Mercury sampling of open ocean waters at the picomolar level

Deep-SeaResearch.Vol.32, No.3, pp.287to 297, 1985. PrintedinGreatBritain.

0198-0149/85$3.00+ 0.00 O 1985PerllamonPressLtd.

Mercury sampling of open ocean waters at the picomolar level GARY A. GILL* and WILLIAM F. FITZGERALD* (Received 29 November 1983; in revised form I I April 1984; accepted I I April 1984)

Abstract--Severe mercury contamination (30 times ambient levels) was observed in open ocean seawater samples collected at depth using a tested polyvinyl chloride (PVC) sampler in a classical hydrographic manner. Consistently smaller Hg concentrations (0.4 to 2.0 ng I-s) were obtained with an improvised technique using synthetic line suspended below the metal hydrographic cable; results agreed favorably with collections of the mixed layer obtained by hand and with samples collected at depth using the Schaule-Patterson sampler. These observations and additional tests with a synthetic hydrowire indicate that reactive Hg concentrations in the open ocean are, for the most part, considerably less than previously reported. While Hg distributions in the open ocean display no distinctive water column features, higher concentrations were observed in the northwest Atlantic Ocean (~1.0 ng 1-I) compared to the North Pacific (,-4).35 ng 1-1). First-order geochemical modell".ragpredicts a short (,-,500 y) oceanic residence time for Hg, indicating that it may follow pathways in the oceans analogous to other very reactive elements, such as lead and manganese.

INTRODUCTION

GEOCHEMICALmodelling and pollution assessment, whether on a global scale or local basis, require sufficient and reliable data for the major reservoirs, principal pathways, and material fluxes. It is clear, therefore, that accurate measurements o f mercury in the oceans, the largest natural water reservoir, are important. Consequently, a substantial number o f investigations concerned with the amounts and distribution o f Hg in seawater have been conducted (FITZGERALD, 1979). The reported range o f Hg concentrations in various oceanic regimes is broad (0 to > 1 0 0 0 ng l-I),t and coherent distributional patterns have rarely emerged. It has been suggested that the high values o f Hg observed in some oceanic investigations result from magmatic Hg introductions associated with submarine tectonic activity (CARR et al., 1974; ROBERTSON, 1976; WILLIAMSet al., 1974). In a study o f Hg in the Gulf Stream, MUgHEg~I and KESTER (1979) reported concentrations o f reactive Hg between 3 and 6 ng !-t and a depth pattern that appeared related to the marine silicate cycle. In general, however, the vertical and spatial variation o f Hg in the oceans has not been linked to documented marine biological and geochemical cycles, nor to physical mixing processes. This suggests that accurate measurements of Hg in seawater have not been realized (FITZGERALD, 1979). The variety o f problems that have plagued trace metal studies in the oceans is now well known (PATTERSON, 1974; MEETING REPORT, 1974). Indeed, significant improvements in

* Department of Marine Sciences and The Marine Sciences Institute, The University of Connecticut, Groton, CT 06340, U.S.A. 1" 1 ng H'g I-! = 5 pM. 287

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G^RYA. GILLand WILLIAMF. FITZOm~.LD

