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Chemical and strontium isotopecharacterization of rainwater inFrance: influence of sources andhydrogeochemical implicationsPhilippe Négrel a , Catherine Guerrot a & Romain Millot aa Service Métrologie Monitoring Analyse, Metrology MonitoringAnalysis Department , Bureau de Recherches Géologiques etMinières (BRGM) , 3 Avenue Claude Guillemin, BP 36009, 45060Orléans Cedex 2, FrancePublished online: 03 Sep 2007.

To cite this article: Philippe Négrel , Catherine Guerrot & Romain Millot (2007) Chemicaland strontium isotope characterization of rainwater in France: influence of sources andhydrogeochemical implications, Isotopes in Environmental and Health Studies, 43:3, 179-196, DOI:10.1080/10256010701550773

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Isotopes in Environmental and Health StudiesVol. 43, No. 3, September 2007, 179–196

Chemical and strontium isotope characterization ofrainwater in France: influence of sources and

hydrogeochemical implications

PHILIPPE NÉGREL*, CATHERINE GUERROT and ROMAIN MILLOT

Bureau de Recherches Géologiques et Minières (BRGM), Service Métrologie Monitoring Analyse,Metrology Monitoring Analysis Department, 3 Avenue Claude Guillemin, BP 36009,

45060 Orléans Cedex 2, France

(Received 23 January 2007; in final form 4 June 2007)

Strontium isotope ratios and Ca2+, Na+, K+, Mg2+, Cl−, SO2−4 , NO−

3 and Sr2+ concentrationswere measured in rainwater samples collected in four stations in France (Brest, Dax, Orleans andClermont-Ferrand) over a period of 1 year. Each sample represented a monthly series of rain events.The chemical composition and the 87Sr/86Sr ratios of the rainwater samples varied considerably.Using Na concentrations as an indicator of marine origin, the proportion of marine and crustal ele-ments was estimated from elemental ratios. Strontium isotopes were used to characterize the differentsources using data from the four stations and the literature. Such sources include sea salts, crustalsources (carbonates, silicates and volcanic rocks) and anthropogenic sources (fertilizers, automobileexhausts, incinerators and urban heating).

Keywords: France; Hydrochemistry; Major ions; Rainwater; Strontium isotope ratios

1. Introduction

Atmospheric aerosols including sea salts (SSs), crustal dust, volcanic dust, biogenic materialand anthropogenic emissions are the main sources of chemical elements in rainwater [1, 2].The chemical composition of rainfall is strongly affected by the chemical composition of theatmosphere. Determination of the chemical composition of rainwater provides an understand-ing of the source types that contribute to rainwater chemistry and enhances understandingof the dispersion (i.e. both local and regional scales) of elements, whether pollutants or not,and their potential impact on eco-hydrosystems through deposition processes [3]. Conse-quently, precipitation chemistry has been the subject of intense research and study over thelast few decades for determining the source types that contribute to rainwater chemistry [4–7],for characterizing the origin of acidic rain [8] or for serving as primary input in catchmentareas [9–11].

*Corresponding author. Tel.: +33-2-3864-3969; Fax: +33-2-3864-3711; Email: p.negrel@brgm.fr

Isotopes in Environmental and Health StudiesISSN 1025-6016 print/ISSN 1477-2639 online © 2007 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/10256010701550773

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180 Philippe Négrel et al.

Applications involving Sr isotopes in rainwater have been relatively scarce but have providedinsights into the source of base cations, particularly Ca, which is not well constrained fromconcentration data alone [12–14]. Because SS, the major source of dissolved species in rain,has a very homogeneous 87Sr/86Sr ratio, Sr isotopes have been used to decipher the origin ofaerosols, either in individual rain or snow falls [11, 15–17], during a complete rain event [18]or in precipitations collected over long periods [19, 20].

The aim of this study is to present the results of the first investigation of Sr isotope ratiosin rainwater collected over a longer time period (i.e. all rainfall over 1 year) at a largerscale. The study includes an overview of the Sr isotope signature in the rain input, whichshould be identified either in surface or groundwater since the sources of Sr in water areatmospheric input, dissolution of Sr-bearing minerals and/or anthropogenic input. For massbalances estimation of pollution, it is essential to be able to distinguish such sources. It isworth noting that this study represents the first long-term monitoring of Sr isotopes in rain andthat France is likely an ideal place for this study due to its central role in Europe in terms ofcontinental and ocean sources of precipitation (i.e. Alps, Massif Central, Atlantic Ocean andMediterranean Sea).

Grosbois et al. [10] have reported an input of Sr from rainwater to the dissolved load of theLoire River of around 3.8 ± 1.1% over a 1-year survey. Négrel et al. [9] have reported a rangefor Sr derived from rainwater in different tributaries of the Congo River from 4 to 18%, whereasthe rain input ranged between 1 and 4% in the tributaries and the main river in theAmazon Basin[21] and ranged between 8 and 30% for surface and ground water in the Maroni catchment areain French Guiana [22]. Petelet-Giraud et al. [23] have reported a range from 1 to 35% in surfacewater draining granites of theVienne District (France) but only 3% of Sr can be assigned to raininput in surface and groundwater in the Somme catchment area in France [24]. These studieswith variable results indicate the need to precisely characterize rain input with regard to Sr iso-tope systematics. The aim of this work was to characterize, with regards to Sr isotope systemat-ics, rain input at a large scale (i.e. Metropolitan France) by compiling the data from the literatureand adding a new data set from four sites (two close to the ocean and two further inland).

