Seasonal air–water exchange fluxes of polychlorinated biphenyls in the Hudson River Estuary

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Environmental Pollution 152 (2008) 443e451 www.elsevier.com/locate/envpol

Seasonal airewater exchange fluxes of polychlorinated biphenyls in the Hudson River Estuary Shu Yan a, Lisa A. Rodenburg a,*, Jordi Dachs b, Steven J. Eisenreich c a

Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901, USA Department of Environmental Chemistry, IIQAB-CSIC, Jordi Girona 18-26, Barcelona E-08034, Catalunya, Spain c European Chemicals Bureau, Institute for Health and Consumer Protection, Joint Research Center, TP 582, European Commission, Ispra 21020, Italy b

Received 11 April 2007; received in revised form 25 May 2007; accepted 2 June 2007

Investigation of the airewater exchange of PCBs in the Hudson River Estuary suggests that PCBs with 5 or fewer chlorines undergo net volatilization. Abstract Polychlorinated biphenyls (PCBs) were measured in thePair and water over the Hudson River Estuary during six intensive field campaigns from December 1999 to April 2001. Over-water gas-phase PCB concentrations averaged 1100 pg/m3 and varied with temperature. DissolvedP phase PCB concentrations averaged 1100 pg/L and displayed no seasonal trend. Uncertainty analysis of the results suggests that PCBs with 5 or fewer chlorines exhibited net volatilization. The direction of net air/water exchange could not be P determined for PCBs with 6 or more chloP rines. Instantaneous net fluxes of PCBs ranged from þ0.2 to þ630 ng m2 d1. Annual fluxes of PCBs were predicted from modeled gasphase concentrations, measured dissolved-phase concentrations, daily surface P water temperatures and wind speeds. The net volatilization flux was þ62 mg m2 yr1, corresponding to an annual loss of þ28 kg/yr of PCBs from the Hudson River Estuary for the year of 2000. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Colloids; Partitioning; Henry’s law; Mass balance

1. Introduction Airewater exchange describes the processes of absorption to, transfer across, and volatilization from the interface between the air and water. Many studies have demonstrated that airewater exchange is an important process that controls concentrations, mass fluxes, and residence times of semivolatile persistent organic compounds (POPs) in aquatic ecosystems such as the Great Lakes and the Chesapeake Bay (Baker and Eisenreich, 1990; Hornbuckle et al., 1994; Gustafson and Dickhut, 1997; Nelson et al., 1998; Zhang et al., * Corresponding author. Tel.: þ1 732 932 9800x6218; fax: þ1 732 932 8644. E-mail addresses: [email protected] (L.A. Rodenburg), [email protected] (J. Dachs), [email protected] (S.J. Eisenreich). 0269-7491/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2007.06.074

1999). Gas absorption is one of the major atmospheric inputs of POPs to surface waters of large lakes and oceans, and thus contributes to the food web contamination in natural waters (Baker and Eisenreich, 1990; Zhang et al., 1999; Jeremiason et al., 1999; Iwata et al., 1993). Volatilization from contaminated waters represents a source of organic pollutants to the ambient atmosphere (Hornbuckle et al., 1994; Nelson et al., 1998; Zhang et al., 1999; Achman et al., 1993; Totten et al., 2001; Rowe et al., 2007). The Hudson River Estuary (HRE) has been heavily impacted by inputs of POPs from the New York/New Jersey metropolitan area. The main source of polychlorinated biphenyls (PCBs) to the HRE is the upper Hudson River. Other sources include stormwater runoff, municipal and industrial discharges, and sediment resuspension (Farley et al., 1999; Totten, 2005). Major PCB losses include sediment burial, volatilization, and outflow to the Atlantic Ocean (Farley et al., 1999; Totten,

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S. Yan et al. / Environmental Pollution 152 (2008) 443e451

