The fate of trace metals in Narragansett Bay, Rhode Island: Radiotracer experiments in microcosms

Estuarine and Coastal Marine Sclozce (I98O) xoj 635-654

The Fate of Trace Metals in Narragansett Bay, Rhode Island: Radiotracer Experiments in Microcosms °

Peter Hans Santschi, Yuan Hui Li and Steven Robert Carson Lamont.Doherty Geological Observatory of Columbia University, Palisades, Nezv York xo964, U.S.A. Received 28 August I978 and in revisedform 30 May. !979

trace elemenis; roetals; uptake; radiotracers; laboratory experiments; Narragansett Bay

Keywords:

Experiments, designed to determine removal rate constants and removal mechanisms of various radioactive trace metals (6aCr, S ' i n , tsCo, ~gFe, eSZn, T6Se, lt~mCd; ts4Cs, tl°Po, tt°Pb, tSIRa, ttSTh) in controlled ecosystems simulating Narragansett Bay conditions in spring and early summer seasons are descHbed.-Overall removal could adequately be described with a model assuming first order removal to Several reservoirs. Initial removal rates in two identical tanks replicated well during the early summer experiment. The removal b~i~avior of Mn, Cr and Se changed after the first x-2 weeks of the same experiment. Although adsorption to walls and other tank parts wassignificant, that fract.ign" ~eemed to be effectively removed from further interaction and would be accounted for in the model. The major mechanisms for metal uptake that would be characteristic of the natural system were found to be scavenging by particles that settled from the water column (mostly resuspended sediment) and direct adsorption within the bioturbated layer of the sediment. By comparing the relative importance of these sinks and considering the similarities in respective half removal times, the I2 elements studied were grouped according to general behaviour: (I) hydrolysable elements including Fe, Th, Po, Cr (-III), and Se (as SeOst-), with half removal times ranging from 4 to 40 days; (2) particlereactive elements Mn and Co with half removal times ranging from 2 to x4o days; (3) Zn and Cd which were removed more slowly, with half removal times ranging from 5o to 4oo days and about equally by settling particles and by adsorption within the bioturbated layer of the sediment; (4) Cs and Ra, which were removed most slowly, with half removal times ranging from 600 to 800 days, mostly by adsorption within the bioturbated layer of the sediments. Generally slower removal was observed during the early summer relative to the spring and was ascribed to a combination of trace metal association with low molecular weight organic compounds and increasing rates of return from the sediments. Bioturbation rates in the top 3 cm of the sediments in the spring experiments were similar to other experimental values for Narragansett Bay and other coastal regions: a mixing coefficient of 2 - 5 × 1 o -7 cm t s -1 was calculated. •

Lamont-Doherty Geological Observatory Contribution No. 2870 635

o3o2-3524/8o/o6o635+2o ~2.oo/o

~) x98o Academic Press Inc. (London) Ltd.

636

P. H . Santschi, IT. H. L i ~ S. R . Carson

Introduction The pathways and rates of removal of trace metals from aquatic systems are poorly understood. The use of radiotracers is an ideal technique for studying these processes without disturbing the ecosystem. In most instances, however, such studies can only be carried out in microcosms designed to model the natural system as closely as possible. Such experiments have been carried out in artificial ecosystems representing Narragansett Bay, Rhode Island.

Artificial Light Pulsed inflow of Bay water

Pulsed outflow of

Roy,_wo_.~r

I I

fast flow through I of Bay water I =

-3

',

II 'l l = Figure r. A schematic of the experimental setup, simulating temperature, light, residence time (35 days) and mean water depth (xSo 1/I69 cm:~9 m) turbulence (half paddle, as described in Perez et al., x977) and sediment resuspension rate (7"4-x8"3 kg m-* yr -I, Oviatt and Nixon, 1975) of Narragansett Bay.

The microcosms used in these experiments are located at the Environmental Protection Agency Laboratory in Narrangansett, Rhode Island (Perez et al., x977). A schematic illustration of one microcosm is presented in Figure I. It consists of a polyester resin-coated tank measuring z m in depth and 5o cm in diameter, and it contains about i5o 1 of water. Three times each week xo 1 of water is removed and replaced with xo I of Bay water freshly collected with a bucket, to give a residence time of the water in the tank similar to that of the Bay, -.,35 days. Bay temperatures are maintained by a continuous flow of Bay water around the experimental tanks. Suspended in each tank is a closed, opaque PVC box containing a box core collected shortly before the beginning of each experiment from a sampling station north of Jamestown , R.I. Sediment properties and benthic communities at the sampling station are described by Hale (i975) and Nixon et al. (i976). Circulation of water over the sediments in the box is produced by pumping water into the box in pulses (using an alrpump) at a frequency of one pulse per minute, an average flow rate of o. 71 rain -1 and a mean velocity within the box of I. 4 cm s -1. The sediment surface area is i69 cm 2 giving a water volume to sediment surface area ratio of 9 m, which is close to the average depth of Narragansett Bay. Turbulence in the tank is produced by the rotation of a plastic paddle. Fluorescent light is delivered with an intensity adjusted to the average daily intensity falling on the Bay

637

Trace metals in Narragansett ;Bay

during the time of the experiment. T h e tank can satisfactorily replicate the phyto- and zooplankton biomass and production rates, benthic N H s fluxes and benthle 0 2 consumption rate of Narragansett Bay (Perez et aL, x977). One artifact associated with the system used in these experiments is that particles resuspended within the sediment box are removed from further contact with the sediment by the water passing through the box. These particles later settle to the bottom of the tank. This differs from the Bay where particles suspended from the bottom would return to the sediment upon settling. T w o sets of metal experiments were run in these tanks. I n each set two tanks were used. T h e first set of experiments was carried out during the spring of x977 (x 7 M a r c h - 2 o April) and the second during the summer of x977 (27 ~ I a y - 4 August). I n the spring experiment, the two tanks were identical, except that the sediment box in one tank contained no sediments (tank B, hereafter), whereas the other included sediments (tank A, hereafter). T h e purpose of this experimental design was to study (x) the importance of sediment contact and rcsuspension in the removal pathways of trace metals, and (2) the extent of adsorption by tank walls and other surfaces. T h e temperature during this experimental period was 4 4-x °C, except during the last week when it rose to Io °C (Figure 4, bottom). In the summer experiment two identical tanks (designated tank C and tank D, hereafter) with sediment in both sediment boxes were used. T h e purposes of this set of experiments were (Q to test replication of metal behavior between two identical tanks, and (z) to make a comparison of metal behavior during summer vs. spring. T h e temperature during the summer experiments ranged from I3-X6 °C in the first month to ~3-+ x °C in the last month. During the last month of the summer experiment I. 5 g sodium azide and x. 5 1 of an antibiotic mixture (containing penicillin, streptomycin and a fungicide) were added to tank D and tank C, respectively. These manipulations were performed in order to test the importance of biologically mediated processes on metal removal T^BL~ x. Gamma- and alpha-emittlng isotopes used in EPA-microcosms A,B, C,D (Volume ,,, xso 1)

