Geochemistry of trace metals in the Gironde estuary

Geochimica et Cosmochimica Acta, Vol. 61, No. 7, pp. 1421-1436, 1997 Copyright ~ 1997 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/97 $17.00 + .00

Pergamon

PII S0016-7037(97)00016-1

Geochemistry of trace metals in the Gironde estuary ANNE M. L. KRAEPIEL, J JEAN-FRANCOIS CHIFFOLEAU, 2 JEAN-MARIE MART[N, 3 and FRANCOIS M. M. MOREL 4 ~Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544, USA 2Centre de I'IFREMER, Centre de Nantes, B.P. 1049, 44037 Nantes, France 3Institut de l'Environnement, Centre Commun de Recherche, 21020 Ispra (VA), Italy 4Department of Geosciences, Guyot Hall, Princeton University, Princeton, New Jersey 08544. USA

(Received May 9, 1996; accepted in revised.fbrm JanuaO' 2, 1997 ) A b s t r a c t - - U s i n g clean techniques, we measured the dissolved, particulate, and (by cross-flow filtration ) colloidal fractions of Cd, Ni, Zn, Cu, Pb, Mn, and Fe in the Gironde, an estuary in southwestern France. The tractions of the particulate riverine metals that are apparently mobilized in the estuary vary from > 9 0 % for Cd to less than 2% for Pb. Observed mid-salinity m a x i m a for Cd, Ni, and Zn are well reproduced by a simple steady-state conservation model which accounts for the inorganic complexation of the metals by seawater anions. The concentration profiles of other metals, except Fe, can also be modeled by choosing an appropriate desorbable fraction and maintaining equilibrium between particles and solution. While colloidal iron decreases rapidly at low salinities, the colloidal concentrations of the other metals are quasi-conservative in the estuary. It appears that the colloidal fraction contains both iron oxide particles that separate from the rest by coagulation and organic macromolecules which bind most of the other metals and remain in solution. Copyright © 1997 Elsevier Science Ltd confluence of the Garonne and the Dordogne rivers. The former to the south originates in the Pyrfn~es and flows through Toulouse and Bordeaux and its famed vineyards. The latter to the north originates in the Massif Central. The Garonne River is the major tributary, contributing two-third of the water and particles entering the estuary. The physics of the Gironde have been well-characterized in a series of studies (Allen et al., 1974; Jouanneau, 1982; Jouanneau et al., 1983; Martin et al.. 1986; Gibbs et al., 1989; Li et at., 1994). It is a partially mixed to well-mixed estuary, with a significant tidal amplitude ( 1.5 5 m) and a stable turbidity maximum with a longitudinal extension reaching usually several tens of km. The residence time for the water is estimated between 20 and 90 days and the particles have a residence time of approximately 5 y. The estuary is continuously dredged to maintain the navigation canal which runs along the left bank up to Bordeaux. The Gironde drains the Aquitaine basin, one of the least industrialized regions in France and perhaps Western Europe. The main anthropogenic inputs come from the town of Bordeaux and the oil refineries in Pauillac, about halfway between Bordeaux and the mouth of the estuary. Photosynthetic activity is kept low by the high turbidity of the water, and primary production is usually assumed to be negligible (Fontugne and Jouanneau, 1987), as shown by low chlorophyll and phytoplankton concentrations (Romana and Breton, 1983a,b).

1. INTRODUCTION There have been m a n y studies of the geochemistry of trace metals in estuaries with the objective both to elucidate the physical, chemical, and biological processes that determine the fate of the metals and to estimate the net riverine contribution to the metal budget of the world oceans. The central issue in all these studies is that of the conservative or nonconservative behavior of the dissolved metals in the estuary: does some of the dissolved fraction b e c o m e part of the particulate phase through adsorption or flocculation? Or is some of the particulate fraction mobilized? Various authors have reached contradictory conclusions regarding the conservative b e h a v i o r of a particular metal; some of those disagreements may be traced to analytical problems, some to the different hydrographic regimes of various estuaries. W e have in most cases insufficient understanding of the m e c h a n i s m s that cause a metal to behave conservatively or not. The question is further complicated by the presence of an important colloidal phase which is poorly characterized and usually included in the dissolved fraction (because of the quasi universal use of 0.4 # m filters). In this study, we measured with clean techniques the concentrations of the dissolved and particulate fractions of Cd, Ni, Zn, Cu, Pb, Mn, and Fe along the salinity gradient in a macrotidal estuary with a well characterized turbidity m a x i m u m . W e also attempted to study the composition and behavior of the colloidal fraction separated by cross-flow filtration. The data are interpreted with a simple model that accounts for the variable partitioning of the metals between particulate and dissolved phases as a function of the salinity.

2.2. Sample Collection and Analysis In February 1994, surface samples were collected by hand from a small rubber boat, except the high salinity samples which were collected aboard the N/O La Thalia by pumping water into acidcleaned containers. The hydrological conditions were of high and ascending river discharge. The collection and filtration of the samples were carried out using clean techniques (Dai et al., 1995). Twenty-four stations were selected along the salinity gradient for collection and analysis of the particulate and dissolved phases. For each sample, 500 mL were pressure-filtered ( 1.5 atm N2 pressure) on 0.4 ,urn Nuclepore filters ( Q3 47 mm). At seven of these twentyfour stations, a 10-15 L duplicate sample was obtained to collect the colloidal phase, following exactly the same experimental procedure as described in Dai et al. ( 1995 ). These samples were pressurefiltered on 0.4/~m Nuclepore filters ( Q~ 142 mm) and an aliquot ( 1 L) of the filtrate collected (as total dissolved) for analysis. The filtrate was then processed through a cross-flow ultrafiltration system (Pellicon cassette, Millipore Corp.) containing a polysulfone filter with a molecular weight cut-off of 104 Daltons and 1 L of the

2. BACKGROUND AND METHODS 2.1. The Gironde The Gironde is a 72 km long estuary in southwestern France (see Fig. l), which begins downstream of the Bec d'Amb~s at the 1421

1422

A . M . L . Kraepiel et al. duced in the value of S at the low salinities (S < 2) is not relevant to this study. 3. R E S U L T S

o £$

Fig. 1. Map of the Gironde estuary. (a) upper limit of salt intrusion. (b) approximate upper limit of tidal node. (c) upper limit of tides.

permeate collected (as the truly dissolved fraction). We note that there is now growing evidence that the separation of colloids by cross-flow ultrafiltration is subject to a number of artifacts and that the results depend on the particulars of the techniques (Gustafsson et al., 1996). Cadmium, nickel, zinc, copper, lead, manganese, and iron in the total dissolved and truly dissolved phases were analyzed. For each metal, triplicate analysis of the samples were performed on a graphite furnace atomic absorption spectrophotometer (GFAAS). Prior to that step, the samples were preconcentrated in a class 100 clean room using a method modified from Danielsson et al. (1982) (complexation by ammonium pyrrolidine dithiocarbamate/diethylammonium diethyl dithiocarbamate (APDC/DDDC), followed by extraction into freon before back extraction by HNO3 into water). The blanks were for Cd: 1.2 pM; for Ni: 0.06 nM; for Zn: 0.8 nM; for Cu: 0.1 riM; for Pb: 46 pM; for Mn: 0.1 nM; for Fe: 0.4 nM. When compared to the NASS-4 and SLRS standard water references, recoveries were for Cd: 115%; for Ni: 92%; for Zn: 95%; for Cu: 91%; for Pb: 99%; for Mn: 86%; for Fe: 130%. Suspended particulate matter (SPM) concentrations were determined by a separate filtration under vacuum on 0.4 #m Nuclepore filters ( Q 47 mm). The dry nuclepore filters were weighted before and after filtration of the samples. The particulate phase was digested in a mixture of nitric, hydrochloric, and hydrofluoric acids at 120°C for 180 min, using a method from Loring and Rantala (1990). The metals were then analyzed by GFAAS. For the determination of POC concentrations, separate samples were collected in glass bottles before filtration on precombusted glass fiber filters (Whatman GF/ F, 0.7 #m). Carbon was analyzed with a CHN autoanalyser (Carlo Erba) after decarbonation by HC1 vapor. The chlorinity was measured by titration with AgNO3 and the salinity S was calculated according to S (g/l) = 1.8 [CI ] (g/l). The uncertainty so intro-