marine trace metal geochemical acumen and analytical practice have yielded trace metal distributions that display oceanographicaHy reasonable behavior (SCHAULEand PATTERSON, i 981 ; BOYLe-et aL, 1977; BRULAND,1980). Careful scrutiny of the methods and materials used for sample collection and handling has become a major factor in the amelioration of oceanic trace metal investigations (PATTERSON and SETTLE, 1976; BENDERand GAGNFR, 1976; BRUL~N'Det al., 1979; BEWERSand WINDOM, 1982). For example, surface water collections from small rubber rafts away from waters contaminated by the research vessel are slowly becoming an accepted practice. For sample collection at depth, metallic hydrowires, weights, and sampling devices have generally been considered potential sources of contamination. However, contamination from these sources has not been rigorously demonstrated to be a serious problem, except possibly for iron measurements (B~rZER and PILSON, 197S). In an effort to obtain accurate information concerning the vertical distribution of Hg and to examine Hg variations with respect to hydrographic parameters, nutrient concentrations, and several other trace metals (BRULANDand FRANKS, 1983; SCHAULEand PATT~XSON,1983), we participated in a joint sampling station in the northwest Atlantic Ocean (34°1 I'N; 66°06'W) during July 1979 and also in the North Pacific (14°40'N; 160°07'W) in October 1980. Therefore, we had the opportunity to compare several collection methods, including surface collections from a small rubber raft and deep-water collections with the Schaule-Patterson common lead sampler and with a modified Go-rio sampler (General Oceanics, Inc.) attached to Kevlar° (DuPont Corp.) hydrocable (Whitehill Manfacturing Corp.), operated by K. Bruland of the University of California at Santa Cruz (UCSC). The test procedures were similar to those described by BRULANDet al. (1979). The major purpose of this communication is to document, unequivocally, that a metal hydrographic cable is a serious source of Hg contamination. The unnatural Hg enhancement observed when a tested polyvinyl chloride (PVC) sampler was attached directly to a metal hydrowire was so severe (~30 times ambient levels) that it places in jeopardy any Hg study that has used this classical oceanographic water sampling procedure. Geochemical interpretations regarding Hg behavior and cycling in the oceans will only be briefly discussed in this work, but will be the subject of future reports. EXPERIMENTAL METHODSAND ANALYSES Collection methods

The procedures used for seawater collection and sample processing for Hg determinations are, in general, a refinement of an earlier methodology (FITZOERALDand LYONS, 197S). Briefly, surface waters were carefully sampled from a rubber workboat (Zodiac) about 0.8 to 1.5 km away from the research vessel. Surface samples were obtained by hand (using armlength polyethylene gloves) offthe bow, while the workboat was either rowed, or gently driven forward, into the wind. Precautions were taken to avoid sampling the microlayer by uncapping the collection bottle below the surface. Samples from the surface mixed layer (to 40 m) were also collected from the workboat using a 5.5 1 PVC sampler (described later) attached to a polypropylene line, held taut by a 15 Ib epoxy-coated hydroweight. Samples at depth in the northwest Atlantic were collected from the R.V. Endeavor using three different sampling systems. Aliquots were obtained with the California Institute of Technology (CIT) deep-water common Pb sampler which was designed and constructed by SCHAULEand PATTERSON(1981). The CIT sampler operates suspended from the end of a

Mercury s=,mpling of open ocean waters at the picomolar level

289

hydrographic cable and collects a sample while it is slowly lowered over approximately 50 m of the water column. Simultaneously, a conventional type collection was made using the PVC sampler attached to the hydrowire 10 m above the depth at which the CIT sampler began collecting. Finally, an improvised sampling method was devised and tested whereby it was possible to collect water at depth that was unaffected by immediate contact with the ship's metal hydrowire and weight. This was accomplished by attaching and operating the PVC sampler at the end of a 15 m polypropylene tippet, suspended below the ship's hydroweight, kept taut with the 15 ib epoxy-coated weight. A simple trip mechanism that bypassed the ship's hydroweight was used to operate the PVC sampler at depth.

Sample handling and preservation Seawater samples were preserved and surface samples collected in acid-cleaned (l and 2 1, FEP) Teflon® bottles (Naigene®, Sybron Corp.) acidified with high purity concentrated nitric acid to yield a 0.5% acid solution (pH 1.2). Sample acidification was conducted aboard ship in a Class 100 clean air bench. Comparison of the Hg contents of samples simultaneously collected into pre-acidified bottles and those acidified several hours after collection indicated no detectable Hg difference. Teflon bottles were stored before and after use, double-bagged in acid-washed polyethylene bags to prevent deleterious contacts with the rubber workboat, dust, and the ship. During the northwest Atlantic cruise, sample transfer from the CIT collector was carefully conducted in the main laboratory under the protection of a polyethylene tent. Strict control of airborne contamination was maintained during draining of the sampling devices for the North Pacific collections by employing an enclosed work area containing fdtered air from a cleanbench. Sample transfer from the PVC collector was carried out on deck, on the windward side of the ship, in clean marine air.

Preparation of high purity acid High purity, low Hg content, nitric acid was prepared by sub-boiling distillation (SBDHNO3) in an all Teflon apparatus similar to that described by MATrrNsoN(1972). By repeating the distillation process twice the Hg content of reagent grade acid is typically reduced below 20 ng 1-~ . Once prepared, the SBD-HNO3 is stored, double-bagged, in tightly-capped Teflon bottles until use. Acid was dispensed using Eppendorf pipets with acid-cleaned tips. The blank contribution from the SBD-HNO a used in these studies was 0.06 ng Hg 1-~ of sample.

