Modelling the baseline geochemistry of groundwater in a Chalk aquifer considering solid solutions for carbonate phases

Applied Geochemistry 25 (2010) 1564–1574

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Modelling the baseline geochemistry of groundwater in a Chalk aquifer considering solid solutions for carbonate phases M. Gillon ⇑, P. Crançon, J. Aupiais CEA DAM DIF, F-91297 Arpajon, France

a r t i c l e

i n f o

Article history: Received 25 January 2010 Accepted 8 August 2010 Available online 11 August 2010 Editorial handling by W.M. Edmunds

a b s t r a c t The Chalk aquifer of Champagne (France) baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution. The water–rock interactions are modelled by the speciation code CHESS from field data and Ca, Mg and Sr aqueous concentrations in groundwater. The stoichiometries of solid solutions are defined in each stratigraphic unit of the Chalk aquifer from bulk geochemistry and Chalk mineralogy of samples taken from boreholes. The initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The aquifer that has developed in the Upper Cretaceous Chalk of France, Belgium and England has a large extent, a high productivity and a high water storage capacity that makes this resource essential but highly vulnerable to pollution (Edmunds et al., 1987; Crampon et al., 1993b; Kloppmann et al., 1998; MacDonald et al., 1998). The European Community formulates, in the groundwater directive (EC, 2006), the importance of characterising all groundwater bodies in Europe in order to assess any pollution impacts, and to ensure the protection of water reservoirs. The determination of the natural baseline geochemistry at the local scale is thus fundamental for the environmental management, the monitoring of the eventual anthropogenic impact (Edmunds, 2009), the prediction of spatial and temporal variations of groundwater composition (Edmunds et al., 2003) and the solute transport (natural or anthropogenic). Water–rock interactions are major processes that should be taken into account since they control the solute transfer due to the dissolution of primary rock minerals and the precipitation of secondary phases. Since in a Chalk aquifer the rock matrix is dominated by carbonate minerals, a relative and fast homogeneity of the baseline geochemistry is expected (Edmunds et al., 1987; Langmuir, 1997; Moral et al., 2008). However, spatial heterogeneity in groundwater chemical composition can occur in relation to:

⇑ Corresponding author. Tel.: +33 1 69 26 40 00; fax: +33 1 69 26 70 65. E-mail address: [email protected] (M. Gillon). 0883-2927/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2010.08.006

 Water–rock interaction processes in the geological framework. Local variations of the rock mineralogy and its composition likely occur in calcareous or chalk deposits due to the presence of dolomitic bodies, clay-rich beds or phosphatic layers (Jarvis, 1980; Deconinck et al., 2005; Richard et al., 2005).  The residence time of groundwater in the aquifer. For instance, the incongruent dissolution of carbonate has been evidenced in several carbonate aquifers from the evolution of Mg/Ca, Sr/Ca ratios and isotopic contents of Sr–C along the flow line (Edmunds et al., 1987; Dennis et al., 1997; Kloppmann et al., 1998; Elliot et al., 1999; Edmunds and Smedley, 2000).  The mixing between waters of different origins: freshwater and residual connate water with ancient marine or diagenetic signatures (Edmunds et al., 1987; Dennis et al., 1997; Valdes et al., 2007).  The redox processes, for example when the aquifer is confined below an impermeable overlying clay (Edmunds et al., 1987; Kloppmann et al., 1996). The goal of this paper is to characterise the natural groundwater baseline by focusing on the major elements in carbonate environment (Ca, Mg, Sr) and their evolution. The approach is based on water–rock interaction modelling in the context of an unconfined Chalk aquifer developed in the Upper Cretaceous formations in the Champagne region (Marne, France) with particular attention to field data. This site had been previously investigated for the transport of natural radionuclides (Hubert et al., 2006) but refinement was necessary to fully elucidate the migration of U in the presence of dissolved carbonates. The present work is intended to provide new information for the study of radionuclide transfer in this aquifer by taking into account the precise role of the carbonate matrix.

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2. Materials and methods 2.1. Site description The study area is located on the Champagne mounts, 20 km east of Reims (Marne, France; Fig. 1) and covers approximately 160 km2 between the Suippe river (as the north boundary of the study area) and the Prosne river (as the south boundary). Its geological and

2482000

Study site Reims

N

C6

Saint Masmes

C5

Paris

FRANCE

C5 oy e

C6

Ep

2480000

hydrogeological description can be found elsewhere (Hubert et al., 2006; Renard and Jeannée, 2008; Bourdon et al., 2009). The Cretaceous geological formations are constituted by: Turonian (C3), Coniacian (C4), Santonian (C5) and Campanian (C6) Chalks (Fig. 1). The Chalk formation is locally overlain by colluvium, Quaternary formation resulting from the in situ alteration of Chalk and by clayey deposit (from the Tertiary) at the summit of the Mont Haut (Fig. 1). In addition, the first 10–20 m of Chalk

Pontfaverger-Moronvilliers Suipp e

C5

2478000

colluvium Spring of Moncet

C5 Saint Hilaire-le-Petit

C6

A21

C6

2474000

Y (m)

2476000

C5

C5

B3

C4 2472000

clay

Teton Le casque

Mont Haut

C6 2470000

flow line

A20

2468000

colluvium

2466000

Spring of Prosne (low water level)

736000

738000

740000

Prosnes

A30 A8

742000

744000

colluvium

springs

clay

boreholes (groundwater sampling)

C6 Chalk

boreholes (groundwater and rock sampling)

C5 Chalk C4 Chalk

city mounts

water level (m NGF), equidistance=10m

A25b

746000

A23

A10 to A13

A14 A7 A16

X (m)

Legends

100

A25 A26

C6

e Prosn

A19

Soil samples

Spring of Prosne (high water level)

A9 A1 to A4 A15

A29

A17 A28

A24

A27 flow line

groundwater divide line X, Y: Lambert II extended coordinates Fig. 1. Geological and hydrogeological synthetic map recorded at high water level (March 2005; Renard and Jeannée, 2008) of the study site, with the localities of boreholes and soil samples. C4, C5 and C6 Chalks correspond to Coniacian, Santonian and Campanian Chalks, respectively.

