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Sulfur and Oxygen Isotopes as Tracers of the Origin of Sulfate in Lake Créteil (Southeast Of Paris, France)
141
SULFUR AND OXYGEN ISOTOPES AS TRACERS OF THE ORIGIN OF SULFATE IN LAKE CRETEIL (SOUTHEAST OF PARIS, FRANCE) A. CHESTERIKOFF, P. LECOLLE, R. LETOLLE and J.P. CARBONNEL
Dkpartement d e Giologie Dynamique, Universitk Pierre e t Marie Curie, 75230 Paris Ckdex 05 (France) (Accepted for publication June 23,1981)
ABSTRACT Chesterikoff, A., Lecolle, P., Letolle, R. and Carbonnel, J.P., 1981. Sulfur and oxygen isotopes as tracers of the origin of sulfate in Lake Cr6teil (southeast of Paris, France). In: W. Back and R. L6tolle (Guest-Editors), Symposium on Geochemistry of Groundwater - 26th International Geological Congress. J. Hydrol., 54: 141-150. Lake Cr6teil is located “10 km SE of Paris and the preservation of its water quality has prompted a thorough interdisciplinary study, for the lake itself and for the nearby feeding aquifer. Although previous hydrogeological and hydrochemical studies have given indications of the mechanisms involved, uncertainties remained as for the origin, path and history of the waters. This isotopic study of dissolved sulfate (34S, 0)has been carried out in order to find new information about the system.
LOCATION AND GEOLOGICAL SETTING OF THE LAKE
Lake Creteil is located SE of the confluence of the rivers Marne and Seine (Fig. l), in the small alluvial plain limited by these two rivers and the Tertiary outcrops of the Mt. Mesly hill (NE) and the Bois de la Grange Plateau (SE). The geological cross-sections (Fig. 2) show several superposed aquifers: the perched aquifer of the karstic Brie Limestone, then the semiconfined aquifer of the Champigny Limestone which is hydraulically linked with the alluvial aquifer of the small River Yerres in the south and less sharply with alluvium of the Cr6teil plain [these are presently investigated (Fig. 2)]. The “Calcaire Grossier” (Lutetian) aquifer is found -30 m below the latter plain. The lake and other ponds in this plain are located in a former sand pit. Its area is -0.4 km2, the mean depth 4 my and the total volume 1.6 Mm3. At site 2 a small trough exists (-1000 m2 ), which is 6 m deep (Fig. 3). For several years development programs have intended for it to be arecreation area. However, the alluvial aquifer which feeds the lake is completely devoid of oxygen. Natural alluvium, dug out in the past, has been replaced by various infillings, made of clinker, spoiled earth and refuses, which produce a strongly reducing medium.
142
1
Dissolution
( M' Merly
OF
Montmartre's
, plaster ).
Mixing op groups
&
1
and
2
gypsum
.
Sulfate OF Seine , Morne ond /or Chompigny limestone aquifers
Fig. 1. Principal features o f the Creteil area.
Waters are reoxygenated in the lake, while at the bottom and especially during summer the oxygen deficiency still exists: the reducing character of bottom waters is not due to an excess of organic matter ( 5 d a y biochemical oxygen demand, BOD,, and 5-day chemical oxygen demand, COD,, are low, -3 mgl-' and 30 mgl-l , respectively), nor t o an eutrophic state of the lake, which is in fact mesotrophic (Chesterikoff and Testard, 1981). HYDROGEOLOGICAL DATA
From piezometric studies (Chesterikoff, 1980) it is known that the major
143
W
m y1
N
SENART
FOREST
1
kF G=
c
Loom.
36
Stampion marl . Sonnoision limestone
Ludian
Alluvlum Lower Ludban Monlrnortr? gypsum (in Mt Mesly ).
.
_ _ -
m
Lower
marl.