The measured concentration of suspended particles in the Gironde is highly variable but remains roughly constant from the upstream end of the salt intrusion to a salinity of 20 (Fig. 2i). At higher salinity, the concentration decreases markedly. Those results are expected from what is known of mixing and resuspension patterns in this estuary. The development of a turbidity maximum in the saline waters of the Gironde is well-documented (Etcheber, 1978; Jouanneau et al., 1983; Elbaz-Poulichet et al., 1984; Gibbs et al., 1989) and has been demonstrated to be strongly related to the turbulent energy field (Li et al., 1994). The composition of the suspended particles is remarkably constant throughout the salinity range with only the river and seawater endmembers exhibiting different trace metal and organic carbon concentrations (Fig. 2 a - h ) . Particulate concentrations of Fe, Pb, and Mn show some variations at low salinity. The particulate Fe concentration increases slightly at very low salinity and then decreases gradually over the length of the estuary. In contrast, the particulate Mn and Pb concentrations drop initially before increasing back to their upstream value between 2 and l0 salinity. The behavior of these two metals in suspended particles may reflect a sorting mechanism that concentrates fine particles in some parts of the estuary (Elbaz-Poulichet et al., 1982). It is also possible that Pb and Mn are affected by upstream dissolution and downstream reprecipitation as a result of redox reactions but their dissolved concentrations do not obviously reflect such phenomena. When compared to the world average riverine concentrations as reported in Martin and Windom ( 1991 ), the particulate concentrations in the Dordogne river are lower for all the metals studied (see Table 1 ). The particles of the Garonne river are significantly enriched in Cd (20 vs. 11 nmol/ g) and Pb (232 vs. 169 n m o l / g ) . This may be an indication that particulate Cd (and to a lesser degree, Pb) is for a large part of anthropogenic origin. The uniform metal composition of the suspended particles throughout the estuary undoubtedly reflects the fact that they have a long residence time ( > 1 year) and are well-mixed over that timescale (Elbaz-Poulichet et al., 1982). Their composition thus represents the interim fate of river particles which accumulate in the estuary. Published 6 ~3C data show indeed that the particulate C (and hence probably the particles) in the estuary are chiefly of terrigenous origin (Letolle and Martin, 1970; Fontugne and Jouanneau, 1987). Further, these estuarine particles have an organic carbon concentration of 1.69% ( = 1.4 m m o l / g ) compared to 3.54% ( = 2 . 9 retool/g) for those of the river thus demonstrating that they are on average oxidized and old (see Table I ). The total dissolved concentrations of several metals follow an approximate dilution line from the river to the seawater endmembers, although most of the data are sufficiently noisy that they do not provide proof of conservative behavior, and some metals (Ni, Zn, and maybe Cu) appear to have a midsalinity maximum (Fig. 3). Cadmium is the one metal that definitely exhibits such a mid-salinity maximum in dissolved

Geochemistry of trace metals in an estuary

1423

Table 1. Dissolved and particulate metal concentrations in the rivers and in the estuary. Particles

Cd Ni Zn Cu Pb Mn Fe organic carbon

Dissolved phase

x,,.~

x(;

.q~

x.,

x~..,

p,mol/g

pmol/g

~tmol/g

pmol/g

#mol/g

Me3 nM

Me~ nM

0.01 I 1.53 3.8 1.57 0.17 19 859

0.020 0.94 3.8 0.78 0.23 16 882

0.005 0.75 2.5 I. 15 0.17 15 852

0.0167 0.90 3.5 (I.86 0.22 16 876

0.0048 0.88 4 0.58 0.27 14 887

(I.355 6 18 13 0.26 57 139

184 II 35 3I 0.26

2950

Percentage of metal desorbed from the particles (Me~ Cp.,, = 42.5 m g ' L

Me~] )/(x., ~C,, ,~,) ,,i, Cp,. = I00 m g - L

209c/, 13(; I I q4

J(2,

89~.~ 6q 5%

1410

x,vr~ is the average composition of world river particles as reported by Martin and Windom (1991). .q, is the weighted average of the measured metal concentration in the particles of the Garonne river (GI and G2, see Fig. 2). xD is the measured metal concentration in the particles of the Dordogne river. .rr~ is the weighted average of the measured metal concentration in the particles of the Garonne and Dordogne rivers. x ~ is the measured metal concentration in the estuarine particles. Me~ is the weighted average of the measured dissolved metal concentration in the Garonne and Dordogne rivers. Me~° is the total reactive metal concentration at S = 0 (see text and Fig. 6). ' Cr,~ - 42.5 rag" L ' is the weighted average of the particulate concentration in the Garonne and Dordogne rivers as measured during this study. -~Cpr,~ - 100 m g " L ' is the weighted average of the particulate concentration in the Garonne and Dordogne rivers as reported by Jouanneau (19821 from a 10 year study.

concentration (Fig. 3a) as has been observed by others in different estuaries (Edmond et al., 1985; Elbaz-Poulichet et al., 1987). The most convincing conservative mixing lines are those for dissolved Pb and Ms. The dissolved Pb concentration measured for the seawater endmember (=100 pM) is comparable to the few other measurements of Pb concentration in the Atlantic ocean (e.g., 70 pM at Bermuda in 1987; Sherrell et al., 1992). Dissolved Fe concentrations decrease rapidly at low salinity before following an apparent mixing line (Fig. 3g), which is a well-known behavior for Fe in many estuaries (Boyle et al., 1977; Dai and Martin, 1995). In the Garonne river, when dissolved Fe was separated into colloidal and truly dissolved fractions, a majority of the dissolved Fe (ca 70%) was found to be in fact in the colloidal phase (Fig. 4f). This is not true for most of the other metals, for which the colloidal fraction represents on average 30% and never more than 60% of the total dissolved concentrations. The concentration of each metal in the colloidal fraction exhibits on average a weak decreasing trend toward higher salinity in the estuary. We compared the total dissolved concentrations obtained in the data of Fig. 3 with those obtained in Fig. 4 (colloidal + truly dissolved), both of which are the result of filtration of a water sample through a 0.4 #m filter. In general, the results agree reasonably well, as shown in Fig. 5a for Cd, although the data are noisier for other metals. In the case of Fe, however, the data obtained in Fig. 4 were about 50% lower than those obtained in Fig. 3 (see Fig. 5b). The difference between the two types of filtration is the volume filtered: for the data of Fig. 4 the volume filtered is 10 L instead of 500 mL for the data of Fig. 3. The much longer filtration time necessary for the higher volumes (up to 5 h vs. 30 min) apparently resulted in a coagulation of colloidal iron and a much more efficient retention by the filter. This is not the case for the others metals, except possibly Mn (see regression data in caption of Fig. 51.

4. DISCUSSION Our quasi-synoptic measurements of trace metals concentrations in the dissolved, colloidal, and particulate phases allow us to examine the estuarine biogeochemistry of these metals with more data constraints than are often available. We first focus on Cd whose large variations in dissolved concentrations provide us with an opportunity to develop a simple conservation model for trace metals in the estuary. We then discuss the behavior of the other elements and conclude by examining the nature and behavior of the colloidal phase.

