Philippe Négrel
email: p.negrel@brgm.fr

Romain Millot
email: r.millot@brgm.fr

Emmanuelle Petelet-Giraud

Gerard Klaver
email: gerardklaver3@gmail.com

Li and δ7Li as proxies for weathering and anthropogenic activities: Application to the Dommel River (meuse basin)

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Lithium is a mobile element that preferentially tends to go into the fluid phase during water/rock interactions [START_REF] Huh | Lithium and its isotopes in major world rivers: implications for weathering and the oceanic budget[END_REF]Kisakürek et al., 2004). Lithium has two stable isotopes of mass 6 (natural abundance 7.5%) and 7 (natural abundance 92.5%), the relative mass difference between the two isotopes being considerable (17%). This generates significant mass-dependent fractionation during geochemical processes in the context of water/rock interactions [START_REF] Huh | Lithium isotopes as a probe of weathering processes: Orinoco River[END_REF][START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF][START_REF] Rudnick | Extreme lithium isotopic fractionation during continental weathering revealed in saprolites from South Carolina[END_REF][START_REF] Pogge Von Strandmann | Riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain[END_REF], 2014;[START_REF] Millot | Multi-isotopic (Li, B, Sr, Nd) approach for geothermal reservoir characterization in the Limagne Basin (Massif Central, France)[END_REF]Millot et al., , 2010a,b;,b;[START_REF] Dellinger | Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion[END_REF][START_REF] Dellinger | [END_REF][START_REF] Henchiri | Riverine dissolved lithium isotopic signatures in low-relief central Africa and their link to weathering regimes[END_REF]. These studies have clearly shown that isotopic fractionation supports the enrichment of the heavy isotope ( 7 Li) in solution, the light isotope ( 6 Li) being preferentially retained in secondary weathering minerals. Variation in the δ 7 Li values of water was scarcely used for distinguishing between natural and anthropogenic inputs, even if lithium isotopes can be a probe for tracing anthropogenic activity [START_REF] Négrel | Lithium isotopes as tracers of groundwater circulation in a peat land[END_REF].

We analysed both the suspended and dissolved loads of the major tributaries of the Dommel basin, in order to determine and quantify the anthropogenic influence. We used δ 7 Li and mixing equations in the impacted parts of the catchment for deducing the behaviour of Li isotopes during the weathering of sandy deposits. The Dommel is a small river that has its origin in north-eastern Belgium and runs through the southern part of the Netherlands before joining the Meuse north of Hertogenbosch. The water quality of the Dommel is strongly influenced upstream by discharge from urban-wastewater treatment plants [START_REF] Petelet-Giraud | Natural versus anthropogenic sources in the surfaceand groundwater dissolved load of the Dommel River (Meuse basin): constraints by boron and strontium isotopes and gadolinium anomaly[END_REF]. Also upstream along the Dommel lies the Umicore zinc smelter in Overpelt (Belgium); discharge from the smelter contains exceptionally high concentrations of chemical elements, including lithium and its isotopes, which can be used as effective tracers of industrial discharge into the river [START_REF] Petelet-Giraud | Natural versus anthropogenic sources in the surfaceand groundwater dissolved load of the Dommel River (Meuse basin): constraints by boron and strontium isotopes and gadolinium anomaly[END_REF].

Site description

The Dommel catchment in northern Belgium and the southern part of the Netherlands drains 380 km² with three main tributaries (Eindergatloop, Keersop and Run, Fig. 1), discharging into the Meuse River north of Eindhoven (Fig. 1). The mean annual rainfall in the Dommel area is about 740 mm/yr, the reference evapotranspiration for grassland is 560 mm/yr and the average annual runoff is 190 mm/yr, thus groundwater seepage is estimated to contribute up to 70% to the annual runoff for the Dommel catchment [START_REF] Pieterse | Contribution of point sources and diffuse sources to nitrogen and phosphorus loads in lowland river tributaries[END_REF][START_REF] Rozemeijer | The groundwater contribution to surface water contamination in a region with intensive agricultural land use (Noord-Brabant, The Netherlands)[END_REF] with groundwater levels usually within 1 to 3 m below surface. Agriculture occupies 56% of the Dommel catchment and consists mainly of intensive livestock farming. About 20% is urbanized [START_REF] Pieterse | Contribution of point sources and diffuse sources to nitrogen and phosphorus loads in lowland river tributaries[END_REF].