Ludion
Champigny limestone
Upper Lutetian marl
iyc Aquifer Lutetian PL
limestone “Calcaire grassier*'
81
Fig. 2 . Geologic cross-sections along lines indicated in Fig. 1.
water input comes from the superficial aquifer in the alluvium and recent infillings, from east to west around the south of Mt. Mesly (Fig. 1).It has been shown that there is no contribution from the nearby confined Lutetian “Calcaire Grossier” aquifer. The water itself in the superficial aquifer comes from several sources: (a) The River Marne, of which the water level is rigorously regulated for navigation to the 31.4m MSL height mark. The level of Lake Creteil oscillates from -I-30 to 30.5 m within the season. (b) The semiconfined Champigny Limestone aquifer in the south is a possible contribution, although the input is minimal during dry periods, as most of the output discharges in the River Yerres.
+
144
w
E
ARTIFICIAL
201 I - '
AQUIFER
NNW
PROGRESSION
2
1
SSE
Fig. 3 . Cross-sections of Lake Creteil.
(c) Rains in the Creteil and La Fosse aux Moines alluvial plains southeast of the lake (6 km2 ), which are mostly responsible for the water-table fluctuations. A part of rain water running down the slopes of.the surrounding hills may also contribute to the recharge of the superficial aquifer. Waters from the superficial aquifer and from the lakes in the Creteil plain flow to the west and NW, towards the River Seine, of which the mean watermark level is -2 m lower than the level of the River Marne.
CHEMICAL DATA
Table I presents the mean chemical composition of the lake for the major ions. In this paper only the sulfate ion will be considered. The Mt. Mesly sequence contains the southernmost occurrence of the Bartonian gypsum beds, formerly excavated for plaster manufacturing. Dissolution of gypsum considerably enhances the sulfate concentration of waters in the northern part of the alluvial Creteil plain. Mean samples were taken from piezometers which are shown in Fig. 1 (A-G, L and 201). It was possible to sample separately surface and bottom water from piezometer 201 (201 and 201 ) as well as from piezometer L (surface Ls , bottom L, and medium height L , ). TABLE I Mean chemical composition of Lake Creteil waters
so:c1-
HCO;
590 230 120
2.0
20.8
Ca2+ Mg2+ Na+ K+
250 31 116 38
12.5
52:;
1 .o
1 J
21.0
145 TABLE I1 Characteristics of the waters of this study
Marne Darse Marne
20 40
6 8
9.4 11.6
Seine
18
5
10
Lake Crkteil
1 2
M B
590 620
26.6 32.5
15.7 16.7
Piezometers
201
S B M S B M M M M M S M B
1,760 1,780 1,400 165 785 295 500 1,120 265 460 415 67 0 710 -
27.2 28.0 23.4 15.6 15.3 13.8 12.5 17.0 8.6 12.8 17.6 25.5 26.4
15.2 16.2 15.6 14.4 12.8 11.1 11.3 13.8 11.2 10.7 16.3 17.8 17.0
16.2
14.9
A B
Mt. Mesly gypsum
B = bottom; M = medium height of the water column; S = surface.
TABLE111 Sulfate content of piezometers 203, 206,209 and 211 Piezometers
203
206
209
211
SO:-
1,150
1,510
820
1,500
(mg 1-'
Water from Lake Crkteil sampled at site 1 where the water depth is of the uniform type (4m), and at the 6 m deep site 2. An aliquot of samples was precipitated in the form of BaS04 for isotope analyses. Sulfate concentrations are shown in Table 11. Chemical data do not show seasonal variations except for dilution by rain. Piezometer data may be divided into three groups: (A), SO4 concentration >1400 ppm; (B), from 600 to 1120ppm; and (C), <500 ppm. The low concentrations for piezometers B, and L, relative to B , and LB respectively, are due to dilution by rain, as shown by the concentrations of other cations which are insensitive t o microbial action (C1; Na', etc.).