2005). Most of the research studies have been focused on the fate and transport of PCBs in the Hudson River. Very few studies have been conducted to investigate the occurrence of PCBs in the waters of the HRE water and the role of airewater exchange in this complex estuary. Totten et al. (2001) and Gigliotti et al. (2001) estimated net volatilization airewater exchange fluxes of POPs in the HRE for field campaigns conducted in the summer of 1998. This study is an extension of that work. The objectives of this study are to provide seasonal measurements of PCB concentrations in the atmosphere and water of the HRE during six intensive field experiments from 1999 to 2001, to provide the first seasonal estimates of the magnitude and direction of instantaneous airewater exchange fluxes for PCBs in the HRE, to estimate the annual airewater exchange flux of PCBs in the HRE, and finally to examine the role of airewater exchange compared with other transport processes as a source or sink of PCBs to the HRE. 2. Experimental methods

(Axys Environmental Systems, Sydney, BC, Canada) at a flow rate of w300 mL min1 yielding volumes of 18e50 L. Glass fiber filters (GFFs, 0.7 mm pore size, Whatman) were used to collect particles and XAD-2 resin (Amberlite) was used to capture the operationally defined dissolved phase. Prior to deployment, w30 g of XAD-2 resin was wet-packed into 2.5  30 cm Teflon columns and spiked with PCB surrogate standards [PCB 23 (3,5-dichlorobiphenyl), PCB 65 (2,3,5,6-tetrachlorobiphenyl), and PCB 166 (2,3,4,4’,5,6-hexachlorobiphenyl)]. The depth profiles of water temperature, salinity, dissolved oxygen, and pH were obtained by CTD-transmissometerefluorometer casts on each sampling date and the stratification of the water column was characterized. Additional water samples at 1.5-m depth were collected using a Neeskin bottle (5-L capacity) for total suspended matter (TSM), dissolved organic carbon (DOC), particulate organic carbon (POC), and particulate organic nitrogen (PON).

2.2. Analytical method Methods for the measurement of PCBs in PUF, QFF, XAD, and GFF samples were as described previously (Totten et al., 2001; Brunciak et al., 2001). PCBs were analyzed on an Agilent 6890 gas chromatograph equipped with a 63Ni electron capture detector using a 60 m by 0.25 mm i.d. DB-5 (5% diphenyl dimethyl polysiloxane) capillary column with a film thickness of 0.25 mm. Fifty-seven peaks representing 90 congeners were quantified.

2.1. Sampling and site characterization

2.3. Quality assurance

During six intensive cruises from 1999 to 2001, simultaneous air and water samples were taken aboard the research vessel Walford in the lower Hudson River Estuary (HRE) (Fig. 1). Morning (08:30e12:30) and afternoon (13:00e17:00) samples were taken in the HRE west of Sandy Hook (40.30 N, 74.05 W) during October 20, December 3 of 1999, April 19e 21, August 21e23, October 25e27 of 2000, and April 24 of 2001. Additionally, during April 2001, a sampling campaign was conducted at the coastal Atlantic Ocean site (CAO) northeast of Sandy Hook (40.30 N, 73.58 W). Air samples were also collected at two locations on land: Jersey City (JC) (40.71 N, 74.05 W) and at the Gateway National Park at Sandy Hook (SH) (40.46 N, 74.00 W). Integrated 4-h air samples were collected using modified high-volume air samplers (Tisch Environmental, Village of Cleves, OH, USA) with a calibrated flow rate of w0.5 m3 min1. The gas phase was captured on polyurethane foam plugs (PUFs), and the aerosol phase was collected on quartz fiber filter (QFFs, 0.7 mm pore size, Whatman). Air temperature and wind speed data were recorded every 15 min throughout the sampling. Surface water samples were collected in situ at a depth of 1.5 m using two Infiltrex 100 sampling units

Samples were corrected for surrogate recoveries using PCB 23 to correct congeners eluting before PCB 45, PCB 65 to correct those eluting from PCB 45 to PCB 110 þ 70, and PCB 166 to correct all those eluting thereafter. The average  standard deviation surrogate recoveries for PCBs 23, 65, and 166 were better than 84%. The average percentages of laboratory blank P mass relative to the sample mass for PCB were less than 3%, therefore sample PCB concentrations were not corrected for laboratory blanks. Additional quality assurance parameters can be found in Yan (2003).

