G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 9, Number 12 4 December 2008 Q12007, doi: /2008gc ISSN: Estuarine distributions of Zr, Hf, and Ag in the Hudson River and the implications for their continental and anthropogenic sources to seawater L. V. Godfrey and M. P. Field Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA (godfrey@marine.rutgers.edu) R. M. Sherrell Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901, USA Department of Geological Sciences, Rutgers University, Wright-Rieman Laboratories, 610 Taylor Road, Piscataway, New Jersey 08854, USA [1] Zirconium and Hf in seawater are derived from continental weathering and are transported to the oceans by rivers, but the modification of this flux with estuaries is largely unknown. Furthermore, there is no information as to whether there is a pollutant source of these metals that might affect their budget calculations in the modern ocean. To address the riverine flux and potential anthropogenic sources, the distributions of dissolved Zr and Hf have been measured in surface and bottom water through the Hudson River Estuary. The distribution of Ag is included because it is sensitive to urban sources and has partial similarities in its distribution to those of Zr and Hf. There is about 50% removal of Zr and Hf in surface water relative to their freshwater end-member concentrations of 243 and 3.1 pmol/kg as salinity starts to increase, but an urban source of these metals midestuary may obscure greater removal. Silver concentrations in surface water are below the detection limit until salinity of 3.5 is reached. The concentrations of Zr and Hf 0.5 m above the bottom are 1.5 to 3 times higher than their surface water concentrations throughout the estuary and may result from input of groundwater, diffusion from pore fluids, or inclusion of colloidal phases in our analyses. Downstream of the turbidity maximum, bottom water Ag concentrations are high due to release from particles that originally formed within the estuary and were moved inland following the spring freshet; this process may also affect Zr and Hf. The increase in the freshwater atomic Zr/Hf ratio from 70 to 80, characteristic of terrestrial rocks, to the elevated Zr/Hf (>110) ratio of seawater takes place at middle to high salinity due to enhanced release of Zr from resuspended particles and slower removal rates compared to Hf. A main implication of this study is that estuaries are a sink for Zr and Hf. The reduction of the predicted export of continental Zr and Hf to the oceans increases their previously calculated seawater residence by as much as 2 to 4 times, assuming rivers dominate the supply of Zr and Hf to seawater. Components: 8186 words, 4 figures. Keywords: hafnium; zirconium; silver; estuaries; seawater; Hudson River. Index Terms: 1065 : Major and trace element geochemistry; 4808 Oceanography: Biological and Chemical: Chemical tracers; 0442 Biogeosciences: Estuarine and nearshore processes (4235). Received 10 June 2008; Revised 26 August 2008; Accepted 2 October 2008; Published 4 December Copyright 2008 by the American Geophysical Union 1 of 13

2 Godfrey, L. V., M. P. Field, and R. M. Sherrell (2008), Estuarine distributions of Zr, Hf, and Ag in the Hudson River and the implications for their continental and anthropogenic sources to seawater, Geochem. Geophys. Geosyst., 9, Q12007, doi: /2008gc Introduction [2] Weathering and erosion of the continents is the major source of most metals in seawater and is the expected major source of seawater Zr and Hf [Pettke et al., 2002]. The importance of different transport pathways of continental Zr and Hf to the oceans is debated [Albarède et al., 1998; Bau and Koschinsky, 2006; van de Flierdt et al., 2004], but the modification of their river fluxes within estuaries has not been determined and could drastically alter the importance of their dissolved river fluxes. Furthermore, the industrial use of Zr and Hf suggests they could have a pollutant source to the environment [Gobeil et al., 2005], but low concentrations have largely hampered their study. Auxiliary materials. 1 [3] Zirconium and Hf are chemically close relatives. Their similar geochemistry causes the atomic Zr/Hf ratio of most terrestrial rocks as well as chondrites to vary within a narrow range of which is used to describe Bulk Silicate Earth (BSE) [Jochum et al., 1986; David et al., 2000]. Rarely, Zr/Hf ratios vary beyond this narrow range: partial melting can produce values that are slightly high (72 90) relative to BSE and fluid interaction with highly evolved magmas can form granites with Zr/Hf ratios as low as 30 [Bau, 1996; Zhu et al., 2001]. The greatest deviation of Zr/Hf ratios from the terrestrial rock ratio is observed in seawater and authigenic precipitates which reflect the very high Zr/Hf ratios of seawater, up to 500 [Godfrey et al., 1996; McKelvey and Orians, 1998]. In common with other lithophile metals, Zr and Hf are insoluble in water and readily adsorbed onto particle surfaces which results in very low dissolved concentrations in river and seawater [Boswell and Elderfield, 1988; McKelvey and Orians, 1993, 1998; Godfrey et al., 1996]. In neutral ph water, Zr and Hf are complexed by hydroxyl ions allowing them to be complexed with organic acids which can increase their solubility [Viers et al., 1997, 2000; Pokrovsky and Schott, 2002; Tosiani et al., 2004; Braun et al., 2005]. 