Amotmt/tank Halfiife (d=days) Means of 7 Energy Branching (~tCi) Tank A Tank B Tank C Isotope ()'=years) detection [keV] ratio slCr 5'Mn s~Co mFe

28 d 303 d 72 d 45 d

esZn 75Se

z44 d xzo d

7

45 d

al~mCd a~Cs

z'o6 y

7 7

Y

320 835 8xx xo99, xz9z xxx5 I36, "265 934

o.xo x 0"99 0"56, 0"44 o'5I 0"57, 0"57 o'oz

7

6o5,

0"98,

796 .

0"80

7

7 7

*toPo

x38 d

u

za°Pb

zz y

u*

2t6Ra **STh

x6oo y x'9 y

(13) u (Rn) ct

.

.

.

. .

. .

2

. .

. .

. .

. .

3

3

7

2

9

25 3 6"5 x2

Tank D

Chemical form

25 3 6"5 xz

Cr Cls Mn CI~ Co C12 Fe Ci~

3

8

8

8

--

--

3

3

Zn C12 Nan SeO,

x9

--

8r

o

Cd CI,

--

--

3

3

C s CI

o'oozx

--

Po (NOs)4

o'ooxz

--

P b (NOa)s

0"00077 0"00065

---

IRa C1, Th (NOs),

*Measured by alpha counting the ~a°Pogrown in after o'5-z year after Pb-separation (isotope dilution method). The methods used for alpha counting are the same those described in Santschi et al. (I979).

638

P. H. Santschl, Y. H. Li & £I. R. Carson

pathways and on the chemical forms of the metals. The radionuclide spikes used in the spring and summer experiments are listed in Table I. Several types of samples were routinely collected for 7-counting in order to study the removal and the distribution of chemical forms of trace metals. A ioo ml acidified total water sample was collected three times each week. Less frequently, 5oo ml samples were collected and were passed through a chemical separation column which consisted of o.45 lam millipore filter, activated charcoal (3 ml), and chelex resin (3 ml) in series. The fractions produced in this process were: (x) metals associated with particles larger than o.45 Ilm, (2) metals removed by activated charcoal, probably associated with inorganic and/or organic colloids, (3) metals removed by chelex resin (Na-form) probably in cationic forms and labile complexes of transition metals, (4) metals that passed through the filter-charcoal-chelex columns (effluent) which may include anions, heavy metals associated with low molecular weight organic molecules, and the cationic forms of the alkaline and alkaline earth series metals. Reversing the positions of chelex and charcoal in this sequence showed that chelex could retain some of the charcoal-extractable fraction as well All tracer measurements were carried out by gamma ray spectroscopy utilizing a Ge-Li crystal detector and a 4o96 channel multi-channel analyzer. Each zoo ml water sample was counted in a xz5 ml polyethylene bottle, filters in petri dishes, and chelex and charcoal fractions in the separation columns (syringes). Standards were counted in the three different geometries in order to convert the concentration obtained in one geometry to another geometry. Replicate countings of the same sample gave reprodueible results with errors comparable to counting errors. The amount of sediment resuspension, an important experimental parameter, was measured by sampling aliquots of all the particles settled on the bottom of the tanks (resulting from resuspension of sediments inside the sediment box) at the end of the experiment. They were dried, weighed and counted on the gamma counter. The total amount of zo g dry sediment in tank A after 34 days is equivalent to a sediment resuspension rate of 13 kg m -2 yr - t . Sediment resuspension rates of 9 and z 3 k g m - 2 y r -x were obtained in tank C and tank D, respectively. These values are similar to values reported by Oviatt and Nixon (x975) for the middle (I 3 kg m -2 yr -1) and lower (x8 kg m -2 yr -x) regions of Narragansett Bay and are equivalent to erosion rates of z- 4 mm of top sediments in the sediment box per month. Sediments of tank A were sampled by coring three times (x2, 20 and 34 days after the spiking) and sectioning shortly afterwards in o.5-x-o cm intervals. The sediments of tank C and D were carefully frozen in liquid nitrogen at the end of the experiment and sectioned soon afterwards into x cm intervals. Radioisotope profiles in the sediments were used in the study of bioturbation (sediment mixing by benthic maerofauna). Selected samples of wall material and other tank parts as well as leachates from these were also counted at the end of the experiments to characterize the amount of adsorption on the tank walls. Results and discussion (a) 3lass balance and removal rates of radlonuclides (i) Spring experiment. In both tank A (with sediment) and tank B (without sediment), concentrations of radionuclides in the water column decreased exponentially with time

Trace metals in Narragansett Bay

.L. . . . . . . . .

639

L~ne

~ir~;on

65Zn 54Mr~ 58Co

oJ o3 ~ o

..J

23

'~gFe (Tank A)

76

Bl

~ ,

,

85

91

3"~i

96

101

1

lOG Jution D~y

Figure 2. Decay corrected activities of radioisotopes in the water column of tank A (with sediments) and tank B (without sediments), displayed in arbitrary logarithmic units, are plotted as a function of time (Julian days). Error bars indicate one standard deviation due to counting statistics only (if not graphically represented, they have the approximate size of a symbol).

