trace-metal behaviour in natural granitic waters

Journal of Contaminant Hydrology, 13 (1993) 309-325 Elsevier Science Publishers B.V., Amsterdam

309

Trace-metal behaviour in natural granitic waters

G. Alaux-NegreP, C. Beaucaire b, G. Michard a, P. Toulhoa t b and G. Ouzounian c

aUniversit~; Paris VII, Laboratoire de G~ochimie des Eaux, Tour 54-53, 2 Place Jussieu, F-75005 Paris, France

bCommissariat a l'Energie Atomique, Centre d'Etudes de Fontenay aux Roses, DCC/DSD/SCS/LCASH, B.P. No. 6, F-92265 Fontenay-aux-Roses Cedex, France

~ANDRA, Division d'J~tude des Sites, Route du Panorama R. Schuman, F-92266 Fontenay-aux-Roses Cedex, France

(Accepted for publication January 8, 1993)

ABSTRACT

Alaux-Negrel, G., Beaucaire, C., Michard, G., Toulhoat, P. and Ouzounian, G., 1993. Trace- metal behaviour in natural granitic waters. In: J.I. Kim and G. de Marsily (Editors), Chemistry and Migration of Actinides and Fission Products. J. Contam. Hydrol., 13: 309-325.

Alkaline thermal waters were sampled in granitic areas of southern Europe. In this study attention is focused on the behaviour of trace elements likely to be present in a radioactive waste disposal. We demonstrate that, in this type of waters:

(1) most transition metals are limited to very low concentrations probably by sulfide minerals (COS, MoS3, ZnS, Sb2 $3);

(2) uranium is limited in solution by uraninite; (3) for tri- and tetravalent metals such as Zr, Hf, etc., and rare-earth elements (REE), we

observe that they are well correlated with Th, with a slope of 1 in log-log diagrams. These elements are associated with particles suspended in solution. The different Th to REE or Th to Zr, Hf, etc., ratios are constant in solution and in the particulate fraction.

1. INTRODUCTION

Within the framework of deep high-activity waste disposal projects, an extensive analysis was made of deep groundwater in granitic rocks. Emphasis was put on alkaline waters since they can be considered as reacting with granite minerals at equilibrium with respect to mineral assemblages (Michard, 1987; Grimaud et al., 1990). 150 water samples were studied with emergence temperatures ranging between 15 ° and 70°C. Neutron activation analyses (NAA) gave accurate trace-element values (Alaux-Negrel, 1991). Previous studies have shown (Michard and Fouillac, 1980; Michard et al., 1986, 1989)

310 (~ A I , 1 1 ; X ~ N ~ ( ; R I A I:l x~

that the major factors controlling the chemical composition of these waters are the temperature of water-rock equilibria and the amount of dissolved mobile elements (C1, F, SO4). From the knowledge of alkaline waters, one ca n predict the concentrations of major elements (Na, K, Mg, Ca) and some trace elements (Li, Sr, Rb, Cs, Fe, Mn) in solution, at equilibrium with secondary minerals in granite (Michard, 1990; Michard et al., 1991). In this study, the attention is focused on the behaviour of trace elements likely to be present ira a radioactive waste disposal, i.e. chalcophile elements, U, Th and lanthanides.

2. GEOCHEMISTRY OF ALKALINE WATERS

2.1. Geological environment

A complete study was carried out of the chemical composition of deep waters in granitic areas, on many sites in southern Europe:

French and Spanish Pyrenees. where the springs emerge either in feldspar- rich gneiss called G1 (Guitard. 1965) or in calc-alkaline granites. These thermal waters were extensively studied mainly for geothermal purposes (Michard et al.. 1980: Ouzounian et al.. 1980).

Italian Alps. where the springs are principally located in the Mercantour- Argentera crystalline massif. Previous studies of the geochemical characteristics of these springs have shown the importance of chloride in the mobilization of many elements (major and trace) in solution (Michard. 1990).

Corsica, Western Mediterranean: all the sampled waters well up in the granitic southwestern part of the island. See previous studies (Michard and Fouillac, 1980: Ouzounian et al.. 1980) for more detailed data.