2. Sampling sites

All of the sampling sites are illustrated in figure 1. Three sampling sites were monitoredbetween 2003 and 2004 during the course of this study. The first, located in the north-west ofFrance, was in Brest (48.24 N, 04.31W), less than 5 km from the Atlantic Ocean; the secondwas in Dax (43.44 N, 01.03W), located in the south west of France, 30 km from the AtlanticOcean; the final one was in Orleans (47.54 N, 01.52E), located in the centre of France, 400 kmfrom the Atlantic Ocean.

One sampling site from the literature was near Clermont-Ferrand (45.46 N, 03.04E) in theMassif Central, 200 km upstream from Orleans, which was fitted with an automatic sampler[20] that functioned from March 1994 to March 1995. The Mediterranean Sea is 300 km tothe south and the Atlantic Ocean is 380 km to the west. The nearest towns, Clermont-Ferrandand Issoire, are located 15 km to the north-west, and 14 km to the south, respectively.

Another sampling site was at Tours, 110 km downstream from Orleans, and was operatedduring each rain event from September 1996 to January 1998 [10].

Seimbille et al. [25], Roy [26] and Négrel (unpublished data) collected individual rainsamples between 1988 and 1995 in the city of Paris using a polypropylene funnel, as didBenOthman et al. [27] in the city of Montpellier. Rain samples were collected in three dif-ferent locations in eastern France [11], marked by rather contrasting climatic and pollution

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Hydrochemistry and strontium isotopes of rainwater in France 181

Figure 1. Location map of the rainwater collection sites from this study (Brest, Dax, Orleans) and from the literature.Data from the literature are from Négrel and Roy ([20], Clermont-Ferrand), Chabaux et al. ([11], eastern France);Grosbois et al. ([10], city of Tours); BenOthman et al. ([27], city of Montpellier); Seimbille et al. [25], Roy [26] andNégrel (unpublished data) for the city of Paris. The different x–y diagrams display monthly mean rainfall (mm) inthe collection sites (Brest, Dax, Orleans and Clermont-Ferrand).

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182 Philippe Négrel et al.

conditions. Rain samples were collected with an automatic wet-only precipitation collec-tor (Esch-sur-Alzette and part of those from Strasbourg). The other precipitation samplesfrom Strasbourg and all of the samples from Aubure were collected with permanently opencollectors.

3. Sampling and analytical procedures

A polycarbonate funnel [28], 12 cm in diameter and with an area of 400 cm2, was used tocollect the rainfall in the Brest, Dax and Orleans sites. This apparatus enabled precipitationmeasurement by direct reading, without the need for preliminary transfer into a measurementtest tube. The rainwater was stored in a polypropylene flask (12 l) to avoid any evaporation ormodification of the sample. The accumulation of daily samples allowed a monthly sample ofrain to be obtained.

An automatic precipitation sampler was designed for collecting rain at the site nearClermont-Ferrand [20]. The basic requirements for our collector were automatic detectionand collection of rainfall, elimination of dry fallout, collection of frozen precipitation andavoidance of sample contamination. A removable PVC lid covers the funnel when no pre-cipitation is falling. The cover exposes the funnel when electrical contact is made in the raindetector. A polypropylene funnel 45 cm in diameter was used to collect the rainfall.

A polypropylene funnel 45 cm in diameter was also used to collect the rainfall in the site atTours, but in this case it was manually operated during each rain event [10].

The water samples were filtered through pre-cleaned 0.45 μm acetate filters using a pre-cleaned Nalgene filter apparatus and the filtrate was separated into two aliquots. Of this:(i) 100 and 250 ml were acidified with double-distilled nitric acid and stored in pre-cleanedpolyethylene bottles for major-cation analysis as well as strontium isotope ratio and elementalSr determinations and (ii) 100 ml were stored unacidified in polyethylene bottles for anionanalysis. The water samples were chemically analysed by atomic absorption spectrometry(Ca2+, Na+, K+, Mg2+, precision 5–10%), ion chromatography (Cl−, SO2−

4 , NO−3 , precision

5–10%) and inductively coupled plasma mass spectrometry (Sr, precision 5%).Chemical separation of Sr was performed using an ion-exchange column (Sr-Spec), with

total blank <0.3 ng for the entire chemical procedure.After chemical separation, around 150 ngof Sr was loaded onto a tungsten filament and analysed with a Finnigan MAT262 multiplecollector mass spectrometer. The measured 87Sr/86Sr ratios were normalized to an 88Sr/86Srof 0.1194 and then adjusted to the NBS 987 standard value of 0.710240. An average internalprecision of ±0.000010 (2σ ) was obtained during this study; reproducibility of the 87Sr/86Srratio measurements was tested through duplicate analyses of the NBS 987 standard and themean value was 0.710228 ± 0.000022 (2σ ; n = 34).

4. Results

4.1 Amount of rainfall

Thirteen samples of rainwater were collected between March 1994 and March 1995 inClermont-Ferrand, with each sample representing all the rainwater that fell during a period of1 month. During this period, 970 mm of rainwater was collected and the rainfall breakdown,illustrated in the diagram of figure 1, showed the highest rainy periods during the spring(each between 80 and 120 mm).

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Hydrochemistry and strontium isotopes of rainwater in France 183

Twelve samples of rainwater were collected between July 2003 and May 2004 in Brest,representing 841 mm of rainwater. The breakdown showed the main rainy season occurredbetween November 2003 and January 2004 (figure 1). In the Dax station, the 12 rainwatersamples collected between July 2003 and June 2004 represented 1219 mm, with the maximumrainfall occurring in October 2003 (203.1 mm), January 2004 (187.3 mm) and May 2004(155.1 mm). In the Orleans station, only nine samples of rainwater were available betweenJanuary 2004 and December 2004 due to technical problems with the rain collector. Theyrepresented 572.5 mm of rainwater, with the maximum observed rainfall in January 2004 andAugust 2004 (120 and 118 mm, respectively).