Cleaning procedures In general, we employ cleaning procedures similar to those developed by PATTER,SOSand SETTLE(1976) to study Pb in the environment. Thus, all Teflon-ware was initially washed with ordinary laboratory detergent (Alconox), rinsed with distilled-deionized water (DDW), then sequentially with chloroform, DDW, and concentrated reagent grade nitric acid. Teflon sample bottles were leached in reagent grade nitric acid at 55°C for at least 3 days. Items that required immersion for leaching were soaked in a vat of concentrated reagent grade acid at ambient temperatures. The 3-day acid leaching process was repeated using hot (55°C) 1 N HNO3, prepared from reagent grade HNO3 and DDW. Following this step, all Teflonware was rinsed with quartz sub-boiling distilled DDW (Q-water), and the hot acid digestion continued for 3 days, but with 0.5% (v/v) high-purity SBD-HNO 3 (prepared in Q-water).

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After this final step, the dilute acid was dis~rded, the Teflon-ware completely rinsed (4 times) with Q-water, and then refilled with 0.5% high purity acid.

PVC sampler The commercially available 5.51 PVC Niskin* sampler (General Oceanics, Inc.) was modified by replacement of the nylon spigot, O-ring, and vent screw with Teflon pans. The Teflon-coated stainless steel spring closure previously used (FrrZtEI~LD and LYONS, 1975) was replaced with amber latex rubber tubing. The sampfing bottle was cleaned with detergent, rinsed with DDW, filled with 1 N HC 1 (prepared from reagent grade HC 1 and DDW), and allowed to soak for 7 to 10 days. It was then rinsed with DDW and stored in a polyethylene bag until use.

A nalytical procedures Mercury analyses were carried out on board the research vessels using a two-stage~Au amalgamation modification of the SnCI2 reduction-aeration flameless atomic absorption procedure of FnrZOV.XALVet al. (1974), but with a Au-coated glass or quartz bead collection column instead of their cold-trap preconcentration stage. A significant improvement in precision and sensitivity has been achieved with this two-step amalgamation technique. For example, using a 500 mi sample, the precision of analysis reported as a coeff~ient of variation for the determination of 1.0ng Hg is 10%, and 0.12 ng Hg l-m (0.6 pM) can be confidently measured. Increased sensitivity can be achieved for very low-level Hg analyses by increasing the sample ~/olume to 11 or greater. Analytical details associated with this method can be found in FrrZOE~LD and GXLL(1979)and GILL(1980). Experimentally, Hg measurements in seawater are often divided into two fractions--reactive and total Hg (FrrZOERALDand HUNT, 1974; OLA~SON, 1983). In this present study, the reactive Hg fraction represents the amount of Hg measured (within 24 h of collection) in unfiltered seawater acidified to a pH of 1.2. Reactive Hg measurements are therefore broadly defined to represent those Hg species that are readily reducible with SnCI 2 at the sample pH. Thus, the reactive Hg determination includes dissolved inorganic Hg species, labile organoHg associations, and Hg that is readily leachable from any particulate matter present. Operationally, the difference between a reactive and total Hg determination represents, in general, a reasonably stable organo-Hg association, e.g., methyl- and dimethylmercury, requiring photochemical or prolonged acid treatment to cleave covalent carbon-Hg bonds and liberate inorganic Hg for detection (FrrZOERALDand LYONS, 1973; BAKER, 1977; OLAFSSON, 1978; GILL, 1980). RESULTS AND DISCUSSION

Northwest Atlantic sampling tests To evaluate the various sampling methods described, we compared collections from the modified PVC sampler, attached directly to the metal hydrowire, with aliquots from the CIT collector. The reactive Hg measurements obtained from this initial test experiment as well as additional observations from other sampling methods are presented in Table 1. Substantially higher Hg values (>30 times) were obtained with the PVC sampler attached directly to the metal hydrowire. This sampling disparity is illustrated in Fig. 1. There are several plausible causes for this disagreement. An anomalously low Hg signal might have resulted from adsorption of Hg by the polyethylene collection bag, which is an

291

Mercury sampling of open ocean waters at the picomolar level

Table I.