146
In Table I11 sulfate concentrations are shown for some of the piezometers which were destroyed before sampling for isotope studies could be performed. ISOTOPE DATA
The BaS04 precipitate has been processed following the classical methods in stable-isotope geochemistry (Mizutani, 1971; Sakai and Krouse, 1971; Filly et al., 1975) which will not be discussed in detail here. Data are given in per mil deviation from an international standard, which is the standard mean ocean water (SMOW) for '*O, and Canon Diablo Troilite (CDT) for 34S: (isotope ratio for sample) (isotope ratio for standard)
-
j
3. x 1000
-
Accuracy and reproducibility are ? 0.2yoOfor both' "0 and 34S. Variation in the heavy-isotope content of sulfate ion varies with the origin of sulfate: (a) Marine sulfate shows large variations through the geological epochs and even in the same basin it may vary in its 634Scontent (Claypool et al., 1980) and t o a lesser extent in l80(Holser et al., 1979). The present authors have data for the Bartonian gypsum beds (NW of Paris) (Fontes and Letolle, 1976) of the same age as the Mt. Mesly gypsum, for which they had the possibility to study one sample from a trench. This sample is shown in Fig. 3 together with the position of isotope sulfate values for all gypsum samples from the area (data taken from Fontes and Letolle, 1976). (b) Sulfate from fertilizers: this comes from the sulfuric acid used in the chemical treatment of phosphorites, and its isotope content is lower than for marine sulfate. The authors possess some data from a previous study of a nearby aquifer in the Brie and Beauce areas, south of Paris (Berger et al., 1977). (c) Sulfate from sulfide reoxidation: sulfides (or elemental sulfur), restored under oxic conditions, are oxidized and the sulfate formed usually shows a low isotope content. In the studied area, such a possibility occurs in the top soil, since waters become quickly anoxic with depth. The dissolved S2- content in the aquifer is highly variable and may be as high as 34 mg 1-' (piezometer 201), but the highest value observed in the lake is only 300 pgl-' , even at the deepest site (2). This implies that local reoxidation of hydrogen sulfide is not significant. (d) Sulfate from rains: this source cannot be discounted in this area since Megnien (1976) has shown that local rains contain from 7 to 1 6 mg 1-' SO:-, with an isotopic composition between -3 and 7%0. This source may contribute from 1%to 10% of the sulfate content of the aquifer, and may represent a potential component, a t least for the less mineralized waters.
+
147 I
20
0
10
6“s
I
I
20
30
I
%o/CDT
Fig. 4.6180-634S diagram for SO$- samples.
(e) Finally, water from the nearby rivers, of which the 634S fluctuates from 3 to t 8yoO(Mkgnien, 1976; unpublished data of the present authors, 1977), does not intervene locally, which is demonstrated by piezometry. I t is known that the lighter-isotope sulfate is more easily reduced by anaerobic bacteria than the sulfate of the heavier isotope. This effect has been studied by various authors (see, e.g., Thode et al., 1961, for 34S; Mizutani and Rafter, 1969,1973, for l80). I t has been shown that, on the whole, the sulfate remaining through reduction becomes enriched in l80as well as in 34S. The change may be described, in a 6180-634S diagram (Fig. 4), as going from left t o right and from bottom to top in Fig. 3; the slope being taken as first approximation between 2 and 4 (see Zak et al., 1980, for comments). Finally, let us mention that sulfate of a given isotopic composition may result from the mixing of initial batches with different isotopic composition. In a 6180-634S diagram, this appears as straight “mixing lines”. A t first glance, the isotopic data, as shown in Fig. 4,would favor a straightforward interpretation based on the reduction process from two sources: the first should be the gypsum bed from Mt. Mesly (or plaster imbedded in recent infillings), the second from an unknown source of sulfate with low l80 and 34S contents, which could be fertilizer sulfate or sulfate coming from the superficial oxidation of sulfur, always present in clinker. However, this interpretation does not fit with the distribution of sulfate concentrations (Table I) and a more elaborate model must be found. Fig. 4 shows that samples may be divided into four groups: (a) Group 1 comprises the lake samples 1 and 2 and piezometer samples A and 201, for which it is clear that sulfates come from Mt. Mesly.