1 Auxiliary materials are available in the HTML. doi: / 2008GC [4] Industrialization and changes in land use have increased the metal loadings of many rivers, estuaries, and coastal areas. Over the last 50 years efforts to reduce contamination of inland and coastal waters have been largely successful, but for many metals, a paucity of data has led to uncertain baseline concentrations for unperturbed systems and a limited understanding of spatial and temporal variations. Zirconium and especially hafnium are two metals which exemplify this state. There are a limited number of estuarine studies that include dissolved Zr data [Pokrovsky and Schott, 2002; Gobeil et al., 2005] and none that include dissolved Hf. Silver is another metal which has only been studied in a limited number of rivers and estuaries [Miller and Bruland, 1995; Wen et al., 1997; Breuer et al., 1999; Flegal et al., 2005; Buck et al., 2005]. The interest in Ag stems from anthropogenic sources overwhelming its natural sources making it a sensitive tracer of urban pollution. [5] Silver enters the environment from wastewater facilities that have acquired silver from its many industrial and commercial uses such as X-ray and photographic processing, solders for plumbing and electronics, and as a bactericide and algaecide in water purification systems. In urban estuaries silver exhibits nonconservative mixing due to its association with Fe-Mn oxyhydroxide/sulfide phases, organic macromolecules, and colloids [Wen et al., 1997; Reinfelder and Chang, 1999]. It has a high affinity for particle surfaces, and combined with a major urban source from wastewater treatment plants has made it a sensitive indicator of sewage [Sañudo-Wilhelmy and Flegal, 1992; Bothner et al., 1994]. The industrial use of Zr as an additive to alloys and foundry castings and as a catalyst or catalyst support in industrial processes and vehicle emission controls suggests that there could be an anthropogenic source of this metal to the environment. Hafnium is used in control rods of nuclear reactors, to make superalloys, and as scavenger metal of oxygen and nitrogen. However, the near identical properties of Zr and Hf means that for most applications it is not necessary to separate the two metals ( pubs/mcs/1997/) and they would be introduced into the environment with a Zr/Hf ratio similar to that of terrestrial rocks. Concentrations of dis- 2of13

3 Figure 1. Map of the Hudson River Estuary showing sampling stations 1 9 occupied in November 2002 and CH53 occupied in June Abbreviations are as follows: LIS, Long Island Sound; NRWTF, North River water treatment facility; GWB, George Washington Bridge. solved Zr at the mouth of the St Lawrence (78 pmol/l) and an outlet from the Montreal wastewater facility (240 pmol/l) and water fluxes indicate that 1% of the Zr flux of the St Lawrence is derived from urban effluent [Gobeil et al., 2005]. The outflow from the St Lawrence River is about 25 times higher than that of the Hudson River; with a population 10 times that of Montreal 10%, and a flux from wastewater treatment facilities exceeding 15% of the natural river discharge instead of <1% [Brosnan and O Shea, 1996; Gobeil et al., 2005], the probability that anthropogenic sources contribute more than 1% of the river s Zr flux is high. [6] The Hudson River Estuary (HRE) is a large urban estuary which has received pollutant metal inputs from industrial plants and water treatment facilities both upriver and within the lower estuary in the New York metropolitan area. Over the last decade efforts to abate sewage discharge have improved water quality [Sañudo-Wilhelmy and Gill, 1999], however, particle reactive pollutant metals accumulate with sediment that can be trapped in the lower HRE (between the Battery at the southern tip of Manhattan and Stony Point), and the New York Harbor [Mueller et al., 1982] making these sediments a major repository of pollutant trace metals. Sediment redistribution occurs during the spring freshet and by strong tidal currents later in the year, allowing some metals to be released back to the water column in different parts of the estuary (Y. Rosenthal et al., Toxic metal inputs to the lower Hudson River estuary during sediment resuspension, unpublished report, 2004 available at