Fig. 1. Map of coastal New Jersey showing the Hudson River Estuary, the coastal Atlantic Ocean, the Jersey City, and the Sandy Hook sampling site. Shaded areas represent regions with dense urban populations.

3. Results and discussion 3.1. Measured atmospheric PCB concentrations Four-hour (4-h) morning and afternoon air samples were taken for most of the sampling dates with a few exceptions when 8-h daytime samples P were collected at HRE site. Gasand particulate-phase 90 PCB concentrations at HRE, CAO, JC, and SH are shown in Table 1. Paired t-tests showed that gas-phase concentrations measured at the HRE were statistically higher than those at the CAO ( p-value < 0.05), whereas no significant difference was observed for the particulate-phase concentrations ( p-value > 0.1). Both gas- and particulatephase concentrations over water at the HRE were not statistically different from those at JC. However, paired t-tests showed that the gas- and particulate-phase concentrations at JC and HRE were significantly higher (by a factor of 2) than those at SH ( p < 0.05).P Gas-phase 90 PCB concentrations varied significantly throughout the year at the HRE. The average concentrations in August and October 2000 were 1420 pg/m3 and 1670 pg/m3, respectively, which were significantly higher than the average value of 600 pg/m3 for all other sampling seasons ( p < 0.05). No significant difference was observed among concentrations from October 1999, P December 1999, April 2000 and April 2001. Gas-phase 90 PCB concentrations measured

S. Yan et al. / Environmental Pollution 152 (2008) 443e451 Table 1 P Atmospheric 90 PCB concentrations in the HRE, CAO, at JC and SH in 1999e2001 P Sampling Gas-phase PCB concentration (pg/m3) a date HRE CAO JCb SHb 10/20/99 a 10/20/99 p 12/3/99 p 4/19/00 d 4/20/00 a 4/20/00 p 4/21/00 d 8/21/00 d 8/22/00 a 8/22/00 p 8/23/00 a 8/23/00 p 10/25/00 a 10/25/00 p 10/26/00 a 10/26/00 p 10/27/00 a 10/27/00 p 4/24/01 a 4/24/01 p 4/25/01 a 4/26/01 a 4/26/01 p

890 750 670 550 540 550 270 2100 3200 820 490 460 1200 1700 620 770 3000 2700 720 600 n/a n/a n/a

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 250 250 240

Average

1100

250

a b

850 480 1200 970

n/a n/a n/a n/a 180

630 1400 2000

800 1300 750

1600

280

1800

550

1500

440

2400

1300

1300

n/a n/a n/a n/a n/a

670 540 1200

700

Particulate-phase

445

P PCB concentration (pg/m3) CAO

JCb

SHb

n/a n/a n/a 120 140

n/a n/a n/a n/a 13

36 23 75

15 37 8.4

27

7.1

140 120 110 39 n/a n/a n/a

n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a 67 37 68

55

57

HRE 52 61 76 27 46 37 14 42 85 16 b 5.0 78 29 b 17

42 50

14 8.2

190

34

n/a n/a 120 74

n/a n/a n/a n/a n/a

82

17

‘‘a’’, ‘‘p’’, and ‘‘d’’ represent morning (08:30e12:30), afternoon (13:00e17:00), and daytime (08:30e16:30) samples, respectively. 8-h daytime (08:30e16:30) samples.