+
148
(b) Group 1’ is for piezometer L alone. Sulfate comes from gypsum, but not necessary from the above sedimentary source, as this piezometer is located on a small mound built from refuses containing plaster indisputably manufactured years ago, from Parisian gypsum (“platre de Paris”) (roasting gypsum to make plaster does not modify the isotopic composition of the SO:- ion) (Fig. 3). (c) Group 2: piezometers C, D ,F and G show 6 l 8 0 - and 634S-valueswhich cannot be ascribed to a regional sedimentary origin, whatever the mechanism is involved t o modify the isotope content. Intrusion by Marne river water for piezometers C, F , D, and by the Seine river water for piezometer G, cannot be excluded on the basis of isotope data. Here the hypothesis of the intrusion of fertilizer sulfate, from the south may be introduced, as well as sulfate from sulfide reoxidation. Data from Berger et al. (1977) show 634Sof fertilizers with a mean value of +8%0. Waters from the Beauce aquifer (SW of Paris) exhibit a mean SO:- concentration of 40 ppm (+40),with a large scatWhatever the exact sources of this ter of 634S-values from -10 t o +12x0. sulfate (fertilizer, rain, or pyrite reoxidation) may be, these values could be considered as representing approximatively the second end-member for the mixing process postulated for the waters sampled with the southeastern piezometers. (d) Group 3 ( B andE) could correspond to a mixing of waters from groups I and 2. Although the isotopic composition would indicate the same origin as for group 1 (dissolution of sedimentary gypsum, but without further
30
I-
n
20
0
u)
$
W
10
PARIS
0
500
GYPSUM
1000 b4=1
1500
2000
m9/e
Fig. 5. Interpretation of data through the 6?3---[
SO:-] representation.
149
evolution), the direction of the flow of the aquifer, as indicated in Fig. 1 from piezometric measurements, prevents us from such an interpretation. In defining group boundaries, both isotopic composition and concentration of sulfate were considered, so that they correlate to establish genetic relationships: group A group I group B groups 1' group C + group 2 --f
--f
+3
Moreover, the reduction phenomenon is clearly shown, since there is no possible local source with 634Shigher than 20yoO(see Fig. 4). In Fig. 5 the model built upon the above observations is summarized. l8O measurements fit exactly the same model.
+
CONCLUSION
Two end-members at least may be defined:
Source I : Interstitial water or a confined low-conductivity aquifer in which solutions develop to high sulfate concentrations. The sulfate which may have originated in Paris (Montmartre) gypsum has a heavier isotopic composition, being probably the sulfate remaining partly through reduction. These are found t o the west of Mt. Mesly, down the piezometric level. Source II: Waters of low salinity and isotopic light sulfate are found down the Bois de la Grange Plateau (sample F , group 3 ) . This light sulfate should originate from oxidation of sulfides or fertilizers. The piezometry forbids the possibility of sulfate originating from lateral infiltration from the two nearby rivers. The data of the isotope study have permitted the improvement of the hydrogeological model of the Creteil plain and ascribe more precisely the origin of water and of its mineral content. Moreover, it has been shown that a study based on isotope data alone can lead t o improper conclusions without the information concerning other parameters. ACKNOWLEDGEMENTS
Thanks are due to G. Shearer and I. Zak for their most helpful comments and criticisms, and to A. Dindeleux and M. Grably for technical assistance. This work was performed through a grant of C.N.R.S. (ERA 604).
150