(Figure 2, activities ate all decay corrected to the beginning of the experiment). It seems that the sum of all the removal mechanisms of the radionuelides from the water column to various reservoirs is first order. Therefore, the change of the number of radionuclide'atoms in the water column of the tank can be expressed as d Nw

d~

=

--2wNw--2aNw

(xa)

or in the integrated form: Nw = N~° exp [--(2,~+2a) t]

(xb)

where N ° and Nw are the number of radionuclide atoms in the water column at time zero (i.e. the time of the spiking) and time t, respectively. 2a is the radioactive decay constant of the radionuelide, and 2w is the overall first order removal rate constant of the radionuclide from the water column. 2w's for various radionuclides in tanks A and B were determined from the slopes in Figure 2 and half removal times tw ( = In2/2w) are given in Table 2. If one assumes that the removal mechanism of radlonuclides from the water column into various individual reservoirs is also first order, then the change in the number of radionuclide atoms, Nt, in a reservoir i other than the water column, is

64 °

P. H. Santschl, Y. H. Li ~.~ S. 2?,. Carson

TABLE2. Fractions (in %) of radionuelides in various reservoirs (Fz) at the end of the spring experiment and the total half removal time, tw (days) of radionuclides from water eolumn

Nuclide

Tank

t,, (=In2/2,,)

Fp

F,,

F. (Sediments)

(Settled particles)

(Water) Column)

~gFe

A B

6.2 6"3

14 o

62 66

5tMn

A B A

4"8 xx'9 4"8

6 o x2

*sZn

B A B

x3" 3 x5"9 2.0.2

11staCd

A

I5"6

~sCo

F,,~,~*

/;'out (Outflow)

(Adsorption on Wall)

0"5 xo

24 24

~ o ~o

30
2 36 4

20 39 2o

4z "24 4°

o xz o

(i to
33 22 46

37 49 44

8

3

22

5o

"29 7 "9 x7

~Calculated by mass balance.

aN ! =

dt and

~.,N.--).dNl

(2a)

X),t = 2w.

where hi = the first order removal rate constant of a radiortuctide from the water column into reservoir i. The integrated form of equation (2a) is N, = 21 N ° exp ( - - 2 d ) [x--exp (--gwt)] / 2w

(zb)

or by rearranging equation (2b): )q =

).wN, exp (2at) = 2~/;': ] [ I - - e x p (--3.wt)] = N ° [ I - - e x p (--2wt)]

ln21t,

(2c)

where Fl = Nt exp (2at)/N$ = the fraction of the radionuclide, corrected for decay to the beginning of the experiment, in reservoir i at time t. From the mass balance consideration, onealso obtains the following relationship:

or

A~ = (X 2Vt+Nw) exp ().at)

(3 a)

I = (~' N , + N w ) exp ().at)/N~ = Z F , + F w

(3 b)

The summation over all reservoirs does not include the water column. In tank B (without sediment in the sediment box) the major reservoirs of the radionuclides at the end of the experiment (22 days duration) were: (i) water column (Nw); (2) outflow of water (Nout); (3) a small amount of settled particles (Np) on the bottom of the tank and (4) wall surfaces of the tank (Nwau). The fraction of various radionuclides in each reservoir (Ft) fo/tank B is given in Table 2. Fw is obtained by rearrangement of equation (Ib) and Four by rearrangement of equation (2b) since 2out (the reciprocal of the mean residence time of water in the tank, i.e., 35"3 days) is known. Fp is obtained from the measured particle reservoir and Fwan is estimated from the mass balance equation (3b). In the tank A experiment (34 days duration), the sediment, an additional reservoir, (Ns) was included in the mass balance. F / s in each reservoir are given in Table 2 following the same procedure described above in the tank B experiment with the addition that Fs is

64z

Trace metals in ~rarragansett )Bay

~,~

~

[

~

~nlmvvv alooS

~o-I

,'(.~o.~l'q,~v

!

!

t~ntoo n t

w

I~ --

~,r~

~'~"6

•

r~ ~ ,,~ c~ o "

. o ,.,., ~,

".~, ='F> '~ -

-~I

,~I

/

I=,.o

o

~ ¢~ 0 ~ ;.~

,

/

= ~Loo$ Bo-1 .(zoJl!q~v

642

P. H. Santschl, 7*'.H. Li & S. R. Carson

obtained from the integrated radionuclide activity in a sediment core at the end of the experiment. Considering the settled particles and sediment box as a single reservoir as would be true in the Bay, the removal rate constant of a radionuclide onto settled particles and sediments, 2sv, should be equal to (Fs+Fp) tw/[x--exp (--t~t)]. The calculated results are given in Table 4 in terms of t,p = In~/l,p. It is evident from Table 2 that adsorption of radionuclides (except 59Fe) onto the wall materials of the tanks was considerable. Attempts to leach the adsorped radionuelides from vaU material with detergents and eomplexing agents (Na~EDTA, Na-citrate) were only •artially successful. Furthermore, a desorption study of radioactive trace metals from tagged .ank walls to freshly introduced Bay water showed that the first order desorption rate constants from the walls were one to several orders of magnitude smaller than the adsorption rate constant to the walls for all the trace metals studied here (the details will be published in the future). This strongly indicates that most adsorbed metals adhere tightly to the wall materials and are removed from further interaction with the rest of the system during the period of the experiments. The mass balance indicates that, excluding wall adsorption, the most important mechanism for removal of Fe, Co and ~In from the water column in the tank simulation of the natural system is adsorption of metals onto resuspended particles. Direct uptake by bottom sediments is of importance only for metals with longer half removal

(TANK C)

Dilution Line

226R¢I

21opb

mOpo

ZZSTh

146

156

166

176

186

196

AM

206

Julian Dcy

Figure 4. Decay corrected activities of radioisotopes measured, after applying standard chemical separation and purification techniques, by alpha counting (see Santschi et al., x979), displayed i/~ arbitrary units, are plotted as a function of time (Julian days). Error bars indicate one standard deviation due to counting statistics only (if not graphically represented, they have the approximate size of a symbol).

Trace metals in Narragansett Bay

643

times (Zn, Cd). Fe was removed at the same rate in both tanks even though tank A contained sediment and tank B did not. This suggests that the removal of iron was not a result of adsorption onto suspended material alone. The most likely removal mechanism is the floeculation of colloidal iron hydroxides (Aston & Chester, 1973; Sholkovitz, I978; Sholkovitz et al., I978), which could occur in both tanks. The half removal times of 13 days for both Win and Co is somewhat slower than that for Fe. According to Murray (I975) , Co tends to coprecipitate with Win-oxide-hydroxides or to adsorb tightly onto Win oxide surfaces. Therefore, the similar removal behavior of Win and Co is not unexpected. (ii) Sutnmer experiment. Figures 3 and 4 show the disappearance of the radioisotopes from the water column with time. In the later part of the experiment, manipulative additions of NaN3 ('SA'), and an antibiotic mixture ( ' A M ' ) are marked with arrows. Concentrations of Fe, Co, Zn, Cs and Cd decreased exponentially throughout the experiment in tank D, and until adding antibiotics in tank C. The replication of the removal curves between tank D and tank C before adding antibiotids was excellent (Figure3). Wieanwhile, Cr, Se and M n were rapidly removed during the first one to two weeks of the experiment. After that period there was a change in the behavior of these metals as indicated by the change of slope in Figure 3. After the change, Cr in tank C showed no net removal and Win and Se in tank C actually showed a net increase in concentration with time, while all three elements in tank D showed slower removal than before. The replication of the removal curves between the two tanks was poor after the initial fast removal period (Figure 3)22STh, 21°Po, ~t°pb and 226Ra added to tank C were removed from the water column exponentially except that some 2t°Po was returned to the water column after addition of antibiotics (Figure 4)-