4.2 Compositional variations of major ions

Table 1 illustrates the mean weighted concentration of major ions and Sr2+ in the collectionsites (Brest, Dax, Orleans and Clermont-Ferrand).

4.2.1 Clermont-Ferrand. Négrel and Roy [20] have presented and discussed the results ofeight major elements, the trace element Sr and the 87Sr/86Sr ratio in 13 samples of rainwater(970 mm). To summarize, the order of cation abundance was Ca2+ > Na+ > K+ > Mg2+,with the Ca2+ and Na+ species giving mean weighted concentrations of 18 and 14 μmol l−1

respectively. The cation concentrations showed large variations during the sampling period: theNa+ concentrations varied 100-fold (0.4 μ mol l−1 in October–November 1994 to 43 μ mol l−1

in February–March 1995), the Ca2+ concentrations varied 80-fold and the K+ concentra-tions varied 20-fold. The order of anion abundance was NO−

3 > SO2−4 > Cl−, but their mean

weighted contents were very similar (respectively 26, 23 and 19 μ mol l−1). Their variationswere less than those of the cations, between 5- and 10-fold.

The Sr concentration in the rainwater greatly fluctuated from 0.008 μ mol l−1

(October–November 1994) to 0.121 μ mol l−1 (June–July 1994). The 87Sr/86Sr ratio rangedfrom 0.709198 (September 1994) to 0.71314 (October 1994) with a mean annual average of0.7097. This ratio increased and decreased several times during the sampling period.

4.2.2 Orleans. The order of cation abundance was Na+ > Ca2+ > K+ > Mg2+, with theNa+ and Ca2+ species giving mean weighted concentrations of 49 and 16 μ mol l−1, respec-tively. The Na+ concentrations varied 2.5-fold (97 μ mol l−1 in January 2003 to 39 μ mol l−1

in September 2003) and the Ca2+ concentrations varied 4.4-fold (39 μ mol l−1 in January 2003to 9 μ mol l−1 in September 2003). The order of anion abundance was Cl− > SO2−

4 > NO−3 ,

with Cl− giving a mean weighted concentration of 36 μ mol l−1, being twice that of SO2−4

Table 1. Mean weighted concentration (μ mol l−1) of major ions andSr2+ in the collection sites (Brest, Dax, Orleans and Clermont-Ferrand).

Brest Dax Orleans Clermont-Ferrand

Na+ 226.99 194.94 48.74 13.84Mg2+ 26.94 21.69 9.28 3.02K+ 6.36 27.4 11.51 6.21Ca2+ 9.12 40.52 15.81 17.23Sr2+ 0.04 0.13 0.06 0.02Cl− 245.25 183.26 35.92 18.09SO2−

4 31.06 29.61 14.99 23.13NO−

3 36.17 22.81 8.81 26.67

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184 Philippe Négrel et al.

(15 μ mol l−1) and four times that of NO−3 (9 μ mol l−1). Their variations were higher than

those of the cations, between 5- and 6-fold.The Sr concentrations varied greatly (12-fold) between the minimum (0.013 μ mol l−1 in

January 2004) and the maximum (0.16 μ mol l−1 in September 2004) giving a mean weightedconcentration of 0.06 μ mol l−1. The 87Sr/86Sr ratio ranged from 0.71079 (March 2004) to0.71119 (December 2004).

4.2.3 Brest. The order of cation abundance was Na+ > Mg2+ > Ca2+ > K+, with theNa+ and Mg2+ species giving mean weighted concentrations of 227 and 27 μ mol l−1, respec-tively. The Na+ concentrations varied 5.3-fold (394 μ mol l−1 in February 2004 to 74 μ mol l−1

in September 2003), and the Mg2+ concentrations varied 5.8-fold (63 μ mol l−1 inAugust 2003to 11 μ mol l−1 in September 2003). The order of anion abundance was Cl− > SO2−

4 ∼ NO−3 ,

with Cl− giving a mean weighted concentration of 245 μ mol l−1, being 7- to 8-fold that ofSO2−

4 (32 μ mol l−1) and NO−3 (36 μ mol l−1). The variations in concentration were similar to

that of cations for Cl− and SO2−4 (6- and 5.4-fold, respectively), but this variation was very

high for NO−3 (around 25-fold; 172 μ mol l−1 inAugust 2003 and 7 μ mol l−1 in January 2004).

The Sr concentrations varied greatly (10-fold) between the minimum (0.015 μ mol l−1 inSeptember 2003) and the maximum (0.15 μ mol l−1 in August 2003) giving a mean weightedconcentration of 0.04 μ mol l−1. The 87Sr/86Sr ratio ranged from 0.70926 (February 2004) to0.70977 (September 2003).

4.2.4 Dax. The order of cation abundance was Na+ > Ca2+ > K+ > Mg2+, with the Na+and Ca2+ species giving mean weighted concentrations of 195 and 41 μ mol l−1 respectively.The Na+ concentrations varied 3.1-fold (274 μ mol l−1 in October 2003 to 90 μ mol l−1 inAugust 2003) and the Ca2+ concentrations varied 5.5-fold (114 μ mol l−1 in June 2004 to21 μ mol l−1 in January 2004). The order of anion abundance was Cl− > SO2−

4 > NO−3 , with

Cl− yielding a mean weighted concentration of 183 μ mol l−1, being 6-fold that of SO2−4

(30 μ mol l−1) and 8-fold that of NO−3 (23 μ mol l−1). The variations in concentration were

similar to that of cations for Cl− and SO2−4 (5.4 and 2.4-fold, respectively) but, as for the

Brest site, the variation was large for NO−3 (around 42-fold; 11 μ mol l−1 in October 2003 and

3 μ mol l−1 in September 2003).The Sr concentrations varied greatly (2.5-fold) between the minimum (0.08 μ mol l−1 in

January 2004) and the maximum (0.22 μ mol l−1 in April 2004) giving a mean weightedconcentration of 0.13 μ mol l−1. The 87Sr/86Sr ratio ranged from 0.71025 (February 2004) to0.71076 (September 2003).