hudsonriver.org). Suspended sediments are concentrated by frontal processes leading to high levels of total suspended matter (TSM) between 10 and 35 km north of the Battery [Woodruff et al., 2001] with a turbidity maximum zone (TMZ) occurring as a result of vigorous physical mixing associated with the salinity front close to the George Washington Bridge, about 16 km north of the Battery. A second smaller permanent turbidity zone occurs at the salt front in Haverstraw Bay, about 60 km north of the Battery. Tidal flow in the lower estuary ranges between 60 and 120 cm/s in near surface water and between 40 and 80 cm/s in near bottom water, nontidal flow is seaward between 0 and 20 cm/s in near surface water and landward between 0 and 40 cm/s in near bottom water [Geyer et al., 2001]. Fourteen kilometers north of the Battery is the North River wastewater treatment facility (NRWTF) which is a potential source of metals and organic material which are not removed during the treatment process [Mackie et al., 2007; Sañudo-Wilhelmy and Taylor, 1999; Rosenthal et al., unpublished report, 2004]. However, it is important to remember that there are numerous points of discharge to the Hudson River in the metropolitan area, including numerous combined sewage overflows, but the NRWTF has the largest point source discharge into the lower Hudson. Sampling at nine stations (Figure 1) took place in mid-november 2002 during a neap tide when the position of the salt front at the time of sampling was about 70 km north of the Battery (Figures 2 and 3). We present dissolved Zr, Hf, and Ag concentration data through the Hudson Estuary in surface and bottom water. 2. Methods [7] Water samples were collected midriver through the Hudson estuary in November 2002, starting at 3of13

4 Figure 2. Geographical distributions of salinity (closed circles), suspended material (open circles), Zr (squares), Hf (diamonds), Zr/Hf ratio (triangles), and Ag (crosses) dissolved concentrations in surface water. The Battery, at the southern tip of Manhattan is located at 0 km, the North River water treatment facility is located 14 km above the Battery (indicated by hatched pattern) and the turbidity maximum zone by the speckled pattern. The dashed line on the Zr plot is the calculated concentration if Zr/Hf ratios are maintained through the estuary, the addition of Zr due to release from particles in the TMZ is indicated by the arrow. Where Ag concentrations are below the detection limit of 10 pmol/kg, data is not plotted. the northern end of the transect. At the start of sampling the tide was in flood, and by the time the Battery was reached, it was in ebb. Samples were collected 0.5 m below the water surface and 0.5 m above the river bottom using weighted Teflon 1 - lined polyethylene tubing connected to a bottom lander equipped with a pivoting mast and vane to direct the sampling tube into the current. The sampling tube for the near-bottom samples was held at a fixed depth by insertion through the mast and water was delivered on board at 5 L/min using an in line Teflon diaphragm pump with a polypropylene body (Rosenthal et al., unpublished report, 2004). The sample locations are included in Figure 1. Water was filtered through acid washed 0.45 mm polycarbonate filters (Poretics 1, GE Osmonics Inc, 4of13

5 Figure 3. Geographical distributions of salinity, suspended material, Zr, Hf, Zr/Hf ratio, and Ag dissolved concentrations in water 0.5 m above the sediment surface. Where Ag concentrations are below the detection limit of 10 pmol/kg, data is not plotted. Symbols as in Figure 2. Minnetonka, MN) in a Class 100 HEPA laminar flow bench following the protocol of established trace element clean methodology [Cullen and Sherrell, 1999]. Following filtration, 500 ml volume samples were collected in acid cleaned low-density polyethylene bottles and acidified immediately with 2 ml 50% HCl. Other water samples used in separate studies were collected from the shore of Hudson estuary close to station 1 in October 2005 and seaward of Raritan Bay on R/V Cape Henlopen in 1999 are included to complement the data set presented here. These samples were filtered using in-line 0.45 mm sealed capsule filters. The subsequent treatment and analytical protocols were identical for all samples. All acids that were used to stabilize samples during storage and in the analysis were Baseline 1 Seastar Baseline grade, and all Teflon 1 -ware was cleaned with a sequence of strong acids (aqua regia, 50% HNO 3 and 50% HF). 5of13