Geologically, the upper Dommel catchment is divided into two parts. The southwestern part is on the Campine High and the rest of the catchment lies in the Roer Valley Graben (RVG), which is bounded by the Feldbiss and Peel faults (Schokker, 2003, Fig. 1).

The RVG is covered by the Boxtel Formation, a sandy deposit with small amounts of micas and feldspars, and by heterogeneous loam and peat layers (maximum thickness ~35 m). The Boxtel is underlain by the Sterksel Formation, consisting of Pleistocene sand and gravel deposited by the Meuse River. On the Campine High, the Sterksel Formation is the top unit.

Regionally, the sandy soils of the Boxtel and Sterksel formations are vulnerable to leaching due to the acidifying conditions caused by an absence of carbonates, a small amount of clay minerals (<1%), and low contents of organic matter [START_REF] Wilkens | Accumulation of cadmium and zinc from diffuse emission on acid sandy soils, as a function of soil composition[END_REF]. Upstream from the confluence with the Keersop and Run streams, the Dommel receives water from the Hageven nature reserve, which is partly fed by seepage from the Bocholt-Herenthals canal (~0.3 m 3 .s -1 ), the latter containing water from the Meuse River. The basin also receives contaminated industrial and agricultural waters. The anthropogenic impact of past and present zinc smelting is first as a diffuse source through past atmospheric emissions, now concentrated in the soil and clearly higher than the natural background in streams, and as sulphur dioxide (SO2) emissions. Secondly, it can be considered as a point source, as the treated wastewater from the Umicore smelter discharges into the Eindergatloop stream. These effluents are the final product from the physico-chemical treatment of smelter wastewater through several pH adjustments (using amongst others chalk hydrate) where metals, fluorides and sulphates are removed [START_REF] Ovam | Actieplan Cadmium -February[END_REF]. Improvements in the treatment process (op. cit.) have led to a reduction of more than 90% for Cd and 60% for Zn in the

Methods

Sampling

The water sampling locations are schematically shown on were also sampled for a "Snapshot" campaign on 17 th July 2007. In addition to monitoring by the Waterboard de Dommel, additional monitoring stations were selected around the confluence of the Dommel with its tributary Run (Figs. 1 and2) as the latter flows only through an agricultural area and is assumed to be representative of a "diffuse-source" input in the upper Dommel catchment. Water was also sampled from an artificial pond (240013AV) isolated from the streams and fed only by groundwater and rainwater. Water was also sampled from a lake, hereafter referred as the Lake Veldhoven, located along the Run River.

Analytical method

Surface-and groundwater samples were collected in polyethylene bottles and separated into several aliquots. Two of these were filtered through pre-cleaned 0.45 µm acetate filters with a pre-cleaned Nalgene filter apparatus; the filtrate of one of the aliquots was further separated into two additional aliquots (100 and 1000 ml). The latter was acidified with Suprapur HNO3 (pH ~2) and stored in pre-cleaned polyethylene bottles for major-cation analysis and for lithium-isotope and Li-content determinations.

The water samples were analysed by ion chromatography for major ions (accuracy 5 to 10%), by inductively coupled plasma mass spectrometry for Li (accuracy 5%), and by Gran's method (HCl titration) for alkalinity [START_REF] Négrel | Strontium isotope as tracers of groundwater-induced floods: the Somme case study (France)[END_REF].

Lithium-isotope compositions were measured with a Neptune Multi-Collector ICP-MS (Thermo Fisher Scientific) in the BRGM Isotope Laboratory (see [START_REF] Millot | Accurate and high precision measurement of lithium isotopes in two reference materials by MC-ICP-MS[END_REF] for more details) and are reported in delta units as follows:

( ) ( ) ( )

3 6 7 6 7 7 10 1 / / ‰ ×         - = -SVEC L sample Li Li Li Li Li δ (Eq. 1)
The 7 Li/ 6 Li ratios were normalized to the L-SVEC standard solution (NIST SRM 8545) following the standard-sample bracketing method. Typical in-run precision on the determination of δ 7 Li is about 0.1-0.3‰ (2σm, [START_REF] Millot | Accurate and high precision measurement of lithium isotopes in two reference materials by MC-ICP-MS[END_REF]2010a). Lithium was chemically separated from the matrix before mass analysis using 3 mL of cationic resin (BioRad AG® 50W-X12, 200-400 mesh) and HCl (0.2N) for 30 ng of lithium. Blanks for the total chemical extraction were ≥20-30 pg Li, which is negligible since it represents a 10 -3 blank/sample ratio. The accuracy and reproducibility of the total method (purification + mass analysis) were tested by repeated measurement of a seawater sample (IRMM BCR-403) after separation of lithium from the matrix, for which we obtained a mean value of δ 7 Li = +30.8‰±0.4 (2σ, n=15) over the duration of the analyses. This mean value is in good agreement with our long-term measurement (δ 7 Li = +31.0‰±0. 5, 2σ, n=30, Millot et al. 2004).