TABLE3. Fractions (%)b of radionuelides in various reservoirs (Ft) at the end of the summer experiment and the total half removal time of radionuclides from the water column tw

Element Tank Fc

C

Th

D C

Po Cr ~{n Co

C C D C D C

t,4-xa (=In2fil,,) (days) 9-04-0"4 6"34-0-2

Fp Fs (Settled (Sediment) particles) 3"3 3"o

z6'z 30-7

6"z 4-o'5

54-2

26

7"34-0"4 4"x 4-o.x (xx4-x)

54-2 7"5 5"9 x'3 x'3 0"9 1.2 5"3 3"6 3"4 2"5

x5 (2x) 26-x 29"x 8"2: 44"4 x'7 (9"9) 5"9 4"7 7"5 !"5 3"0

(2"74-0"2) 2.z4-o- 5 x8"x4-0"4 x7"24-o'2 x3"o4-2 z7"o4-3 I9"84-o"4" x8.64-o-3

F,, (~Vater column) 0"9 o.z ~,o

Fw.tt° /;'out (Adsorption (Outflow) on walls) 37"o 28"9 25

44

4 (o) 2"4 o 30"6 0.2 14"o (7"0) 5"9 x5"8 9"6 x6"4 8"5

33 (30) 22.6 27"7 34"x 5"5 7o'2 (69) 64"5 57 56"2 75"5 75'2

44 4x'4 37"3 25"8 48"7 x3"2 x7"6 I7"2 23"z 3"2 to-8

80.8

o

0"5 5"8 5

Cd

D C D C D C D C

24"04-2"3

3"4

Cs

23"3 4-x'o

2"x

x'5 0"3

x4"4 x6"3

D

23.34-0" 5

x'8

0"7

x2"3

80"8 79"4

C

22.o+x.o

x4

78

Se Zn

Ra

N2


42"7 37'3

"Calculated by mass-balance. bNumbers in brackets reflect the calculated percentage without the effects caused by the additions of antibiotics (see text).

644

P. H. Santschfi Y. H. Li ~ S. R. Carson

The initial first order removal rate constants of metals from the water column (2w) were obtained from the initial slopes in Figures 3 and 4; overall half removal times of each of the metals, t~ ( = ln2/).w) are given in Table 3. The mass balance for tanks C and D at the end of the experiment again indicates (Table 3) that a substantial fraction of metals was adsorbed on the wall material (Fwan, Table 3). The importance of wall adsorption of metals was confirmed by direct analysis of wall material (the agreement was within 5o,-, 1oo%, depending on the isotope). Excluding wall adsorption, the resuspended sediments which settle to the bottom of the tanks are the major sink for ~in, Co, Fe, Cr, T h and Po. On the other hand, the bioturbated top layer of sediments in the sediment box (see Fs column in Table 3) is the major sink for Cs and Ra. Se, Zn and Cd are about equally distributed between the sediment and settled particle reservoirs. T h e much lower fraction of INIn and Co in the settled particle reservoir in tank C compared to tank D appears to have been caused by the addition of antibiotics which released ~in and Co from the settled particles to the water column. The fraction of various metals in the sediment reservoir, F~, is very similar for both tanks. Whenever the removal of a radionuclide from the water column decreased exponentially throughout the experiment, t,p was calculated by equation (2c). Also, since the concentration vs. time curves for iMn and Se in tank D and Cr in tank C showed changes of slope after I to z weeks indicating no net removal, it was assumed that the values of the parameters F,, Fsp and Fwau, determined at the end of the experiment, could be used in the calculation of t,p during the initial removal period. The results of all t,p calculations are summarized in Table 4TABLE 4" The half removal time t,p (days) of metals from the water column to the settled particle reservoir and sediment reservoir (4-In) Tanks A C D

Fe

Th

Po

Cr

Mn

Co

So

Zn

8--bx x34-x x34"x 564-6 414- 4 204-2 274"3 44-0"4" -1564-24 ~ 3694-37 x94-2 24-o'5 a x35+x4 I74-4 3134-3x

Cd

Cs

Ra

8504-85 8~64-82

'~65o

IiO4-2o 4244-42

at,~ for these elements apply only to the first 2-3 weeks of the experiment. Since the removal behavior of the added trace elements was not influenced by the addition of NaN3 to tank D, the removal rates calculated from tank D data (Tables 3 and 4) can be regarded as reliable and tank D can be considered as a control tank. In the case of tank C, the addition of antibiotics might have induced a small release of Cs and Zn from the settled particles and sediment reservoir, therefore, the calculated half removal time, tsp, for Cs and Zn can be regarded as an upper limit. The addition of antibiotics to tank C did cause a release of Co and Po from various reservoirs back into the water column (Table 3 and Figures 3 and 4). Therefore, the estimate of tsp or F~+Fp for Co and Po, if there were no addition of antibiotics, was done by a mass balance, i.e., F~+Fp = I--Fw--Fout--F~all, where Fw= exp (--2wt), Four = (2out/).w) [I--exp (--J~wt)],/'walt for Co in tank C is known from tank D and Fw~ll for Po was assumed to be equal to Fwall for T h in tank C. The simple exponential removal behavior and similar half removal time (20 to 4 ° days) among Fe, T h and Po indicate their similar removal pathways in the tanks, e.g., direct adsorption onto surfaces of pre-existing suspended particles, or co-precipitation and floeculation of metal hydroxides (Sholkovitz, x978; Sholkovitz et al., 1978 ) or heterogeneous nucleation of colloidal hydroxides onto any particleg (Aston & Chester, I973). The removal of Fe may have been controlled by the amount of sediment resuspension, which was 2-5 times