6 Figure 4. Dissolved concentrations of Zr, Hf, Zr/Hf, and Ag plotted versus salinity; closed symbols for surface water and open symbols for water 0.5 m above bottom. If these metals were unreactive across salinity gradients and had no additional sources along the estuary, conservative mixing would lead to straight lines when their concentrations are plotted against salinity. The Zr/Hf ratio range for terrestrial rocks is from Jochum et al. [1986] and for coastal Jersey seawater from an unpublished compilation of data by Godfrey (range is 1 standard deviation for salinity and Zr/Hf). The position of the turbidity maximum zone in surface and bottom water is indicated by the speckled pattern behind the relevant data point. [8] Isotope spikes ( 91 Zr and 177 Hf) and FeCl 3 were added to the samples in the laboratory and equilibrated for two days. The ph was then raised to 8 with dissolved ammonia and the samples left 24 h to allow the Fe precipitate with which the Zr and Hf had coprecipitated to settle. The supernatant was decanted, and the residual centrifuged to isolate the Fe precipitate. The precipitate was 6of13

7 dissolved with 6N HCl and a small amount of HF which was then diluted to 1N HCl. Zirconium and Hf were separated from the majority of the matrix (Fe) and REE using a simplified and scaled down version of the procedure described by Patchett and Tatsumoto [1981]. Sample solutions were dried and redissolved into a small volume (1 ml) of 1N HNO 3 /0.1N HF for analysis using HR-ICP-MS (Thermo Element 1, Bremen). The precision for this method of analysis from replicate samples is ±6.5% (1 standard deviation) for Hf and Zr, and blanks contribute <3% to the Zr and Hf concentrations. We were unable to determine the accuracy of the measurements because there were no established river or seawater standards for Hf or Zr concentrations at the time of analysis. [9] Dissolved Ag concentrations were determined on high-resolution inductively coupled plasma mass spectrometer (HR-ICP-MS, ELEMENT-1, Finnigan MAT, Bremen, Germany) set at low resolution (Dm/m = 300). Samples were introduced to the plasma using a microconcentric desolvation system (MCN-6000, Cetac Tecnnologies, Omaha NE) in free aspirating mode using a microflow nebulizer (Elemental Scientific, Omaha NE) [Field and Sherrell, 1998] to reduce water-derived interferences and increase sensitivity. Working standard mixtures were made in ultrapure 3% HNO 3 (vol/vol) from 10 ppm primary standards solutions (High-Purity Standards, Charleston, SC). Samples were diluted 10 times with a 10 ppb indium (In) 3% HNO 3 (vol/vol) and standard additions were performed to selected samples covering the complete range in salinity. Variations in sensitivity due to salinity based matrix effects are corrected for by In normalization and calculation of concentrations use standard addition curves from samples of similar salinity. On the basis of our replicate analysis this internal standardization combined with pseudo standard additions provides approximately ±5% (1 standard deviation) precision and accuracy for Ag. 3. Results [10] Midstream surface water concentrations of Zr (243 pmol/kg) and Hf (3.1 pmol/kg) are highest in the freshwater end of the estuary, 67 km above the Battery (Figure 2). As salinity increases to 2 (at 52 km north of the battery) surface water dissolved concentrations of Zr (133 pmol/kg) and Hf (1.7 pmol.kg) decrease by 60% while maintaining a constant Zr/Hf (78.4 ratio) (Figure 2). Concentrations of silver across this section of the estuary were below detection limits (10 pmol/kg). Zirconium, Hf, and Ag have a broad midestuary maximum concentrations (230 pmol/kg, 2.7 pmol/kg, 102 pmol/kg) between 10.8 and 16.2 km above the Battery (Figure 2). Surface water concentrations of Zr, Hf, and Ag decrease by 20, 40, and 70%, respectively, at the Battery from their midestuary maxima; beyond Raritan Bay their concentrations decline further by 5, 30, and 25%. The ratio of Zr:Hf shows almost no change through the midsection of the estuary until 16.2 km above the Battery where it begins to increase to reach 111 at the Battery; beyond Raritan Bay the Zr/Hf ratio increases further to 153. [11] Bottom water concentrations of dissolved Zr and Hf are higher than they are in surface water throughout the estuary. The highest concentrations of dissolved Zr (847 pmol/kg) and Hf (12.4 pmol/ kg) in bottom water occur at the freshwater end of the estuary and the concentrations decrease by 60% as salinity increases to 3.1 (Figure 3). The dissolved Zr/Hf (68) ratio in bottom water is slightly lower than it is in surface water (78), and increases slightly with salinity (Figure 3). The bottom water concentration of dissolved Ag is below detection limit (10 pmol/kg) in the freshwater end-member. Bottom water Zr and Hf concentrations get lower through the estuary until 34.2 km above the Battery where they reach 305 and 4.0 pmol/kg. There is an increase in the Zr/Hf ratio to There is a small local maximum in the dissolved concentrations of Zr (402 pmol/kg) and Hf (5.3 pmol.kg) 24.9 km above the Battery, downriver of which, Zr and Hf concentrations fall and then slightly increase (Figure 3). The TSM maximum occurs 8 km downstream of the small Zr and Hf concentration maxima, and here there is a sharp increase in Zr/ Hf ratios from 76 to 91. The concentration of Ag increases to reach a broad midestuary plateau of pmol/kg 24 to 40 km above the Battery. At the TMZ, 16 km above the Battery, the Ag concentration doubles and then decreases to 60% of this maximum concentration as the Battery is reached. 4. Discussion 4.1. Removal of Zr and Hf at Low Salinity [12] Surface reactive metals can be initially removed from solution when particles form or later removed by adsorption to their surfaces. Largescale removal of metals, the rare earth elements for example, from river water occurs as Fe-C org col- 7of13