Results

Dissolved lithium concentrations in the Dommel catchment (Table 1) span one order of magnitude, ranging from 1.55 to 39.20 µg/L with a mean concentration of 6.58 µg/L (SD=6.80, N=50), higher than the worldwide riverine average of 1.9 µg/L [START_REF] Huh | Lithium and its isotopes in major world rivers: implications for weathering and the oceanic budget[END_REF][START_REF] Gaillardet | Trace elements in river waters[END_REF]. The surface-and groundwater samples from the Run and Keersop The suspended-matter samples (Fig. 4) from the Dommel-Down and the Run diverge mainly by their Li concentrations rather than the δ 7 Li.

Discussion

Sources of dissolved Li: Conservative behaviour of Li in the catchment

First of all, we observe that the Cl concentrations are well-correlated with Na concentrations (Fig. 5) for the main stations of the Dommel Basin; the equation of the linear relationship is In the absence of direct rainwater-input data for the Dommel catchment, we could use the data for Lake Veldhoven, a small lake in the Run watershed (Fig. 2) mostly fed by rainwater, but sometimes also by Dommel floodwater. Though this lake could be considered-in a first approximation-as a rainwater reference for low Li, Cl and Na concentrations, it was decided, because of the potential flooding risk, to use rainwater data from France (Millot et al. 2010). This point will be discussed in more detail in section 5.2, hereafter. for surface-and groundwater samples, and those of Na by a factor 1.3 and 1.1 for surface-and groundwater samples, respectively. If Cl can be related to atmospheric input through cycling salts and anthropogenic input [START_REF] Meybeck | Atmospheric inputs and river transport of dissolved substances[END_REF], Na is related to atmospheric input through cycling salts and water/substratum interaction [START_REF] Dellinger | Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes[END_REF].

Atmospheric inputs correction: methodology and results

Rainfall can supply an important fraction of dissolved elements in river water [START_REF] Meybeck | Atmospheric inputs and river transport of dissolved substances[END_REF]. Subtraction of the portion of elements carried by rainwater is the aim of atmospheric correction [START_REF] Négrel | Erosion sources determined from inversion of major, trace element ratios and strontium isotopic ratio in riverwater: the Congo Basin case[END_REF], when quantifying the chemical composition of river water deriving from water/substratum interactions and anthropogenic input [START_REF] Petelet-Giraud | Variability of 87Sr/86Sr in water draining granite revealed after a double correction for atmospheric and anthropogenic inputs[END_REF]. Such quantification requires knowledge of the chemical composition of rainfall [START_REF] Meybeck | Atmospheric inputs and river transport of dissolved substances[END_REF]. Normally, in the absence of evaporites, which is the case for the Dommel, Cl ions in surface water can be assumed to originate from wind-blown sea salt-the North Sea is only about 100 km to the northwest-and human activity. The correction of atmospheric contribution to water [START_REF] Meybeck | Atmospheric inputs and river transport of dissolved substances[END_REF], for any element Z, is estimated by reference to the Cl concentration called Clref multiplied by the Z/Cl ratio of rainwater. Clref is calculated from each mean weighted chloride content for the rainfall, multiplied by a concentration factor F.

This factor F is the concentration effect of evapotranspiration and is related to the total quantity of rainwater P and the evapotranspiration process E by the equation: F = P / (P-E).

Eq. 4

The mean annual rainfall in the Dommel area is approximately 740 mm/yr, the reference evapotranspiration for grassland is 560 mm/yr and the F factor is thus close to 4.1.