Trace metals in Narragansett Bay

645

greater in tank D than in tank C, resulting in the faster removal of Fe in tank D. Se in the first part of the experiment exhibited a removal behavior similar to Fe (half removal times tsp in tank D: 17 days for Se, 19 days for Fe). Indeed, other results (Geering et al., 1968; Lakin, 1973) suggest a strong association of SeO32- with Fe 3 +. The fast removal rate of M n during the first 2 to 3 weeks of the experiment and its subsequent fast baekdiffusion rate (equal to or greater than the removal rate) indicate the fast cycling of Mn between sediments and overlying water column. The apparent conservative behavior of M n in estuaries (e.g. Graham et al., I976; Holliday & Liss, x976) may reflect the dynamic balance between removal and backdiffusion rates as observed here. The initial half removal time of 2 days for Mn in tank D agrees well with the estimates obtained from field data, e.g., 3 days in Narragansett Bay (calculated from McCaffrey et aL, who used data from Graham et al., 1976), 1.5 days in Saanich Inlet of British Columbia (Emerson et al., 1979) and 1- 5 days reported by Hunt & Smith (198o) for stable Mn in M E R L microcosms (Pilson et aL, 1977) simulating Narragansett Bay conditions. In contrast to the spring, Co in the summer experiment did not show the same removal behavior as Mn. Its removal remained first order throughout the experiment, at a rate which was considerably slower than in the spring (half removal time tsp 135 days in summer vs. 13 days in spring). The 2x°Pb in tank C was removed from the water column at a similar rate to Co (half removal time t , for Pb: 14-5-4-o.5 days, for Co: 18.14-o-4 days in tank C). However, the mass balance for 2t°Pb could not be completed and no firm conclusion, therefore, can be dra~al on the removal mechanism of this element. The drastie change in the removal behavior of Mn, Cr and Se after their initial fast removal can be related to the change in their chemical forms during the experiment. The details will be discussed in the section on chemical forms. The half removal times for Zn and Cd are of the order of 3oo-400 days, while for Ra and Cs they are 65o-850 days. Zn and Cd have been found to have profiles similar to nutrients in the open ocean (Boyle et aL, 1976; Knauer & Martin, 1973; Bruland et al., 1978), which indicates that their removal pathways may be partially mediated by the biological processes of plankton. Since the bioturbated top layer of sediments is the major sink for Cs and Ra, these two elements may be simply removed by adsorption processes within the sediments, which are enhanced by the pore water pumping activities of benthic fauna. McCaffrcy et al. (198o) estimated a biopumping rate (i.e. volume of water exchanged between the'pore water and the overlying water column per unit area and time) of about o.74-o.4 cm 3 cm -~ day -1 in cores taken from the same location in Narragansett Bay as the sediments used in these experiments. The distribution coefficients of Ra, Cs and other trace metals between water and sediments are always greater than lO2 (in g HzO/g sediment, Duursma & Gross, 1971); thus the transfer of these radionuclides from pore water to sediments is quantitative ( > 995/o). Therefore, considering the ratio of water volume to sediment area of 9 m in the tanks, it would take about 8904-360 days, (=

90o'cmZ/cm2 × In2 = 8904-360 days) (o.74-0.4) cm3/cmz day

to flush half of the tank water through the burrows and pore space of the bioturbated top sediments and thereby remove half of the metal tracer in the water column. This value is similar to the half removal time of Ra and Cs obtained in the tank experiments. The reasons that Fe, CO, Zn and Cd gave much longer half removal times in the summer experiment than in the spring again seem to relate to the difference in the chemical forms of these elements in different seasons (see next section).

646

P. H. SantscM, II. H. Li ~ S. R. Carson

B. Chemlcal forms of trace metals and their relationship to removal mechanisms

As mentioned in the introduction, each radionuclide in the water column was differentiated into four operational fractions, i.e. (x) particles >0.45 lam on filter, (z) colloids on charcoal, (3) cations on chelex resin, and (4) the rest in the effluent. Only the results from tanks A and D are displayed in Figures 5 and 6. The results from tanks B and C are very similar to tanks A and D, respectively. Several features from these studies are worth mentioning here: (I) Despite the fact that all metals except Se were added in cationic forms, the different chemical forms of each metal were established quite rapidly within the water column. The similarity in chemical forms of the five trace metals added to tank A (Figure 5) with sediment and tank B without sediments (not displayed here) indicate that the presence of sediment

°°HiH140o20o UEIULW, I

2 0

o

#

i llll li ii il t-

1itt Ht 11 tt

'0' ' / 1 o

80

90

!

I00

I10 ,AJLtAN DAY

"

9

TEMPERATURE

ps

? 6

° 76

,~-~,,

80

t 90

IO0

t I10 ~Jt_IAN DAY

Figure 5- The different chemical forms of the dements studied in tank A are displayed as % in each fraction: ( 0 metals associated with particles >o'45 lun caught on a millipore filter (solid), (z) metals extracted by activated charcoal (dotted), (3) metals extracted by chelex resin (diagonal Iines) and (4) metaIs found in the effluent, after passing (I) through (3). On the bottom of the figure, the water temperature is displayed for comparison.

Trace metals in Narragansett Bay

647

does not immediately control the chemical forms of the trace elements in the water column. I n many cases the partitioning among these forms remained relatively constant throughout the experiment. T h u s it is not surprising that removal rates of most metals remained constant during the experiments. I n other cases, such as ~'ln, Cr and Se in the summer, however, the chemical forms showed a change that can be correlated to a change in removal rates (discussed further below). r^^^, usCd

65Zn

58Co ::=,, o Ifie i.l.. -1-o

75Se

w z

51Cr

59Fe

~4Mn

$A

SA SA

n Day

Figure 6. The different chemical forms of the elements studied in tank D are displayed as % in each fraction: (z) metals associated with particles ) o ' 4 5 pm, caught on a millipore filter (solid), (z) metals extracted by activated charcoal (dotted), (3) metals extracted by chelex resin (d~agonal lines) and (4) metals found in the effluent after passing (z) through (3). SA indicates the time when the sodium azide was added.