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formation below the topsoil are usually cryoturbated. The fracturing of the Chalk decreases with depth and depends on the proximity of surface drainage valleys or paleo-valleys (Bourdon et al., 2009). The upper unconfined aquifer has developed in the C5 and C6 Chalk formations. The total porosity of the Chalk is about 40% vol. (Crampon et al., 1993a), including about 1% vol. related to fractures (Vachier et al., 1987). About 15% of the groundwater recharge takes place through fractures (Price et al., 1993). The permeability of the Chalk matrix ranges between 109 and 107 m s–1, but the permeability of the bulk Chalk may reach 106 m s–1 due to the presence of fractures (Bourdon et al., 2009). The top of the C4 Chalk formation is constituted by a compact and impermeable layer (C4c Chalk), where the fractures seem to be absent, which is responsible for a significative difference in piezometric head between the upper and the lower aquifers. It is thus considered to be the substratum of the upper aquifer. A piezometric dome is located below the Champagne mounts region in the centre of the study area (Fig. 1). The effective flow velocity ranges between 30 and 1200 m a1 (Hubert et al., 2006) but preferential flow can occur under dry valleys (Crampon et al., 1993a). The thickness of the unsaturated zone varies from 10 to 50 m. The infiltration rate is set to 150 mm a1 with vertical water flow velocities estimated to be between 0.4 and 0.8 m a1 (Vachier et al., 1987; Renard and Jeannée, 2008). The lower aquifer has developed in the Coniacian (C4) and Turonian (C3) formations and is confined with a downward piezometric head gradient with regard to the upper aquifer. Its substratum is constituted of the clayey formations of Mid and Lower Turonian (C3a–b). The borehole A25 (235 m deep; Fig. 1) allows water sampling in the lower aquifer. 2.2. Sampling and analytical methodologies The rock samples in the Upper Cretaceous Chalk formations were collected thanks to 5 drill cores (A17, A21; A23; A24 and A25b; see Fig. 1 for locations). The sampling of the surface soil (0–15 cm deep) was performed in the rendosols (predominant soil) (Table S1; Supplementary data). Major and trace elements have been determined in both bulk soils and rock samples (Table S1) by X-fluorescence spectrometry and ICP-mass spectrometry after complete digestion by alkaline fusion. The mineral phases have been determined by X-ray diffraction (XRD). The mixed-layer minerals in the <2 lm clay fraction have been identified by XRD after orientation of this mineralogical fraction. The ethylene–glycol

0.16

3. Baseline characterisation 3.1. Rock geochemistry and mineralogy The CaCO3 content of the Chalk is generally above 95% whereas it ranges between 60% and 80% in colluvium and clayey carbonates. The remaining fraction is composed of silicate, clay minerals (mainly kaolinite, illite, chlorite, quartz, micas, feldspars) and Al– Fe oxide or hydroxide mixtures. Pyrite, apatite and a few particles of barite have been observed by SEM in combination with X-ray analysis. The relative abundance of mineral phases in each geological formation has been estimated from both bulk chemical compositions and XRD results, on the basis of mass balance calculation and correlation analysis (Fig. 2; Table 1). The three main geological formations present in the study site have some significant differences in their respective chemical and mineralogical properties (Table 1, Fig. 2): 1. Santonian and Campanian Chalks are mainly composed of Cacarbonates containing significant amounts of Sr and Mg (Fig. 2a). The purity of Chalk increases with depth whereas the cryoturbated Chalk in subsurface levels is enriched in Si, Mg, Al and Fe. 2. Clayey carbonates related to Tertiary deposits are characterised by higher Si, Mg, Al and Fe contents than in the Chalk group, with the presence of silicate minerals and Fe oxides or hydrox-

1.00

a

Colluvium

b

0.90

presence of iron oxides

0.80

0.12

Fe (mol.kg-1rock)

Mg (mol.kg-1rock)

0.14

exposure method has been used for the determination of expandable phyllosilicates. In order to complete the mineral phase identification, several samples of Chalk have also been characterised using a Scanning Electronic Microscope (SEM) in combination with an energy dispersive X-ray spectrometer. Groundwater samples were collected in 24 piezometers by pumping after purge of the water column, and in two springs using Nalgène™ bottles (Nalge Nunc International Corporation, Rochester, New-York, USA). The sampling occurred mainly from 2002 to 2007, twice a year, during high and low water level periods, respectively. Major elements were determined by ion chromatography and trace elements by ICP-mass spectrometry (Tables S2a and S2b, Supplementary data), after on-site sample filtration through 0.45 lm cellulose filters and acidification with ultrapure 60% HNO3 (Merck KGaA, Darmstadt, Germany). pH, Eh and specific conductivity were measured on-site in boreholes using combined YSI 600 XLM multi-parameter probes (YSI, USA).