4.1. Cadmium Concentrations and Model of the Estuary A large mid-salinity maximum in dissolved Cd concentration is considered to be a general feature in estuaries (Shiller and Boyle, 1991 ) although the maximum is sometimes questionable (Boyle et al., 1982; Elbaz-Poulichet et al., 19961 especially when the particulate load is low and the residence time of water and particles is short. In the Gironde, this welldefined maximum provides a test for our understanding of the processes that control metal geochemistry in the estuary. It has been usually attributed to the release of Cd from the particulate phase because of increasing complexation by chloride at higher salinities, and this explanation has been buttressed by a variety of laboratory experiments (Edmond et al., 1985; Elbaz-Poulichet et al., 1987; Comans and Van Dijk, 1988). For example, Comans and Van Dijk (19881 equilibrated mgCd labeled particles from the Rhone river in media of varying salinity and observed over a few days a reversible desorption of Cd increasing with salinity. Especially in the Gironde, the observed maximum is unlikely to be the result of temporal variations in the fluvial concentrations (because of strong tidal mixing and long water residence time), and there is no evidence of a localized anthropogenic input (Jouanneau et al.. 19901. Moreover, the Cd

1424

A . M . L . Kraepiel et al.

70

[

estuary

rivers

60

g

•

a

0

t

10

Ib•

•

~be

•@•••

••

0

1.5

b

70 .~1.0

E e~

@@

e~

50 0.5

30 @

400 300

4

o

t~

•

200

2

100 t

•

0

100 E

o

tP, e @ I

D G2 GI

4t4

e~ee

@

•

50

•

•

I

I

I

I

I

I

I

0

5

10

15

20

25

30

0

35

$81inity

Fig. 2. Particulate (>0.4 #m) concentration for (a) Cd, (b) Ni, (c) Zn, (d) Cu, (e) Pb, (f) Mn, (g) Fe, (h) organic carbon, (i) suspended load for surface samples in the Gironde estuary. For Zn and Mn, the seawater endmember is missing. Closed circles correspond to the river samples (collected at the limit of the dynamic tide). GI: sample collected at La Reole (Garonne river) on 02/03/94. G2: same location as GI, collected on 02/10/94 (high flow). D: sample collected at Pessac (Dordogne river) on 02/04/94. Open circles correspond to the weighted average of the Garonne and Dordogne rivers according to solid discharge (=x,~,. in Table I). Closed diamonds correspond to the estuarine samples. Salinities were calculated according to S (g/l) = 1.8 [CI-] (g/l).

m a x i m u m in the Gironde has been observed at various times of the year and with different riverine discharges (ElbazPoulichet et al., 1987; Boutier et al., 1989). A striking fact in our data is that the increase in dissolved Cd concentration is not mirrored by a decrease in suspended particulate Cd concentration in the estuary. Conservation of

mass necessitates that some particles must exhibit decreasing Cd concentrations between the salinities of 0 and 15 to make up for the difference of about 1500 pM between the extrapolated dilution line for dissolved Cd and the measured value at low salinities. But the suspended particles exhibit a constant Cd concentration of 4.8 n m o l / g and, given an upper

Geochemistry of trace metals in an estuary

500

rivers

1425

estuary

100

400 300 50

200 100 0 25 20 15

1000

•

O•

•••$

• 500

10 5 1000

If""

900

g.

"•

••$

•

4.8

800

Y,

700

3.8

600

2.8

500

8

0

00 0

300 200 100

tet

@ °

0

0

D G2 G1

I

I

0

5

I-10

I 15

I-

I

20

25

30

I 35

salinity

Fig. 2. (Continued)

limit in SPM concentration of 300 mg/1, they would need to release at least 5 nmol/g to account for the measured increase in dissolved Cd. The obvious explanation is that the suspended particles are only a small fraction of the total particulate load, most of which is in the bed sediments whose total mass has been estimated at ca 1 g/L (Li et al., 1994). The suspended and bed particles are known to exchange with each other under the influence of tidal mixing in the Gironde (Allen et al., 1974; Li et al., 1994). Thus, because

the riverine particles entering the estuary with high Cd concentrations represent only a small fraction of the total particulate load, they can release Cd to solution without affecting measurably the average particulate Cd concentration in the estuary. Only at very low salinities, where these Cd-rich riverine particles likely constitute a sizable fraction of the suspended load, is the suspended particulate Cd concentration elevated compared to downstream (see Fig. 2a). The constancy of the particulate Cd concentration also

26

A . M . L . Kraepiel et al.

rivers

estuary

1000 + []

500 JE

E

0 10T

5.

[]

•

---"--e

I

ol 40=

[]

i

e=

i lO!

E

0 [] 25

20 []

15 [] 10

[]

i

D G2 G1

0

5

10

15

20

25

30

35

salinity

Fig. 3. Dissolved ( < 0 . 4 #m) concentration for (a) cadmium, (b) nickel, (c) zinc, (d) copper, (e) lead, ( f ) manganese, and (g) iron. GI, G2, D: see Fig. 2. Open squares correspond to the weighted average of the Garonne and Dordogne rivers according to water discharge. Closed diamonds refer to the estuarine samples. The lines were Me,. calculated according to Me~ = (see Fig. 6 for notation). For Cd, Ni, Zn and Pb, the values l~r SW-S 1 I + R - SW K' K' are given in Table 2; the fitted parameters are for (a) cadmium R = 18, Cd ° = 1840 pM, (b) nickel R = 0.85, Ni~ = 11 riM, (c) zinc R = 2.5, Zn~ = 35 nM. For Cu, K' is assumed to be constant; the fitted parameters are R / K ' = 0.7, Cu~ = 31 nM. For lead, two lines were calculated according to two different scenarios: ( 1) no particulate Pb is mobilized in the estuary; the fitted parameters are R = 0, Pb ° = 260 pM (solid line); (2) 2% of the particulate Pb is mobilized; R - 21.23, Pb~J = 436 pM (dotted line).

Geochemistry of trace metals in an estuary rivers

1427

estuary

350 300 250 & D~

200 150 100 50 0

+ ±i

100

[] 50

il • 0

•

$ 300 25O 150

T

[]

I

IO0 5O 0

l"

D G2 G1

l --I

I 5

I

10 15 salinity

010

I 20

25

30

-I 35

Fig. 3. (Continued)

brings up the question of equilibrium between solid and solution. If all the particles were at equilibrium with the free Cd 2+ activity, { Cd 2+ }, which decreases with increasing salinity because of increasing complexation by seawater anions, we should see a decrease in particulate Cd concentration downstream. Since this is not the case, we are lead to hypothesize that only a small fraction of fresh particles are actually reacting with Cd and at equilibrium with { Cd 2+ } and that the great majority of old particles in the bed or in suspension have too low an affinity for Cd to bind it effectively, except for a residual Cd concentration of 4.8 nmol/g. To model the geochemistry of Cd in the Gironde, we only need to take into account the flux of reactive riverine particles and can ignore the much larger mass of unreactive parti-

cles. For simplicity we consider only one type of such reactive particles and take those to be well-mixed in the estuary (i.e., the rate of aging of reactive riverine particles is assumed slow compared to the longitudinal mixing rate, such that we can consider them as a conservative homogeneous particle population of average age and Cd affinity). We assume further that equilibrium is maintained between the reactive particles and the dissolved phase with the result that Cd will be released from the particles downstream because of the increasing complexation of Cd 2+ at increasing salinity. In accord with the interpretation of previous authors, we neglect the possible organic, complexation of Cd and take dissolved Cd in the estuary to be dominated by chloride complexes. The resulting steady-state mathematical model as shown in Fig. 6 then consists of four equations: ( 1 ) con-

428

A . M . L . Kraepiel et al.

Garontle

estuary

&

000 t

&

500

&

m

'°t: 0

A

A

A

A

A

A _A •

.Ik

0

20I

A

15 10

A []

5 0

•

IT=

d t~

10

A

A

&

&

&

5

0 30 20

[]

& A

10 0

& •

t

•

A_

-10 40

--

A truly dissolved • colloidal

35 - - I I 30252015-

[]

10

A

A

ol 132

I

I

{

I

I

I

I

0

5

10

15

20

25

30

~

I 35

salinity Fig. 4. Colloidal (10 4 D - 0.4/zm) and truly dissolved ( < 1 0 4 D) concentrations for (a) Cd, (b) Ni, (c) Zn, (d) Cu, (e) Mn, and (f) Fe. Open symbols refer to truly dissolved concentration while closed symbols correspond to the colloidal concentratic, n, calculated by subtracting the truly dissolved from the total dissolved concentration. G2: see Fig. 2. No colloidal data are available for the Dordogne river. For Cd, Ni, Zn, Cu, and Mn, the line was obtained by a linear fit of the colloidal concentrations using the least square method; for Fe, the colloidal data points are connected by straight lines.