648

P. H . Santschi, Y. H. L i ~ S . R . Carson

(Z) Elements that are removed to the sediment-particle reservoir most rapidly tend to have significant fractions on suspended particles. This can be seen in Figures 5 and 6 in the cases of Fe in tanks A and D and Cr, Mn and Se during the first 2- 3 weeks of the experiment in tank D. In tank C (not illustrated) Po and T h show filterable fractions of a5% and 2O3/o, respectively, in a single measurement I week after the experiment began (with i5% charcoal and 65% effluent for Th, and 7o% charcoal and 5% chelex+effluent for Po). Cr, IVIn and Se in tank C showed smaller particulate fractions than in tank D in the first few weeks of the experiment, reflecting the smaller sediment resuspension rate in tank C than in tank D. Thus it appears that the rapid removal of these metals is caused by association with suspended particles through adsorption, and]or coprecipitation on particle surfaces followed by settling. Co and B,1n in tank D (Figure 5) and Fe in tank C showed a small filterable fraction initially, but that fraction increased in the later weeks of the experiment. However, removal at the beginning of the experiments was rapid, and comparable to that of metals with significantly larger particulate fractions. Therefore, the most likely removal mechanism for Mn and Co in tank A and Fe in tank C is a mechanism which involves flocculation of colloidal hydroxides. (3) The chemical forms determined in the summer experiment contrast strongly with those of the spring in that nearly all metals show significant fractions (ranging from 2o to 95%) in the effluent of the separation system. Since all the metals except Se were introduced in cationic forms, this indicates a major change in speciation after introduction to the seawater. In all cases except Ra and Cs the cationic forms should be retained by the chelex resin. In the spring experiment, on the other hand, the effluent fracti0rt contained less than to% of any of the five metals studied during the period when the water temperature remained below 5 °C. Near the end of the spring experiment when temperatures increased from 5 to xo °C the effluent fraction of all metals increased as well (Figure 5). Such an increase in the fraction passing through chelex resin was also observed for stable Fe in the waters of Narragansett Bay in the spring of I978 (Hunt, personal communication). Several lines of evidence indicate that the effluent fraction of heavy metals are probably associations of the metals with negatively charged low molecular weight organic molecules: (i) Irradiation of water with ultraviolet light (). N25oo-3ooo A), which would affect organic molecules only, resulted in a significant reduction in the effluent fraction for Fe, Zn, Co and Mn, as shown in Table 5. TABLE 5- Effect of UV-irradiation ().~a5oo to 3000 A) on chemical forms* (day t95, tank (2) % in each fraction Element Beforeirradiation Fe Zn Co ~1n Afterirradiation Fe Zn Co

Mn

Filter

Charcoal

Chelex

Effluent

66 5 x 37

8 z7 x3 o

o x9 5 26

a6 59 8t 37

75

22

3

o

3 z 25

4° 30 3

46 58

xz xo o

7z

*For Se and Cs there was no noticeable change in chemical forms after UV" irradiation. For Cr tile same was true due to a lack of precision.

Trace metals in Narragansett Bay

649

(ii) All of the non-filterable species (the colloidal fraction caught on charcoal, the cationic fraction retained by chelex and the metals in the effluent fraction) can penetrate through 0"03 lam filters and through dialysis membranes with a molecular weight cutoff in the range IO ooo--I2 ooo. This indicates that even the colloidal fraction has molecular weights smaller than IO ooo. According to Kerr & Quinn (in press), up to 50% of the dissolved organic matter in Narragansett Bay water indeed has a molecular weight smaller than 700. Although we have no direct evidence of the nature of the organic fraction that the metals associate with, it seems likely that fulvie acids, which have molecular weights of xooo and less (Gamble & Schnitzer, I973) and are negatively charged in natural waters, are responsible for the observed behavior of the metals in the effluent fraction. Fulvie acids can tightly adsorb trace metals (e.g. Hg in the Mississippi Delta and Florida Everglades, Andren & Harriss, x975) or can indirectly stabilize inorganic colloids (Eppler et al., cited in Singer, x975, Boyle, I976 ) and their adhering trace metals. Andren & Harriss (1975) have found that a large part of the dissolved mercury in the Mississippi Delta is associated with dissolved organic matter of molecular weights less than 5oo, having properties of fulvic matter found in soils, regardless of salinity. Unpublished results from later microcosm experiments indicate that the removal behavior of Hg strongly resembles that of Fe. (iii) Less than 5% of Cr, Co, Mn, Fe and Zn could be extracted into hexane, benzene and carbon tetrachloride, whereas more than 6% of Co, 34% of l-Mn and 19% of Zn could be extracted into n-butanol, indicating a polar character of those heavy metal compounds in the effluent fraction. (4) Any two radionuclides which show similar removal behavior, e.g. Zn-Cd, Co-~in pairs in tank A and Zn-Cd, Cs-Ra in tank D, also show very similar patterns in the partitioning of the four fractions throughout the experiments. Similar colloidal and chelex extractable fractions of Cd in seawater have previously been described by Batley & Gardner (1978). (5) As mentioned before, the drastic changes in the removal behavior of ~In, Cr and Se during the course of the summer experiment are related to the changes in chemical forms of these elements. For example, the dominant fractions for Mn, Cr and Se on the second day of the experiment were chelex (cationic), filter (particulate) and effluent (anionic SeO]-) fractions, respectively. As the experiment proceeded, the dominant fractions became the effluent for Mn and Cr and the charcoal for Se. The real chemical nature of Se in'the charcoal fraction is still uncertain. Cr in the effluent fraction was most likely CrO42-, the thermodynamically stable chemical species in seawater first proposed by Goldschmidt (1954) and Krauskopf (I956) and experimentally verified by Cutshall et al. (I966), Elderfield (x97o), Emerson et al. (i979) and others. For t~,in, the change in chemical forms and removal behavior is clearly related to the rapid cycling of 13,in between the water column and underlying sediments, through the oxidation and reduction cycle of Mn in the system (e.g. Graham et al., x976; Aller, x977; ~lcCaffrey et aI., x98o; Hunt & Smith, x98o; Emerson el al., x979). C. hnportance of biologically mediated removal pathways In the open ocean the major removal pathway for many trace metals has been found to be 'scavenging' by suspended particles (which is the sum of processes such as adsorption onto surfaces, flocculation of colloidal matter, incorporation into plankton, coprecipitation with Fe and Mn hydroxides), which subsequently are filtered by zooplankton and removed from the water column by fast settling zooplankton fecal pellets (e.g. Higgo et aL, x977; Beasley et al., x978). In contrast, the main removal mechanisms in the experimental tanks were found to be (i) scavenging by adsorption or flocculation processes by particles which were resuspended