0.10 0.08 Chalk 0.06 Clayey carbonates 0.04

Clayey 0.60 carbonates 0.50 0.40 0.30 0.20 Chalk 0.10

Mg associated to Chalk

0.02

0.70

0.00

Colluvium

0.00 0

2

4

6

8

0

2

Si (mol.kg-1rock)

4

6

8

Si (mol.kg-1rock)

soils

colluvium

C6 Chalk

C5 Chalk

clayey carbonates

cryoturbated Chalk

C5-C6 interface (A21)

mixing

Fig. 2. Geochemical properties of the three main geological formations recognised on the site, highlighted by their Mg and Fe contents relative to the Si concentration. In colluvium and clay carbonates, the Mg concentrations associated with carbonate is equal to the value in the absence of non-carbonate minerals.

M. Gillon et al. / Applied Geochemistry 25 (2010) 1564–1574 Table 1 Mean chemical (measured) and mean mineralogical (calculated) compositions of geological formations. Colluvium

Carbonate (weight%)

Mean 58.8

Clayey carbonates Mean 66.1

Cryoturbated Chalk Mean 93.9

C6 Chalk Mean 96.6

C5 Chalk Mean 98.5

9.32 0.46 0.25 0.046 0.057 0.014 <0.035 0.032 0.003 0.006 0.012 0.006

9.62 0.28 0.16 0.030 0.052 0.014 <0.058 0.014 0.003 0.006 0.014 0.013

9.76 0.17 0.12 0.021 0.049 0.013 <0.064 0.010 0.003 0.005 0.016 0.015

9.30 0.03 0.04 0.22 0.02

9.59 0.01 0.03 0.12 0.02

9.74 0.01 0.02 0.05 0.01

0.00 0.04

0.00 0.04

0.00 0.03

1

Element concentrations ðmol kg rock Þ Ca 5.77 6.70 Si 4.44 2.41 Al 1.00 1.48 Fe 0.281 0.509 Mg 0.110 0.073 Sr 0.006 0.004 Na 0.080 <0.032 K 0.143 0.050 Mn 0.011 0.010 Ti 0.045 0.043 P 0.029 0.020 S 1

Mineral concentrations ðmol kg rock Þ Carbonates 5.74 6.54 Clay/Micas 0.14 0.05 Kaolinite 0.16 0.35 Quartz 3.12 0.86 Hematite/ 0.05 0.22 pyrite Feldspars 0.08 0.00 Other 0.10 0.20

ides (Fig. 2b). The clayey mineral phase is characterised by the relative abundance of kaolinite (Table 1). 3. Colluviums are characterised by a larger non-carbonate fraction than in the Chalk formations, but with Al and Fe concentrations lower than the ones of the clayey carbonates. The silicate mineral fraction is characterised by the relative abundance of quartz together with some detrital phyllosilicates such as illite. 3.2. Water geochemistry Evolution of water chemistry in groundwater from the upper aquifer along the flow lines for some representative ions is given in Fig. 3. The composition of the bulk groundwater in the upper aquifer is fairly homogeneous and consistent with a Ca–HCO3-rich reservoir representative of the Chalk aquifer: all groundwaters are at (or close to) saturation with calcite (Tables 2 and S2b). The groundwater baseline characteristics are summarised in Table 2. A statistical analysis based on cumulative frequencies is used to define the main properties of baseline chemistry (Fig. 4) (Edmunds et al., 2003). Bicarbonate, Ca2+, Mg2+ and Sr2+ concentrations follow a normal distribution characterised by a predominance of concentrations near to the median values (Fig. 4). The Ca2+ and HCO 3 concentrations corresponding to the equilibrium between water and calcite are rapidly reached and hardly evolve along flow lines due to the dissolution of carbonate minerals in the unsaturated zone. The sharpness of the SiO2(aq) cumulative distribution supports a fast dissolution of silicate mineral up to the solubility limit in the unsaturated zone and is followed by little evolution of its concentration along the flow line in the saturated zone (Edmunds et al., 1987). Thus, for Ca2+, HCO 3 and SiO2(aq) species, the spatial heterogeneity of rock composition in the geological substratum is not likely to have a major effect on the aqueous chemistry of this system. These concentrations correspond to values measured in the recharge area of Chalk aquifers (Edmunds et al., 1987; Hiscock et al., 1996; Elliot et al., 1999). It is consistent with the location of the present study area below a piezometric dome. In addition, the Mg2+ concentrations in groundwater samples are also found to be