2.2. General considerations

Alkaline waters are generally slightly mineralized and characterized by high pH (8-9) and very low CO2 partial pressure. There is no predominant anion, but chloride can reach a high level of concentration, e.g. at Vinadio, Italy (0.03 mol L ~ ). As these waters are often confined in granite, redox potential can be low, and they contain sulfide (a few 10 -4 to 10 -3 mol L-~). Na + isthe dominant cation and Mg 2+ is generally very low. In Table 1, the principal chemical features of alkaline waters are summarized. For each thermal area, we have chosen the most representative waters.

2.3. Analytical techniques

Water samples were filtered in the fietd ~through membranes of different pore size: 450 nm for major elements, l0 nm for minor elements such as A1, to

TRACE-METAL BEHAVIOUR IN NATURAL GRANITIC WATERS 311

avoid polynuclear forms and acidified to pH ,~, 1. Measurements ofpH, T(°C), Eh and sulfide were carried out on site. Sulfur species (HS- , $2 0 3 - , SO ]- ) were analysed by potentiometric titration with HgC12 and a specific electrode (sulfide). By this method (Boul6gue, 1981), we can estimate quite accurately in the 1 0 - 4 - 1 0 -3 mol L -~ range of S concentration.

Major cations and anions, some trace elements (Li, Rb, Cs, Sr, Fe, Mn) are analysed by atomic absorption spectrometry (AAS; flame and flameless) and/or ionic chromatography. Time-resolved laser-induced spectrofluorimetry (TRLS) (Moulin et al., 1990) is used for the determination of uranium at very low levels of concentration.

NAA (Albert P., 1973) allowed us to obtain an important data set for trace elements: Cs, Rb, Sr, Ba, La, Ce, Sm, Eu, Lu, Sc, Co, Zn, As, Zr, Mo, Ag, Sb, Hf, Ta and W. Among these elements, some are also analysed by tradi- tional methods (see above) in order to compare the different approches. A great advantage of NAA is that it reduces contamination risks because only elements present during irradiation contribute to the final analyses. A method with several chemical separations by coprecipitation and ion-exchange chromatography was developed. Detection limits make it necessary to evaporate 125 mL of water before irradiation. The residue is irradiated in a quartz ampoule for 17 h with a maximum flux of 2.5.1014 n cm -2 s -~ . After a decrease of activity during 8 days, the sample is divided into three aliquots. Most elements are directly measured by gamma spectrometry. Lanthanides, Th, U, Hf, Sc and Ta in the second aliquot are coprecipitated with ferric hydroxide before ?-counting. In a third aliquot, As is removed by passing it through an ion-exchange column in HNO 3 medium and then counted by gamma spectrometry.

In this method, which is applied to trace elements at very low levels of concentration, the study of blanks is very important. The different sources of contamination of each element in a dilute water sample were studied. We distinguish the blank of reacting agents (acids) and the blank of container without adding of reacting agents. Except for Au, Cr and Ni whose blanks are not negligible, it is thus possible to measure trace elements at very low levels after a blank correction. Yb, Cd, Sn and In are beyond the limit of detection, but an appropriate chemical separation could allow them to be measured. Although NAA requires a heavy protocol with several chemical separations, it compares favourably with other methods.

2.4. Global equilibrium

During water-rock interaction, many elements are mobilized from minerals in thermodynamic disequilibrium at the temperature and pressure conditions

312 (~ AIAliX-NJ:(~R[ ' I I I ;",[

FABL[ I

Chemical composition of alkaline waters

Site 7~ 1~, pH Eh Alka- HS CI F SO42 SiO, Na ~ K + Ca 2 {°('1 (°CI (mV) linity (x lO 4) (x IO 411×10 4 ) (x lO 4)(×1-0 4 l l x l O 4 ) ( x I 0 41(×I0 4)

1 × i 0 3)

(or~-i¢a. g'estcrn Mediterranean:

(iuitera 43 60 9.61 310 1.374 0.24

Caldanella Chaude 33 I I0 9.22 260 1.88 0.6

Pietrapola 7 33 110 9.65 354 2.009 0.83

[¢tub.~-le.~-Buin,s. lzttslt,rtl prren~;~',~:

Cascade 75 IO0 9 3811 23 1.84

Exalda 4 61 IO0 9 317 2,3 I

Entr,~c 45 I00 9 2.3 1,6

Beaute 66 103 906 38(I 2.3 1.88

]:'~l~l~'rtl Plrctlt;cs:

Coneordc 61.1 IO9 8.75 338 2.092 1.39

Amblie 65.6 11)9 8.61 367 21 1.37

Dorr(.s / 40.3 n.d, 9.53 3~5 116 1.4

( 'entral Pvr~:m;e~:

Cautcrcts 46.5 82 9.34 38(I 102 17

kuchon I-I 69 103 8.5 213

Luchon F2 65.1 !)8 8.55 2.25

Spuni,~h Prrem;cs:

Panlicosa l iberio 465 92 926 3113 11.775

Arti,Ss 39 4 98 9.4t 4IN 1.66 3.42

LOs 28", 811 9.74 3:t8 0.967 121

('atalonttt, .~ll~titl:

St Climcns 27.8 41) 9 6 355 t.86

Cantallops 18 10:~ 9.71 ~51 2.45

It,dian .4lps:

Vinadio b.h. 3 50.5 121) 833 0.45

Vinadio b.h. 4 41.5 120 8.38 0.43

Vinadio hh. S 472 12t1 8.31 0.47

'gardinia. Western Mediterranean:

Oddini 328

3.74 1 5 3.55

14.07 6 52 I 1.42 14.5

8.09 1 98 33)9 14

2,84 32 2.4 13.9

2.9 32 2.9 14

2.8 31S 2.3 135

2.84 ~15 232 I~,7

2558 0.298 114

51 I 289 t l 8 7

30.85 0 8 2 ; 038

28. I 0.62:~ 0 42

27.9 0.62 0.5

28 OL6t 0.43

28.3 0614 041

6.75 IO.65 4.35 45.69 1.097 0.806

5.6I 9.45 3 3 145 44.64 0.99 11.57

17.5 185 16 9 0.29 t).92

9.2 3.87 9.6 2293 04119 0.84

8 3 15.3 3,3 12,8 42.68 t.038 1) 57

8.23 14.7 3.55 125 42118 11138 1) 7

2 3 054 252 I11

8.98 279 3.33 12.5

1.42 0,3 13 6.6

13.4 02~1 1

30.2 0552 0559

11).7 t t l 7 1) 76

1.82 4.82 1.48 0.61 81 22.3 0.174 0.257

342 7.8 571 1.31 12 ~73 1!.482 0.251

2.08 296 5.1 3.3 279 ,~ 39 16

175 332 51 2.95 297 9 5 21

1.27 312 5 I 3.4 283 9 191

0.68 1.06 70 3 3 6 794 0.652 3 4

7 = lemperamrc at the emergence: 11~ ~ calculated temperaIure al the reservoir; pH := measured pH: concentrafons are gl~,en in 10 '~ tool L I n d no da ta

T R A C E - M E T A L B E H A V I O U R IN N A T U R A L G R A N I T I C W A T E R S 313

Mg 2+ AI Fe aa Ag Co Zn Sb W U Mo Th Sm Eu { x l 0 -4 ) ( x 1 0 - 7 ) ( x 1 0 - 7 ) ( x 1 0 - 9 ) ( x 1 0 - 9 ) ( × 1 0 - 8 ) ( × 1 0 - 9 ) ( × 1 0 - 8 ) ( x l 0 9) ( x l 0 7 ) ( × 1 0 % ( × 1 0 - 1 ° ) ( x l 0 - I I )

0.26 0.11 1 0.08 7.6 1.7 0.33 0.015 - 0.016458 6.5 0.7 0.37 0.7 1.6 0.6 25 0.04 3.6 0.4 0.2 0.15