Geochemistry of trace metals in an estuary

1250

3OO

y = 0.94x

1429

y = 0.61x R 2 -- 0.99

R 2 = 0.94

20o

750 50O .~

loo

250 0

I

I

I

I

0

I

250 500 750 1000 1250

0

Cd small volume (pM)

loo

200

300

Fe small v o l u m e (riM)

Fig. 5. Comparison of the total dissolved concentrations for (a) Cd, (b) Fe, obtained in two different experiments. The small volume concentration was obtained by filtering 500 mL of sample (1 min. < filtration time < 1 hour). The large volume concentration was obtained by filtering 10 L of the same sample (4 h < filtration time < 9 h). The lines were obtained by a linear fit of the data using the least square method, forcing the lines to pass through the origin. For Ni, Zn, Cu, Pb, and Mn (not shown), the slopes of the straight lines were: Ni: 1.07; Zn: 1.02; Cu: 0.79; Pb: 0.93; Mn: 0.67.

servation of salt; (2) conservation of reactive particles whose rate of aging (diagenesis) is considered slow compared to mixing, and whose mass flux Q at any salinity is thus taken to be constant and equal to the riverine flux Qriv; (3) conservation of total reactive Cd (dissolved + reactive particulate) ; and (4) an equilibrium expression between dissolved and (reactive) particulate Cd. The resulting formula (S is the salinity, S W is the salinity of seawater, Cd~ is the total reactive cadmium concentration, Cdd is the measured dissolved Cd concentration, qr~vis the river water flow, k is the affinity of the reactive particles for Cd) Cde - Cdd × SW Cdd SW- S

k \ q~iv /

known, but likely constant, parameter (R = Q~a___zk) and a qriv side reaction coefficient, K', that can be calculated as a function of salinity (see Table 2). As seen on Fig. 7, log Y, calculated from the field data and log( 1 / K ' ) , calculated from published thermodynamic constants, track each other closely over the salinity range 0-12. Above this salinity, the difference between Cd~ and Cdd is too small (all the desorbable Cd is desorbed) to allow meaningful calculations. The coefficient R is thus indeed effectively independent of salinity. This is what we expect since the reactive particle flux and water flow from the rivers (Qriv and qr~v)are salinity-independentand the affinity of the X = { Cd2+ }

/

x K-';

equates a ratio of measurable estuarine concentrations and Cde - Cdd SW salinities, Y × - to the ratio of an unCdd SW- S

reactive particles for Cd

vary by the same factor over a similar salinity range, provides strong support for the plausibility of our simple model of Cd geochemistry in the estuary. Taking the numerical value of R from the difference of the two curves in Fig. 7 (R = 18), we can calculate the predicted dissolved concentration at any salinity from the values of K' and Cde ( = total reactive Cd). As expected, the calculated line follows the experimental data rather well (Fig. 3a). The desorption of Cd from the particulate phase is confirmed by the much higher particulate concentration of Cd in the river than in the estuary (see Table 1 ). To account for the total reactive Cd at salinity 0 (which is five times the measured dissolved

should be constant

if, as postulated, the average age (and hence the degree of oxidation and Cd affinity) of the reactive particles is constant throughout the estuary, or at least between salinities of 0 and 12. The similarity of the two curves of Fig. 7, which

\

Qnv

Cd in the rivers), the riverine particulate load ( ) must \ qr~ / be higher than our measured value (42.5 mg/L). Indeed the long-term average value reported by Jouanneau (1982) is 100 mg/L. A more quantitative comparison between the parameters of the model (Cd ° or equivalently R) with the riverine data is not warranted in view of the paucity of such data. 4.2.

Nickel

and Zinc

Aside from Cd, the two metals that most clearly appear to have a mid-salinity maximum in dissolved concentrations are Ni and Zn. Dissolved Ni is usually conservative in estuaries (Boyle et al., 1982) though desorption of Ni is sometimes observed at low salinities (Edmond et al., 1985; Windom et al., 1991 ). Like Ni, Zn is usually found to be conservative (Shiller and Boyle, 1991 ) but mobilization (Klinkhammer and Bender, 1981) or removal (Windom and Smith, 1985) have also been shown. For both Ni and Zn, we can apply the same model that we used for Cd, assuming also that their desorption is caused chiefly by increasing inorganic complexation with the major seawater anions, C1- and SO] (see Table 2). By choosing reasonable riverine endmembers for the total reactive metal concentrations (Ni °

1430

A.M.L. Kraepiel et al. e__xko

- y 6

Cd..,

) salinity distance downstream

Equations SW Conservation of salt : q = S---W~S "q"

(1)

Conservation of reactive particles : Q = Qn, (2) Conservation of total reactive cadmium :

q.Cdd + Q . C d ~ " = q.Cd, Cadmiumequilibriumbetween phases : Cdp"~'~ 2 Cd d

K'

where 2 =

and K' = Cd.4..~. lCd ÷}

By substituting(i) (2) and (4) into (3) : Cd,-Cd¢. S W =Qn*.,~.1

Cd+

SW-S

I

qa, i

Y

K'

I R

IU I/K'

Symbols q,, (L.yr') is the river water flow.

(3) Q~ (mg.yr') is the particulateriverineflux. q (L.yr') is the flux of estuarin¢ water.

(4) Q (mg.yr') is the flux of reactive particles. S is the salinity. [(21"](M) = 0.0157 S (%o). SW is the salinity of the seawater endmember. SW=32.09. Cd; " ~ is the re,active particulate cadmium (pznol/g). CAd (pM) is the measured dissolved cadmium concentration. Cd, (pM) is the total reactive cadmium concentration. (5) Cd,* is the total reactive cadmium concentration at S = 0. {Cd2.} is the free cadmium activity. Y and R are nondiraonsional groupings defined in equation (5).

Fig. 6. Cartoon of the estuary with the parameters and the equations of the model developed to explain the midsalinity maximum in Cd concentrations.

= 11 nM, Zn ° = 35 n M - - o b t a i n e d by trial and error since the variability in the data hardly justifies a nonlinear fitting procedure) and calculating the parameter R to fit the measured dissolved concentration at low salinity, we obtain rather good fits of the experimental data (Fig. 3b and c). To account for the relatively high total reactive metal concentrations needed to fit the data (Nie° = 11 nM, Zn ° = 35 n M ) , compared to the measured dissolved concentrations in the rivers (Ni ° = 6 nM, Znd° = 18 n M ) , we need only about 6 - 1 3 % of the particulate nickel and 5 - 1 1 % of the particulate Zn from the rivers to be mobilized in the estuary (see Table 1 ). Indeed, unlike those of Cd, the particulate concentrations of Ni and Zn in the estuary and in the rivers are approximately equal but a more precise comparison is precluded by the small number of riverine samples. 4.3. C o p p e r Both conservative (Boyle et al., 1982; Shiller and Boyle, 1991 ) and nonconservative ( W i n d o m and Smith, 1985; Windom et al., 1991 ) behaviors have been observed for dissolved Cu in estuaries. From the estuarine profile in dissolved Cu concentrations, which show an initial plateau if not a midsalinity maximum, it seems that Cu is partly mobilized from the particles in the Gironde. Unlike Cd, Ni, and Zn, however, we do not expect the inorganic speciation of Cu to change

with salinity since Cu forms only weak chloride or sulfate complexes. Inorganic species of Cu in natural waters are dominated by carbonate complexes, and those should change little in the Gironde whose pH is roughly constant and equal to that of seawater (Etcheber, 1978; Jouanneau et al., 1983; Elbaz-Poulichet et al., 1984). Further, dissolved Cu is usually considered dominated by organic complexes in estuaries as in all natural waters (e.g., Coale and Bruland, 1988; Dai et al., 1995). Thus, provided that the concentration and affinity of the organic ligands for Cu are constant, the dissolved Cu concentration should stay proportional to the free cupric ion activity (i.e., K ' = constant at all salinities). Copper should then desorb from the reactive particles strictly as a result of dilution. As for Ni and Zn, we thus chose a reasonable river endmember for the total reactive copper (Cu ° = 31 n M ) and adjusted the parameter R / K ' to fit the dissolved Cu measured in the estuary at low salinities. As seen in Fig. 3d, the resulting fit of the data is certainly reasonable. In contrast to the model calculations for Cd, Ni, and Zn, the dissolved Cu concentrations are predicted to decrease monotonically with salinity. Thus, the increase in side reaction coefficients with salinity is the feature of the model that allows it to fit mid-salinity maxima for Cd, Ni, and Zn. Indeed while desorption of metals from the particulate phase may be achieved by simple dilution, an actual increase in dissolved concentration requires an increase in the dissolved