As mentioned before, we might use Lake Veldhoven for characterizing rainwater. This small lake in the Run watershed is mostly fed by rainwater (Fig. 2), but occasionally also by potentially contaminated Dommel floodwater. It could be considered, in a first approximation regarding the low Cl and Na concentrations (Fig. 5), as a rainwater reference. However, because of the possible Dommel flooding of the lake, we preferred to use another rainfall reference. Thus, we will use hereafter the characterization of rainwater by Négrel and Roy, (1998), [START_REF] Négrel | Chemical and strontium isotope characterization of rainwater in France: influence of sources and hydrogeochemical implications[END_REF] and Millot et al. (2010b) at the national scale of France. They also reported major ions, Li and its isotopes. The stations range from near-ocean locations (Brest, Dax) to more continental stations (Orléans). The most continental station in Orléans has the lowest lithium concentrations (0.37 µg/L) and δ 7 Li value (16.1‰), and in view of its continental character, this station is certainly the one that best matches the rain input over the Dommel catchment.

There, the Clref was estimated at 7 µg/L; lower chloride contents were assigned an atmospheric origin. However, when the chloride content was higher than Clref, an atmospheric correction was applied with Clref as both rainwater input and residual chloride (Clres) were attributed to human activities (here WWTP and smelter input). We applied the atmospheric correction to Na, Li and δ 7 Li values as earlier done for other studies on Li isotopes (Millot et al., 2010a;[START_REF] Clergue | Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles)[END_REF][START_REF] Dellinger | Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes[END_REF][START_REF] Henchiri | Riverine dissolved lithium isotopic signatures in low-relief central Africa and their link to weathering regimes[END_REF][START_REF] Négrel | Behaviour of Li isotopes during regolith formation on granite (Massif Central, France): Controls on the dissolved load in water, saprolite, soil and sediment[END_REF][START_REF] Gou | Li isotopes in the middle Yellow River: Seasonal variability, sources and fractionation[END_REF].

After rain-input correction, all samples (except Lake Veldhoven) showed a Cl concentration higher than Clref. The lowest Clres was observed for Run groundwater (0.2 to 3.4 mg/L) and for the Run river (4.3 to 8.6 mg/L), whereas the highest Clres was for the Dommel-DBR and Dommel-Down samples (57.6 to 119.4 mg/L and 46.5 to 148.2 mg/L, respectively) and Eindergatloop-Up ones (112 mg/L).

For Na, 34 to 37% and 29 to 38% originate from rainfall in Run groundwater and the Run River, respectively. This percentage of Na from rainfall was 6 to 12% for the Dommel-DBR and Dommel-Down samples, 5% in the Eindergatloop-Up and around 25% in the Dommel-Up and Keersop. We estimated an atmospheric origin of the Li in waters as 36.9 to 100% in Run groundwater and 30.4 to 72.7% in the Run River. Values for the Dommel-DBR and Dommel-Down samples were similar (8 to 47 and 8 to 55%, respectively).

Thus, 46% and 59% of Li derive from rainfall in the Dommel-Up and Keersop, respectively, and about 71% of the Li in the Eindergatloop-Up originates from rainfall. For the δ 7 Li values, once corrected (δ 7 Li*), the range is between -12.9 and +7.1‰ for Run groundwater, and between -19.9 and +5.3‰ for the Run river. The δ 7 Li values for the Dommel-DBR and Dommel-Down samples are similar (+20.1 to +28.8‰ for DBR and +19.9 to +28.2‰ for the Dommel-Down). The Dommel-Up and Keersop show a δ 7 Li* of around -3.6‰ and +0.0‰, respectively. The δ 7 Li value of the Eindergatloop-Up is -20.7‰.

Figure 7a shows the distribution of Li concentrations vs. δ 7 Li, all corrected for atmospheric input. The data are scattered into two groups, but these are different from the data without rain correction (Fig. 3). The Dommel-DBR and Dommel-Down samples constitute a first group, with the highest δ 7 Li that fits well with an influence of smelter or WWTP effluents. The larger variation is in Li* concentrations rather than in δ 7 Li*. Figure 7b, where δ 7 Li values are plotted against Li/Cl* ratios, confirms this influence, all Dommel-DBR and Dommel-Down points plotting close to the smelter effluents. We will further interpret this anthropogenic input in the discussion.

The second group on Figure 7a consists of samples from the Dommel-Up, the Keersop, the Eindergatloop-Up, Run groundwater and the Run River. Here, the larger variation is in the δ 7 Li* values, ranging from -20.7 to +7.1‰, rather than in the Li* concentration, reflecting the effect of different increasing stages of water rock interaction (WRI). These different stages of increasing weathering are marked by an increase of δ 7 Li* associated with higher Li/Cl* ratios. This trend of increasing weathering (Fig. 7b) may reflect the preferential intake of 6 Li into secondary minerals during weathering. The positive trend between the δ 7 Li* values and the Li/Cl* ratios seen for data from this group on Figure 7b are related to an increase in the WRI, because the variation in the Li/Cl* ratios is related to an increase in the Li* concentration and not to a variation of the Cl* concentration. Again, we will further discuss this water-substratum interaction process hereafter.