1567

lower than values given in the literature and suggest some variations in geochemical composition of the Chalk forming the aquifer. The concentrations of Mg2+ and Sr2+ in the lower aquifer are similar to those generally found in the literature but are higher than the ones in the upper aquifer (Fig. 3 and Table S2b). The difference can be explained by variation in the chemistry and mineralogy of Chalk in the lower aquifer or by kinetically controlled reactions like the incongruent dissolution of carbonates. Indeed, the modifications of groundwater chemistry associated with water–rock interactions increase with time (Kloppmann et al., 1998; Elliot et al., 1999). The lower aquifer being confined, its recharge occurs outside the studied site and the residence time of groundwater in the lower aquifer is therefore expected to be higher than in the upper aquifer. A temporal variability of water chemistry was observed in each piezometer especially for the upper aquifer (Fig 3). The vegetation heterogeneity and seasonal climatic variations lead to a spatial variability of CO2 partial pressures and consequently changes in pH, solubility of carbonates and water alkalinity (Reardon et al., 1979, 1980; Schoeller, 1980; Lee, 1997). Moreover, the double porosity of the Chalk leads to mixing between waters of different residence times: freshly percolated water, water from fissures and water from the matrix. The contributions of each water evolve with time and depend on the sampling conditions (well purging rate, sampling depth, season, etc.). During the congruent dissolution of carbonate, the Mg/Ca and Sr/Ca ratios are representative of the composition of the parent rock and independent of pH. The low Mg/Ca and Sr/Ca ratios found in water samples closed to the groundwater divide line, display this trend (Fig. 5). However, a slight increase of Mg/Ca and Sr/Ca ratios exists along the flow lines and is independent of pH variabilities (Figs. 3 and 5). An agricultural contribution related to the use of fertilizers does not explain this trend since this evolution also occurs below the uncultivated soils. It does not seem to be directly associated with the spatial heterogeneity of rock composition because the chemical homogeneity between Santonian (C5) and Campanian (C6) Chalk samples should not induce significant differences in the groundwater composition (Table 1). However, despite the relative homogeneity of Chalk, the presence of impurities in the calcite leads to specific water–rock interaction processes. Incongruent dissolution of carbonate associated with precipitation of pure calcite occurs in the Chalk aquifers and is probably the main process that explains the increasing of Mg/Ca and Sr/Ca ratios (Edmunds et al., 1987; Kloppmann et al., 1998; Elliot et al., 1999; Dogramaci and Herczeg, 2002; Schürch et al., 2004). However, some authors suggest other types of water–calcite interaction. McGillen and Fairchild (2005) describe the incongruent processes as a preferential leaching of Mg and Sr during the dissolution of carbonate rather than the precipitation of pure calcite and suggest a dependence of the Mg/Ca and Sr/Ca ratios on the water/rock ratios and with the dissolution kinetics. VillegasJiménez et al. (2009) evidenced ion exchange behaviour of calcite, and thus the evolution of the Mg/Ca and Sr/Ca ratios could be a consequence of an ion exchange onto the surface of calcite rather than an incongruent process. Other processes may be involved such as water mixing between the upper aquifer groundwater and freshly percolating water. Indeed, the spatial heterogeneity of the rock composition of the superficial geological formations influences the composition of the freshly percolating water. The differences in chemical composition between these two water bodies are not sufficient to induce a representative variation of Mg2+ and Sr2+ concentrations in groundwater, but lead to dilution, disequilibria and slight changes in calcite and carbonate saturation indexes. A new congruent dissolution can occur modifying the initial equilibrium and support secondary reactions such as incongruent dissolutions.

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Spring

Altitude (m NGF)

300

piezometric dome

Southern watershed

S

Northern watershed

Spring water table N lower limit of cryoturbated Chalk

clay

250 200

Prosnes

150

colluvium

cryoturbated Chalk

Suippe

C6 C5

100

C4c

50

C4ab

Sr2+ (mmol.L-1)

Mg2+ (mmol.L-1)

0 0.24 0.16 0.08 0.00 0.02

0.01

0.00

Mg/Ca

0.04

0.02

0.00

Sr/Ca

0.006 0.004 0.002 c ultiv ated s oil

c ultiv ated s oil

0.000 -5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

Distance (km) upper aquifer

low er aquifer

Fig. 3. Chemical evolution of Mg2+, Sr2+, Mg/Ca and Sr/Ca ratios in groundwater along the flow line in the form of a box and whisker plot (the geological profile is schematic). The top and bottom of the boxes define the 75th and 25th percentiles. The lines through the middle of the boxes show the medians. The ends of the two ‘‘whiskers” define the minimum and maximum values. Some values of Mg/Ca and Sr/Ca ratios in groundwater from the lower aquifer are not represented because they are above the present scale. See Table S2 in see greater precision for these samples.

One could also consider exchanges between the two aquifers since previous studies on the Chalk aquifer in the Champagne area do not give evidence of the presence of two groundwater levels (Crampon et al., 1993a; Kloppmann et al., 1996, 1998; Hubert et al., 2006). The C4c level could thus be locally a semi-permeable formation with structural and textural properties varying in space. However, such contributions are not realistic since the hydraulic head gradient between the upper and lower aquifers is not favourable to upward water movement through the C4c level.

4. Modelling of the rock–water interactions Batch models using the code CHESS were applied to model water–rock interaction and mineral dissolution (Van der Lee and de Windt, 2002). In the Chalk aquifer, it is assumed that there is thermodynamic equilibrium between water and minerals since carbonate dissolution is usually fast (Reardon et al., 1980; Edmunds et al., 1987; Moral et al., 2008).

Since a variability of pH occurs naturally in the groundwater, calculations were performed to reproduce the experimental observations at 12 °C (mean temperature of groundwater samples) for different CO2 partial pressures ranging from 0.001 to 0.4 atm. Since the carbonate dissolution is fully controlled by the CO2 partial pressure in an open system (infinite CO2 reservoir) (Langmuir, 1997), the CO2 partial pressure is kept constant during the calculation. These conditions usually occur in the unsaturated zone where the chemical equilibrium between water and minerals is quickly achieved. Precipitation of secondary minerals is not considered here. Rock composition is assumed homogeneous for all Chalk formations. Compositions and relative abundances of minerals considered in the rock model were deduced for each geological formation from results of Section 3.1. (Table 1). Carbonates, quartz (SiO2), kaolinite (Si2Al2O5(OH)4) and muscovite (KAl3Si3O10(OH)2) are taken as dominant mineral phases. Two different situations have been considered for the modelling of water–rock interactions: an assemblage of pure carbonate

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M. Gillon et al. / Applied Geochemistry 25 (2010) 1564–1574 Table 2 Mean, minima, maxima, deviation and other statistical parameters of chemical concentrations and physical parameters in water.