Geochemistry of trace metals in an estuary

1431

Table 2. Inorganic side reaction coefficients for cadmium, nickel, zinc, and lead. Salinity K'

Cd Ni Zn Pb

0.00

0.06

0.26

0.45

0.63

1.09

3.15

4.21

6.06

9.31

9.95

12.06

15.56

17.03

24.75

28.71

29.92

32.09

1.00 1.00 1.00 31.4

1.25 1.17 1.17 30.6

1.76 1.40 1.39 30.1

2.20 1.56 1.53 29.8

2.59 1.68 1.64 29.7

3.57 1.94 1.88 29.7

7.93 2.76 2.61 30.5

10.3 3.08 2.89 31.2

14.6 3.55 3.30 32.6

23.0 4.21 3.87 35.3

24.8 4.33 3.96 35.9

31.0 4.67 4.24 37.9

42.7 5.15 4.63 41.5

48.2 5.33 4.78 43.2

83.5 6.15 5.43 53.4

107 6.53 5.74 59.8

ll5 6.65 5.83 62.0

130 6.85 6.01 66.1

For a metal Me, the side reaction coefficient K ' is defined as the ratio of the total inorganic metal concentration to the activity of the free metal ion. For cadmium: K ' =

1 + 102 Tcl [El '] -? 1026 ("/cl)2 [CI ]2 + 102.4 (TCl)3 [El- ]3 + t017 (Tcl)4 [ E l ] 4 Tcd:+ "YCdCE" TcocI: TCdCl, TCdClj

+ 100.6 Yc~ [CI ] + 10=", (7so]) [SO] ]

For nickel:K' = I '~Ni ~+

For zinc: K' = 1

')/NiCl*

"YNiSO4

+ 10o, Tc, [CI ] + 10°2 (r~,)2 [CI ]: + 10~' r_~oj [SO4 ] + 10~ (~'~?~)~[SO~ ]~

7z,,:'

7z.c~*

7z,,%

7z,so,

7z.,¢so.,~

For lead: K' =

1

+ 100.3 Tort. [OH ] + 10 '°') ("/OH)2 [OH-] 2 + 1016 Tel [CI ] + 10 ~8 (Yc~)2 [CI ]2 + 1017 (TcI)! [CI ]3 TPbOH + TPb(OHI 2 "Yl~Cl~ "YPhC'2] TPhCI~ + 10,4(Yc~)4[C1 ]4 + 10as Ysoj 1SO4 ] + 10 ° ~ Yco~ [co~ ~/PbCI~ '~PbSO~ TPbCO~

ICI l

S (M); 1.8 x 35.45

28 2 [504- ] = ~ [El ] (M) 343

]

"~OH[OH ] = l0 6 (M)

The actual pH and carbonate concentrations were not measured. Since the pH was found to be approximately constant in the Gironde (Elbaz-Poulichet et al., 1982), we assumed an average constant value of pH = 8 and [HCO3 ] = 2.2 10 3 M. The stability constants for formation of complexes were taken from Smith and Martell (1976, 1989). The activity coefficient of the species i (charge number z~) is calculated according to the Davies expression: ]lp. In(T,) = - 1 . 1 7 z ) ( i - " - ~ 7 - - 0.3•)

( 1 = 0.02S)

to particulate ratio with increasing salinity. Thus the good match between the relative increases in dissolved concentrations observed for Cd, Ni, Zn, and Cu in the estuary (Cd > Zn ~ Ni > Cu) and the relative increases in their side reaction coefficients with salinity provides further confidence in the plausibility of the model of Fig. 6. In the Seine estuary, Chiflbleau et al. (1994) observed mid-estuarine maxima for the dissolved concentrations of Cd, Ni, Zn, and Cu (Cd > Zn > Cu > Ni). The relative higher mobilization of Cu when compared to the Gironde could be due to the steady increase of pH and hence the increase in carbonate or organic complexation with salinity in the Seine estuary. About 2 0 - 5 0 % of the particulate Cu in tile tributaries of the Gironde must be reactive and eventually desorb to yield the extrapolated total reactive copper at S = 0 (see Table 1 ). Indeed, unlike the case for Zn or Ni, the particulate Cu concentration in the estuary is markedly lower than in the rivers (0.58 vs. 0.86 #mol.g-~). This contrast between the behaviors of Zn, Ni, and Cu serves to illustrate what may be a counterintuitive point: the sharpness of a mid-salinity maximum in dissolved concentration does not directly reflect the absolute desorbability of an element (i.e., its reactive particulate fraction), but rather its increasing complexation by seawater anions (as reflected in our model by the side reaction coefficient K ' ) . For example, dissolved Cu, which is not extensively complexed by C1- or SO42 , exhibits no clear concentration maximum in the estuary but its total dissolved load is increased - 2 . 4 times (Cue/Cud 0 0 = 2.4) by

desorption from the particulate phase whose concentration decreases ca. 30% from the river to the estuary. Dissolved Ni and Zn, which are both complexed by C1- and SO 2-, exhibit clear concentration maxima but their total dissolved load are only increased 1.8-1.6 times and their particulate phase concentrations only decrease by a few percent. The importance of the riverine particulate contribution to the ultimate dissolved input of an element to the oceans is governed chiefly by its total reactive particulate (i.e., desorbable) fraction which is presumably fixed by the nature of the particles and not by the complexing properties of seawater. 4.4. Lead Few estuarine datasets have been published for Pb. The study by Windom et al. (1985) in the Savannah river shows removal of dissolved Pb on one sampling date while the other campaigns made in the Savanna and Ogeechee rivers show no clear trend. The dissolved Pb data in the Gironde follow one of the more convincing conservative dilution lines. Further, the dilution line extrapolates to a river endmember which is very close to the actual Pb concentration measured in the rivers. Thus, surprisingly, dissolved Pb probably behaves quasi conservatively in the estuary. While Pb, like Cd, forms rather stable chlorocomplexes, its speciation in the Gironde is dominated by a carbonatocomplex, PbCO3, up to a salinity of S = 20. Overall, the side reaction coefficient K ' increases by only a factor of two

1432

A.M.L. Kraepiel et al. et al., 1994) and the long residence time of the particles in the Gironde.