REFERENCES Berger, G., Bosch, B., Desprez, N., Letolle, R., Marce, A., Mariotti, A. and Megnien, C., 1977. La mineralisation des eaux souterraines de la Beauce et le renouvellement de la nappe: application des marquages isotopiques (3H, "N, 34S). In: Colloq. Protection des eaux souterraines captees. Orleans. Bur. Rech. Geol. Min., Spec. Publ., 2: 21-34. Chesterikoff, A., 1980. Les eaux du lac de Creteil; leur origine; leurs caracteristiques. Actes 256me Congr. Assoc. Fr. Limnol., pp. 134-146. Chesterikoff, A. and Testard, P., 1981. Le lac de Creteil. Etude d'un plan d'eau artificiel en milieu en voie d'urbanisation. (In prep.) Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H. and Zak, I., 1980. The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol., 28: 199-260. Filly, A., Letolle, R. and Pusset, M., 1975. L'analyse isotopique du soufre; aspects techniques. Analusis, 3( 4) : 197-200. Fontes, J.C. and Letolle, R., 1976. l80and %S in the Upper Bartonian gypsum deposits of the Paris Basin. Chem. Geol., 18: 285-296. Holser, W.T., Kaplan, I.R., Sakai, H. and Zak, I., 1979. Isotope geochemistry of oxygen in the sedimentary sulfate cycle. Chem. Geol., 25: 1-17. Megnien, C., 1976. Hydrogdologie du Bassin de Paris. Mem. Bur. Rech. GQol. Min., 98, 532 pp. Mizutani, Y., 1971. Improvement in the carbon reduction method for the oxygen isotopic analysis of sulphates. Geochem. J., 5 : 69-77. Mizutani, Y. and Rafter, T.A., 1969. Bacterial fractionation of oxygen isotopes in the reduction of sulphate and in the oxidation of sulphur. N.Z. J. Sci., 1 2 : 60-68. Mizutani, Y. and Rafter, T.A., 1973. Isotopic behaviour of sulphate oxygen in the bacterial reduction of sulphate. Geochem. J., 6 : 183-191. Sakai, H. and Krouse, H.R., 1971. Elimination of memory effects in 180/'60 determination in sulfates. Earth Planet. Sci. Lett., 11: 369-373. Thode, H.G., Monster, J. and Dunford, H.B., 1961. Sulphur isotope geochemistry. Geochim. Cosmochim. Acta, 25: 159-174. Zak, I., Sakai, H. and Kaplan, I.R., 1980. Factors controlling the l 8 0 / l 6 O and % S / j 2 S isotope ratios of ocean sulfates, evaporites and interstitial sulfates from modern deep sea sediments. In : Isotope Marine Chemistry, Uchida Rokakuho, Tokyo, pp. 339-373.
SULFUR AND OXYGEN ISOTOPES AS TRACERS OF THE ORIGIN OF SULFATE IN LAKE CRETEIL (SOUTHEAST OF PARIS, FRANCE) A. CHESTERIKOFF, P. LECOLLE, R. LETOLLE and J.P. CARBONNEL
Dkpartement d e Giologie Dynamique, Universitk Pierre e t Marie Curie, 75230 Paris Ckdex 05 (France) (Accepted for publication June 23,1981)
ABSTRACT Chesterikoff, A., Lecolle, P., Letolle, R. and Carbonnel, J.P., 1981. Sulfur and oxygen isotopes as tracers of the origin of sulfate in Lake Cr6teil (southeast of Paris, France). In: W. Back and R. L6tolle (Guest-Editors), Symposium on Geochemistry of Groundwater - 26th International Geological Congress. J. Hydrol., 54: 141-150. Lake Cr6teil is located “10 km SE of Paris and the preservation of its water quality has prompted a thorough interdisciplinary study, for the lake itself and for the nearby feeding aquifer. Although previous hydrogeological and hydrochemical studies have given indications of the mechanisms involved, uncertainties remained as for the origin, path and history of the waters. This isotopic study of dissolved sulfate (34S, 0)has been carried out in order to find new information about the system.
LOCATION AND GEOLOGICAL SETTING OF THE LAKE
Lake Creteil is located SE of the confluence of the rivers Marne and Seine (Fig. l), in the small alluvial plain limited by these two rivers and the Tertiary outcrops of the Mt. Mesly hill (NE) and the Bois de la Grange Plateau (SE). The geological cross-sections (Fig. 2) show several superposed aquifers: the perched aquifer of the karstic Brie Limestone, then the semiconfined aquifer of the Champigny Limestone which is hydraulically linked with the alluvial aquifer of the small River Yerres in the south and less sharply with alluvium of the Cr6teil plain [these are presently investigated (Fig. 2)]. The “Calcaire Grossier” (Lutetian) aquifer is found -30 m below the latter plain. The lake and other ponds in this plain are located in a former sand pit. Its area is -0.4 km2, the mean depth 4 my and the total volume 1.6 Mm3. At site 2 a small trough exists (-1000 m2 ), which is 6 m deep (Fig. 3). For several years development programs have intended for it to be arecreation area. However, the alluvial aquifer which feeds the lake is completely devoid of oxygen. Natural alluvium, dug out in the past, has been replaced by various infillings, made of clinker, spoiled earth and refuses, which produce a strongly reducing medium.
142
1
Dissolution
( M' Merly
OF
Montmartre's
, plaster ).
Mixing op groups
&
1
and
2
gypsum
.