in August 2000 were comparable with those reported by Totten et al. (2001) (1000 pg/m3) at the same site in a field campaign conducted in July 1998. Occasional concentrations above 2000 pg/m3 measured in the HRE (Table 1) are higher than those measured over other aquatic systems such as the Great Lakes (Baker and Eisenreich, 1990; Hornbuckle et al., 1994; Zhang et al., 1999; Achman et al., 1993; Simcik et al., 1997) and the Chesapeake Bay (Nelson et al., 1998; Offenberg and Baker, 1999), and are comparable to those observed in the atmosphere of heavily impacted areas, such as Jersey City (Brunciak et al., 2001; Totten et al., 2004) and Chicago (Simcik et al., 1997). Temperature dependence of gas-phase PCB concentrations is P well studied and a linear relationship between 1/T and ln½ PCBs is often observed (Wania et al., 1998; Carlson and Hites, 2005). When air P temperature was high in August and October 2000, higher PCB concentrations were observed compared with other seasons. A similar trend was found for the P two land sites. However, the regression of 1/T against ln½ 90 PCBs was not significant ( p-value > 0.1) for over-water PCB concentrations probably due to the limited number of samples collected (n ¼ 35), the narrow range of temperature observed throughout the sampling (8.2e25  C) and the role of volatilization in supporting air concentrations. Wind speed and wind direction measured on the R/V Walford averaged 3.6 m/s during the 14 sampling dates (Fig. 2), which is lower than the typical wind speed observed in this region of about 5 m/s, obtained from the National Oceanic and Atmospheric Administration meteorological stations located

at John F. Kennedy International Airport and the Newark International Airport.

3.2. Water PCB concentrations P Dissolved and particulate-phase 90 PCB concentrations are reported in Table 2. Concentrations reported are averages of two Infiltrex samples collected simultaneously. Measured P dissolved-phase PCB concentrations were consistent 90 throughout the year at the HRE, averaging 1100  240 pg/L (n ¼ 34), which was significantly higher than the average concentration of 420 pg/L at the offshore site, but similar to earlier measurements at the HRE site (Totten et al., 2001). The relatively constant dissolved PCB concentrations are consistent with a system in which volatilization of the dissolved PCB is buffered by desorption from contaminated sediments (Jurado et al., 2007). Indeed, Schneider et al. (2007) have shown that desorption of PCBs from Hudson River sediment is rapid and is not necessarily a function of molecular weight or diffusivity of the congeners. Asher et al. (2007) conducted chiral analysis of these samples and concluded that a significant fraction of the dissolved phase penta- and hexachlorinated PCBs arose from desorption from Hudson River sediments. Heavier PCBs could be buffered by airewater exchange, as their fugacity ratios are close to one (see below). Measured dissolved concentrations in HRE were higher than those in the northern Great Lakes (Hornbuckle et al., 1994; Zhang et al., 1999), but were comparable to other industrially

S. Yan et al. / Environmental Pollution 152 (2008) 443e451

446

Fig. 2. Gas-phase 2001.

P

90

PCB concentrations, wind speed, air temperature, and instantaneous net airewater exchange fluxes in the Hudson River Estuary in 1999e

impacted estuaries, such as the Chesapeake Bay (600e 1400 pg/L) (Nelson et al., 1998). Constant seasonal water concentrations were unexpected considering the proximity to major sources such as the New York City metropolitan area, upstream loadings from the Hudson River, wastewater treatment plant discharges, rainfall runoff, and tidal mixing effect with the open Atlantic Ocean. The estuary was well mixed throughout the water column in all sampling seasons, which may contribute to the observed stable water concentrations. Details regarding water column parameters such as

temperature, salinity, dissolved and particulate organic carbon, can be found elsewhere (Yan, 2003).P Particulate phase water column 90 PCB concentrations (pg/L) varied widely in the HRE throughout the year (averaged 1600  1200 pg/L, n ¼ 34). The particulate-phase P PCB concentrations were significantly correlated with 90 TSM concentrations (r2 ¼ 0.73, p < 0.05) and particulate organic carbon (POC) concentrations (r2 ¼ 0.47, p < 0.05) in the water. When normalized to TSM, particulate concentrations were less variable (400  150 ng/g, Table 2).