0.16 0.1 I 1.41 13 0.15 1.2 0.02

0.0036 29 0.48 0.6 2.2 2.7 9.4 0.25 0.4 5.5. 3.7 0.6

0.012 25 2.24 0.42 0.6 1.2 3.9 13 0.16 0.5 I.I 0.74 3.4

0.003 14 0.36 0.45 0.9 2.1 7,7 12.6 0.15 0.42 2.2 I I

0.0018 22 0.32 0.49 0.4 I 2.3 13 0.04 0.5 I.I 0.34 0.3

0.0012343 9.75 3.1 1.9 8.8 0.6 30 0.03 0.3 0.03 0.05 I).15

0.011 9 0.9 0.39 0.2 4.8 0.45 37 0.7 0.025

0.004114 6.65 0.3 0.2 0.9 0.78 I I 0.3 0.07

0.0085 1.4 0.45 0.35 0.9 23 15 0.1 0.9 0.2

0.0055 8.75 0.82 0.4 1.4 3.9 28 0.2 2.7 1.8 1.7

0.028 0.45 0.2 1.2 0.45 46 0.09 0.05 0.06 0.1

0.027 10 0.48 0.3 0.4 6.4 0.7 6.4 0.025 I 0.05 0.004 8.3 I.I 0.3 0.34 4 4 0.2 0.09 0.2 0.09 0.3 0.3

0.002 10.25 0.2 0.3 5.05 4.7 0.02 0.04 0.04 0.06 0.05

0.004 6.7 0.58 - 1.6 9 0.01 0.02 0.13

0.013 10 I 0.2 0.2 3.55 0.76 31.7 0.02 0.18 0.2 0.27 0.17

0.168 14,7 - 55 0.6 0.09 0.2

0.126 13.2 14 0.8 48 0.12 4

0.144 9.7 8.57 14 0.4 61 0.12 0.15 0.4 0.7

0.3

0.001 8.3 0.3 0.5 1.1 0.09 10 0.15 0.09

314 ~i. ALAI~X N I ( i R [ [ . I I \ i

of the solution. In a closed system, the net rock-to-solution mass transfer process stops when the solution becomes saturated with respect to a ther- modynamically stable assemblage of minerals, including all the components of the primary rocks. This is called isochemical recrystallization (Giggenbach, 1984). Alkaline waters can be defined in terms of global equilibrium, i.e. all the measured elements in solution are at equilibrium with respect to the different aluminosilicates in the deep reservoir, except for some elements such as AI, which is re-equilibrated during its ascent from the deep reservoir to the surface.

2.5. Control of the chemical composition

As this problem has been extensively addressed in Michard (1987), only a brief presentation is given here.

In the case of global equilibrium, major cations are controlled by alumino- silicate solubility (Na, K, Ca, Mg), Si and A1 are controlled by oxides and hydroxides or kaolinite for A1. The equilibrium reaction can then be written as follows:

2NaAISi308 + H 2 0 + 2 H ~ ~ 2 N a + + AI2Si2Os(OH)4 + 4 SiO 2 albite kaolinite quartz

One can readily see that for a given temperature the ratio (Na + )/(H + ) is fixed. It can be demonstrated (Michard, 1982, I987) that for all simple Me ~+ ions

or complex ions (Me(OH)p) (:-p)+ a general equation can be obtained for each controlled element:

(Me:+ )/(H+ ): = Ki(T)

(Me(OH)p)~:-pl+/(H +)l: p~ = ~ ( T )

(la)

(lb)

Conversely, the concentration of mobile elements (such as chloride), i.e. elements completely taken up in the aqueous phase, is considered as an external variable and taken into account in the electroneutrality equation:

Z [ M e l - [ A ] = 0

with

[Me] = (Me :~ )/7 + Z(Me(OH)p (:-p~+)/7

(2)

(3)

where 7 is the activity coefficient of the ions; and [A- ] indicates the concentration of mobile elements.

These two relations allow the calculation of the pH and the chemical

TRACE-METAL BEI-IAVIOUR IN NATURAL GRANITIC WATERS 315

Cunkis -2

-3

-4

~ - 5

o - 6

-7

-8

- 9 I I H No K Co Mg Si AIc.

Fig. 1. Modelling of the chemical composition of groundwater: case of Cuntis (Galicia, Spain).

composition of the water if T (°C) and A- are known:

ZzKi(H + ):/?i+ E(z-p)Kj(H + )u-P)/7 j - [A- ] = 0 (4)

For each of the studied waters, deep temperature is calculated from the Na/K geothermometer (White, 1965; Ellis, 1970) and the chemical composition of groundwaters is estimated from the adequate mineral assemblage and the measured concentrations of anions. In the same way, it can be shown that some trace elements (Rb, Cs, Sr, Mn, etc.) can be incorporated into solid solutions and behave in the same way as major elements. Their concentrations can be also estimated from empirical relations with other major elements. This is illustrated ida Fig. 1, by the case of Cuntis, Galicia, Spain (Michard and Beaucaire, 1993), where calculated concentrations at equilibrium and measured concentrations of some elements are compared. These results are in a good agreement.