Comparison of reactive Hg measurements in the northwest Atlantic (34011'N, 66006'W) by various collection methods (R. V. Endeavor, cruise 039, 13 to 26 July 1979)

Conventional collection (with PVC sampler)

CIT common Pb sampler

Depth (m)

Hg Cone. (ng I-t)

Depth (m)

Hg Cone. (ng I-')

30 800 1000 1500 2070 2500

45 50 36, 37 43 37 38

35 1000 3500

7.6t 1.5 2.8"t

PVC sampler attached to polypropylene tippit* Depth (m)

oat 5§ 10§ 35 75 150 300 500 750 1000

Hg Cone. (ng I-I)

0.7 _+ 0.2n 0.8 0.4,0.7 0.7, 0.9 0.6 0.6, !.0 I.I 2.0 0.8 1.3

* Sample depths of 0, 5, and 10 m were obtained at 30°08'N; 66°33'W. "t"Sampler operated improperly, collecting some near-surface water contaminated by ship and hydrowire or from a faulty internal seal, as evidenced by elevated levels of Pb and other metals (SCHAULE and PATTERSON. 1983). :[: Collected by hand from the rubber workboat directly into a Teflon sample bottle. § Sampler attached to polypropylene line and suspended from rubber workboat. IIMean and I s.d. for four collections.

integral part of the CIT sampler. Elevated Hg concentrations could have occurred from contamination introduced directly from the PVC sampler through inadequate sample handling procedures aboard ship or from the sampler becoming contaminated during transit through near-surface waters befouled by the ship. A more probable explanation, however, is that elevated Hg levels resulted from the collection of water contaminated by the metallic hydrographic cable. Adsorptive losses of Hg in the CIT collector were not considered suspect for several reasons: (1) adsorptive losses of common lead, a very reactive trace metal in seawater, do not occur at detectable levels in the CIT collector (C. C. PATTERSON,personal communication, 1983); (2) we have not observed detectable losses of Hg onto the walls of our PVC collector or Teflon bottles over short (few hours) periods of time; (3) if adsorptive losses were a serious concern in the CIT collector, then collections that remain in the sampler for longer periods of time should show lower Hg levels. This phenomenon was not observed. Based on previous experience, contamination problems associated with either the PVC sampler or the handling operations aboard ship were not considered likely. Hand-held collection tests of the PVC sampler vs Teflon bottle collections in both local coastal waters and during the sampling experiments indicate that any measurable differences are within the analytical precision of the method (+0.2 ng 1-1). Further, comparison of hand-collected surface samples using Teflon bottles (0.7 + 0.2 ng 1-1, n = 4) and the PVC sampler attached to the polypropylene line, suspended from the rubber workboat at 5 m (0.8 ng 1-~) and at 10 m (0.4, 0.7 ng 1-~), showed f'me agreement. This suggests that contamination was introduced from the hydrowire or by passage of the open sampler through contaminated surface water near the ship. Fortunately, we were able to devise a test of these hypotheses at sea. By placing the PVC sampler on a polypropylene line attached to the bottom of the hydrowire, we could sample water at depth unaffected by

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REACTIVE 0 0

i

Hg

50

100

150

200

250

300

pM

10

20

30

40

50

60

ng/J

200

400

600

E "r

a

8OO

lOOO

~0 0

200C

0 0

3000

4000

Fig. I. Reactive Hg determinations from surface and deep waters of the northwest Atlantic (34011'N; 66006'W) using several different collection methods to test for contamination during sampling, R.V. Endeavor, 13 to 26 July 1979. 1:3,Collections with the CIT sampler suspended at the end of the hydrowire;O, conventional collection using a previously te~ed PVC sampler attached directly to the metal hydrowire;~, PVC sampler attached to a polypropylene line, either suspended below the metallic hydrographic cable or handbeld for col|ections (at 5 and lO m) from a rubber workboat: A, subsurface samples (0 m) collected directly into Teflon storage bottles from a rubber workboat, hydrowire contamination. However, if the sampler was contaminated during transit through surface waters, then a high result would still be obtained. As summarized in Table 1, and illustrated in Fig. 1, the Hg concentrations found in the seawater collected using the polypropylene tippit arrangement were indeed small. Moreover, the Hg concentration o f the 1000 m sample is similar to the CIT aliquot (1.3 vs 1.5 ng l-I). Also, the concentration found in the mixed layer sample at 35 m compares very favorably with the sub-surface collections and with hand-operated collections of the mixed layer using the PVC sampler at 5 and 10 m. This is unequivocal evidence documenting, in this case, severe Hg contamination from a metal hydrographic wire using a conventional-type collection for water sampling protocol.