4.2.5 Values from the literature: Tours. An identical survey to that performed inClermont-Ferrand was carried out in the city of Tours from September 1996 to September1997 (figure 1 [10]). Seven samples of rainwater (190 mm) were collected each month. Aswith the automatic sampler, one sample represented all the rainwater that fell during onemonth.

The cation abundance was Na+ > Mg2+ > Ca2+ and K+. The mean weighted Na+ con-centration was 203 μ mol l−1, with the mean weighted Mg2+ concentration being 30 timeslower at 6.7 μ mol l−1 and the mean weighted Ca2+ concentration being 2.7 μ mol l−1. TheK+ concentrations were often below the detection limit of the analysis (between 5 and10 μ mol l−1). The anion abundance was Cl− > SO2−

4 > NO−3 , except for January 1997 when

NO−3 was the most abundant species. The mean weighted Cl− concentration was 104 μ mol l−1,

with the mean weighted SO2−4 concentration being six times lower at 16 μ mol l−1 and

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Hydrochemistry and strontium isotopes of rainwater in France 185

the mean weighted NO−3 concentration being of the same order of magnitude as SO2−

4at 15 μ mol l−1. The inversion of the Cl− and NO−

3 abundances could be related to dif-ferent air-mass origins. During January 1997, the dominant wind directions and air-masstrajectories were clearly easterly, passing over all the Eastern European forested areasto give dominant NO−

3 . Moreover, during November 1996, the Cl− concentration varied2-fold higher because of dominant westerly wind directions with an oceanic origin for theair-mass [10].

The Sr concentrations were fairly uniform, except for the January 1997 rainwater. Sr con-centrations varied from 0.012 μ mol l−1 (December 1996) to 0.04 μ mol l−1 (January 1997).The 87Sr/86Sr ratio ranged between 0.70901 (May 1996) and 0.71006 (March 1997) with aweighted mean of 0.70943.

4.2.6 Values from the literature: the east of France. Chabaux et al. [11] measured majorand trace element concentrations, as well as Sr isotope ratios in rainwater samples collectedin three different locations in the east of France and Luxembourg: a mid-altitude mountainsite (Aubure), an urban site (Strasbourg) and a peri-urban site located in an area of well-developed industrial activity (Esch-sur-Alzette in Luxembourg). Major element concentrationsconfirm that the chemical composition of these rainwater samples departs from seawaterchemical composition, implying the contribution to these rainwater samples of continentalcomponents of natural and/or human origin. In the Esch-sur-Alzette rainwater, the dominantcations and anions were Ca2+ and HCO−

3 , respectively, Ca2+ is the dominant cation in Aubureand Strasbourg rainwater, and SO2−

4 and NO−3 the dominant anions. The origin of the high

level of Ca2+ and HCO−3 in Esch-sur-Alzette rainwater is related to the input of carbonate

particles into rainwater from neighbouring industries.The Sr isotope ratios of the rainwater in Esch-sur-Alzette varied between 0.70879 and

0.70903 (mean value and standard deviation close to 0.70895 ± 0.00013, n = 3), lower thanthe seawater value. They are consistent with an input of Sr from dissolution of carbonate dustscoming from old marine sediments, as used by local industry (iron and steel works, cementworks). The Sr isotope ratios of the Aubure rainwater (between 0.71078 and 0.71457; meanvalue and standard deviation close to 0.71257 ± 0.0016, n = 4) are characterized by radio-genic values higher than seawater, which indicates the contribution of a radiogenic Sr source.The simplest explanation would be the contribution of Sr input from dusts of local graniticbedrocks and soils that are radiogenic and/or plant exudates as Ca isotopes suggest [14]. TheStrasbourg precipitations are marked by 87Sr/86Sr ratios between 0.70858 and 0.70902 (meanvalue and standard deviation close to 0.70884 ± 0.00013, n = 8), suggesting the contributionof at least two different Sr sources to the Strasbourg rainwater (seawater end-member andinput of aerosol of carbonate origin).

4.2.7 Values from the literature: Montpellier. Seven individual rain samples were col-lected in the city of Montpellier between 1995 and 1996 (south France, figure 1) and presentedby BenOthman et al. [27]. Only three samples were analysed for chemistry and five for Srisotopes. Located 30 km from the Mediterranean Sea, the rainwater showed variable Cl con-centrations from 32 to 100 μ mol l−1, NO−

3 was the other main anion with a concentrationthat ranged from 56 to 100 μ mol l−1. Ca2+ was the main cation but the concentrations variedlargely from less than 1 up to 156 μ mol l−1. This reflects an influence of both continental dustand/or seawater aerosols since seawater could also provide Ca-rich aerosols (CaSO4).

The Sr concentrations fluctuated from 0.01 up to 253 μ mol l−1 and the 87Sr/86Sr isotoperatio ranged between 0.70834 and 0.70980, with a mean value of around 0.70897.

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4.2.8 Values from the literature: Paris. Seimbille et al. [25], Roy [26] and Négrel (unpub-lished data) have reported Sr2+ concentrations in Parisian rainfall (600 km from the Atlanticocean) in the range of 0.30–0.60 μ mol l−1, with a 87Sr/86Sr ratio ranging from 0.70846 to0.70895 and a mean value close to 0.7087.

