2.2. Benthic diffusion fluxThe diffusive flux of HgD was estimated from the mean concen-

tration gradients of HgD over the pore water and seawater at thesediment–water interface using Fick’s first law (1):

J ¼ �UDsdcdz

� �ð1Þ

where J = diffusive flux in ng cm�2 s�1, U = porosity (dimension-less), Ds = diffusion coefficient in cm2 s�1, dc = difference in the con-centration of HgD in the uppermost sediment layer and in near-bottom water in ng g�1, and dz = depth difference between theuppermost sediment layer and the near-bottom water in cm. Thediffusion coefficient, Ds, was defined assuming sediment porosity

<70%, such that Ds = h2D0, where h = sediment tortuosity (dimen-sionless) and D0 = molecular diffusion coefficient of Hg in seawaterof 5.0 � 10�6 cm2 s�1 (Gobeil and Cossa, 1993; Mason et al., 1993).For the current study, we used the measured porosity (U) of thesandy mud sediments of 51% (0.51), which was estimated fromthe bulk sediment density of 1.53 g cm�3 and particle density of3.1 g cm�3 based on the following equation (Avnimelech et al.,2001): U = 1- (bulk density/particle density) � 100. The tortuosityof the sediment, h = 1.53, was derived by applying the relationshipbetween porosity and tortuosity, h =

p1 – ln(U2) (Boudreau,

1996). The mean concentration of HgD in pore waters, extractedfrom a sediment depth of 0–4 cm and in overlying water, was17 ± 2 nmol m�3 (n = 4) and 1.9 ± 0.63 nmol m�3 (n = 4), respec-tively. The uncertainty term for the benthic flux was estimatedbased on the standard deviations associated with the mean valuesof the HgD concentrations in pore water and overlying water. Over-all, the benthic flux was 1.2 ± 0.2 mol yr�1 with an area normalizedflux of 14 ± 2 nmol m�2 yr�1 (Table 3).

3.2.3. Groundwater fluxThe magnitude of the HgD input through SGD was calculated by

multiplying the mean groundwater flux by the mean concentra-tions of HgD in coastal groundwater (2.4 ± 0.5 pM). The SGD fluxin the literature, (7.6 ± 4.6) � 109 m3 yr�1, is the combination ofthe recirculating seawater and the terrestrially derived freshgroundwater (Waska and Kim, 2010). The magnitude of the HgDinput through SGD was estimated to be 18 ± 12 mol yr�1 and anarea normalized value of 212 ± 135 nmol m�2 yr�1 (Table 3). Tocalculate the magnitude of the HgD input through fresh SGD, themean fresh groundwater flux was multiplied by the mean concen-trations of HgD (3.2 pM) determined for the May fresh groundwa-ter (n = 2). Waska and Kim (2011) estimated the freshwaterfraction in the overall SGD by dividing the offshore and groundwa-ter salinity difference by the offshore salinity, which produced a

Table 2Concentrations of total Hg (HgT), dissolved Hg (HgD), and particulate Hg (HgP), along with salinity, dissolved organic carbon (DOC), particulate organic carbon (POC), andsuspended particulate matter (SPM) in coastal seawater of Hampyeong Bay, Korea.

Sampling month Sites Salinity HgT (pM) HgD (pM) HgD/HgT (%) HgP (pmol g�1) DOC (mg L�1) POC (%) SPM (mg L�1)

May 2010 Inner bay SW1 29.4 3.4 1.0 31 146 1.4 2.1 16SW2 29.2 1.7 0.91 53 132 1.5 3.9 6.2SW3 29.3 4.4 1.1 25 96 1.3 2.1 34SW4 29.4 1.9 0.83 44 108 1.1 2.9 9.7Mean ± SD 29.3 ± 0.1 2.9 ± 1.3 0.96 ± 0.12 38 ± 13 121 ± 23.6 1.3 ± 0.2 2.8 ± 0.9 17 ± 12