S. Yan et al. / Environmental Pollution 152 (2008) 443e451

447

Table 2 P Water 90 PCB concentrations in the Hudson River Estuary in 1999e2001 Sampling datea

Sampling site

Measured P dissolved [ PCB] (pg/L)

Measured P particulate [ PCB] (pg/L)

Measured P particulate [ PCB] (pg/g)

Calculated truly P dissolved [ PCB] (pg/L)

12/3/99 p 4/19/00 a 4/19/00 p 4/20/00 a 4/20/00 p 4/21/00 d 8/21/00 a 8/21/00 p 8/22/00 a 8/22/00 p 8/23/00 a 8/23/00 p 10/25/00 a 10/25/00 p 10/26/00 a 10/26/00 p 10/27/00 a 10/27/00 p 4/24/01 a 4/24/01 p 4/25/01 a 4/26/01 a 4/26/01 p

HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE HRE CAO CAO CAO

850 1300 1000 1100 1100 1300 1600 1300 1100 1400 900 850 850 800 1100 820 1300 960 1100 620 390 490 370

1700 3200 5000 3400 1800 3100 1500 1300 1100 1300 1200 1500 590 540 580 590 590 490 1200 1500 470 530 740

620 350 250 390 250 480 670 650 390 600 250 260 430 400 450 410 400 330 270 150 220 190 100

610 900 660 800 730 870 1200 870 840 960 690 600 440 510 610 450 800 530 570 400 260 310 260

Avg.  SD

HRE CAO

1100  240 420  65

1600  1200 580  140

400  150 170  61

700  210 280  30

a

‘‘a’’, ‘‘p’’, and ‘‘d’’ represent morning (08:30e12:30), afternoon (13:00e17:00), and daytime (08:30e16:30) samples, respectively.

3.3. Water column partitioning

CT ¼ Cd þ CDOC þ CP ¼ Cd ð1 þ KDOC $DOC þ KOC $TSM$fOC Þ

Partitioning of PCBs between the truly dissolved and particulate phases are frequently described in terms of the organic carbonewater partition coefficient, Koc (L kg1): KOC ¼

Cp Cd $POC

ð1Þ

where Cp and Cd are the concentrations (ng L1) of PCBs in the particle and dissolved phases, respectively, POC is the particulate organic carbon (kg L1). Because the sampling protocol used here cannot distinguish the truly dissolved PCB concentration (Cd) from the apparent dissolved concentration (Cd,a), the apparent KOC value (KOC,a) was estimated. The slope of the log KOC vs. log KOW relationship should be w1 when partitioning is at equilibrium (Hornbuckle et al., 1993). In contrast, slopes ranged from 0.34 to 0.80 in this study. This deviation from 1 suggests that a significant portion of the PCB burden is associated with colloids (Karickhoff et al., 1979). This discrepancy may lead to an overestimation of PCBs in the dissolved phase, which will affect the calculated airewater exchange fluxes by increasing the volatilization fluxes. Thus a three-phase partitioning model was used to calculate the truly dissolved PCB concentrations (Cd). The total concentration of PCBs in the water column (CT) is equal to the sum of the concentrations in each of the phases: the truly dissolved (Cd), the colloidal (CDOC), and the particulate (CP):

ð2Þ where KDOC is the partitioning coefficient between the compound and dissolved organic carbon (L kg1) and DOC is the concentration of the dissolved organic carbon (kg L1). Here, KDOC was assumed to equal 0.1$KOW, as in previous studies (Farley et al., 1999; Totten et al., 2001). On average, the relative contribution of colloids to the total water column PCB mass was 12%, 35%, and 53% for colloidal, truly dissolved, and particulate phases, respectively. When truly dissolved concentrations (Table 2) were used to calculate KOC, the slopes of the log KOC vs. log KOW plots were close to 1 (0.91e1.43), suggesting that partitioning of PCBs in the water column was close to equilibrium and variations largely due to the confounding factor of colloidal OC. 3.4. Fugacity ratios The first step in evaluating the air/water exchange of contaminants is to ascertain the direction of the net flux by examining the fugacity ratios in the system. The fugacity ratio ( f ) is defined as:

f¼

Cg Cd Kaw

ð3Þ

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S. Yan et al. / Environmental Pollution 152 (2008) 443e451

where Cd is the truly dissolved (DOC-corrected) water concentration, Cg is the gas-phase concentration and Kaw is the dimensionless Henry’s Law constant. Here, if f > 1, there is net absorption from the air to the water and if f < 1, there is net volatilization from the water to the air. If f ¼ 1, then the system is at equilibrium. As per the method of Rowe et al. (2007), fugacity ratios were calculated under the most conservative assumptions: minimum literature value of Kaw, temperature correction of Kaw via the enthalpy values of Bamford et al. (2000, 2002), and correction of dissolved PCB concentration for sorption to DOC. The log of the fugacity ratios were then examined to determine whether they were less than 0 at the 95% confidence level using the single-sample t-test. This analysis suggested that PCBs containing 5 or fewer chlorines, with the exception of the two co-eluting pentachlorinated congener groups (87 þ 81 and 107 þ 123 þ 149), undergo net volatilization in the HRE. These 44 congeners comprise 31 chromatographic peaks. For the heavier congeners, the direction of air/water exchange could not be determined. This does not imply that they are at equilibrium or that their fluxes are not significant, only that the uncertainties (primarily the uncertainty in Kaw) are too large to allow an accurate determination of the net direction of airewater exchange. Not enough samples were collected at the CAO site to allow an analysis of the fugacity ratios there. 3.5. Airewater exchange model The gas exchange of PCBs across the airewater interface of the HRE was calculated using a modified two-layer resistance model previously described elsewhere (Eisenreich et al., 1997; Nelson et al., 1998; Zhang et al., 1999; Totten et al., 2001; Gigliotti et al., 2001). The net diffusive gas exchange flux Fnet (ng m2 d1) is defined as the product of the overall mass transfer coefficient (vaw, m d1) and the fugacity gradient between water and air:   Ca Fnet ¼ vaw Cd  ð4Þ Kaw Recent published values of Kaw and its temperature dependence by Bamford et al. (2000, 2002) are considered to be the most appropriate (Totten et al., 2001) and were used for the calculation. Kaw was also corrected for salinity (Totten et al., 2001; Gigliotti et al., 2001). Details on the estimation of vaw can be found elsewhere (Achman et al., 1993; Eisenreich et al., 1997; Totten et al., 2001; Rowe et al., 2007). 3.6. Instantaneous airewater exchange fluxes The instantaneous fluxes were calculated using the model described above for the 31 chromatographic peaks (representing 44 PCB congeners) that undergo net volatilization. The P magnitude of the instantaneous net fluxes of 44 PCBs ranged from þ0.2 to þ630 ng m2 d1 (Fig. 2). The HRE showed significantly ( p-value < 0.005) higher fluxes (averaging þ170 ng m2 d1) than those at the CAO (þ37 ng m2 d1).