North Pacific deep-water sampler intercomparisons The previous test experiments clearly indicate that to sample seawater at depth for Hg requires avoidance of water directly influenced by metallic hydrowires and weights. The non-

Mercury sampling of open ocean waters at the picomolar level Table 2. Comparison of reactive Hg determinations from the North Pacific (14°40'N, 160°07'W) by several collection methods (R.V. T.G. Thompson, cruise TT-153 (leg 2), l to 25 October 1980) Sample depth (m)

Sampling method

Hg Cone. (ng I-n)

0 245 940

Teflon bottle* C IT:I: CIT

1000 1500

Go-rio§

0.37 + 0.131" 0.39 0.38 0.25 0.23 0.26

2509

Go-rio CIT

3000

Go-rio

0.33

4000

Go-rio

0.28, 0.39

* Collections from rubber workboat directly into a Teflon sample bottle. 1. Mean and I s.d. for five collections. California Institute of Technology (CIT) deep-water common lead sampler designed and constructed by SCHAULEand PATTERSON(1981). § Sample collected by K. Bruland, using a modified 301Go-rio sampling bottle attached to Kevlar line (for sample handling and processing procedures, see BRULAND e! a/., 1979). REACTIVE 0 •

2 i

•

0.0 0

4 i

0.4 i

i

v

6 w

0.8 i

|

Hg

i

8 ,

1.2 i

|

|

10 •

1.6 i

w

pM

•

2.0 n g / I w

(3

500 D

1000

A

1500

&

¢) 0

E

2000

"7" I-O.

2500

D

UJ

r~ A

3000

3500

4000

A

A

Fig. 2. lntercompadson of proven trace metal sampling methodologies for the determination of Hg at an open ocean sit,. in the central North Pacific (14°40'N; 160o07'W), R.V. 7". G. Thompson, 1 to 25 October 1980. Samples at depth: El, CIT deep-water common lead sample; zx, modified Go-flo bottle attached to Kevlar hydrocable. Samples of sub-surface water: O, collected directly into Teflon storage bottles from a rubber workboat (value shown is the mean for five collections taken over a 4day period).

293

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GARYA. GILLand WILLIAMF. FITZGERALD

contaminated collections obtained with the polypropylene tippit arrangement suggest that a non-metallic hydrographic cable might provide a suitable means for collecting water at depth while utilizing wire-attached samplers. We had the opportunity to assess this methodology in October 1980 at a station in the tropical North Pacific (14°40'N, 160°07'W) in conjunction with a trace metal geochemical study being conducted on the R.V.T.G. Thompson. For intercomparison, two proven deep-water sampling methodologies for trace metals were employed to collect a Hg profile with depth. Aliquots of seawater were obtained from the CIT sampler and from a modified 301 Go-rio bottle attached to Kevlar cable held taut with a polypropylene-coated hydroweight. The Hg profile obtained using the two methods, as well as surface collections of the mixed layer obtained by hand, are given in Table 2 and illustrated in Fig. 2. Here, mercury measurements from the CIT collector represent the reactive fraction as described previously for unfiltered seawater. However, reactive mercury measurements obtained from the Go-rio bottles were conducted on filtered samples (filter pore size = 0.30 pm). These filtered and unfiltered collections can be directly compared because extremely low levels of particulate matter are normally present in these open ocean oligotrophic waters. Intercomparison results indicate that the deep-water collection methods yield Hg concentrations that are in good agreement. Mercury concentrations obtained from surface seawater collections (directly into Teflon sample bottles) are also consistent with the low levels of Hg observed at depth. Further, the concentration levels obtained at this site agree well with those obtained previously in the northwest Atlantic, although they are lower in concentration by about half. Clearly, a non-metallic hydrographic cable, such as Kevlar, is suitable for obtaining Hg collections at depth with wire-attached samplers.