8 loids flocculate as salinity increases at the onset of an estuarine environment [Eckert and Sholkovitz, 1976; Boyle et al., 1977; Sholkovitz et al., 1978; Hoyle et al., 1984; Sholkovitz, 1995]. Zirconium and Hf exist as M(OH) 5 complexes, are transported by Fe and/or organic colloids, and therefore are likely candidates for removal by flocculation processes and by scavenging by hydrous Fe and Mn oxides [Cabaniss, 1987; Byrne, 2002; Pokrovsky and Schott, 2002; Pokrovsky et al., 2006]. Dissolved concentrations of Zr and Hf decrease similarly (40%) in surface and bottom water up to salinity of 2 3, indicating removal at low salinity is a product of aggregation of Zr- and Hf-bearing colloids. It is possible that Zr and Hf are removed by complexation with particle surfaces, but the minimal change in Zr/Hf ratios up to salinity 8 10 are more consistent with a physical aggregation process. Furthermore, Zr/Hf ratios close to those of BSE suggest that Zr and Hf behave similarly during weathering and transport in this watershed. The fractionation of Zr and Hf that is observed between rocks (70 78) and seawater (>100) does not appear to be produced by colloid aggregation in the low salinity region of estuaries [Jochum et al., 1986; Godfrey et al., 1996; McKelvey and Orians, 1998] Anthropogenic Sources of Zr and Hf [13] There are broad midestuary maxima in surface water dissolved Zr, Hf, and Ag concentrations (Figure 2). The maximum concentration in dissolved Hf and Ag concentrations occurs a few kilometers upriver of the outfall from the NRWTF which is located between stations 6 and 7. Input of dissolved chemicals (including Zr, Hf, and Ag) as well as fine particles from the NRWTF may form a broad maximum around its outfall due to reversible tidal flow. Dye release experiments to bottom water in the Hudson River estuary have demonstrated that the core of the dye released is transported more than 10 km from its point of release and horizontal dispersion can allow it to spread more than 20 km [Geyer et al., 2008]; tidal flow is 1.5 to 2 times higher in near surface water [Geyer et al., 2001] allowing greater dispersion of materials in surface water. While the dye is unreactive, removal to particle surfaces can reduce the lateral spread of metals such as Ag (Figures 2); the increased spread in elevated concentrations of Zr and Hf compared to Ag implies that they are less actively removed from solution. The midestuary concentration maxima of Zr and Hf are a surface rather than bottom water phenomenon further supporting surface input rather than disaggregation of particles that had formed when salinity first increased. [14] Zirconium, unlike Hf and Ag, exhibits its maximum 5.4 km further downstream and is coincident with the TMZ. While some particles are released to the river from the NRWTF, resuspension of bottom sediments from a midestuary depocenter around the George Washington Bridge (close to station 6) dominates the particle population in the TMZ [Woodruff et al., 2001; W. R. Geyer, Final report: Particle trapping in the Lower Hudson Estuary, unpublished report, 1995]. The spatial separation of the TMZ and Zr maximum from the Hf and Ag maxima derived from the NRWTF may occur because particles are injected into surface water as fronts develop during ebb tides, whereas water released to the river through the NRWTF is continuous. Sampling occurred at stations 6 and 7 around high tide, when chemicals from the NRWTF start to move down river and particles from bottom sediment start reaching surface water. [15] The approximate 40% reductions in dissolved Zr and Hf concentrations as salinity first increases are considerably less than the 80% reduction in dissolved Fe and Pb concentrations (Rosenthal et al., unpublished report, 2004). The stronger effect that flocculation has on dissolved Fe concentrations compared to those of Zr and Hf suggests Zr and Hf may be associated with different or additional ligands that give Zr and Hf different adsorption properties to Fe-organic colloids or that the speciation of Zr and Hf in the freshwater Hudson gives them a low K d relative to that