3. BEHAVIOUR OF TRACE-METAL CATIONS

3.1. General discussion

Many trace elements are known to form very insoluble hydroxides. This phenomenon explains why they reach very low concentration levels in natural waters. Among these elements, we studied two groups. These two groups were well identified in Thu6s-les-Bains (Eastern Pyren6es). The first group includes Co, Zn, Ag, U, W and Mo; their concentration in solution is globally indepen- dent of the emergence temperature, in groundwater series. Their behaviour

316 ~; ,~,i ,',,I X - N I ' . J R I l I I , \ i

seems to indicate that they are limited in solution by a very insoluble phase, so we have called them "solubility-limited trace elements". The second group includes tri- and tetravalent elements (lanthanides, Zr, HI', Th, Sc, Ta), the concentrations of which are highly variable.

3.2. Solubility-limited trace elements

One of the most variable parameters in alkaline groundwaters is the temperature of equilibration which ranges between 20 ° and 120°C, it might

"-S

10 -7

lO-a f- 10 -9

10 - l o _

10-1~ b

i 10 -12 L_

20

- - - 1 - - - - - ~ q - ~

i

o

o

40 60 80 100 120

a)

E

0 7

O-a I 0--9

10 --10

10--11

10 - ! 2 2O

J T I ~ I

o o

o

40 50 80 100 120

b)

lo =~ T . . . . . ~-- T -T----~--- i

1 0 - 5 0

i 0 - 7 t'~ ~ N 0

t - - . 0 o

~ - % 0 0 0 ,

c-, 0 - 9 L-'-q 1

1 0 - 1 ~ I _ _ I c )

20 ~o 60 80 1oo ~2o

Deep Temperature ( 'C)

Fig. 2: Evolution of: (a) Ag; (b) Co; and (c) Zn concentrations vs, deep temperature in alkaline waters. The straight lines indicate the saturation with r e s ~ i to sulfide minerals for a bulk composition.


therefore be interesting to examine the influence of this parameter on the distribution of metal ions. These elements (Fig. 2) show very weak variations of concentration vs. temperature.

Alkaline waters have high pH-values (8.5-9) and are strongly reduced. We can consider that most metal ions may exist in the hydroxo-complex form (Baes and Mesmer, 1981). In the presence of sulfide, which is a competing ligand in such waters, mono- and divalent metals generally form sulfide

T A B L E 2

F o r m a t i o n l o g K a t 2 5 ° a n d 1 0 0 ° C o f t h e d i f f e ~ n t c o m p l e x e s i n s o l u t i o n a n d minerals

log K

25°C 100°C

Reference*

Co 2 + + H S - ~ C d ( H S ) ÷ 5.7

Co 2+ + 2 H S - ~-~Co(HS)2 ° 8.7 CoS(s) + H + ~ C o 2+ + H S

D = 0.44 (a) - -7 .09 (a)

n = -1.3 (b) -5.35 (b) A g + + H S - ~--Ag(HS) ° 14.1

A g + + 2 H S - ~.-~Ag(HS)f 18.5

Ag2 S¢s) + H + ~.~-2Ag ~- + H S - f~ = 0.1 - 3 6 . 2

ZnS + H 2 0 ~ Z n ( O H ) ( H S ) °

f~ = - 1 . 6 - 6 . 7 5 [3] - 6 . 4 [4] U O 2 + H 2 0 + O H - ~.~U(OH)5 -

- 5.75 - 5.77

< - 7 . 8 9 UO2 + 2 H 2 0 ~ U ( O H ) 4 0

f~ = - 0.9 - 9.47 C a 2+ - ] - W O 4 2 ~--CaWO 4

f~ = - 1 . 5 - 1 1 . 1 - 1 0 . 1 M o S 3 + 4 H 2 0 ~ - M o O 4 2 + 3 H S - + 5 H ÷

f~ = - 2 . 9 - 6 8 . 1 - 5 8 . 3 FeS + H20~.-~-Fe(OH) ÷ + H S -

f~ = 3.3 - 14.97 - 13.79 Sb2 $3 + 6H2 O~-2Sb(OH)30 + 3 H S + 3H +

- 6.5 < f~ < - 2.9 (very undersaturated) - 60.2 - 51.1

- 5 3 . 1 1 - 4 3 . 4 2 - 5 8 . 2 5 - 4 7 . 4 4

- 7 . 1 3

- 5.39

[1] [1]

[11 [21 [1] [1]

[1]

[3], [4]

[5] [63

[6]

[7]

[8]

[9]

[lOl, [111

The saturation index, fl, with respect to minerals is calculated for Thu6s-les-Bains springs.