T pH Conductivity Eh Alkalinity HCO 3 Ca2+ 2+ Mg Sr2+ Na+ K+ Cl NO 3 SO2 4 F SiO2(aq) S.Icalcite S.Idolomite S.Istrontianite

Unit

Mean

Median

Standard deviation

95th percentile

Minimum

Maximum

Number of data

°C – lS cm1 mV meq L1 mmol L1 mmol L1 mmol L1 mmol L1 mmol L1 mmol L1 mmol L1 mmol L1 mmol L1

11.7 7.2 466 437 4.2 3.8 2.2 0.030 0.004 0.168 0.024 0.268 0.218 0.068

11.7 7.3 448 423 3.8 3.7 2.2 0.026 0.004 0.139 0.018 0.155 0.152 0.045

1.3 0.3 147 61 1.4 1.2 0.8 0.020 0.002 0.087 0.019 0.249 0.192 0.049

14.0 7.7 708 517 8.4 5.9 3.8 0.061 0.008 0.353 0.074 0.762 0.587 0.166

6.7 6.5 64 315 2.2 1.5 0.4 0.006 0.001 0.078 0.006 0.051 0.001 0.007

16.6 8.0 919 668 8.4 8.3 6.6 0.195 0.012 0.526 0.084 1.345 0.726 0.188

100 100 101 69 16 63 167 167 167 81 50 81 80 80

mmol L1 mmol L1 / / /

0.019 0.067 0.1 2.1 1.9

0.016 0.065 0.0 2.1 1.9

0.013 0.015 0.2 0.5 0.2

0.043 0.091 0.3 1.3 1.5

0.005 0.021 0.5 2.8 2.5

0.081 0.128 0.4 1.1 1.5

71 52 57 57 57

The difference S.Icalcite  S:Isolid solution ranges between 0.01 and 0.02. * colluvium, clayey carbonates, cryoturbated Chalk, C5 Chalk or C6 Chalk. S.I = log(IAP/K) with IAP the ion activity product.

Cumulative frequency (%)

100 90

HCO3

80

Ca Mg

70

Sr 60

SiO2

50 Median 40 30 20 10 0 0.001

0.01

0.1

1

10

Concentration (mmol.L-1) Fig. 4. Cumulative frequency of Ca2+, Mg2+, Sr2+, HCO 3 and SiO2aq concentrations. All data available on the study site corresponding to statistical values presented in Table 2 have been used. The chemical compositions of water samples with a more complete data set are detailed in Table S2.

phases (calcite–dolomite–strontianite), and a composite carbonate built as a solid solution CaxMgySrz(CO3). In the latter case, the stoichiometry of the composite carbonates is deduced from the bulk composition of the different geological formations (colluvium, clayey carbonates, cryoturbated Chalk, C6 Chalk and C5 Chalk), assuming that Mg and Sr are all included in the carbonate phase in C5, C6 and cryoturbated Chalks. For colluvium and clayey carbonates, the siliceous fraction is more important, and the previous assumption concerning Mg cannot be supported. In that case, the Mg concentration associated with the composite carbonate phase 1 is deduced from Fig. 2 (0:047 mol kgrock ). The determination of x, y, z in CaxMgySrz(CO3) is deduced from Eqs. (1)–(4):

½CO3  ¼ ½Ca þ ½Mg þ ½Sr

ð1Þ

x¼

½Ca ½CO3 

ð2Þ

y¼

½Mg ½CO3 

ð3Þ

z¼

½Sr ½CO3 

ð4Þ

with [i] the concentration of element i in rock (i = CO3, Ca, Mg or Sr; 1 mol kgrock ) and x + y + z = 1. The calculated stoichiometries of composite carbonate phases for each geological formation are given in Table 3. Carbonate solubility constants from the compilation of available thermodynamic data in Hummel et al. (2002) are selected. For other minerals, the solubility constants are taken from the CHESS database (Table 4). The solubility constants of composite carbonate solid solutions can be deduced from the solubility constants of all pure mineral phases forming the solid solution (Bruno et al., 1998). However, several authors working on ‘‘aragonite–strontianite” or ‘‘strontianite–witherite” systems have demonstrated that the dissolution constant of the solid solution does not correspond to a simple mixing between the two end-members (Plummer and Busenberg, 1987; Plummer et al., 1992; Kiseleva et al., 1994; Casey et al., 1996). Here, Mg and Sr represent minor components of the carbonate phase and Ca largely dominates (>99%). Therefore, the dissolution constant is assumed to be equal to that of calcite (Eqs. (5) and (6)):

Cax Mgy Srz ðCO3 Þ þ Hþ ! xCa2þ þ yMg2þ þ zSr2þ þ HCO3

ð5Þ

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M. Gillon et al. / Applied Geochemistry 25 (2010) 1564–1574

0.006

Sr2+/Ca 2+ (molar ratio)

0.005

0.004

0.003

0.002

0.001

ratios in Chalk samples 0 0

0.01

0.02

0.03

0.04

Mg2+/Ca2+ (molar ratio) Solid samples: Chalk samples (from A17 A21 A23 A24 A25b) Groundwater samples: piezometric dome (0-0.5km)

north watershed (<1km)

north watershed (>1km)

south watershed (<0.5km)

south watershed (>0.5km)

spring of Moncet

spring of Prosne

in brackets: distance from the groundwater divide line

Fig. 5. Variability of Mg/Ca and Sr/Ca ratios in groundwater from the upper aquifer. White circles: Mg/Ca and Sr/Ca ratios of solid Chalk samples; cross: uncertainty associated with the measurement.