Geochemical significance There are several items of geochemical significance in our findings. Mercury concentrations in open ocean waters are considerably less than those of most previous investigations. Recent work by BLOOMand CRECEUUS(1983) and OLAFSSON(1983) report Hg concentrations in coastal northeast Pacific and North Atlantic waters, respectively, that are in fine agreement with our data. While our Hg data are limited, a significant (two-fold) concentration difference between northwest Atlantic and North Pacific waters is evident. Also, Hg distributions with depth show no distinctive water column features. This distribution pattern is markedly diff~ent (both vertically and geographically) from the majority of trace metal distributions in the ocean (BgULAr~D, 1983). It is important to note that small but geochemically significant water column features might not be revealed clue to the paucity of data and the precision associated with the measurement (Tables i and 2). At present, it is not possible to provide a geochemically rigorous explanation for the observed vertical and geographic distributions in seawater, though we can demonstrate that these Hg distributions are 'oceanographically consistent'. A first-order estimate for the oceanic residence time for Hg can be obtained from our seawater measurements and by assuming that Hg is introduced to the oceans principally through the atmosphere. An oceanic Hg burden of 0.8 x 1012 g is calculated using an average seawater Hg concentration of 0.6 ng Hgi -I. FITZGERALD(1982)estimated the atmospheric introduction of Hg to the marine environment at 1.7 x 109 g y-i from measurements of Hg in the atmosphere and rainfall at remote tropical Pacific and coastal northwest Atlantic study sites (FOGG and FITZGERALD,1979; FITZGERALDet al., 1983). Therefore, based solely on the estimated atmospheric flux of Hg to the oceans, a relatively short (--,470 y) residence time for Hg in the oceans is obtained.

Mcrcurysampling of open ocean watersat the picomolarlevel

295

A reasonably short residence time for Hg in seawater should be anticipated given the demonstrated involvement of Hg in many biogeochemical processes (NRIAOU, 1979). However, the short oceanic residence time predicted for Hg by this simple calculation departs considerably from previous modelling estimates (NAS, 1978; ANOREN and NRIAOU, 1979; MATSUNAGA, 1981). More importantly, it indicates that Hg should be removed rapidly from the water column on time scales less than (or perhaps equal to) oceanic mixing. This would preclude its distribution resulting from conservative mixing processes or by significant involvement in 'nutrient-type' cycling (BRULAND, 1980). The residence time for Hg of ,-,500 y falls between the very reactive elements, such as Pb and Mn, which have extremely short residence times (< 100 y), and Cu which has a residence time near 5000y (BOYLE et al., 1977; BaULANO, 1983). By inference then, one might anticipate Hg to share common oceanographic and geochemical behavior with these very reactive elements. Indeed, elevated levels of Pb and Mn have been observed in the North Atlantic compared to the North Pacific (SCHAULEand P^TrERSON, 1983; BRULAND and FRANKS, 1983), analogous to the geographical distribution of Hg. This interocean variability results from larger introductions of these elements into the North Atlantic combined with a very short water column residence time. Thus, the vertical and spatial oceanic Hg distributions may reflect both the magnitude of atmospheric input and the intensity of the water column removal process. At present, additional water column investigations in other oceanic regions, as well as studies on Hg input and removal, are being conducted to test and refine this present hypothesis. CONCLUSIONS it was fortunate that in the original test experiments the hydrowire contamination was pronounced and readily identified. This insidious problem has not been recognized in most oceanic Hg investigations and may be the source of much of the reported variability. It is evident that a metal hydrographic cable is not acceptable for wire-attached type samplers, including those devices which pass closed through surface water influenced by the ship. Also, there is no guarantee that samples obtained with a rosette-type sampler will not be compromised from mounting collection bottles on a metallic frame and also from the close proximity to the metallic hydrowire. From the present study, it was not evident whether the Hg contamination came from iron rust, associated shipboard oils and grease, foreign matter (dirt), or other sources. Using this Hg study as an analog, it is suggested that measurements based on similar water-sampling procedures, whether for trace metals or for organic studies, may be affected in a similar manner. We recommend that water sampling procedures for Hg studies be conducted using a nonmetallic hydrocable, such as Kevlar, with a non-metallically coated hydroweight and associated davits and sheaves. Precautions also should be taken to prevent oil, grease, or rust from contacting the hydrocable. While there is no evidence that Hg contamination occurred during passage of an open sampler through the surface waters, collectors that can be opened after transit of the upper 5 to 10 m of ship-influenced water would be desirable. Finally, it is clear that a collector, such as the CIT sampler, is satisfactory for sampling ocean water for Hg. Acknowledgements--The authors are indebtedto the Captain and crew of the R.V. Endeavor and the R.V. 7".G. Thompson and to our colleagues in the SEAREX program for their assistance. We are particularly grateful to