*References: [1] = N a u m o v et al. (1971); [2] = Karapet'yants and K a r a p e t ' y a n t s (1970); [3] = Gubel~ and Ste Marie (1987); [4] = Bourcier and Barnes (1987); [5] = Tremaine et al. (1981); [6] = Parks and Pohl (1988); [7] = Kiseleva et al. (1980); [81 = Barner and Schuerman (1978); [9] = He lgeson (1969); [10] = W a g m a n et al. (1965); [11] = Spycher and R e a d (1989).

~ ] ~ (; A L A I : X * N t ( ; R I i I I AL

complexes that are mostly uncharged or monovalent. Chalcophile elements are essentially present as sulfide complexes, for sulfide concentrations around 10 4 mol L '. Tri- and tetravalent elements are generally in the hydroxo- complex form. For elements with a higher oxidation number, oxo-hydroxo or oxo complexes (MoO~-, WO~-) are dominant.

For each element, we studied hypothetical solution-mineral equilibria by calculating the degree of saturation with respect to minerals. For every element, the dominant complexes are taken into consideration (Table 2). Some results must be considered with caution, because all appropriate thermodynamic data are not always available and we do not always possess enthalpy of formation data for these different compounds. Under these conditons, the involved calculation for each type of spring water at this range of emergence temperature (20-80°C) cannot be very precise. Thus, we have only illustrated this calculation in the case of Thu~s-les-Bains (Table 2), where the saturation indexes (f~) are calculated with respect to the concerned minerals. In Fig. 2, metal concentrations are plotted vs. the calculated temperature of the reservoir. In this figure, the calculated solubility curves are only indicative of the domain of existence of the different mineral phases taken into account. In this first approach, we can assume that Co, Ag and Zn are probably limited by sulfide minerals.

In the case of Mo, we found (Fig. 3) a correlation between Mo and H S - , but the calculated slope does not indicate (slope of 0.5) a possible equilibrium according to the following reactions:

MoS3 + 4 H 2 0 ~ M o O 4 2 + 5 H + + 3 H S -

MoS2 +4H20~-MoO42 - + 2 H S - + 6 H + +2e

0-6 r r

'i ~ 0 I E, I <)

1 ~,

0

lO -g ______

10 -6

C: L

0

]0 -5 ~0 -~ 10 -3 lO

HS- mo]/l

Fig. 3. Inverse correlation of Mo vs. HS, in alkaline waters~


Moreover, calculations of saturation with respect to MoS2 and MoS 3 show that the different spring waters are hardly undersaturated. According to previous studies (e.g., Vandelannoote, 1984) Mo might rather be incorporated into secondary minerals such as FeS or FeAsS than into pure phases.

A disadvantage of the present approach is that the sampled waters are not completely representative of the waters of the deep reservoir. The lack of variation for many elements in the different springs emerging from the same reservoir suggests that the representativity is satisfactory for the first group of elements. However, some differences appear for sulfides and are related to the fast oxidation of this compound by air. For the waters from some areas, especially where only one spring is available, the measured value of sulfide can be significantly lower than the value in the deep water. This limitation can be overcome if bore holes are available.

The controlling mechanisms are not even well demonstrated, the control at very low concentration levels of these elements in solution is pointed out.

Sb and Fe do not show a well-defined behaviour. In solution, they can be limited to very low levels by sulfide minerals but they can also be mobilized by particles. These elements are often associated with Th, which is found in the particulate fraction. This problem will be discussed later in detail (p.320). Most waters are very undersaturated with respect to Sb2S 3 except in a few cases (e.g., at Vinadio in the Italian Alps) which are close to saturation. Fe shows two different behaviours depending on how it is measured: in solution (AAS) or as total Fe, by NAA. Total Fe, contrary to soluble Fe is well correlated with Th and can be associated to a colloidal fraction. Many authors (e.g., Ouzounian, 1978; Vandelannoote, 1984) have shown that soluble Fe can be controlled by sulfide minerals such as pyrrhotite, but according to them one cannot explain why the solutions are systematically oversaturated with FeS or FeS2. Boul6gue (1978) has reported an equilibrium at depth with FeS and a small oversaturation in emergence conditions.