Table 3 Calculated chemical compositions and stoichiometry of composite carbonate solid solutions in each geological formation. mol kg rock

Colluviuma

Clayey carbonatesa

Cryoturbated Chalk

C6 Chalk

C5 Chalk

Ca Mg Sr CO3

5.71 0.047 0.006 5.76

6.52 0.047 0.004 6.57

9.27 0.057 0.014 9.34

9.57 0.052 0.014 9.64

9.72 0.049 0.013 9.78

1

The stoichiometry is relative to 1 mol of CO3: Colluvium Ca0.9908Mg0.0082Sr0.001(CO3) Colluvium + H+ ? 0.9908Ca2+ + 0.0082Mg2+ + 0.001Sr2+ + HCO 3

Cryoturbated Chalk Ca0.9924Mg0.0061Sr0.0015(CO3) Cryoturbated Chalk + H+ ? 0.9924Ca2+ + 0.0061Mg2+ + 0.0015Sr2+ + HCO 3 Chalk C6 Ca0.9932Mg0.0054Sr0.0014(CO3) Chalk C6 + H+ ? 0.9932Ca2+ + 0.0054Mg2+ + 0.0014Sr2+ + HCO 3 Chalk C5 Ca0.9937Mg0.005Sr0.0013(CO3) Chalk C5 + H+ ? 0.9937Ca2+ + 0.005Mg2+ + 0.0013Sr2+ + HCO 3 Log K = log Kcalcite (1.84 at 25 °C). a nMg(carbonate) = 0.047.

½Ca2þ x ½Mg2þ y ½Sr2þ z ½HCO3 

10pH 2þ ½Ca ½HCO3  ¼ at 25  C 10pH

Minerals

Equations

Calcite Dolomite Strontianite Solid solutions Quartz Kaolinite Muscovite

Calcite + H+ ? Ca2+ + HCO 3 Dolomite + 2H+ ? Ca2+ + Mg2+ + 2HCO 3 Strontianite + H+ ? Sr2+ + HCO 3 + 2+ 2+ CaxMgySrz(CO3) + H ? xCa + yMg + zSr2+ + HCO 3 Quartz + 2H2O ? H4SiO4aq Kaolinite + 6H+ ? 2Al3+ + 2SiO2aq + 5H2O Muscovite + 10H+ ? K+ + 3Al3+ + 3SiO2aq + 6H2O 2 + HCO 3 ? CO3 + H

Clayey carbonates Ca0.9922Mg0.0072Sr0.0006(CO3) Clayey carbonates + H+ ? 0.9922Ca2+ + 0.0072Mg2+ + 0.0006Sr2+ + HCO 3

K¼

Table 4 Principal solubility constants of minerals used in the model.

¼ 101:84 ð6Þ

The values of x, y and z depend on the composition of the considered composite carbonate phase (Table 3).

Log K to 25 °C 1.84 3.56 1.05 1.84 4.00 6.81 13.59 10.33

Calculations for water–Chalk interaction modelling from both pure mineral as well as composite phase cases and aqueous concentrations measured in the upper and lower aquifer groundwaters are compared in Fig. 6. The aqueous concentrations of Ca2+, Mg2+, Sr2+ and HCO 3 are plotted as a function of pH. It should be noted that results are not influenced by the presence or absence of silicates. Calculations using pure carbonate phases give results greatly different from calculations using composite carbonates, and overestimate the Mg2+ and Sr2+ concentrations in groundwater. In carbonate systems, Bruno et al. (1998) suggested an overestimation of trace element concentrations in solution when pure solid phases are dissolved. It means that the concentrations of Mg2+ and Sr2+ found in the groundwater give information about the nature of the mineral phases in contact with water. Based on the experimental results, saturation with respect to dolomite or strontianite is not achieved which means either

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100

Ca 2+

solid solutions

pure carbonates

HCO3- (mmol.L-1)

Ca2+ (mmol.L-1)

10

1

0.1

HCO3solid solutions pure carbonates

10

1 6.2

6.7

7.2

7.7

8.2

6.2

6.7

7.2

pH 10

pure carbonates

1

Mg2+

8.2

pure carbonates

Sr2+

0.1

Sr2+ (mmol.L-1)

Mg 2+ (mmol.L-1)

1

0.1

0.01

7.7

pH

0.01

0.001

solid solutions

0.001

solid solutions

0.0001 6.2

6.7

7.2

7.7

8.2

6.2

6.7

7.2

7.7

8.2

pH

pH pure carbonates model: calcite+dolomite+strontianite solid solutions model: the 5 types of solid solutions are considered. For Ca2+ and HCO3- concentrations, the 5 solid solutions give similar results, as consequence the surface is replaced by a line.

samples from the upper aquifer (C5-C6) samples from the lower aquifer (C4ab) piezometric dome samples: "initial" water of the upper aquifer located close to the groundwater divide line