of Fe. It is also possible that the midestuary influx of Zr and Hf from the NRWTF may overprint the true extent of removal of Zr and Hf by flocculation of particulate Fe. Dissolved organic carbon (DOC) and low molecular weight organic carbon (LMW OC) have a mid salinity excess in the Hudson Estuary arising from inputs of wastewater and surface runoff [Sañudo-Wilhelmy and Taylor, 1999] and could provide the organic colloid and macromolecule substrate to which is bound the Zr and Hf that is not associated with Fe. The association between Zr, Hf, and DOC previously found by Pokrovsky and Schott [2002] and Tosiani et al. [2004] in combination with the seemingly less efficient removal of Zr and Hf compared to Fe, suggests that organic complexation sustains dissolved Zr and Hf concentrations through the zone of colloid flocculation. Inclusion of DOC measurements and a study of Zr and Hf removal in a less impacted estuary 8of13

9 would help to assess the importance of binding to organic ligands in the transport of Zr and Hf through estuaries Bottom Source of Zr and Hf [16] Bottom water dissolved Zr and Hf concentrations are higher than surface concentrations by a factor of throughout the estuary and bottom water Ag concentrations are higher than surface concentrations at salinities above 15. Metal concentrations of water from different depths within an estuary usually plot to form a single trend against salinity, and the changes in concentration with depth occur because of a vertical salinity gradient. This does not happen for Zr and Hf anywhere in the Hudson River, and while the concentration of Ag in surface and bottom water form a single trend when plotted against salinity lower than 15, above 15 the concentration in bottom water is much higher than it is in surface water (Figure 4). [17] The higher concentrations of Zr and Hf in bottom water compared to surface water throughout the estuary implies either a benthic source of these metals by diffusion from pore water or advected with groundwater flow or inclusion of colloidal forms that passed through the 0.45 mm pores of the filters used. A bottom nepheloid layer is often formed (J. Cole, personal communication, 2007) which would favor a colloidal source of these metals if sediment-derived colloidal material is resuspended along with particulate surface sediments. The localized increase in Zr and Hf concentrations 24.9 km north of the Battery and 9 km upstream of the bottom water TMZ may result from a colloidal suspension. Side-scan sonar has revealed deposits of material with low dry weight density that migrate inland after the spring freshet [Woodruff et al., 2001] and if the surface of these deposits is semifluid, it is possible that disturbance of a fluid mud layer could have occurred during sampling, increasing the suspension of sub-0.45 mm particles. Redox-driven metal mobilization in a fluid mud [e.g., Robert et al., 2004] may create a flux of some metals, possibly Zr and Hf, into the river, but the elevated concentrations of Zr and Hf are not accompanied by changes in the Zr/Hf ratio (Figure 3). It is more likely that the increases in Zr and Hf concentrations at this location result from physical suspension and inclusion of Zr and Hf in sub-0.45 mm particles that have Zr/Hf ratios typical of terrestrial rocks to the measurement of their dissolved concentrations. [18] While there is less direct evidence for a benthic source of dissolved Zr and Hf, it should not be discounted. Elevated concentrations of these metals measured in a nearshore water sample collected in 2005 close to Station 1 may support this as a source. The location of this nearshore sample was in a sheltered embayment north of Iona Island at the base of Bear Mountain. This sample has very high dissolved Zr and Hf concentrations, 2060 and 23.5 pmol/kg, respectively, which are double the midriver bottom water concentrations and may indicate that these riverside wetlands are a source of Zr and Hf or that groundwater flow is focused toward the river and transports Zr and Hf. The low dissolved oxygen concentrations that occur in deep water in the tidal freshwater sections of the river due to respiration rates exceeding productivity [Howarth et al., 1992, 1996] may allow Zr and Hf to escape immediate removal from solution as they enter the river. It is also possible that Zr and Hf are released from organic-rich particles that wash into the river. The role of organic molecules as sources, sinks, or transport vehicles of Zr and Hf within estuaries warrants further investigation Effect of Sediment Redistributon and Resuspension on Dissolved Zr, Hf, and Ag Distributions [19] Particle populations in estuaries consist of siliclastic weathering products of rocks in the rivers drainage basin, particles that form from colloidal sized material as a response to increases in