Anthropogenic influence: δ 7 Li and mixing processes

Within the upper reaches of the Dommel catchment, the effects of anthropogenic activity are dominated by industry effluents and wastewater. The Umicore smelter created atmospheric Zn-Cd emissions, now concentrated in the soil, and a point source of treated wastewater that still discharges into the Eindergatloop stream (Fig. 2). In addition, four wastewater treatment plants discharge effluents into the Eindergatloop-Up and the Dommel-Up. The Keersop and the Run are not affected by industrial anthropogenic activity. Downstream, the Dommel-DBR and Dommel-Down samples integrate all these anthropogenic contributions. In addition, agricultural activity changed from a mainly diffuse source to mostly intensive livestock farming, the manure of which leads to a heavy load of nitrogen and phosphates.

Li-isotope contents in this specific system seem to vary due to a source effect, and the smelter addition to the surface-and groundwater system can be considered as a "spiked solution" that may, or may not, mix with the rest of the water input for which the Dommel outlet is considered as the output. Figure 3 illustrates the isotopic signature versus the Li concentration in the dissolved phase, showing that the Dommel River (before the Run confluence and downstream) is significantly marked by smelter influence (δ 7 Li = +26.5‰, mean Li concentration 288 µg/L).

The Run basin has a distinct signature in both surface-and groundwater samples, there being no anthropogenic input from the smelter. The Li concentration in the groundwater is very similar to that of the Run, but the δ 7 Li isotopic ratios slightly diverge, especially during winter (Table 1).

However, a positive gadolinium anomaly (Gd*) is defined by comparison with samarium (Sm) and terbium (Tb), its two neighbouring rare-earth elements [START_REF] Petelet-Giraud | Natural versus anthropogenic sources in the surfaceand groundwater dissolved load of the Dommel River (Meuse basin): constraints by boron and strontium isotopes and gadolinium anomaly[END_REF]. The most significant anomalies were found in WWTP effluents due to the use of gadolinium in medical magnetic-resonance imaging as a contrast agent. Gd is not metabolized by the patient, and is not adsorbed, co-precipitated, or undergoing any ion exchange when passing through a WWTP. Therefore, as illustrated on Figure 9a vs. Comparing the distribution of lithium between the dissolved load-corrected for atmospheric input-and the suspended load on Figure 11 shows several features. First, the values of the dissolved and suspended loads of the smelter samples are comparable and plot close to the 1:1 line. In the Dommel-Down samples, the δ 7 Li value of the suspended load is lower than that of the corresponding dissolved load, while the values in the Run samples for suspended and dissolved loads are more comparable. Such differences may be related to the transport capability of the river, particularly in the case of a transport-limited context. Several studies have shown that, in this case, low isotopic Li signatures in the dissolved phase of rivers are observed as a result of equilibrium reactions between water and clay [START_REF] Huh | Lithium isotopes as a probe of weathering processes: Orinoco River[END_REF][START_REF] Pogge Von Strandmann | Riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain[END_REF][START_REF] Vigier | The relationship between riverine lithium isotope composition and silicate weathering rates in Iceland[END_REF][START_REF] Clergue | Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles)[END_REF], like for the Run samples. If the physical erosion rate increases, as in the Dommel Down, the dissolved river load has a relatively high δ 7 Li content as a result of kinetic reactions [START_REF] Huh | Lithium isotopes as a probe of weathering processes: Orinoco River[END_REF][START_REF] Clergue | Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles)[END_REF].

Millot et al. (2010a) demonstrated that Li incorporation into secondary minerals can

be verified using the Li concentration and that of a 'conservative' element less sensitive to weathering such as Na or Mg. It has been argued (Millot et al. 2010a;[START_REF] Verney-Carron | Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering[END_REF][START_REF] Dellinger | Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes[END_REF] that Na and Mg are most released in solution close to Li during rock weathering. Sodic feldspars are commonly involved with K-feldspar in controlling aqueous Na and K concentrations, and therefore in the control of Li in geothermal waters as suggested by [START_REF] Sanjuan | Use of two new Na/Li geothermometric relationships for geothermal fluids in volcanic environments[END_REF]. In hot water, however, lithium behaves as an incompatible element, not co-precipitated with any secondary mineral. In (near-)surface environments, Li has a strong affinity for clay minerals and oxyhydroxides, but Na and Mg are not significantly incorporated into secondary weathering products (Millot et al. 2010a;[START_REF] Dellinger | Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion[END_REF][START_REF] Wang | Behavior of lithium isotopes in the Changjiang River system: sources effects and response to weathering and erosion[END_REF].