Sulfate OF Seine , Morne ond /or Chompigny limestone aquifers
Fig. 1. Principal features o f the Creteil area.
Waters are reoxygenated in the lake, while at the bottom and especially during summer the oxygen deficiency still exists: the reducing character of bottom waters is not due to an excess of organic matter ( 5 d a y biochemical oxygen demand, BOD,, and 5-day chemical oxygen demand, COD,, are low, -3 mgl-' and 30 mgl-l , respectively), nor t o an eutrophic state of the lake, which is in fact mesotrophic (Chesterikoff and Testard, 1981). HYDROGEOLOGICAL DATA
From piezometric studies (Chesterikoff, 1980) it is known that the major
143
W
m y1
N
SENART
FOREST
1
kF G=
c
Loom.
36
Stampion marl . Sonnoision limestone
Ludian
Alluvlum Lower Ludban Monlrnortr? gypsum (in Mt Mesly ).
.
_ _ -
m
Lower
marl.
Ludion
Champigny limestone
Upper Lutetian marl
iyc Aquifer Lutetian PL
limestone “Calcaire grassier*'
81
Fig. 2 . Geologic cross-sections along lines indicated in Fig. 1.
water input comes from the superficial aquifer in the alluvium and recent infillings, from east to west around the south of Mt. Mesly (Fig. 1).It has been shown that there is no contribution from the nearby confined Lutetian “Calcaire Grossier” aquifer. The water itself in the superficial aquifer comes from several sources: (a) The River Marne, of which the water level is rigorously regulated for navigation to the 31.4m MSL height mark. The level of Lake Creteil oscillates from -I-30 to 30.5 m within the season. (b) The semiconfined Champigny Limestone aquifer in the south is a possible contribution, although the input is minimal during dry periods, as most of the output discharges in the River Yerres.
+
144
w
E
ARTIFICIAL
201 I - '
AQUIFER
NNW
PROGRESSION
2
1
SSE
Fig. 3 . Cross-sections of Lake Creteil.
(c) Rains in the Creteil and La Fosse aux Moines alluvial plains southeast of the lake (6 km2 ), which are mostly responsible for the water-table fluctuations. A part of rain water running down the slopes of.the surrounding hills may also contribute to the recharge of the superficial aquifer. Waters from the superficial aquifer and from the lakes in the Creteil plain flow to the west and NW, towards the River Seine, of which the mean watermark level is -2 m lower than the level of the River Marne.
CHEMICAL DATA
Table I presents the mean chemical composition of the lake for the major ions. In this paper only the sulfate ion will be considered. The Mt. Mesly sequence contains the southernmost occurrence of the Bartonian gypsum beds, formerly excavated for plaster manufacturing. Dissolution of gypsum considerably enhances the sulfate concentration of waters in the northern part of the alluvial Creteil plain. Mean samples were taken from piezometers which are shown in Fig. 1 (A-G, L and 201). It was possible to sample separately surface and bottom water from piezometer 201 (201 and 201 ) as well as from piezometer L (surface Ls , bottom L, and medium height L , ). TABLE I Mean chemical composition of Lake Creteil waters
so:c1-
HCO;
590 230 120
2.0
20.8
Ca2+ Mg2+ Na+ K+
250 31 116 38
12.5
52:;
1 .o
1 J
21.0
145 TABLE I1 Characteristics of the waters of this study
Marne Darse Marne
20 40
6 8
9.4 11.6
Seine
18
5
10
Lake Crkteil
1 2
M B
590 620
26.6 32.5
15.7 16.7
Piezometers
201
S B M S B M M M M M S M B
1,760 1,780 1,400 165 785 295 500 1,120 265 460 415 67 0 710 -
27.2 28.0 23.4 15.6 15.3 13.8 12.5 17.0 8.6 12.8 17.6 25.5 26.4
15.2 16.2 15.6 14.4 12.8 11.1 11.3 13.8 11.2 10.7 16.3 17.8 17.0
16.2
14.9
A B
Mt. Mesly gypsum
B = bottom; M = medium height of the water column; S = surface.