4.5. Manganese 4, >.. I

1 0

0.1

5

•

I

I

010 •

15

•

-L-

salinity

In the Tamar estuary, Morris et al. (1982) found dissolved Mn to be either conservative or mobilized, with a pronounced temporal and spatial variability, while Windom and Smith (1985) observed removal of dissolved Mn in the South Atlantic Bight. Like that of Pb, the dissolved Mn data in the Gironde appear to follow a simple dilution line. This apparent conservative behavior of Mn is interesting. Because of the long residence time of water in the estuary and the dissolution of insoluble Mn (IV) to Mn 2+ in anoxic sediments, Mn is one element for which one might expect sedimentary input. This is not what is observed. 4.6. Iron

x.-

°.1I

G I 5

•

I

I

10

15

0.01

ullnlty

Cd~ - Cdd S W - - , calculated Cdd SW- S from field measurements and (b) K', calculated from published thermodynamic data (see text for notation). Fig. 7. Comparison of (a) Y

from low to high salinities (see Table 2) and, like Cu, Pb should thus desorb from the particles chiefly as a result of dilution rather than an increase in inorganic complexation with salinity. Indeed, the concentration profiles calculated from our model do not exhibit a maximum even if a sizeable fraction of the particulate Pb is assumed to be reactive. The best fit for the data, however, is a simple dilution line (solid line in Fig. 3e) which is obtained by considering that none of the particulate Pb is desorbable. As shown by the dotted line in Fig. 3e, if even a small fraction of the very large particulate Pb concentration is considered desorbable (in this case 2%), the model line diverges markedly from the data. Thus, our dissolved Pb data imply that the great majority of the particulate Pb is effectively unreactive. If we consider the particulate Pb concentration, we see that the suspended particles in the estuary are actually enriched (20%) compared to the particles in the rivers. Only a small fraction of that enrichment can be accounted for by the loss of particulate mass due to decrease in the organic carbon concentration. This estuarine enrichment is not explicable in the context of a steady state mass balance model. It may simply result from atypically low Pb concentrations in our few riverine samples. Alternatively, it may reflect a continuous decrease in the Pb pollution in the area (Nicolas

At low salinities, the particulate Fe concentration increases rapidly by nearly 10%. The increase expected as a result of coagulation of colloids (Fig. 4) is only a very small fraction of this enrichment, which is mostly accounted for by the loss of particulate mass as a result of decrease in the organic carbon concentration. Since it is not mirrored by an increase in dissolved Fe, the downstream decrease in particulate Fe concentration is presumably the result of dilution of riverine particles by marine particles. As noted above, the large fraction of the total dissolved Fe that is actually colloidal precipitates at low salinity (S < 4). The coagulation of colloidal iron in estuaries is well-documented (Sholkovitz, 1976; Figueres et al., 1978). Truly dissolved Fe concentrations vary between 16 and 3 nM with a decreasing trend seaward. These concentrations are clearly in excess of the published solubilities of amorphous Fe(OH)3~ (Byrne and Kester, 1976; Smith and Martell, 1976) and must then represent either colloidal or organically complexed Fe, or both.

4.7. Colloids A significant fraction (30% on average) of each metal in the estuary was found in the colloidal fraction. While the data are fairly noisy, the colloidal concentrations of Cd, Ni, Zn, Cu, and Mn can be considered in a first approximation to follow a simple mixing line, at least in contrast with colloidal iron which shows a rapid decrease at low salinities. This behavior of Fe undoubtedly reflects a loss of colloidal iron through coagulation, as noted by previous authors (Sholkovitz, 1976; Figueres et al., 1978). This contrasting behavior of the colloidal fractions of Fe and of the other metals is in accord with the filtration data (see Fig. 5 ). There is a systematic loss of dissolved Fe, but not of the other metals (except perhaps Mn), in large volume filtration compared to small volume filtration, due presumably to coagulation during the extended time of the large volume filtration. Although there is no evidence of nonconservative behavior in the colloidal Mn data (Fig. 4) it is possible, as indicated by the loss of dissolved Mn in the high volume filtration that some of the Mn behaves like Fe, as may be expected from their similar chemistries.

Geochemistry of trace metals in an estuary

25

1433

i

20

Dordogne

15 10

25

10

20

8

15

6

10

4

5

2

0

0

Garonne

e

25 20

30000

70

25000

60 50

20000 15

40

Estuary

15000 30

10

10000

5

5000

10

0

0

0

25

Seawater

20

d

10000

100

20

8000

80

15

6000

60

10

4000

40

5

2000

20

0 Cd/Fe ×106 Cu/Fe XI04

0

0 Mn/Fe Xl0 ~ Ni/Fe X104

Zn/Fe Xl0 ~ Pb/Fe Xl04

Cd/Fe XI06 Cu/Fe xl04

particles

Mn/Fe Xl03 Ni/Fe X104 colloids

Zn/Fe xlO~

Cd/Fe Xl03 Cu/Fe xl0

Mn/Fe

Zn/Fe

Ni/Fe Xl0 truly dissolved

Fig. 8. Metal composition (tool/tool) of the particulate, colloidal, and truly dissolved phases. The Garonne river sample (=G2)

was collected on 02/10/94.

The different behaviors of Fe and other metals in the colloidal fraction, seen in both the field and the filtration data, imply that different types of colloids dominate the Fe and the non-Fe fractions. A clue to the nature and origin of the colloidal fraction is obtained from a comparison of metal compositions among the particulate, colloidal, and dissolved fractions of the rivers, estuary, and seawater (Fig. 8). In the absence of data on colloidal mass, we normalized the metal concentrations in each fraction to that of Fe whose colloidal concentration is the largest and least subject to large relative errors. From examination of Fig. 8, the two trace metal fingerprints that are most similar are those of the colloidal fractions in the Garonne River and in the Gironde. These two also resemble the metal fingerprints of all the truly dissolved

fractions, the major difference in these being a relative Cd impoverishment in the Garonne river and Cu enrichment in seawater. Further, none of the trace metal fingerprints from the particulate fractions resemble any of those from the colloids. The close resemblance of riverine and estuarine colloidal fractions may simply reflect the fact that the later originates in the former. The major difference between these two fractions is the substantial loss of Fe in the estuarine colloids, in accord with the colloidal Fe data. Further, the non-Fe colloidal fraction appears to be somehow part of the dissolved fraction that it resembles and not of the particulate one which it does not. Clearly, these colloids are not continuously formed by peptization of particles and coagulated into

1434

A . M . L . Kraepiel et al.

the particulate phase. A possible interpretation of these data is that Fe colloids are dominated by some inorganic precipitate (probably hydrous ferric oxides with their usual coating of organic matter), while the other metal colloids are dominated by metal complexes with organic macromolecules, presumably humic and fulvic acids. Other authors have indeed found good correlation between colloidal Cu and Ni and colloidal organic carbon (Dai and Martin, 1995; Dai et al., 1995). According to this interpretation, the colloidal fraction separated by cross-flow filtration is made up two distinct subfractions which behave differently: ( 1 ) finely dispersed iron oxides which eventually coagulate and become part of the particulate load (and may include some M n ) ; and (2) organic macromolecules, which bind most of the other metals, are effectively dissolved constituents, and thus behave more or less conservatively. 5. CONCLUSION A simple geochemical model based on inorganic complexation of the metals by seawater anions seems to account reasonably well for the observed profiles of dissolved and particulate concentrations of Cd, Ni, and Zn in the Gironde. This is a surprising result since there is now much evidence showing that Cd and Zn are usually bound to strong organic ligands, at least in seawater (see for example Bruland, 1989, 1992). Perhaps this paradox may be explained by the fact that primary production in the Gironde is extremely low (because of very high turbidity). The dissolved concentration profiles of other metals (Cu, Pb, Mn) which appear to follow conservative mixing lines, can be accounted for in the same model as an equilibrium between reactive particles and ligands, of constant concentration and affinity. If the model is correct, both desorption kinetics and the mixing of particles in the estuary (at least in the low salinity end) must be fast compared to the aging of the particles by oxidation of the organic carbon content. The model is particularly good at reproducing the shape of the observed mid-salinity maxima in dissolved concentrations. However, the most important parameters governing the eventual contribution of the riverine particulate metals to the dissolved oceanic inputs are their total reactive particulate (desorbable) fractions. These (which are to a large extent adjustable parameters in the model) are apparently widely different for different metals ( > 9 0 % for Cd; 10% for Ni; 8% for Zn; 30% for Cu, < 2 % for Pb) and controlled by unknown processes. The colloidal fraction that is separated by cross-flow filtration appears to consist of two distinct subfractions. One which contains most of the Fe (and is thus assumed to be made up principally of iron oxides) behaves like the particulate phase and separates from the total dissolved fraction upon slow filtration of water samples or natural in situ coagulation in the upper reaches of the estuary. The other which contains most of the other metals (and is presumably chiefly organic ) behaves like dissolved macromolecules and is conservative in the estuary. Acknowledgments--This study was conducted in the Institut de Bio-

g6ochimie Marine/Unit6 de Recherche Marine number 6. We are grateful to Cecile Guieu for her various contributions, to Klaus Keller for his review of this manuscript, and to the Port Autonome de