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Kenneth Bruland of the University of California at Santa Cruz for allowing us to participate with his field sampling program in the Pacific. We also thank Amy Ng, presently with Dow Chemical Co., California, and Robert Franks of the University of California at Santa Cruz for conducting the sampling program with the CIT collector in the northwest Atlantic. This work was supported by the National Science Foundation, Grant Nos OCE-77-13071, OCE-77-13072, and OCE-81-12104, as part of the Sea-Air Exchange Program (SEAREX).

REFERENCES A NDREN A. W. and J. O. NRIAGU(I 979) The global cycle of mercury. In: The biogeochemistry ofmerctlr3., in the environment, J. O. NRIAOU, editor, Elsevier/North-Holland Biomedical Press, Amsterdam, pp. I - 15. BAKER C. W. (1977) Mercury in surface waters (of seas) around the United Kingdom. Nature, London, 270, 230-232. BENDER M. L. and C. GAGNER (1976) Dissolved copper, nickel, and cadmium in the Sargasso Sea. Journal of Marine Research, 34, 327-339. BETZER P. R. and M. E. Q. PILSON(1975) The effect of corroded hydrographic wire on particulate iron concentrations in seawater. Deep-Sea Research, 22, 117-120. BEWERS J. M. and H.L. WINDOM (1982) Comparison of sampling devices for trace metal determinations in seawater. Marine Chemistry, I !, 71-86. BLOOM N. S. and E. A. CRECELIUS(1983) Determination of mercury in seawater at sub-nanogram per liter levels. Marine Chemistry, 14, 49-59. BOYLE E. A., F. R. SCLATERand J. M. EDMOND (1977) The distribution of dissolved copper in the Pacific. Earth and Planetary Science Letters, 37, 38-54. BRULAND K. W. (1980) Oceanographic distributions of cadmium, zinc, nickel, and copper in the North Pacific. Earth and Planetary Science Letters, 47, 176-198. BRUi~ND K.W. (1983) Trace e~ements in sea-water. In: Chemical oceanography, Vol. 8, J. P. RILEY and R. CHESTER, editors, Academic Press, New York, pp. ! 57-215. BRULAND K. W. and R. P. FRANKS (1983) Mn, Ni, Cu, Zn, and Cd in the Western North Atlantic. In: Trace metals in seawater, C. S. WONG, J. D. BURTON, E. BOYLE,K. ~RULAND and E. GOLDnERG, editors, Plenum Press, New York, pp. 395-414. BRULANDK. W., R. P. FRANKS, G. A. KNAUERand J. H. MARTIN (1979) Sampling and analytical methods for the determination of copper, cadmium, zinc, and nickel at the nanogram per liter level in seawater. `4nalytica Chimica`4eta, 105, 233-245. CARR R. A., M. M. JONES and E. R. RUSS(1974) Anomalous mercury in near-bottom water of a mid-Atlantic rift valley. Nature, London, 251, 489--490. FITZGERALD W. F. (1979) Distribution of mercury in natural waters. In: The biogeochemistry of mercury in the environment, J. O. NRIAGU, editor, Elsevier/North Holland Biomedical Press, Amsterdam, pp. ! 6 ! - 173. ~'ITZGERALDW. F. (1982) Evidence for anthropogenic atmospheric mercury inputs to the oceans. EOS, 63, 77. FITZGERALDW. F. and W. B. LYONS(1973) Organic mercury compounds in coastal waters. Nature, London, 242, 452-453. FITZGERALD W. F. and C. D. HuNT (! 974) Distribution of l-lg in surface microlayer and in subsurface waters of the northwest Atlantic. Journal de Recherches Atmospheriques, 8, 629-637. FITZGERALD W. F. and W. B. LYONS (i 975) Mercury concentrations in open-ocean waters: sampling procedure. Limnologv and Oceanography, 20, 468--471. FITZGERALD W. F. and G. A. GILL (1979) Subnanogram determination of mercury by two-stage gold amalgamation applied to atmospheric analysis. A nalytical Chemistry, 5 i, 1714-1720. FITZGERALD W. F., W. B. LYONSand C. D. HUNT (1974) Cold-trap preconcentration method for determination of Hg in seawater and in other natural materials. Analytical Chemistry, 46, 1882-1885. FITZGERALD W. F., G.A. GILL and A. D. HEwrrr (1983) Air-sea exchange of mercury. In: Trace metals in sea,,ater, C. S. WONG, E. BOYLE, K. W. BgULAND, J. D. BURTON and E. COLDBERG, editors, Plenum Press, New York, pp. 297-316. FOGG T. R. and W.F. FITZGERALD(1979) Mercury in Southern New England coastal rains. Journal of Geophysical Research, 84, 6987-6989. GILL G. A. (1980) On the geochemistry of mercury in Long Island Sound: an analytical and field study. MSc Thesis, University of Connecticut, 199 pp. MATSUNAGA K. (1981) Oceanic residence time of mercury. Bulletin of The Faculty of Fisheries, Hokkaldo Uuirersity, 32, 199-202. MATrlNSON J. M. (1972) Preparation of hydrofluoric, hydrochloric, and nitric acids at ultra-low lead levels. .4nalytical Chemistry, 44, 1715-1716. MEETING REPORT(1974) i nterlaboratory lead analyses of standardized samples of seawater. Marine Chemistry. 2, 69-84.