TABLE111 Sulfate content of piezometers 203, 206,209 and 211 Piezometers
203
206
209
211
SO:-
1,150
1,510
820
1,500
(mg 1-'
Water from Lake Crkteil sampled at site 1 where the water depth is of the uniform type (4m), and at the 6 m deep site 2. An aliquot of samples was precipitated in the form of BaS04 for isotope analyses. Sulfate concentrations are shown in Table 11. Chemical data do not show seasonal variations except for dilution by rain. Piezometer data may be divided into three groups: (A), SO4 concentration >1400 ppm; (B), from 600 to 1120ppm; and (C), <500 ppm. The low concentrations for piezometers B, and L, relative to B , and LB respectively, are due to dilution by rain, as shown by the concentrations of other cations which are insensitive t o microbial action (C1; Na', etc.).
146
In Table I11 sulfate concentrations are shown for some of the piezometers which were destroyed before sampling for isotope studies could be performed. ISOTOPE DATA
The BaS04 precipitate has been processed following the classical methods in stable-isotope geochemistry (Mizutani, 1971; Sakai and Krouse, 1971; Filly et al., 1975) which will not be discussed in detail here. Data are given in per mil deviation from an international standard, which is the standard mean ocean water (SMOW) for '*O, and Canon Diablo Troilite (CDT) for 34S: (isotope ratio for sample) (isotope ratio for standard)
-
j
3. x 1000
-
Accuracy and reproducibility are ? 0.2yoOfor both' "0 and 34S. Variation in the heavy-isotope content of sulfate ion varies with the origin of sulfate: (a) Marine sulfate shows large variations through the geological epochs and even in the same basin it may vary in its 634Scontent (Claypool et al., 1980) and t o a lesser extent in l80(Holser et al., 1979). The present authors have data for the Bartonian gypsum beds (NW of Paris) (Fontes and Letolle, 1976) of the same age as the Mt. Mesly gypsum, for which they had the possibility to study one sample from a trench. This sample is shown in Fig. 3 together with the position of isotope sulfate values for all gypsum samples from the area (data taken from Fontes and Letolle, 1976). (b) Sulfate from fertilizers: this comes from the sulfuric acid used in the chemical treatment of phosphorites, and its isotope content is lower than for marine sulfate. The authors possess some data from a previous study of a nearby aquifer in the Brie and Beauce areas, south of Paris (Berger et al., 1977). (c) Sulfate from sulfide reoxidation: sulfides (or elemental sulfur), restored under oxic conditions, are oxidized and the sulfate formed usually shows a low isotope content. In the studied area, such a possibility occurs in the top soil, since waters become quickly anoxic with depth. The dissolved S2- content in the aquifer is highly variable and may be as high as 34 mg 1-' (piezometer 201), but the highest value observed in the lake is only 300 pgl-' , even at the deepest site (2). This implies that local reoxidation of hydrogen sulfide is not significant. (d) Sulfate from rains: this source cannot be discounted in this area since Megnien (1976) has shown that local rains contain from 7 to 1 6 mg 1-' SO:-, with an isotopic composition between -3 and 7%0. This source may contribute from 1%to 10% of the sulfate content of the aquifer, and may represent a potential component, a t least for the less mineralized waters.
+
147 I
20
0
10
6“s
I
I
20
30
I
%o/CDT
Fig. 4.6180-634S diagram for SO$- samples.
(e) Finally, water from the nearby rivers, of which the 634S fluctuates from 3 to t 8yoO(Mkgnien, 1976; unpublished data of the present authors, 1977), does not intervene locally, which is demonstrated by piezometry. I t is known that the lighter-isotope sulfate is more easily reduced by anaerobic bacteria than the sulfate of the heavier isotope. This effect has been studied by various authors (see, e.g., Thode et al., 1961, for 34S; Mizutani and Rafter, 1969,1973, for l80). I t has been shown that, on the whole, the sulfate remaining through reduction becomes enriched in l80as well as in 34S. The change may be described, in a 6180-634S diagram (Fig. 4), as going from left t o right and from bottom to top in Fig. 3; the slope being taken as first approximation between 2 and 4 (see Zak et al., 1980, for comments). Finally, let us mention that sulfate of a given isotopic composition may result from the mixing of initial batches with different isotopic composition. In a 6180-634S diagram, this appears as straight “mixing lines”. A t first glance, the isotopic data, as shown in Fig. 4,would favor a straightforward interpretation based on the reduction process from two sources: the first should be the gypsum bed from Mt. Mesly (or plaster imbedded in recent infillings), the second from an unknown source of sulfate with low l80 and 34S contents, which could be fertilizer sulfate or sulfate coming from the superficial oxidation of sulfur, always present in clinker. However, this interpretation does not fit with the distribution of sulfate concentrations (Table I) and a more elaborate model must be found. Fig. 4 shows that samples may be divided into four groups: (a) Group 1 comprises the lake samples 1 and 2 and piezometer samples A and 201, for which it is clear that sulfates come from Mt. Mesly.