65o

1". H. Santscld, Y. H. Li & S. R. Carson

from the sediment and which then settle directly out of the water column and (2) adsorption within the bioturbated layer of the sediments. As shown in the spring experiment, the removal of all metals except Fe was much more rapid in tank A (with sediments) than in tank B (without sediments). Since there was no biological monitoring of the tanks, differences in zooplankton populations between the two tanks which could affect metal removal cannot be ruled out. TABLE6. Effects of additions of NaNs and antibiotics on algae biomass* of control tank c

Date

Tank

Chl. a

ATP

x93 I96

D C D

2x -~

-6z 26

x99

C D C

30 x9 ~5o00 ~

~ ---

D

~2o

--

zx6

Cell counts

-3520

25

*Additions of x'5 g NaN~ to tank D on day x8r, x99, 203, 208. Addition of x'5 1 of antibiotics to tank C on day x94. ~Due to a bloom of chlorococcum which occurred one week after the addition of antibiotics. "The average concentrations in the control tank wel"e similar to those of the Bay water during the time of the experiment. They ranged between 5-Io lag Chl. a/l and ,,~x'3 lag ATP/I, xaoo--x5oocell counts per ml. A dash in the table means that no measurement has been made. The effects of sodium aside addition on productivity, biomass and A T P in tank D compared to a control tank can be seen in Table 6. The biomass in tank D was depressed by 8o% relative to the control tank after the additions of NaNa, a respiratory inhibitor which would affect both plankton and bacteria. There was no visible effect in the removal and chemical forms of the elements studied after these additions. This indicates that biologically-mediated removal of the metals (transport by living plankton) is unimportant relative to the effects of sediment. Unfortunately, there was no biological monitoring of the tanks "prior to the addition of NAN3, so it is possible that biological activity was low throughout the experiment and therefore the addition had no effect. However, control tanks in this and previous periods have duplicated each other and the bay well, which indicates that the lower biological activity in tank D is indeed due to the additions of sodium azide. Addition of an antibiotic mixture (penicillin, streptomycin and a fungicide) to tank C caused a significant release of radioisotopes from the settled particles (Mn and Co) and walls (Mn, Co, Zn, Se, Cs) into the water column, as can be seen from Table 3 and Figure 3- T h e effects on biological parat])eters can be seen in Table 6. The addition of antibiotics induced a huge algal bloom of chlorococcum (which is not a dominant species in Narragansett Bay) 8 days afterwards, probably by eliminating competition for food between this particular algae and bacteria. It is not certain what caused the release of metals. It is possible that metals removed by bacteria were released when the latter were killed. There is considerable evidence (as reviewed by Goldberg & Luckey, x959, for example, or explained in White et al., i973) that penicillin can interfere in the biosynthesis of cell walls material (murin) by microorganisms. This experiment would therefore indicate that bacteria might play an important role in removing trace metals from the water column. Even though the experiments seemed

Trace metals in Narragansett Bay

65 t

to indicate that removal of metals by the filtering action of zooplankton and their subsequent transfer to the sediments in fecal pellets was not an important pathway during our experiments, not enough biological monitoring of our experimental tanks was performed to prove this point.

D. £;ediment mixing by bloturbation Three sediment cores (1.8, 1.8 and 3"5 cm in diameter and 5 to IO cm in length) were taken from the sediment box of tank A at different times (I2, 2o and 34 days after spiking). Concentrations of various radionuclides in the sediment cores decrease exponentially down to a depth of about 3 cm and cores show finite activity below 3 cm (Figure 7). T h e core ACTIVITY (cpm/cm z')

0

IO

3

4 G 0~

1

ACTIVITY ( c p m / c m 3 ) 20

"

12 DAYS '.~ 34 DAYS I

s[

*

t

o°

!

i

4 ~

t

,o

~o

~o

I

i

i

2,i.(n ~ ~ :

0c

:-b'

-

~o

-

"

4

)5

0

'

ZO

~

40

Figure 7. Sediment profiles oPSCo, ~Fe, ==Mnand ~Zn for day z2 (Julian day 88) and 34 (Julian day no) of the tank A experiment. The numbers next to the model cu~'es (see text) are the mixing coefficients in units of to -~ cm2 s -x. taken on day zo (not shown here) has measurable 6SZn and lt~=Cd as deep as 5 cm. These features can be attributed to the activity of burrowing organisms which cause the mixing of sediment particles and irrigation of the interstitial waters in the sediment column. Depending on the organism, both processes can be either random or unidirectional between the sediment surface and a depth of a few cm corresponding to the length of the organism (Aller, i977). The fitted curves in Figure 7 are obtained by solving simultaneous differential equations in a multi-layer box model (Broecker, z966 ). A box thickness of i nun and a time increment of 1 h were used. T h e reduction of box thickness and time increment by one half of the values taken above has no effect on the results. T h e initial condition is that the concentration of radionuclides is zero in the sediment core. The forcing function, F0, is the input of radionuclides at the sediment-water boundary, i.e. F0 = 2~A~ exp [-- ().w+2d) t] where A ° is the initial total activity of radionuclide in the water column per unit area of sediment. The interesting result is that the concentration profiles of various radionuclides can all be fitted by a sediment mixing coefficient of (4-5) × I o - ? cm2 s - t for the core taken on day 12 and of (2-3) × lO -7 cm e s -z for the core taken on day 34 (Figure 7). The difference between the two cores may reflect the uneven nature of benthic faunal activity in different parts of the sediment box. Our unpublished 23tTh and 2Z°Pb data in Narragansett Bay