Higher airewater exchange fluxes were observed in spring and summer of 2000 and 2001 than those in winter of 1999 andPfall of 2000. measured in this study are 44 PCB fluxes in the HRE P comparable to those reported for 90 PCBs in July 1998 (averaged þ400 ng m2 d1) for the same location (Totten et al., 2001). Although we here report fluxes for a sum of only 44 congeners, the fluxes can be compared with those reported in other studies, because higher molecular weight congeners comprise a small portion of the overall flux due to their slower mass transfer coefficients and lower dissolved-phase concenP trations. The fluxes reported here for 44 PCBs are higher than those estimated for the Great Lakes (Jeremiason et al., 1994; Hornbuckle et al., 1994, 1995; Zhang et al., 1999). For example, Hornbuckle et al. (1994) reported fluxes ranging 2 1 from 40 P to þ110 ng m d in Lake Superior in 1988 and 1992. PCB fluxes were reported to range from 63 to þ800 ng m2 d1 in the Chesapeake Bay in 1993 (Nelson P et al., 1998). Recalculation of PCB fluxes by Totten et al. (2003) led to flux estimates of þ170 to þ5300 ng m2 d1 in Green Bay in 1989 and þ0.5 to þ230 ng m2 d1 in southern Lake Michigan in 1994. 3.7. Modeled annual fluxes To model the annual cycle of airewater exchange of PCBs in the HRE, appropriate surface water temperatures, wind speeds, and accurate gas- and dissolved-phase PCB concentrations are needed. Surface water temperatures were obtained from satellite temperature images provided by the Coastal Ocean Observation Lab at Rutgers University. During the period of 12/1/99 to 4/30/01, there were 210 days for which surface water temperatures were collected by satellite imaging. A sine transformation was used to fit the observed data such that daily surface water temperatures could be predicted and used to model the annual airewater exchange flux (Fig. 3). The modeled daily surface water temperatures were in close agreement with those observed during the 6 sampling cruise campaigns (Yan, 2003). Daily average wind speeds were obtained from the National Oceanic and Atmospheric Administration (NOAA) meteorological station located at the nearby Newark International Airport. Weekly averaged wind speeds did not vary seasonally and averaged 4.5 m/s. 3.8. Annual gas-phase concentration estimation The over-water gas-phase concentrations in the HRE would be best modeled using seasonal measurements taken from field sampling. However, in this study, all samples were taken during the day in relatively warm seasons (air temperature ranged 8.2e25  C). Only one sample was collected in December 1999 and the temperature was a mild 11  C relative to a typical ‘‘winter’’ temperature (about 1.4  C). With the absence of measurements in cold seasons and the limited number of samples collected (n ¼ 35), the

S. Yan et al. / Environmental Pollution 152 (2008) 443e451

449

Fig. 3. (a) Observed and modeled daily surface water temperatures, and observed weekly averaged wind speeds in the HRE. (b) Modeled daily gas-phase P concentrations of PCBs vs. field measurements in the HRE.

measured gas-phase concentrations at the HRE were considered to be insufficient for the prediction of annual gas-phase PCB concentrations. Concentrations at the nearby land site, JC, were used in the model because the JC site is close to the Hudson River Estuary, and a paired t-test P showed that the difference between measured gas-phase PCB concentrations over water at the HRE andP over land at the JC was not significant. The regression of PCB concentrations at JC against HRE is significant ( p < 0.005, n ¼ 12) and gave an r2 of 0.58. A total of 65 air samples were collected at JC every 6e12 days from 1998 to 2001 as part of the New Jersey Atmospheric Deposition Network (NJADN). A much wider range of temperatures (6.2 to 30.5  C) were covered during the sampling, and therefore, PCB concentrations observed at JC were considered to be more appropriate for predicting seasonal variations of gas-phase PCBs in the HRE (Totten et al., 2004). Air temperature is the most important factor controlling the gas-phase concentrations of POPs (Wania et al., 1998; Carlson and Hites, 2005). Therefore, a simple linear regression analysis between gas-phase concentrations (ln Ca) and air temperatures (1/T ) measured at JC was performed for each congener and/or co-eluting congener group as follows:

ln Ca ¼ a0 þ

a1 T

ð5Þ

The slopes (a1) ranged from 2356 to 9152 for di- through nona-chlorobiphenyls ( p < 0.005). On average, temperature explained 49% of the variability of ln Ca for all congeners. Congener-specific gas-phase PCB concentrations over the HRE water were then predicted on a daily basis using the corresponding regression coefficients and modeled daily surface water temperatures discussed above. Wind speed and direction were found to be not significant in predicting gas-phase PCB concentrations at JC. P Modeled gas-phase concentrations of 90 PCBs over the 3 HRE ranged from þ230 to þ2300 pg/m from 12/1/99 to P 4/30/01. Predicted concentrations of PCBs were found 90 to be in good agreement with the average concentrations measured over water (Fig. 3), although they do not capture all of the variability frequently observed, both at the land-based and over-water sites. This failure is relatively unimportant for the estimation of fluxes for two reasons. First, since fluxes here are estimated over the course of a full year, much of the variability averages out. Second, because the fugacity ratios suggest a strong water to air gradient, the absorption flux is

S. Yan et al. / Environmental Pollution 152 (2008) 443e451

450

Fig. 4. Predicted weekly net airewater exchange fluxes of

small relative to the volatilization flux, and the gas-phase concentrations therefore have a limited impact on the net flux. 3.9. Annual dissolved-phase concentration estimation P As mentioned earlier, the measured dissolved-phase PCB concentrations did not display seasonal variation and averaged 1100  240 pg/L (n ¼ 34) in the HRE from 1999 to 2001. Similarly, no seasonal trend was observed for dissolved organic carbon (DOC) concentrations in the HRE. Based on the three-phase partitioning model described earlier, the mean truly dissolved-phase concentrations of PCB congeners were calculated using the mean measured dissolved-phase PCB concentrations and the mean DOC concentration. Calculated mean truly P dissolved-phase PCB concentrations (800 pg L1 for 90 PCBs) were applied in the modeling of the annual cycle of airewater exchange. 3.10. Annual flux estimation Daily airewater exchange fluxes of PCB congeners were predicted from modeled daily gas-phase concentrations, the mean calculated truly dissolved-phase concentrations, modeled daily surface water temperatures, and observed average daily wind speeds from 12/1/1999 to 4/31/2001. Only fluxes for the 44 congeners known to be undergoing net volatilization will be presented here. Including the fluxes P for the other 46 congeners has almost no effect on the net 90 PCB flux, since these congeners have net fluxes P that are close to zero. Predicted daily net fluxes of 44 PCBs ranged from 53 to þ660 ng m2 d1 and averaged þ160 ng m2 d1, demonstrating that under some conditions (very high gas-phase

P PCBs in the Hudson River Estuary in 1999e2001.

P concentrations driven by high temperatures) the net 44 PCBs flux can become negative (absorptive) for short periods. Weekly net fluxes were always positive and are shown in Fig. 4. The net airewater exchange fluxes ranged from þ200 P to þ2260 ng m2 wk1 for 44 PCBs. 4. Conclusion P The annual airewater exchange fluxes of PCBs for the year of 2000 were estimated from the sum of 365 daily fluxes multiplied by the surface area of the lower Hudson River Es2 tuary (446 kmP ) (Farley et al., 1999), resulting in a net loss of þ28 kg/yr of 44 PCBs from the surface water P of the HRE. Farley et al. (1999) calculated that 340 kg of PCBs volatilized from the entire NY/NJ Harbor in 1997. TottenP(2005) similarly estimates that between 317 and 846 kg of PCBs volatilize from the NY/NJ Harbor each year. Both of these calculations include additional portions of the Harbor system, most notably thePTappan Zee/Haverstraw Bay area, where dissolved-phase PCB concentrations are on the order of 10 ng/L (Litten, 2003) and the surface area is nearly equal to that of the area we here define as the Hudson River Estuary (Adams et al., 1998). Thus the HRE, which comprises about half of the surface area of the NY/NJ Harbor system, is responsible for less than 10% of the total volatilization flux of P PCBs due to the lower concentrations of PCBs there resulting from tidal inputs of relatively uncontaminated ocean water. References Achman, D.R., Hornbuckle, K.C., Eisenreich, S.J., 1993. Volatilization of PCBs from Green Bay, Lake Michigan. Environ. Sci. Technol. 27, 75e86.

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