2+ 2+ 2+ Fig. 6. Comparison between the measured and the calculated concentrations of HCO of groundwaters (upper and lower aquifers). The model assumes 3 , Ca , Mg and Sr equilibrium between water and a silicate phase composed of an assemblage of quartz, kaolinite and muscovite, and a carbonate phase composed by either an assemblage of pure mineral phases (calcite, dolomite, strontianite), or a composite carbonate containing Mg and Sr as impurities in calcite.

the absence of these two minerals in the aquifer or slow kinetics (Table S2b). Water already saturated with respect to calcite dissolves dolomite slowly (Moral et al., 2008). Considering the solid solution approach, the five composite carbonates corresponding to five geological formations are used to build a domain of possible concentrations (Fig. 6). For Ca2+ and HCO 3 , the calculated concentrations agree with the data. For the samples collected close to the groundwater divide line, the Mg2+ and Sr2+ concentrations are included in the domain given in Fig. 6. The ‘‘initial” composition of groundwater representative of water–carbonate equilibrium rapidly attained in the unsaturated zone can thus be calculated from a simple model of congruent dissolution of calcite containing Mg and Sr impurities. The Mg/Ca and Sr/Ca ratios of groundwater sampled downstream from the watershed are not representative of congruent dissolution of parent rock and highlight incongruent processes (Fig. 5). The incongruent dissolution of (Ca, Mg, Sr) composite carbonates in the C5 Chalk, where the upper aquifer mainly develops, can be written (Eq. (7)):

Ca0:9937 Mg0:005 Sr0:0013 ðCO3 Þ þ 0:0063Ca2þ ! 0:005Mg2þ þ 0:0013Sr2þ þ CaCO3

ð7Þ 2+

2+

Considering this equation, the evolution of Ca , Mg and Sr2+ concentrations in groundwater can be calculated from Eqs (8) and (9).

C xw ðtÞ ¼ C xw ð0Þ þ C xC5

Chalk

 ðt  /Þ

Ca Ca C Ca w ðtÞ ¼ C w ð0Þ  ð1  C C5

Chalk Þ

 ðt  /Þ

ð8Þ ð9Þ

where C xj is the concentration of ‘‘x” (x = Ca, Mg or Sr) in solution (j = w; moli L1 water ) or in the mineral (j = composite carbonate ‘‘C5 1 Chalk”; moli molC5 Chalk Þ, / is the rate of reaction ðmolC5 Chalk 1 1 Lwater a ) and t is the time (a). In order to define the evolution of the Sr/Ca ratio as a function of the Mg/Ca ratio, it is not necessary to know the product (t  /). The initial conditions of calculations (t = 0) correspond to a solution in equilibrium with the composite carbonates of parent rock as previously calculated with CHESS (T = 12 °C; CO2 partial pressure = 0.01 atm). A part of the Mg and Sr can precipitate with secondary calcite (Katz, 1973; Tesoriero and Pankow, 1996; Dogramaci and Herczeg, 2002). Considering the results of Katz (1973) and of Tesoriero and Pankow (1996) for the distribution coefficient of Mg and Sr, the remobilisation of Mg and Sr is assumed to be negligible in the present approach. The expected limiting Mg/Ca and Sr/Ca ratios when the thermodynamic equilibrium between groundwater and both composite carbonate C5 Chalk and calcite is achieved are represented by a square in Fig. 7 (Evolution 1). The calculations of the limiting Mg/Ca and Sr/Ca ratios are performed using the code CHESS under a constant CO2 partial pressure with precipitation of pure calcite; the carbonate phase is represented by the composite carbonate C5 Chalk.

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1

0.006

a

b

0.005 Limiting ratio

1 Sr2+/Ca2+ (molar ratio)

0.1

0.004

Incongruent dissolution of C5 Chalk

0.003 Evolution 0.002 0.001 lower aquifer

0.000 0.00

0.01

0.01

0.02

0.03

0.04

0.001 Evolution Initial mineralisation 0.0001 0.001

2

Limiting ratio

Incongruent dissolution of dolomite

0.01

0.1

1

10

Mg2+/Ca2+ (molar ratio) piezometric dome (0-0.5km)

northern watershed (<1km)

northern watershed (>1km)

southern watershed (<0.5km)

southern watershed (>0.5km)

spring of Moncet

spring of Prosne

lower aquifer

model (cryoturbated Chalk)

model (colluvium)

model (clayey carbonates)

model (C6 Chalk)

model (C5 Chalk)

in brackets: distance from the groundw ater divide line

Fig. 7. (a) Evolution of the calculated Mg/Ca and Sr/Ca ratios in the upper aquifer as a consequence of the incongruent dissolution of C5 Chalk (evolution 1) or as a consequence of the incongruent dissolution of dolomite (evolution 2), and comparison with experimental data. The range related to the calculated ratio (dotted lines) results from the variability of Mg and Sr stoichiometry in the composite carbonate solid solution. (b) Zoom in on some experimental data. When incongruent dissolution of both C5 Chalk and dolomite exist, the Mg/Ca and Sr/Ca ratios should be located at the intermediate position between the equilibrium with dolomite and the C5 Chalk.

Most of groundwaters sampled downstream from the watershed follow the theoretical variation of incongruent dissolution of composite carbonate C5 Chalk (Fig. 7). However, the Mg/Ca ratio appears higher than the calculated ratios, for the waters collected in the north part of the area in the upper aquifer (‘‘northern watershed (>1 km)”) and in the springs (Fig. 7). In fact, since dolomite has been recognised in one sample at 2 m depth, its presence in another part of the aquifer is possible but its distribution in the Chalk must be heterogeneous. The incongruent dissolution of dolomite could explain the increasing Mg/Ca ratio independently of the Sr/ Ca ratio (Eq. (10); Evolution 2 in Fig. 7).