5. Discussion

5.1 Elemental variation with the distance to the ocean

Meybeck [29] has defined the variation in elemental chemistry of rainwater in France in aneastward direction away from the Atlantic Ocean. Average air mass trajectories and weatherpatterns (French Meteorological Office, http://www.meteofrance.com) have enabled air massorigins to be sectorized. The predominant sector is westerly and mainly of marine origin(Atlantic Ocean) and corresponds to 52% of rain events (French Meteorological Office,http://www.meteofrance.com). Among the three other sectors, the SE-SW sector is of marineorigin, possibly carrying natural aerosols from Africa and pollution from Spain and corre-sponds to 27% of rain events whereas the NE-NW sector is of both marine (North Atlantic andNorth Sea) and continental (Great Britain) origin, passes over a more or less industrializedarea and corresponds to 16% of rain events.

As defined by Meybeck [29], the Cl− concentrations in rainwater are close to 790 μ mol l−1

in all coastal locations, decreasing to 150 μ mol l−1 around 50 km eastward and reach70 μ mol l−1 around 100 km farther inland. In an W-E cross-section through France, thesedata as well as the mean values from the different sites of this study (Brest, Dax, Orleansand Clermont-Ferrand) and that from Tours [10] and the east of France [11] are plotted withincreasing distance from the Atlantic coast (i.e. along the main air masses trajectory fromthe westerly sector) in figure 2. The data define a good relationship between the mean Cl−concentration and the distance to the ocean, as illustrated by the R2 coefficient (0.93). Theequation is in the form

Cl− = −47 × ln(distance) + 332

reflecting the decrease in SS aerosols. This evolution has been demonstrated previously byStallard and Edmond [4] in South America with Cl−, originating from the dissolution ofatmospheric SS particles by rainwater, showing concentrations decreasing with increasingdistance from the coast through washout processes [30].

5.2 Origins of major ions in the rainwater

The most usual method of evaluating the contribution of SS to ion contents in precipitationis to compare the Cl−/Na+ ratio in rainwater to that of seawater. SS is considered to be themajor source of both ions, although they may also be emitted from other natural and industrialsources [3, 31]. The regression line between Na+ and Cl− in rainwater is depicted in figure 3a.A 95% confidence level is assigned to the data that fall between the two lines. The seawaterdilution line is also represented in the figure 3a. Chlorine ions show strong correlation withNa+ (R2 = 0.99), all the samples lie near the seawater (SW) line indicating that both Na+ andCl− in these samples came from SS. Brest and Dax show the highest concentrations in Na+and Cl−.

Thus, the similarities in the ratios of Na+ to Cl− in seawater-rainwater indicate that SS dom-inates and the Na+ ion concentrations in each sample can be used to estimate SS contributionof other ions, which is the element generally used as the best tracers of SS input [8, 20, 31, 32].

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Hydrochemistry and strontium isotopes of rainwater in France 187

Figure 2. Variation in Cl− concentration as a function of the distance to the Atlantic coast. Data from this study aswell as Meybeck [29], Grosbois et al. [10] and Chabaux et al. [11].

Differentiation of SS and non-sea salt (NSS) component contributions in rainwater (RW) isessential in order to be able to decipher the precipitation chemistry [6, 14, 20, 31]. To calculatethe contribution for a given element X of the SS component with seawater (SW) characteristics,the following equation is used:

XSS = Na × (X/Na)SW (1)

Na is used as a marine tracer in rainwater. The contribution of the NSS component (NSS) isthe difference between the total composition of rainwater (RW) and the SS contribution:

XNSS = XRW − XSS (2)

NSS sources make substantial contributions to observed concentrations of Ca2+, Mg2+ and K+ions (table 2). Almost all Ca2+ in the Dax, Orleans and Clermont-Ferrand sites (mean values90–97%) was of NSS origin. Rastogi and Sarin [31] in individual rain events collected over 3years (2000–2003) in south India found similar Ca2+ inputs from NSS sources. In Brest, theproportion of Ca2+ from the NSS source was lower (mean value around 51%), in agreementwith the location of the site in the vicinity of the coast. The large variations in Ca2+ contentswhen compared to Na+ as illustrated in figure 3b and the evidence that most of the samples plotabove the seawater line agree with a large input of NSS Ca2+. The Ca2+ in rainwater, mainly

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188 Philippe Négrel et al.

Figure 3. Plot of Cl−, Ca2+, Mg2+, K+ and SO2−4 vs. Na+ contents, expressed in μ mol l−1, in rainwater collected

in Brest, Dax, Orleans (data from this study), Clermont-Ferrand [20], Tours [10] and the east of France (Alsace, [11]).The seawater line and the linear relationship are represented. For figure 3a, a 95% confidence level is assigned to thedata that fall in the envelope defined by the two lines.

Table 2. Mean, minimum and maximum values for the NSS component in the rainwater from Brest, Dax,Orleans and Clermont-Ferrand (see text for calculation).

Ca NSS (%) Mg NSS (%) K NSS (%) SO4 NSS (%) Sr NSS (%)

Brestmin 0 0 0 44 0max 92 58 84 88 71mean 51 12 29 61 9

DAXmin 81 6 58 16 60max 98 46 93 87 88mean 90 25 80 64 74

Orleansmin 88 9 91 80 77max 96 66 88 72 95mean 92 35 97 86 93

Clermontmin 84 19 59 80 37max 99 99 100 99 97mean 97 64 91 95 82

of terrestrial origin (i.e. NSS), comes from the dissolution of CaCO3 dusts [14, 31]. Ca2+ canalso be produced by pollution (burning coal, cement factories), but the lack of correlation(not shown) between NSS Ca2+ and SO2−

4 indicates that Ca2+ comes mainly from soil dustwhile excess SO2−

4 can be commonly related to pollution (mainly due to burning of fossilfuel [3, 31]).