65z

P. H. Santschl, Y. It. Li ~ S. 1~. Carson

sediments, taken from the same location, gi~e a sediment mixing coefficient of 2 X I O - ? cm"-s -1. Adler et aL (x98o) obtained (xq-o-4)×xo -~ in microcosm experiments. These values are similar to mixing coefficients obtained from other coastal areas using 2a:Th as a tracer: Long Island Sound: (x-x2)× xo-~ cm ~ s -1, with winter values of 2 × xo -~ cm ~ s - l ; New York Bight in winter[spring: 5 × x o -~ cm 2 s -1 (Turekian et al., x978). Since the additions of NaN3 (to tank D) and antibiotics (to tank C) could have affected the profiles of the different radioisotopes in the sediments to a variable degree, no attempt has been made to calculate sediment mixing coefficients for those tanks. Conclusions Although quantitative comparisons with Narragansett Bay have not been completed for this study, microcosms of the type used in these experiments do provide qualitative insight into the nature of metal behavior in the waters of Narragansett Bay. The fact that most metals showed exponential decreases in concentration in the water column with time indicates that the uptake by all reservoirs is first order. Thus, even though there is significant loss to tank walls, this removal can be accounted for. Also, once adsorbed to the walls, the metals seem to be removed from further interaction with the system. Therefore, the removal of metals from the water column by reservoirs more characteristic of the natural system (i.e. sediments, suspended material, organisms) can be adequately assessed by means of microcosms. The artifact of the system that resuspended material is separated from the sediment upon settling has helped in the understanding of removal of metals to sediments vs. suspended material. In the experimental system the major removal mechanisms for most metals appeared to be through adsorption to or flocculation on resuspended sediments and adsorption within the bioturbated layer of the sediment. Uptake or mediation by plankton did not appear to be significant for these experiments. However, the release of trace metals to the water column after an addition of antibiotics to tank C water indicated that microorganisms might play an important role in removing trace elements from the water column. This suggests that in systems such as Narragansett Bay, in which the water is in contact with sediment and contains significant concentrations of particulate matter, the removal of many metals from the water column occurs through such adsorption and flocculation processes, with the ultimate sink being the sediments through either direct adsorption or the sinking of resuspended matter. This does not rule out the possibility that uptake by plankton andTor deposition by means of fecal pellets (of zooplankton and other filter feeders) might be important in the Bay. Characterization of removal rates and chemical forms revealed that the metals studied can be classified into four groups of behavior in the simulated Bay system. The most rapidly removed metals were Fe, Th, Po, Cr (III) and Se (IV), which had half removal times ranging from 8 days in the spring (Fe) to x- 4 weeks in the summer. After the first I-2 weeks of fast removal, both Cr and Se showed a marked decrease in removal rates. This change could be related to a decrease in the filterable fraction and an increase in the effluent fraction (after filtration and passing through activated charcoal and chelex resin) of these metals. The carrier phase is thought to be an Fe phase. The presence of sediments and resuspended sediments did not seem to be essential for the removal of Fe in the spring experiment, but could have been a controlling factor in the summer. The major reservoirs in both experiments was in the particles, settled to the bottom of the tanks. M n and Co form a second group of elements with similar behavior. In the spring experiment, the removal rates (N x3 days) and chemical forms (mostly chelex extractable for M n and Co) were very similar. In the summer, however, Mn removal differed from that of Co, with Mn being removed very rapidly initially (half removal time of 2 days) and after 1-2

653

Trace metals in Narragansett Bay

weeks showing no further net removal, possibly indicating extensive remobilization of Mn in the sediments and backdiffusion to the water column at a rate similar to that of removal. This dynamic behavior could explain its apparent conservative character in many estuaries. Co, on the other hand, was removed much more slowly in the summer (half removal time ,-, I4o days), though its chemical speciation was similar to that of Mn, and the major sink was in the settled particle reservoir. Sediments and settled particles were equally important sinks for the third group of elements, Zn and Cd, which were removed more slowly through both experiments. On the basis of other work, their removal is thought to be at least partially controlled by uptake by plankton. The mare removal pathway for the fourth group of elements (Cs, Ra) was adsorption directly within the bioturbated layer of the sediments; particle adsorption in the water column was less important• Half removal times were 6oo--8oo days. The major difference between spring and early summer experimental results turned out to b~ the slower removal rates (except IWIn), and the appearance of an effluent fraction in the chemical fractionation scheme for many trace elements, in early summer. It was concluded that associations of certain trace metals with low molecular weight organie compounds of polar character were responsible for these observations. The study of bioturbation rates revealed rates similar to published values for other coastal regions. A mixing coefficient of (2-5)× xo -~ cm 2 s - t was obtained by numerically modelling the observed trace element profiles in the sediments during the spring experiment. •

I

o

Acknowledgements We are grateful for the stimulating discussions with W. S. Broecker. Furthermore, we wish to thank Dr Ken Perez of the EPA Laboratory in Narragansett, R.I., for use of the microcosms and logistical support. We also wish to thank Joy Bell, Nelson Lugo and Kathy Kawtaluk for assistance in sample collection and processing, Carlton Hunt and Michael Amdurer for their useful criticisms of the manuscript and Rae Pochapsky and Vicky Costello for typing it. This work was supported by Environmental Protection Agency Grant R8o39o2-o2 to the University of Rhode Island.

References Adler, D., Amdurer, M. & Santschi, P. H. 198o Metal tracers in two marine microcosms: sensitivity to scale and configuration. Microcosms in ecological research, DOE Symposium Series Augusta, Ga., Nov. 8-xo I978 (John P. Giesy, ed.), CONF-78x xoz, National Technical Information Service. Aller, R. C. I977 The influence of macrohenthos on chemical diagenesis of marine sediments. Ph.D. Thesis, Yale University. Andren, A. W. & Harriss, R. C. 1975 Observations on the association between mercury and organic matter dissolved in natural waters. Gcochimica Cosmochimiea Acts 39, x253-1257. Aston, S. R. & Chester, R. 1973 The influence of suspended particles on the precipitation of iron in natural waters. Estuarlne and Coastal 2~larine Science x, zz5-a3x Batley, G. E. & Gardner, D. x978 A study of copper, lead and cadmium speciation in some estuarine and coastal marine waters. Estuarine and Coastal 2ilarine Science 7~ 59-7o. Beasley, T. M., Heyrand, M., Higgo, T. T. W., Cherry, R. D. & Fowler, S. $¥. 1978 2t°Poand mPb in zooplankton fecal pellets. 2~Iarlne Biology 44, 325-328. Boyle, E. A., Sclater, F. & Edmond, T. M. 1976 On the marine geochemistry of Cd. Nature 263, 42-44. Boyle, E. A. 1976 The marine geochemistry of trace metals. Ph.D. Thesis, Massachusetts Institute of Technology. Broecker, W. S. 1966 Radioisotopes and the rate of mixing across the main thermoclines of the ocean. Journal of Geophysical Research 7x, (24), 5827• Bruland, K. $¥., Knauer, G. A. & Martin, J. H. x973 Zinc in north-east Pacific water. Nature 27xj 741-743. Cutshall, N., Johnson, U• & Osterberg, C. 1966 Chromium-51 in seawater: chemistry• Science x52, 2o2-2o3. •

654

P. H. Santschi, II. H. Li & ,S. R. Carson

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