+
148
(b) Group 1’ is for piezometer L alone. Sulfate comes from gypsum, but not necessary from the above sedimentary source, as this piezometer is located on a small mound built from refuses containing plaster indisputably manufactured years ago, from Parisian gypsum (“platre de Paris”) (roasting gypsum to make plaster does not modify the isotopic composition of the SO:- ion) (Fig. 3). (c) Group 2: piezometers C, D ,F and G show 6 l 8 0 - and 634S-valueswhich cannot be ascribed to a regional sedimentary origin, whatever the mechanism is involved t o modify the isotope content. Intrusion by Marne river water for piezometers C, F , D, and by the Seine river water for piezometer G, cannot be excluded on the basis of isotope data. Here the hypothesis of the intrusion of fertilizer sulfate, from the south may be introduced, as well as sulfate from sulfide reoxidation. Data from Berger et al. (1977) show 634Sof fertilizers with a mean value of +8%0. Waters from the Beauce aquifer (SW of Paris) exhibit a mean SO:- concentration of 40 ppm (+40),with a large scatWhatever the exact sources of this ter of 634S-values from -10 t o +12x0. sulfate (fertilizer, rain, or pyrite reoxidation) may be, these values could be considered as representing approximatively the second end-member for the mixing process postulated for the waters sampled with the southeastern piezometers. (d) Group 3 ( B andE) could correspond to a mixing of waters from groups I and 2. Although the isotopic composition would indicate the same origin as for group 1 (dissolution of sedimentary gypsum, but without further
30
I-
n
20
0
u)
$
W
10
PARIS
0
500
GYPSUM
1000 b4=1
1500
2000
m9/e
Fig. 5. Interpretation of data through the 6?3---[
SO:-] representation.
149
evolution), the direction of the flow of the aquifer, as indicated in Fig. 1 from piezometric measurements, prevents us from such an interpretation. In defining group boundaries, both isotopic composition and concentration of sulfate were considered, so that they correlate to establish genetic relationships: group A group I group B groups 1' group C + group 2 --f
--f
+3
Moreover, the reduction phenomenon is clearly shown, since there is no possible local source with 634Shigher than 20yoO(see Fig. 4). In Fig. 5 the model built upon the above observations is summarized. l8O measurements fit exactly the same model.
+
CONCLUSION
Two end-members at least may be defined:
Source I : Interstitial water or a confined low-conductivity aquifer in which solutions develop to high sulfate concentrations. The sulfate which may have originated in Paris (Montmartre) gypsum has a heavier isotopic composition, being probably the sulfate remaining partly through reduction. These are found t o the west of Mt. Mesly, down the piezometric level. Source II: Waters of low salinity and isotopic light sulfate are found down the Bois de la Grange Plateau (sample F , group 3 ) . This light sulfate should originate from oxidation of sulfides or fertilizers. The piezometry forbids the possibility of sulfate originating from lateral infiltration from the two nearby rivers. The data of the isotope study have permitted the improvement of the hydrogeological model of the Creteil plain and ascribe more precisely the origin of water and of its mineral content. Moreover, it has been shown that a study based on isotope data alone can lead t o improper conclusions without the information concerning other parameters. ACKNOWLEDGEMENTS
Thanks are due to G. Shearer and I. Zak for their most helpful comments and criticisms, and to A. Dindeleux and M. Grably for technical assistance. This work was performed through a grant of C.N.R.S. (ERA 604).
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