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Hydrochemistry and strontium isotopes of rainwater in France 189

Similarly, this was observed for K+ with 80–97% of NSS K+ in Dax, Orleans and Clermont-Ferrand and only 29% of NSS K+ in Brest, respectively (table 2). This is further illustratedin figure 3d by enrichment of the K+ concentrations when compared to Na+ concentrations.There are various non-marine origins of K+ in continental rain, including soil dust fromsilicate and calcareous soils [3] either with a local or a remote origin. Another source isagricultural soil dust with fertilizer [3]. A third source is biogenic aerosols, even in temperateclimates. For Mg2+, the values were more variable, ranging between 12% in Brest to 64%in Clermont-Ferrand. This is also illustrated in figure 3c with variable Mg2+ concentrationswhen compared to Na+. Most of the SO2−

4 was of NSS origin in the Orleans and Clermont-Ferrand sites (mean values 86–95%) and lower values around 61–64% were observed in Brestand Dax. Plant exudates on leaf surfaces contain soluble salt (K2SO4) that can contribute K+[3, 33] and SO2−

4 to rain. However, the lack of correlation (not shown) between K+ and NSSSO2−

4 indicates that this is not the primary source and K+ from anthropogenic sources doesnot have the same origin as SO2−

4 .

5.3 Sr and its isotope ratios

5.3.1 Variation in Sr concentrations and 87Sr/86Sr. Strontium isotope ratios vary innature because part of the strontium isotopes (87Sr) is formed by radioactive decay of thenaturally occurring element rubidium (87Rb). 87Sr/86Sr ratios are mainly used as tracersof water–rock interaction [24, 34]. The Sr2+ concentrations in rainwater are generally lowand fluctuate according to the aerosol sources [20]. Several workers have reported con-centrations and isotope ratios of Sr in individual rainwater samples over France and theirresults highlight the role of aerosol sources in the isotopic signature of rains. Close to theAtlantic Ocean (50 km inland, latitude of La Rochelle 46.1 N; Négrel, unpublished data),Sr2+ concentrations in individual rain samples were around 0.025 μ mol l−1 with a 87Sr/86Srratio of around 0.70949; a decrease in the Sr2+ concentration was observed 150 km fromthe sea (Sr2+ = 0.017 μ mol l−1) but the 87Sr/86Sr ratio remained similar (0.70954). Seim-bille et al. [25], Roy [26] and Négrel (unpublished data) have reported Sr2+ concentrationsin Parisian rains (400 km from the ocean) in the range of 0.031–0.057 μ mol l−1, with a87Sr/86Sr ratio ranging from 0.70846 to 0.70895. Concentrations of Sr2+ found in this studyranged from 0.02 μ mol l−1 in the Clermont-Ferrand site to 0.13 μ mol l−1 in Dax. The illus-tration of H Sr2+ vs. Na+ content variation in figure 4a for the Brest samples shows thatboth element contents are covariant, reflecting that most of the Sr2+ is of marine origin.Thus, calculation of NSS fractions in the Brest site shows that most of the Sr2+ is of SSorigin, with the exception of a few samples with low values of NSS Sr2+ (0–15%) andone sample with high Sr2+ (71%) of NSS origin (table 2). However, this sample collectedduring August 2003 (during the great heatwave) has the lowest rainfall amount (6 mm),which could explain the dominant origin of NSS aerosols. As illustrated in figure 4a andconsidering the calculation of the NSS fraction, a large part of the Sr2+ (mean valuesbetween 74 and 93%) was of NSS origin in the Dax, Orleans and Clermont-Ferrand sites(table 2).

5.3.2 Characterisation of the 87Sr/86Sr ratio input by rainwater over France. Table 3summarises the mean 87Sr/86Sr in the rainwater from this study and the literature. In Brest, Dax,Orleans and Clermont-Ferrand, the mean 87Sr/86Sr can be calculated either by weighting withthe Sr2+ concentration and/or rainfall amount. For the other sites, only the mean 87Sr/86Srcan be calculated by averaging the data. There are few differences between the mean andthe mean weighted 87Sr/86Sr ratio (6–9·10−5), the largest difference is for Clermont-Ferrand

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Figure 4. Plot of Sr2+ vs. Na+ contents (figure 4a), expressed in μ mol l−1, in rainwater collected in Brest, Dax,Orleans (data from this study), Clermont-Ferrand [20], Tours [10] and the east of France (Alsace, [11]). The seawaterline is represented. Plot of the strontium isotopic ratios in rainwater in the diagram 87Sr/86Sr vs. the inverse of Sr2+content (figure 4b) in Brest, Dax, Orleans (data from this study) and data from the literature ([20], Clermont-Ferrand;[11], eastern France; [10], city of Tours; [27, city of Montpellier; [25], [26], Négrel, unpublished data for the cityof Paris).

Table 3. Mean 87Sr/86Sr and mean weighted 87Sr/86Sr in the rainwater from Brest, Dax, Orleans andClermont-Ferrand. Mean 87Sr/86Sr in the rainwater from Clermont-Ferrand, eastern France, city of Tours, city of

Montpellier and city of Paris.

Mean non-weighted Number of considered 87Sr/86SrSite 87Sr/86Sr samples Mean weighted Reference

Dax 0.71057 4 0.71051 This studyOrleans 0.71099 4 0.71094 This studyBrest 0.70941 4 0.70932 This studyClermont-Ferrand 0.71067 10 0.70978 This study, Négrel and

Roy [20]Tours 0.70943 7 nd Grosbois et al. [10]Strasbourg 0.70884 8 nd Chabaux et al. [11]Aubure 0.71078 4 nd Chabaux et al. [11]Esch 0.70879 3 nd Chabaux et al. [11]Montpellier 0.70897 7 nd BenOthman et al. [27]Paris 0.7087 12 nd Seimbille et al. [25], Roy [26]

and Négrel (unpublisheddata)

(9 · 10−4). Four sites display 87Sr/86Sr ratios higher than 0.710 (Dax, Orleans, Clermont-Ferrand and Aubure) and are geographically dispersed over France. The remainder displays87Sr/86Sr ratios lower than 0.7094, likewise with a large dispersion over the national territory.This means that the 87Sr/86Sr ratios are regionally specific and that the identification of theprecipitation input either in surface water or ground water needs such characterization overthe whole territory, as illustrated in figure 1.