CaMgðCO3 Þ2 þ Ca2þ ! Mg2þ þ 2CaCO3

ð10Þ

In fact, the incongruent dissolution of dolomite and the incongruent dissolution of composite carbonate take place at the same time. Therefore, when both dolomite and composite carbonates are present, the Mg/Ca and Sr/Ca ratios should be intermediate between the equilibrium with dolomite and the C5 Chalk but dependant on the residence time of water in the aquifer. An accurate reconstruction of the groundwater geochemistry as a function of time requires taking into account the kinetics of composite carbonate and dolomite dissolution and of calcite precipitation. Previous approaches on kinetics of carbonate dissolution mainly concern reactions far from saturation and cannot be applied to the present incongruent dissolution (Plummer et al., 1979; Dreybrodt, 1981; Busenberg and Plummer, 1982; Appelo et al., 1984; Buhmann and Dreybrodt, 1985; Chou et al., 1989). Some studies are based on field experimental data but the incongruent dissolution of car-

bonates with impurities other than dolomite is not taken into account (Mercado and Billings, 1975; Plummer, 1977; Deike, 1990). An alternative approach consists of mass-balance modelling based on the groundwater geochemistry of the study site but needs to know the residence time of groundwater (Plummer et al., 1990; Jacobson and Wasserburg, 2005). An attempt was made to calculate residence times in the upper aquifer from hydrodynamic data taking into account the transmissivity heterogeneity. The first results show that the incongruent dissolution occurs on a time scale of several decades. Previous studies have observed incongruent dissolution on a time scale of several thousand years with a greater increase of Mg and Sr concentrations in solution compared to the present study (Edmunds et al., 1987; Elliot et al., 1999).The present data suggest for the first time that it is possible to evidence these processes on smaller temporal and spatial scales. Moreover, although all these studies concern the Chalk aquifer, geochemical differences in groundwater exist and are associated with differences in Chalk geochemistry. It is thus important to characterise each groundwater body on a local scale. The Mg/Ca and Sr/Ca ratios in the lower aquifer follow the theoretical evolution of incongruent dissolution of carbonates developed for the upper aquifer groundwater (Fig. 7). This highlights the control of groundwater chemistry in the two aquifers by the same processes (mainly incongruent reactions) and great similarities between the geochemical compositions of carbonates. The higher Mg/Ca and Sr/Ca ratios suggest a greater residence time of groundwater in the lower aquifer, and a greater chemical modifications by long-term water–rock interaction.

M. Gillon et al. / Applied Geochemistry 25 (2010) 1564–1574

5. Conclusions The baseline geochemistry of groundwater in the study area is defined by: 1. An initial mineralisation during the water infiltration in the unsaturated zone. In carbonate environments, this mineralisation is fast and supports the conclusions of previous studies (1–3 weeks; Edmunds et al., 1987; Moral et al., 2008). Therefore, the chemical composition of the water is representative of the stoichiometry of the superficial carbonate phase in the unsaturated zone (case of water collected close to the groundwater divide line). 2. A variation along the flow path, mainly time dependent. It is a consequence of slow water–rock interaction especially the incongruent dissolution of solid solution and dolomite associated with the precipitation of pure calcite. The incongruent dissolutions occur because of the presence of impurities in the carbonate phase, and locally because of the presence of dolomite and are directly associated with the heterogeneity of rock composition. Moreover, the mixing with freshly percolated water supports these reactions. The variation of the Mg/Ca and Sr/Ca ratios is a consequence of the combination of these different processes where the incongruent dissolutions dominate. These processes have already been described in the literature but in this study the incongruent processes are highlighted on a smaller scale in the recharge area of the aquifer. A specific approach based on carbonate dissolution equilibria, using solid solutions is proposed to characterise the carbonate phases. It allows calculation of the initial chemical concentrations of groundwater and the evolution associated with the incongruent dissolutions. This method requires the knowledge of chemical composition of rocks forming the aquifer in order to define the stoichiometry of composite carbonate solid solutions. Considering the deviations associated with the slight heterogeneity in the geochemical composition of Chalk, it is possible to model the geochemistry associated with carbonate phases of most water samples. This approach applied here for Mg and Sr, could be extended to other trace elements but should take into account specific co-precipitation with calcite. Application of this method to other carbonate aquifers is possible with careful geochemical analysis of some representative host rocks. However, it is not appropriate in the presence of evaporites, abundant silicate or clayey phases since the interaction between water and these minerals changes the Mg/Ca and Sr/Ca ratios in solution (Land and Öhlander, 1997). The present work has allowed characterisation of the main features of the baseline geochemistry and definition of the main processes that control the groundwater geochemistry on the site. It is essential in order to use a dynamic approach that associates the chemical interactions between water and rock with the hydrodynamic flow, the next step for modelling the behaviour of solute transport in groundwater. Finally, as dissolved carbonates form stable complexes with radionuclides such as uranyl ions in groundwater, the incongruent dissolution and precipitation of carbonates must be taken into account for the study of radionuclide transfer in the aquifer.

Acknowledgements We thank Olivier Marie for observations and analyses on the Scanning Electronic Microscope (SEM) combined with X-ray spectrometry and Patricia Gibert, Eric Pili, Amélie Hubert, Christophe Moulin for their helpfulness in the realisation of this work. We

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