In a pH range of 8-9 without complexing ligands, U 4÷ is mainly in the hydroxo-complex form but does not behave like other tetravalent elements (see p.320). PHREEQE (Parkhurst et al., 1980) calculated uranium speciation, with a new set of thermodynamic data (Grenthe et al., 1992). As observed from measured Eh-values, the uranyl carbonate complexes are always minor in these groundwaters and we can assume that U is principally in the tetravalent state. For example, the calculation in the case of Cauterets, Central Pyrenees, gives the following distribution according to the measured redox potential:

U(OH)4(or U(OH)5-) = 7.8.10 -~j

U O 2 ( C O 3 ) 2 2 - -=-- 1 . 1 . 1 0 - l l

U O 2 ( C O 3 ) 3 4 - = 1.10 -11

320 ~ i %1 &t X-NI (}RI - / I{1 ' \[

- -8

- 7 1

' - 1 2

! I - - ]

i i :i_ i

. . . . %

}

I

8 . 0 8.5 9.0

[:~eeaieuiaLed pFt M deep temperature

Fig. 4. Evolution of U concentrations vs. recalculated pH at depth, The straight lines corres- pond to the two possible equilibria with respect to uraninite according to different sets of data.

This type of calculation must be considered with caution, because the redox state in natural solutions is always difficult to evaluate. In the majority of cases, all the redox systems that can be identified in the solution are not always at equilibrium.

In Fig. 4 we show a set of groundwaters which are equilibrated between 80 ° and 110°C; curve 1 corresponds to the field of solubility o f UO2 in this equilibration temperature range according to the following equilibrium:

UO: + 3H20~-~U(OH)5 - + H +

Nevertheless, the recent OECD compilation of thermodynamic data (Grenthe et al., 1992) for U species is rather favourable to the existence o fU(OH) ° (Fig. 4, 2). From Fig. 4, it seems that at such low levels, and in view of the accuracy of the measurements, the U - p H correlation (1) agrees better with the observed trends. Regardless of the complex involved, most waters are thus generally close to equilibrium with uraninite. The points below the solubility curve 1 can be explained by a possible re-equilibration at lower temperature, during the upward movement of the groundwater.

Even the mineral-solution equilibria are difficult to estimate; we have shown that in all cases these elements are controlled in solution at very low levels of concentration, with a weak dispersion.

3.3. Tri- and tetravalent elements

This second series groups together all the heavy metals such as lanthanides, Hf, Zr, Th and Sc. These elements show a great variability of concentration. Various aliquots of water samples were filtered at different pore sizes: 450 nm, 10nm or ultrafiltered, then analysed by NAA. It showed that all these

FRACE-METAL BEHAVIOUR IN NATURAL GRANITIC WATERS

T A B L E 3

Compar i son of d is t r ibut ion of elements in ultrafiitered and 450-nm filtered waters

321

(mol k g - i ) (mol kg - l )

<450 nm < 10nm <450 nm < 10nm

Ba 8.0.10 8 1.0.10-9 part . Nd 7.1.10 9 < 6 . 1 . 1 0 io part . Ca 4 .0 .10 5 4 .0 .10 -5 diss. Rb 3 .0" 10 -7 3.0" l 0 -7 diss. Ce 5.5"10 -9 < 8.5. l0 -11 part . Sc 1.0" 10 -9 5 .5 .10 12 part . Co 5.0.10 10 5.0. i0-10 diss. Sm 3.7.10 l0 5 .6 .10 12 part .

Cs 1.0.10 -7 1.0.10 7 diss. Sr 2 . 8 " l 0 -7 2.8" l0 -7 diss. Eu 3.5"10 ii < l . 5 " 1 0 -12 part . Ta l . l . 1 0 l0 < 2 . 3 . 1 0 12 part .

H f 7 . 7 " 10 -9 <4.7" l 0 -12 part. T b 6.3" 1 0 - " <2 .2" 10 -12 part.