Bordeaux for kindly providing the daily values of the Garonne and Dordogne riverine flows, as well as information about dredging in the Gironde. We would also like to thank E. Sholkovitz, A. Shiller, and an anonymous reviewer for their insightful and detailed comments. This work was supported by grants from I.F.R.E.M.E.R., N.S.F., and O.N.R. and by an I.F.R.E.M.E.R. fellowship to A.M.L.K. Editorial handling: R. H. Byrne

REFERENCES

Allen G.P., Bonnefille R., Courtois G., and Migniot C. (1974) Processus de s6dimentation des vases dans l'estuaire de la Gironde. Contribution d'un traceur radioactif pour l'6tude du d6placement des vases. La Houille Blanche 1/2, 129-136. Boutier B., Chiffoleau J.-F., Jouanneau J. M., Latouche C., and Philipps I. (1989) La contamination de la Gironde par le cadmium. Origine, extension, importance. Ifremer. Boyle E. A., Edmond J. M., and Sholkovitz E. R. ( 1977 ) The mechanism of iron removal in estuaries. Geochim. Cosmochim. Acta 41, 1313-1324. Boyle E. A., Huested S. S., and Grant B. (1982) The chemical mass balance of the Amazon Plume-II. Copper, nickel, and cadmium. Deep-Sea Res. 29, 1355-1364. Bruland K.W. (1989) Complexation of zinc by natural organic ligands in the central North Pacific. Limnol. Oceanogr. 34, 269285. Bruland K. W. (1992) Complexation of cadmium by natural organic ligands in the central North Pacific. Limnol. Oceanogr. 37, 10081017. Byrne R. H. and Kester D. R. (1976) Solubility of hydrous ferric oxide and iron speciation in seawater. Mar. Chem. 4, 255-274. Chiffoleau J.-F., Cossa D., Auger D., and Truquet I. (1994) Trace metal distribution, partition and fluxes in the Seine estuary (France) in low discharge regime. Mar. Chem. 47, 145-158. Coale K. H. and Bruland K. W. (1988) Copper complexation in the northeast Pacific. Limnol. Oceanogr. 33, 1084-1101. Comans R. N. J. and Van Dijk C. P. J. (1988) Role of complexation processes in cadmium mobilization during estuarine mixing. Nature 336, 151-154. Dai M. and Martin J.-M. (1995) First data on trace metal level and behaviour in two major Artic river-estuarine systems (Ob and Yenisey) and in the adjacent Kara Sea, Russia. Earth Planet. Sci. Lett. 131, 127-141. Dai M., Martin J.-M., and Cauwet G. (1995) The significant role of colloids in the transport and transformation of organic carbon and associated trace metals (Cd, Cu and Ni) in the Rhone delta (France). Mar. Chem. 51, 159-175. Danielsson L.G., Magnusson B., Westerlund S., and Zhang K. (1982) Trace metal determination in estuarine waters by electrothermal AAS after extraction of dithiocarbamate complexes into freon. Anal. Chim. Acta 144, 183-188. Edmond J. M. et al. ( 1985 ) Chemical dynamics of the Changjiang Estuary. Cont. ShelfRes. 4, 17-36. Elbaz-Poulichet F., Huang W. W., Jednacak-Biscan J., Martin J.-M., and Thomas A. J. (1982) Trace metals behaviour in the Gironde estuary: The problem revisited. Thalassia Jugosl. 18, 61-95. Elbaz-Poulichet F., Holliger P., Huang W.W., and Martin J.-M. (1984) Lead cycling in estuaries, illustrated by the Gironde estuary, France. Nature 308, 409-414. Elbaz-Poulichet F., Martin J.-M., Huang W.W., and Zhu J.X. (1987) Dissolved cadmium behaviour in some selected French and Chinese estuaries. Consequences on cadmium supply to the ocean. Mar. Chem. 22, 125-138. Elbaz-Poulichet F., Gamier J.-M., Guan D. M., Martin J.-M., and Thomas A. J. (1996) The conservative behavior of trace metals (arsenic, cadmium, copper, nickel, and lead) in the surface plume of stratified estuaries: Example of the Rhone river (France). Estuarine Coastal Shelf Sci. 42, 289-310. Etcheber H. (1978) Etude de la r6partition et du comportement de quelques oligo-616ments m6talliques (zinc, plomb, cuivre et nickel) dans le complexe fluvio-estuarien de la Gironde. These de doctorat, Univ. Bordeaux I.

Geochemistry of trace metals in an estuary Figueres G., Martin J.-M., and Meybeck M. (1978) Iron behaviour in the Zaire estuary. Neth. J. Sea Res. 12, 329-337. Fontugne M.R. and Jouanneau J.-M. (1987) Modulation of the particulate organic carbon flux to the ocean by a macrotidal estuary: Evidence from measurements of carbon isotopes in organic matter from the Gironde system. Estuarine Coastal ShelfSci. 24, 377-387. Gibbs R.J., Tshudy D. M., Konwar L., and Martin J.-M. (1989) Coagulation and transport of sediments in the Gironde estuary. Sedimentology 36, 987-999. Gustafsson O., Gschwend P.M., and Buesseler K. O. (1996) On the integrity of cross-flow filtration for collecting marine organic colloids. Mar. Chem. 55, 93-111. Jouanneau J.-M. (1982) Matieres en suspension et oligoelements metalliques dans le systeme estuarine girondin: Comportement et flux. These doctorat, Univ. Bordeaux I. Jouanneau J.-M., Etcheber H., and Latouche C. (1983) Impoverishment and decrease of metallic elements associated with suspended matter in the Gironde estuary. In Trace Metals in Seawater (ed. C. S. Wong et al.), pp. 245-263. Plenum Press. Jouanneau J. M., Boutier B., Chiffoleau J.-F., Latouche C., and Philipps I. (1990) Cadmium in the Gironde fluvioestuarine system: Behaviour and flow. Sci Total Environ. 97/98, 465-479. Klinkhammer G. P. and Bender M. L. ( 1981 ) Trace metal distributions in the Hudson river estuary. Estuarine Coastal ShelfSci. 12, 629-643. Letolle R. and Martin J.-M. (1970) Carbon isotope composition of suspended organic matter in two european estuaries. Mod. Geol. 1, 275-278. Li Z. H., Nguyen K. D., Brun-Cottan J.-C., and Martin J.-M. (1994) Numerical simulation of the turbidity maximum transport in the Gironde estuary (France). Oceanol. Acta 17, 479-500. Loring D. H. and Rantala R. T. T. (1990) Sediments and suspended particulate matter: partial and total method of digestion. Tech. Mar. Environ. Sci. 9, 14. Martin J.-M. and Windom H. L. (1991) Present and future roles of ocean margins in regulating marine biogeochemical cycles of trace elements. In Ocean Margin Processes in Global Change (ed. R. F. C. Mantoura et al.), pp. 45-67. Wiley.