5.3.3 87Sr/86Sr ratio to constrain NSS components. When coupled with Sr concen-trations, Sr isotope systematics can be used to investigate the mixing of different Sr sources[20, 35, 36]. By plotting the 87Sr/86Sr ratio vs. the reciprocal Sr2+ concentration in component

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Hydrochemistry and strontium isotopes of rainwater in France 191

mixtures, straight lines are produced [37]. The 87Sr/86Sr ratio in water thus reflects the differentsources of Sr2+ (e.g. SSs, dusts from rock weathering and pollution) and certain constraintson the mixing from these sources are provided by Sr isotope systematics.

Figure 4b summarises the 87Sr/86Sr ratio vs. the inverse of Sr content in rainwater fromthis study as well as those from the literature. It is worth noting the lack of direct linearrelationship between the 87Sr/86Sr ratio and the inverse of Sr content, which pleads in favourof at least three sources of Sr in the rainwater. Most of the rainwater samples collected in Paris,Montpellier and in the east of France have a 87Sr/86Sr lower than that of seawater. This reflectsa contribution from at least one non-radiogenic Sr source. Most of the rainwater samples fromDax, Orleans and Clermont-Ferrand have Sr values superior to the seawater value, while thosefrom Brest plot closely to the seawater value. This indicates a contribution to the rainwater ofat least a radiogenic Sr source. Both the low-radiogenic and the radiogenic source(s) can beeither natural (e.g. NSS) or anthropogenic.

NSS contributions can be from dust emitted by the continent [11, 19, 20]. Among the differ-ent crustal sources, limestone has a high strontium content (600–1000 ppm) and low 87Sr/86Srratio (0.706–0.709 [37]). The 87Sr/86Sr ratios of calcium carbonate are similar to those of thewater where deposition took place because of its very low Rb/Sr ratio [37]. On the other hand,silicate rocks (granite and gneiss bedrock) have lower Sr contents than limestone and higher87Sr/86Sr ratios [37]. Basalt has low isotope ratios (0.703–0.705 [37]) but its Sr content is inthe same range as silicate rocks. Thus, the 87Sr/86Sr ratio of silicate dust depends on the ageand Rb/Sr ratio of the parent rocks.

The isotopic composition of Sr in dusts can be estimated by studying 87Sr/86Sr ratios of riverwater [20, 38]. In a carbonate watershed, because of the congruent dissolution of carbonaceousrocks, the 87Sr/86Sr ratio of river water agrees with that of the rocks. Leaching experimentswith doubly distilled water of soils from silicate watersheds have shown that soluble Sr in soilsand Sr in the dissolved load in rivers also have similar isotope ratios [39]. Vegetation extractscations, including strontium, from soil waters and Graustein and Armstrong [12] have shownthat water in soils and plants have identical 87Sr/86Sr ratios, however higher than that of thedissolved load in rivers.

One of the NSS components in rainwater can be related to calcic particles from soil orlimestone dusts. Main geological deposits include Mezozoic and Cenozoic carbonates [40].The study of rivers draining only one carbonate facies provided 87Sr/86Sr ratios indicatingthe possible sources of dusts [24, 41]. The Lower Jurassic, the Lower Cretaceous and theTertiary yield 87Sr/86Sr ratios ranging between 0.7086 and 0.7094; the Upper Jurassic and theUpper Cretaceous yield 87Sr/86Sr ratios ranging between 0.7081 and 0.7084. Other Tertiarydeposits (Miocene and Oligocene marls and limestones) yield high Sr isotope ratios (0.711–0.712 [20, 42]).

The second NSS component in rainwater is related to dust emissions from silicate terrains(clay minerals, mica and feldspar). The Sr isotope ratios of this silicate end-member can beapproached through the dissolved load of rivers draining silicate basement (granite, gneiss andbasalt bedrock). For granite and gneiss basements, the 87Sr/86Sr ratios range between 0.7135and 0.717 [34, 35, 41, 43–45] and for basaltic basement the 87Sr/86Sr ratios range between0.703 and 0.705 [46–48].

The last Sr source is made up of anthropogenic input [3], such as fertilizers with Sr concentra-tions up to 1500 ppm and an 87Sr/86Sr ratio ranging between 0.7079 and 0.7087 [20, 46, 49].Little data exist on Sr concentrations or isotope compositions of other possible pollutants.Recent data obtained in the Paris atmosphere [50] gives, for automobile exhaust, an 87Sr/86Srratio ranging between 0.7077and 0.7083, for urban heating an 87Sr/86Sr ratio ranging between0.7083 and 0.7335, and for incinerators an 87Sr/86Sr ratio ranging between 0.7097 and0.7100.

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Figure 5. Plot of the mean 87Sr/86Sr and standard deviation (1σ ) in rainwater from Brest, Dax, Orleans (data fromthis study) and data from the literature ([20], Clermont-Ferrand; [11], eastern France; [10], city of Tours; [27], cityof Montpellier; [25], [26], Négrel, unpublished data for the city of Paris) as well as the seawater and all NSS sources(see text for description).