La 3.0" l 0 - 9 2.6" 10 TM part. Th 5.5.10 -~° <4 .3" 10 -~2 part . M o 4 .4 .10 8 4 .0 .10 -8 diss. Z r 6.5" 10 7 < 6.2" 10 -8 part . Na 2.5"10 -3 2.5"10 3 diss. Z n 2.1"10 -8 1.2" 10 -s diss.

part . = associated with particles (colloids); diss. = present as dissolved species.

elements are associated with a particulate fraction of < 450 nm (Table 3). The process of ultrafiltration is difficult to control, particularly in alkaline waters, since in high-pH solutions these elements can be adsorbed on the membrane surface. Thus, the retentate is not, as expected, enriched in these elements. In a second step, after ultrafiltration, the retentate obtained from 30 L of water sample was filtered through a 15-nm membrane filter and the particles examined with a scanning electron microscope (SEM). In addition, X-ray

T A B L E 4

Calculated me ta l / t ho r ium ratio in waters and in granit ic rocks

Element Meta l / t ho r ium ratio

in alkaline water in granit ic rock

La [Lal = 14.2.[Th] [La] = 2.42.[Th1 Ce ICe/ = 15"[Th] [Ce] = 4.75.[Th1 Nd [Nd] = 13.6"[Th] [Nd] = 2.4-[Th] Sm [Sm] = 1.5"[Th] [Sml = 0.5.[Th] Eu [Eul = 0.1.[Th] [Eul = 0.09.[Th] Tb [Tb] = 0.1.[Th] [Tb] = 0.07.[Th] Lu [Lul = 0.15. [Th] [Lu] = 0.3. [Th] n f [ n q = 1.7.[Th] [Hf] = 0.4.[Th] Sc [Sc] = 2.4.[Th] [Sc] = 1.3.[Th] Ta [Ta] = 0.15.[Th] [Ta] = 0.2.[Th]

322 ( ; A L A U X - N I : ( ; R I : I . F I ,a,J

E

1 0 e . . . . . . . . . . . r . . . . . , ~ . . . . . . . ; , :

i

J

10 -9 t'z /

v v _ 10-11 i

10 -12 t ~ I I 1 I a)

Th mol/]

F ~ 10 I1

0 !0

1~ -12

10 12

0

0

n

i

C C . /

o / ' © Q ' ( ,

/ ' O b}

! 0 - I ) IO -IC i" ,, q

Th tool/1

Fig. 5. Log-tog diagrams: evolution of Sc and Ta vs. Th (circles = 450-nm filtered waters; triangles = 10-nm filtered waters; black triangles = bore holes; squares = springs without catchment).

diffraction (XRD) analyses were used to allow chemical identification of particles down to 20 nm in size. Particles or aggregates were detected as silica by XRD. Trace elements such as Zr, Hf, etc., which are below the limit of detection ( ~ 10 -1° mol L -~) were not identified.

In a log-log diagram, (Fig. 5) metal concentration is plotted vs. Th concentration on a straight line which has a well-defined slope of 1. This can be compared with previous observations in waters (Kim et al.. 1987), where the metal concentrations are associated with organic colloids, with the same metal/metal ratios as in those of our study. That means that the metal concentration increases with the colloid concentration. The concentration ratios of two elements in this group are very close to the same ratios in the bulk of the rock [from Fourcade's (1981) data; see Table 4]. It means that there is no fractionation between the rock and the colloidal phase. This tack of fractionation makes it impossible to decipher the origin of the particulate phase and we cannot exclude that these particles originate in the vicinity o f the spring.

This is of great importance for forecasting the migration of elements around a waste disposal if radionuclides behave as their natural analogues (Th and rare-earth elements). For each element we can define a quite accurate metal/Th ratio (Table 4). For further studies it may be sufficient to follow the behaviour o f one element of this group.

4. C O N C L U S I O N S

This study shows that even when trace elements are present at very low, level concentrations, they have relatively w e i l = ~ behaviours, One can


distinguish: (1) the controlled trace elements such as Co, Zn, Ag, U, Mo, etc., which are

probably governed in solution by minerals (sulfide minerals, uraninite); (2) other elements (Th, Sc, Ta, Hf, lanthanides, etc.) which are associated

with particulate fractions. The elements in this group show a very homoge- neous behaviour. For each of them, one can define a metal/Th ratio.

Such considerations are very important in estimating the impact of a deep radioactive waste repository on the mobilization mechanisms of these elements and the levels of concentration in the groundwaters.

ACKNOWLEDGEMENTS

Neutron activation analyses have been performed with help of G. Pinte and N. LeFol (Laboratoire Pierre Siie, Centre d'l~tudes de Saclay). P. Mauchien, P. Decambox and C. Moulin gave us access to low-level uranium measure- ment TRLS devices. All spa companies are thanked for their authorization for water sampling.

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