August 2010 Inner bay SW5 25.5 2.1 1.4 64 65 – 3.8 12SW6 25.4 3.5 1.4 40 109 1.7 3.2 19SW7 25.1 3.3 2.0 61 140 1.7 5.5 9.3SW8 25.2 4.6 1.4 31 109 1.6 3.1 29SW9 25.4 3.6 1.5 43 93 1.7 3.8 22SW10 25.7 2.4 1.3 56 100 1.9 3.8 11Mean ± SD 25.4 ± 0.3 3.5 ± 0.9 1.6 ± 0.3 49 ± 13 103 ± 25 1.7 ± 0.1 3.9 ± 0.9 17 ± 8

Outer bay SW11 25.6 2.3 1.3 58 198 1.3 8.7 4.9SW12 26.1 2.9 1.5 53 118 1.2 6.4 12SW13 25.9 2.6 1.2 44 164 1.1 3.7 8.9Mean ± SD 25.9 ± 0.3 2.6 ± 0.3 1.3 ± 0.2 52 ± 7 160 ± 40 1.2 ± 0.1 6.3 ± 2.5 8.4 ± 3.3

Table 3Hg mass flux calculation in the Hampyeong Bay, Korea.

Category Water flux(�109 m3 yr�1)

Hg (pM) Hg flux(mol yr�1)

Contribution(%)

Unit area Hg flux(nmol m�2 yr�1)

Atmosphericdeposition

0.17 ± 0.03a 31 ± 14 (n = 6) 8.5 ± 2.7 31 100 ± 32

Sediment diffusion NA Overlying 1.9 ± 0.6 (n = 4) pore water 17 ± 2(n = 4)

1.2 ± 0.2 4 14 ± 2

SGD (Fresh SGD)c 7.6 ± 4.6b (4.1 ± 2.5)c 2.4 ± 0.5 (n = 18) (3.2, n = 2)c 18 ± 12 (13 ± 8)c 65 (57)c 212 ± 135 (154 ± 94)c

Total input 28 ± 12 (23 ± 9)c 100 326 ± 139 (268 ± 63)c

Values indicate mean ± standard deviation; n = number of samples; NA = not applicable.a Average flux from 1992 to 2010, Korea Meteorological Administration.b Waska and Kim (2011).c Estimated using the freshwater SGD and Hg concentration in fresh groundwater.

MD. Moklesur Rahman et al. / Chemosphere 91 (2013) 320–327 325

freshwater fraction of 46%. Using our salinity values for the Mayand August groundwater and coastal water samples, we estimatedthe fresh groundwater fractions were 50% for May and 58% forAugust, similar to the literature value. The magnitude of the HgDinput through the fresh SGD, (4.1 ± 2.5) � 109 m3 yr�1, was esti-mated to be 13 ± 8 mol yr�1 and an area normalized value of154 ± 94 nmol m�2 yr�1 (Table 3). Here, the uncertainty term forthe SGD flux was estimated based on the variability associatedwith the mean HgD concentration in coastal groundwater andthe variability associated with the SGD rate.

3.2.4. Sediment erosionIn the literature, the seasonal cycle of sediment deposition and

erosion at Hampyeong Bay was monitored at intervals of 2 monthsover a 4 year period (from 1995 to 1999) along the 4 transect lineslocated in the tidal flat (see Fig. 2 in Ryu, 2003). In the same work,the mean annual sedimentation rate on each transect line was�44.1, �3.5, �0.9, and �1.8 mm yr�1, suggesting that the tidal flatof Hampyeong Bay has been eroded on the whole. This positiveerosion rate is unique when compared to the typical sedimentationrate (2.0–8.6 mm yr�1) found in most intertidal zones of the YellowSea in Korea (Alexander et al., 1991). Sediment erosion may be asource of Hg for the bay water, if sediment Hg is easily soluble inthe water column. In the literature, the effect of sediment resus-pension on the water column Hg release has been studied usinga mesocosm simulating tidal resuspension (Kim et al., 2004).Although muddy sediment resuspension significantly increasedHgT concentration, the release of HgD from sediment resuspensioninto the water column was insignificant (Kim et al., 2004). We con-sider that sediment resuspension is not a significant source of Hg

for the bay, but instead contributes to the internal recycling of Hgbetween the bottom sediment and the water column.