salinity, urban dust, and pollution. The development of strong redox gradients in estuarine sediments can affect the chemical stability of some of these particles after they are buried and make them susceptible to dissolution and to release their surface bound metals or to form new particles. Injection of sedimentary particles into the water column during different stages of river and tidal flow results in their redistribution within the estuary and to the release of metals, either by desorption or by dissolution of the particles and/or their coatings which were formed under the different chemical environment of the sediment column. Concurrently, particle reactive metals are removed from solution by adsorption onto particle surfaces, and noticeable decreases in dissolved metal concentrations occur as suspended particle concentrations increase. [20] Zirconium, Hf, and Ag removal from solution by suspended particles in surface water starts at the 9of13

10 TMZ following their midestuary maxima near the NRWTF. Relative to their midestuary concentration maxima, dissolved concentrations down river are much lower for Ag than for Zr or Hf (Figure 2). At the Battery, 16 km down river from the TMZ and with salinity increasing by 7, the concentration of dissolved Ag has decreased 70%, while Zr and Hf have only decreased by 20% and 40% from their maximum concentrations. The more rapid removal of Ag likely arises from faster scavenging of dissolved silver and/or a higher K d than Zr or Hf. [21] The increase in the bottom water concentration of Ag that coincides with the high suspended particulate concentration of the TMZ is indicative of Ag redistribution and release from particles. There are two convex up trends against salinity in bottom water; the first overlies the surface water Ag salinity trend between a salinity of 7 and 10 and the second occurs at higher salinity downstream from the TMZ. The ultimate source of the second increase in Ag in bottom water is urban outflow. The inputs of metals including Ag to surface water from facilities such as the NRWTF and combined sewer overflows in to the Lower Hudson as well as from, for example the East River [Buck et al., 2005], are adsorbed to particles, some of which are trapped in the New York Harbor [Geyer et al., 2001; Woodruff et al., 2001]. Following the spring freshet sediment transported landward carries some of the pollutant Ag upriver in metal oxide or sulfide phases which are susceptible to dissolution when they are resuspended by tidal and river flow and release of Ag forms the second peak in concentration. Repetitive redox cycling according to hydrologic stage has also been the suggested cause for metal distributions in the Gironde Eastuary [Robert et al., 2004], but could be common to most estuaries, and most visible in impacted estuaries where metal concentrations are artificially high. [22] The concentrations of Zr and Hf in bottom water are also influenced by suspended particles. Zirconium and Hf concentrations decrease following local maxima at station 5, 24.9 km north of Battery and discussed earlier, but while the concentration of Hf in bottom water at stations 6 to 8 resumes the trend of decreasing concentration with increasing salinity that was interrupted at station 5, the concentration of Zr stays about 20% higher than it was at stations 3 and 4. While the concentration of Zr may be maintained at a higher value downstream of the TMZ compared to Hf due to slower removal rates of Zr or a lower K d, the higher concentrations of Zr indicate that more Zr may be released from suspended or surface sediment particles. [23] We can calculate the excess dissolved Zr (Zr xs =Zr meas (Zr/Hf) freshwater Hf meas ) relative to Hf by assuming that Zr/Hf ratios should not change if both metals behave and react identically through the estuary. Relative to the initial Zr and Hf freshwater concentrations, Zr xs in surface and bottom water increases through the estuary somewhat independently from suspended particulate concentrations suggesting faster removal rates of Hf or changes in speciation with salinity regulate the changes in Zr/Hf ratios. The loss of Hf from solution as salinity increases could explain the high Zr/Hf ratios of seawater [Godfrey et al., 1996; McKelvey and Orians, 1998] if it is a common feature in all estuaries. The amount of Zr xs could be particularly high in the Hudson Estuary because it is an anthropogenically impacted estuary and Zr added to the river in surface water can be transported upstream on particles following the spring freshet and then rereleased to the water column. The more labile characteristics of Zr compared to Hf or Ag is highlighted by the increase in surface water Zr concentrations and Zr xs downstream of the TMZ. However, neither bottom water Zr or Hf concentrations increase to the extent that Ag