Figure 5 summarizes the mean 87Sr/86Sr and standard deviation (1σ ) in rainwater as wellas seawater and all the NSS sources described above. The main features of this figure for therains over France are:

(1) the large variation in the mean 87Sr/86Sr ratios demonstrates the different sources ofSr2+, the SSs value (0.70917 [51]) never being directly observed, even in near-coast sites(Brest, Dax). This has been shown by Herut et al. [17] in rain samples from Israel withvalues lower than seawater due to the input of soluble aerosols of important non-sea spraysources (chalk and recent marine deposits). For Brest, the mean 87Sr/86Sr ratio, slightlyhigher than that of seawater, can be due to the input of silicated dusts forming the basementof French Brittany with high 87Sr/86Sr ratios [45]. For Dax, different hypotheses will bediscussed below. Furthermore, the 87Sr/86Sr ratios in rain do not tend to increase and/ordecrease going inland.

(2) 87Sr/86Sr ratios lower than the seawater value point to a carbonate source in aerosols,either local or remote as suggested by Han and Liu [52], the most probable candidatesare Jurassic and Cretaceous terrains. For Esch and Strasbourg [11], Paris [25, 26] andMontpellier [27], an input from anthropogenic sources (fertilizers, automobile exhausts,incinerators and urban heating [19]) cannot be entirely rejected.

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Hydrochemistry and strontium isotopes of rainwater in France 193

(3) some high 87Sr/86Sr ratios in Aubure and Clermont-Ferrand relate to crustal sources assuggested by Chabaux et al. [11] and Négrel and Roy [20]. Such influence can be linkedto dusts from silicate rock and associated soils. However, for both sites, an influenceof a low-radiogenic Sr2+ source should also be pointed out since the mean 87Sr/86Srratio is lower than the value for granite and gneiss. For Aubure, a carbonate source ispointed out (Jurassic to Cretaceous, [11]). Similarly, the carbonate source(s) from theTertiary deposits in the Massif Central (figure 5) may explain the low 87Sr/86Sr ratios inClermont-Ferrand [20].

Such silicate influence in the aerosols may also be highlighted in precipitation collected inOrleans and Dax, even if silicated terrains (e.g. granite, gneiss, etc.) are remote from thesesites. Thus, in Orleans, an influence of both urban heating and tertiary deposits (Massif Centraltype) are better candidates for assessing the high 87Sr/86Sr ratios. In Dax, a silicate influencein the aerosols may be suspected but the source area is not yet well defined as only the CentralZone of the Pyrenees can produce such dusts (87Sr/86Sr ratios around 0.712 [53]). Anothersource could be urban heating, which could induce high 87Sr/86Sr ratios. However, the mostprobable source of high 87Sr/86Sr ratios may be a biogenic source [19]. Indeed, there is thepresence of maritime pine forest Pinus pinaster representing 80% of the trees in the ‘Forêtdes Landes’, a large area covering 860,000 ha developed on sands containing chlorite, micas,feldspars and quartz [54]. As illustrated in figure 3d, there is a large K enrichment in rainwaterfrom Dax, agreeing with numerous studies that have reported the presence of K and Rb inparticles released by plants [33]. Thus, the high 87Sr/86Sr ratios in rainwater may be due tothe radiogenic Sr2+ released through the vegetation exudates, as demonstrated by numerousstudies [12, 13, 19], which originates from the surrounding silicated terrains. The presence ofmica and feldspars in the sand yields high 87Sr/86Sr ratios in water and thus in plants butalso identical 87Sr/86Sr ratios water in soils and plants as shown by Graustein [55]. This wastested by sampling one shallow groundwater from an alluvial aquifer in the sand just southof Bordeaux and a river headwater near Dax. Both gave 87Sr/86Sr ratio close to 0.7108 and0.7118, respectively, agreeing with a possible radiogenic value from plant exudates.

6. Summary of the nature and origin of the different sources in rainwater over France

The concentrations of the analysed elements in the rain samples collected over 1 year in fourstations (Brest, Dax, Orleans and Clermont-Ferrand) varied greatly over the sampling period.The 87Sr/86Sr ratios also fluctuated significantly from 0.70920 to 0.71314. These variationsare, for the most part, related to variations in the sources of aerosols. The coupled use of majorelement (mainly Na as marine reference) and Sr isotope systematics enabled us to highlightthe mixture of several components that fingerprint the isotope signature. Such sources includeSS, crustal sources (carbonates, silicates and volcanic rocks) and anthropogenic sources(fertilizers, automobile exhausts, incinerators and urban heating).

Considering the mean 87Sr/86Sr ratios among the sources of aerosols, the sea salt endmem-ber was never directly observed, even in near-coast sites (Brest, Dax). In both locations, themean 87Sr/86Sr ratios were slightly higher than that of seawater, which can be due to theinput of silicated dusts or related to a biogenic source. This was also observed more inlandwith higher values of the mean 87Sr/86Sr ratio in sites located within a silicated areas (Aubure,Clermont-Ferrand) as well in sites located in sedimentary areas (Orleans), suggesting a remotesource of aerosols with a high 87Sr/86Sr ratio. On the other hand, the mean 87Sr/86Sr ratioslower than the seawater value point to a carbonate source in aerosols (Esch and Strasbourg,

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Paris, Montpellier) either local or remote but an input from anthropogenic sources cannot beentirely rejected (Paris).

Results from this study help define the input of Sr either to surface or groundwater and giveconstraints on the 87Sr/86Sr values that should be corrected for atmospheric input in order tocharacterize the anthropogenic signature [23, 24] and/or the water-rock signature [9, 21, 28].

Acknowledgements

The BRGM Research Division financially supported this work. Chemical and isotopic analyseswere performed in the Geochemistry Laboratory of BRGM, France. The work benefited fromthe collaboration of C. Crouzet (BRGM Chemistry team), who provided the major elementalanalyses. The authors thank the BRGM Translation Service for proofreading the English. Twoanonymous reviewers are acknowledged for providing helpful reviews of this manuscript.

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