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MUKHERJI P. and D. R. KESTER(1979) Mercury distribution in the Gulf Stream. Science, Wash,, 204, 64-66. NAS (NATIONAl. ACADEMY OF SCIENCE) (1978) An assessment of mercury in the environment. Washington, D.C., 185 pp. NRIAGU J.O., editor (1979) The biogeoehemist O' of mercury in the environment. Elsevier/North Holland Biomedical Press, Amsterdam, 696 pp. OLAFSSON J. (1978) Report of the ICES international intercalibration of mercury in seawater. Marine Chemistry, 6. 87-95. OLAFSSON J. (1983) Mercury concentrations in the North Atlantic in relation to cadmium, aluminum, and oceanographic parameters. In: Trace metals ht seawater, C.S. WONG, J. D. BURTON, E. BOYLE, K. BRULAND and E. GOLDBERG,editors, Plenum Press, New York, pp. 475--486. PATTERSON C. C. (1974) Lead in seawater (Meeting Report). Science, Wash., 183,553-554. PA't'rERSON C. C. and D. SETTLE (1976) The reduction of orders of magnitude errors in lead analyses of biological materials and natural waters by evaluating and controlling the extent and sources of industrial lead contamination introduced during sample collection, handling, and analysis. In: Accuracy in trace ana~esis: sampling, sample handling and analysis, P. D. LAFLEUR, editor, U.S. National Bureau of Standards Special Publication 422, pp. 321-351. R OBERTSON D. E. (1976) Analytical chemistry of natural waters. In: Accuracy in trace analysis: sampling, sample handling and analysis, P. D. LAFLEUR, editor, U.S. National Bureau of Standards Special Publication 422, pp. 805-836. SCHAULE B. K. and C. C. PATTERSON (1981) Lead concentrations in the northeast Pacific: evidence for global anthropogenic perturbations. Earth and Planetary Science Letters, 54, 97-116. SCHAULE B. K. and C. C. PATTERSON(I 983) Perturbations of the natural lead depth profile in the Sargasso Sea by industrial lead. in: Trace metals in seawater, C. S. WONG, J. D. BURTON, E. BOYLE, K. BRULANO and E. GOLDBERG, editors, Plenum Press, New York, pp. 487-504. WILLIAMS P. M., K. J. ROBERTSON, K. CHEW and H. V. WEISS (1974) Mercury in the South Polar Seas and in the Northeast Pacific Ocean. Marine Chemistry, 2, 287-299.