1435

Martin J.-M., Mouchel J.-M., and Thomas A. J. (1986) Time concepts in hydrodynamic systems with an application to 7Be in the Gironde estuary. Mar. Chem. 18, 369-392. Morris A. W., Bale A. J., and Howland R. J. M. (1982) The dynamics of estuarine manganese cycling. Estuarine Coastal Shelf Sci. 14, 175-192. Nicolas E., Ruiz-Pino D., Buat-Menard P., and Bethoux J.-P. (1994) Abrupt decrease of lead concentration in the Mediterranean sea: A response to antipollution policy. Geophys. Res. Lett. 21, 21192122. Romana L. A. and Breton M. (1983a) Estuaire de la Gironde. Campagne Libellule. Distribution longitudinale des param6tres mesur6s et calculfs. Centre Oc6anologique de Bretagne, Brest. Romana L. A. and Breton M. (1983b) Estuaire de la Gironde. Campagne Libellule. Recueil des donn6es. Centre Oc6anologique de Bretagne, Brest. Sherrell R. M., Boyle E. A., and Hamelin B. (1992) Isotopic equilibration between dissolved and suspended particulate lead in the Atlantic ocean: Evidence from Pb-210 and stable lead isotopes. J. Geophys. Res. 97, 11257-11268. Shiller A. M. and Boyle E. A. (1991) Trace elements in the Mississippi river delta outflow: Behavior at high discharge. Geochim. Cosmochim. Acta 55, 3241-3251. Sholkovitz E. R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and seawater. Geochim. Cosmochim. Acta 40, 831-845. Smith R. M. and Martell A. E. (1976) Critical Stability Constants. Vol. 4. Plenum Press. Smith R. M. and Martell A. E. (1989) Critical Stability Constants. Vol. 6. Plenum Press. Windom H. L. and Smith R. G., Jr. (1985) Factors influencing the concentration and distribution of trace metals in the South Atlantic Bight. In Oceanography of the Southeastern U.S. Continental Shelf(ed. L. P. Atkinson et al.), pp. 141-152. Amer. Geophys. Union. Windom H. L., Smith R. G., and Maeda M. (1985) The geochemistry of lead in rivers, estuaries, and the continental shelf of the southeastern United States. Mar. Chem. 17, 43-56. Windom H. et al. ( 1991 ) Trace metal-nutrient relationships in estuaries. Mar. Chem. 32, 177-194.

436

A . M . L . Kraepiel et al. Appendix. A. Particulate concentration (on 0.4 Wn filter) for cadmium, nickel, zinc, copper, lead, manganese, iron, organic carbon. Suspended particulate matter concentration. - 1- In the Garoane and Dordogne rivers. G 1, G2, D : see caption fig. 2. WAp is the weighted average of the Garonne and Dordogne rivers according to solid discharge (=x,, in table 1). sample

Cd nmol/g 62.6 15.9 5.1 16.7

G1 G2 D WAp

Ni ttmol/g 0.96 0.94 0.75 0.90

Zn pmol/g 6.5 3.5 2.5 3.5

Cu ~ml/g 1.62 0.70 1.15 0.86

Pb nmol/g 325 224 170 219

Cu

Pb nmol/g 305 272 250 260 245 243 277 259 305 292 291 1 69 276 284 284 289 283 277 284 216 146

Mn ~mol/g 20.2 15.5 14.9 15.7

Fe pmoUg 822 888 852 876

POC mmoYg 3.82 2.45 4.38 2.95

SPM mg/I 13 69 28 43

POC

SPM

1.79 1.15 1.25 1.15 1.27 1.20 1.48 1.50 1.46 1.32 1.46 1.61 1.34 1.38 1.48 1.39 1.68 1.54 2.40 2.63 6.88

110 224 219 294 229 128 103 95 350 112 185 35 37 218 145 88 112 69 37 8 1

-2- In the estuary. Salinity

Cd

nmol/g

0.1 0.3 0.5 0.7 1.4 3.2 5.7 5.9 9.2 9.7 10.8 15.3 16.8 17.3 19.1 20.9 23,0 23.3 27.7 29.2 30.9

9.3 6.0 5.7 4.7 4.5 4.5 5.2 5.1 6.0 5.2 5.2 3.4 4.4 4.7 4.4 4.5 4.4 4.9 3.9 4.7 9.3

Ni

Zn

pmoltg

pmol/g

0.90 0.97 0.91 0.94 0.95 0.87 0.88 0.88 0.85 0.94 0.84 0,82 0.98 0.82 0.80 0.84 0.83 0.86 0.98 0.78 0.93

pmoP8

4.2 4.3 3.9 4. l 4.0 4.0 4.0 4.1 4.0 4.2 4.1 3.3 4.0 3.9 3.8 3.9 3.9 3.8 3.7 3.4 no data

0.62 0.56 0.56 0.60 0.56 0.54 0.59 0.59 0.58 0.58 0.58 0.67 0.59 0.56 0.54 0.58 0.56 0.55 0.66 0.51 0.74

Mn

pmol/g 16.3 13.3 12.0 11.0 10.7 11.5 14.2 13.7 17.5 16.2 16.7 8.9 14.4 16.8 15.3 15.4 16.4 15.3 15.4 11.8 no data

Fe

Itmol/g

mmoPg

874 933 902 938 938 938 947 906 863 910 894 695 938 847 854 861 888 870 892 790 550

mWI

Appendix (coat.). B. Dissolved concentration (through 0.4 g m fdter) for cadmium, nickel, zinc, copper, lead, manganese and iron. - 1- In the Garotme and Dordogne rivers. G I , G2, D : see caption fig. 2. WAd is the weighted average of the Garonne and Dordogne rivers according to water discharge ( = Med° in table 1). sample GI G2 D WAd

Cd pM 646 354 159 355

NI nM 6.98 6.44 4.61 5.96

Zn nM 39.5 17.1 5.8 18.3

Cu nM 10.0 15.9 11.6 13.2

Pb pM 249 283 2 aa 261

Mn nM 101.8 39.4 49.3 56.6

Fe nM 66 83 266 139

Zn nM 10.8 15.4 9.3 10.9 7.2 17.9 30.6 24.4 18.8 17,9 20.4 16.6 19.4 12.4 11.0 11.5 8.0

Cu nM 15.1 16.4 21.2 19.6 19.4 15.6 16.8 18.7 16.5 16.0 13.2 19.6 17.9 11.0 9.8 8.9 6.1

Pb pM 293 301 256 244 264 21 4 206 255 173 204 1 58 t 66 t 10 1 28 102 * 99

Mn nM 17.8 35.9 11.8 10.3 18.5 28.0 25.8 19.5 11.3 15.7 13.1 27.7 19.0 6.5 7.5 9.1 3.8

Fe nM 59 51 48 45 33 24 23 2l 17 19 16 25 18 7 8 * 4

- 2- In the estuary. * : contaminated sample. Salinity 0.1 0.3 0.5 0.6 l.l 3.2 4.2 6.1 9.3 10.0 12.1 15.6 17.0 24.8 28.7 29.9 32.1

Cd pM 256 214 179 223 236 488 594 1029 982 1033 960 1165 983 712 501 493 304

Ni nM 6.77 6.27 7.56 7.45 7.42 7.08 8.49 8.06 8,90 9.13 8.04 7.78 7.25 5,14 4.55 6.18 5.04

Appendix (cont.). C. "Total dissolved" and "truly dissolved" concentrations for cadmium, nickel, zinc, copper, manganese and iron. - 1- In the Garonne river. G2 : see caption fig. 2. sample G2

Cd (pM) <0.4 p,m 350

< I 0 ~D 138

NI (aM)

Zn (aM)

Cu (aM)

M n (aM)

Ire (aM)

<0.4 lama < I04 D 7.04 4.73

<0.4 Itm 2.9

< I0' D 1.9

<0.4 l.tm 14.1

< I04 D 8.3

<0.4 l.l.m 24.3

< 104 D 23.2

<0.4 p m 51

< 104 D 16

<0.4 p m 7.92 8,21 9.49 9.24 7.67 4.33

<0A tim 13.3 21.2 19.9 12.8 8.0 7.9

< 10' D 11.7 10.9 15.8 7.1 3.3 7.2

<0.4 lun 17.8 18.7 14.7 15.2 11.7 4.2

< 104 D 10.1 11.9 11.3 11.1 7.0 3.0

<0.4 p m 17.7 19.4 10,1 12.7 4.9 5.2

< 104 D 12.3 12.3 13.5 12.7 4.1 4.6

<0.4 pal 44 21 16 15 8 4

< 10' D 6 9 9 9 6 3

- 2- In the estuary. Salinity 0.5 4.2 6.l 8.4 2•.6 32.1

Cd (pM) <0.4/~m 193 493 816 I056 940 264

< 104 D 91 374 588 851 858 254

< 104 D 5.31 8.15 9.06 7.80 6.63 3.89