does at the TMZ, and this may be related to Ag associating with sediment phases that dissolve in the water column, such as poorly crystalline sulfides, as opposed to simple surface desorption that may be the main Zr supply mechanism Zr and Hf in the Hudson and Implications for the Marine Hf Cycle [24] The Zr/Hf ratio of the Hudson before it enters its estuary is virtually unchanged from that of average terrestrial rocks. Aggregation and settling of colloids removes Zr and Hf, but this physical process does not alter the dissolved Zr/Hf ratio. Sediment resuspension and desorption of Zr and Hf, and the subsequent removal of these metals as salinity increases above 10, drives the dissolved Zr/ Hf ratio toward a higher value that is characteristic of seawater. [25] The concentration of Hf in Hudson River decreases 50% by the time salinity has increased to 15, and 60% by S = 30. If this is typical of all estuaries, the residence time of Hf in seawater, using the average seawater and freshwater river water concentrations of Godfrey et al. [1996] is 10 of 13

11 increased from 1500 years to 2500 years. However, the estuary of the Hudson River is surrounded by a large city and is impacted by urban sources of metals. If we assume that the concentration of Hf is increased following the salinity range where Fe organic colloids coagulate because of anthropogenic inputs, the removal of Hf could be 80% or higher, giving a residence time of up to 7500 years. These estimates of the residence time of Hf are considerably longer than the 600 year residence time of Hf based on a vertical seawater scavenging model of McKelvey [1994]. [26] Our new estimate of a longer seawater residence time for Hf, from a value close to the ocean mixing time [Godfrey et al., 1996] to one considerably longer than the mixing time for an ocean basin has important consequences for variations in Hf isotopes both spatially and temporally. The range in e Hf of terrestrial rocks (50 e units) is close to double that of e Nd (30 e units) [Vervoort et al., 1999], while the range of e Hf in marine authigenic deposits is similar to that of e Nd (10 e units) [Albarède et al., 1998]. Our estimate of 2500 to 7500 years for the Hf residence time in seawater is on the order of five times that of Nd [Tachikawa et al., 1999] and makes it easier for seawater mixing to homogenize e Hf than e Nd. The more homogeneous marine e Hf values relative to terrestrial rocks, and compared to e Nd is considered to reflect weathering of the zircon-free component of the continents [van de Flierdt et al., 2002] possibly leading to a decrease in e Hf variability of material that can be weathered. However, the limited analyses of river water e Hf indicate otherwise, with river water exhibiting a much wider range due to preferential weathering of accessory minerals which have a wide range of Lu/Hf ratios [Bayon et al., 2006; Godfrey et al., 2007]. Wholesale removal of Hf from river water in estuaries could explain the reduction in impact of highly variable river water e Hf on seawater e Hf. 5. Conclusions [27] The Hudson River Estuary is an impacted urban estuary. The export of riverine Zr, Hf, and Ag through estuaries to the coastal ocean is moderated by flocculation and deposition of Fe colloids. Interaction between Zr and Hf with other colloidal and organic macromolecules may keep them from being as efficiently removed from solution as other metals, including Ag. The concentrations of Zr, Hf, and Ag in surface waters of the estuary are increased by urban sources, including water treatment facilities; their concentrations in bottom water close to the Harbor are affected by sediment resuspension processes. Bottom water Zr and Hf are high relative to surface water throughout the estuary due to be a benthic/groundwater flux or bottom nepheloid layer containing colloidal material. [28] The Zr/Hf ratio of surface and bottom water is a useful indicator of sources and sinks of Zr and Hf. At low salinity, the river water Zr/Hf ratio is close to average terrestrial rocks and is not altered by loss of Zr and Hf by colloid aggregation and settling. The Zr/Hf ratio of surface and bottom water increases at salinities above 10 due to their release and adsorption from suspended particles. Removal of Hf from solution within the Hudson Estuary by 50% at salinity of 15, and 60% by salinity of 30 increases the estimated residence time of Hf in seawater from 1500 years [Godfrey et al., 1996] to 2500 years. If we assume that the concentration of Hf is elevated in the midestuary by urban sources, the true removal of Hf in the estuary is greater, giving a residence time of 7500 years or more. Acknowledgments [29] We would like acknowledge the help of Suzanne Perron- Cashman during sampling and the comments of Yair Rosenthal. 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