3.3. Hg sinks in Hampyeong Bay

To calculate the ocean export flux, we assumed that the totalamount of freshwater entering the bay through SGD and directatmospheric wet deposition is the same as the water volume ex-ported out of the bay (Sakata et al., 2006). Using the fresh SGDand the wet precipitation, we estimated the net bay water dis-charge rate was (4.3 ± 2.5) � 109 m3 yr�1. The amount of exportedHg was then estimated by multiplying the mean concentration ofHgT (3.1 ± 1.0 pM, n = 10) found from the inner bay seawater bythe net bay water discharge rate. The calculated annual export fluxof Hg was 13 ± 9 mol yr�1 or 156 ± 104 nmol m�2 yr�1. The overallinput flux, (28 ± 12 mol yr�1) including saline SGD or(23 ± 9 mol yr�1) including fresh SGD, largely exceeds the oceanexport flux. An imbalance between the input and the output couldbe attributed to the surface evasion flux. The large positive evasionflux from the surface water to the atmosphere has been commonlyreported from estuarine/coastal areas (Rolfhus and Fitzgerald,2001; Sakata et al., 2006; Weiss et al., 2007; Kuss and Schneider,2007).

The amount of Hg stored in the bay (1.05 mol) was estimatedusing the average concentration of HgT in the inner bay water(3.1 pM, Table 2) and the bay volume of 3.4 � 108 m3. We also esti-mated the amount of Hg stored in the bay water (0.86 mol) usingthe average freshwater residence time (11.3 d), obtained fromthe literature (2.71 d for spring and 19.97 d for winter; Kim et al.,2011), and the yearly Hg flux (28 mol yr�1) found in Table 3.

326 MD. M. Rahman et al. / Chemosphere 91 (2013) 320–327

Similar results between the two methods confirm that the Hg fluxcalculation in Table 3 is reasonable.

4. Conclusion

The overall Hg input estimation in Table 3 shows that a total28 ± 12 mol of Hg enters Hampyeong Bay annually. SGD and atmo-spheric deposition are the dominant sources of Hg, contributing65% and 31% to the total input, respectively. Compared to theSGD and atmospheric fluxes, the Hg input through the benthic dif-fusion flux is insignificant (1.2 ± 0.2 mol yr�1), being 4% of the totalinput. The total Hg release through the SGD was more than twicethe atmospheric input and 15 times the benthic diffusion input,confirming the potential importance of SGD in the coastal Hg in-put. When we separate out the freshwater fraction from the overallSGD, the Hg flux through fresh SGD was estimated to be13 ± 8 mol yr�1, contributing 72% to the overall SGD Hg input and57% to the total Hg input. We suggest that wet precipitation couldbe the dominant sources of Hg to the groundwater, as the ground-water Hg levels are quite low, similar to those from uncontami-nated areas (Ganguli et al., 2012). Rainwater Hg levels wereapproximately ten times higher than groundwater (Tables 2 and3), which is common for coastal systems (Mason et al., 1998; Lau-rier et al., 2007). Rain-borne Hg might be scavenged by sorptiononto aquifer particles during the water transit. Then, in the terres-trial mixing zone, the HgD levels appear to be simply the result ofconservative mixing of groundwater and coastal water, althoughseasonal variation was observed. Similarly to Hg, SGD was a majorsource of nutrients in Hampyeong Bay: SGD contributed more than50% of the total nutrient fluxes, enhancing the primary productionin the water column as well as benthic environments (Waska andKim, 2011). A series of recent reports revealed that SGD is ubiqui-tous along the west and southeastern coastline of Korea (Kim andHwang, 2002; Kim et al., 2005, 2008; Waska and Kim, 2010). Hgseepage through SGD may influence the overall coastal Hg flux inthe Yellow Sea. Further work is necessary to understand the roleof SGD for Hg flux and its effects on coastal ecosystems of the Yel-low Sea.

Acknowledgements

We thank Hyun-Cheol Kim in the Carbon Cycle Research Labo-ratory in POSTECH for his DOC and POC analysis. This research wassupported by a National Research Foundation of Korea (NRF) grantfunded by the Korean government (2012R1A2A2A06046793).

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