The soils in the Dommel catchment are mostly sandy with feldspars. They are vulnerable to leaching because of acidifying conditions (very low pH in the topsoil, between 3.5 and 4), low organic matter content (mostly present in the topsoil down to 30 cm) and clay [START_REF] Wilkens | Accumulation of cadmium and zinc from diffuse emission on acid sandy soils, as a function of soil composition[END_REF]. The choice of Na as a comparative element is reinforced as in the shallow subsurface of the Netherlands this element is virtually only derived from Na-bearing feldspar [START_REF] Schokker | Patterns and processes in a Pleistocene fluvio-aeolian environment[END_REF], which is susceptible to chemical weathering in the time span and geochemical environment under consideration due to the acidic conditions.

Thus, the δ 7 Li contents and Li/Na ratios in the dissolved load of the Run river and groundwater, corrected for atmospheric input and without any anthropic contribution, are only sensitive to the amount of water/substratum interaction over time. This makes them into useful tracers of the degree of silicate chemical weathering occurring in this mono-lithology catchment. Figure 12 shows the relationship between dissolved δ 7 Li* and the molar Li/Na* ratio, corrected for atmospheric input, for the "non-impacted" samples (Eindergatloop-Up, Dommel-Up, Keersop, Run and Run groundwater) and for the "impacted" samples (Dommel-DBR and Dommel-Down). The most Li-depleted sample relative to Na (the Eindergatloop-Up) has the lowest δ 7 Li* value, and vice-versa. The δ 7 Li* of the Run surface-and groundwaters positively correlates with the dissolved Li/Na* ratio, with an R-squared of about 0.86 for the Run River and 0.66 for Run groundwater, and slopes of around 2.5 and 2.1, respectively (Fig. 12). The δ 7 Li* in the Run River is significantly higher than the δ 7 Li* of Run groundwater.

In view of the homogeneous sandy substratum, the difference can be explained by water/substratum interaction in terms of both residence time and/or weathering intensity (Millot et al., 2010a). Groundwater with its relatively longer residence time causes more intense weathering, which is reflected in the formation of secondary weathering minerals.

Surface runoff has a much shorter residence time and a lower weathering intensity, confirming that Li-isotope fractionation in river water plays a major role in riverine δ 7 Li.

Li/Na ratios, tracing the water/substratum interaction, do not allow discriminating a natural (Run basin) from an anthropogenic (smelter) source, whereas the clearly different isotopic signatures show distinct sources. For the Dommel-DBR and Dommel-Down samples, the lack of, or weak, correlations (R-squared of around 0.12 and 0.58, respectively), in addition to very low slopes of around 0.3 and 0.8, respectively, suggest that no Li-isotope fractionation can be related to Li incorporation in solid weathering products. The variation is restricted to Li/Na* ratios, as illustrated on Figure 12. This variation in the Dommel-DBR and Dommel-Down samples is framed by the two smelter-effluent samples with Li/Na* values between 0.00011 and 0.00076 (molar ratio) being the two extremes of the variations in the river.

Conclusions

In the Dommel catchment, a large range of δ 7 Li values is observed despite the small size of the area. Li-isotopic composition in water samples varies from +5.4‰ to +27.8‰, which is explained by several processes like interaction between water and the substratum and anthropogenic input, in addition to atmospheric influence.

In this catchment with its sandy-feldspar substratum, silicate weathering is considered to be a major source of Li in surface-and groundwaters. However, Li and major-element concentrations as well as Li-isotopic compositions attest that precipitation is another important source of Li in surface-and groundwaters of the Dommel catchment, 8 to 100% of Li in waters being derived from rain (100% means no interaction with rocks, no anthropogenic disturbance).

Once corrected for atmospheric input, the distribution of Li concentrations and δ 7 Li shows a scatter of the data into two groups:

-The main Dommel River samples (Dommel-DBR and Dommel-Down) form the first group with the highest δ 7 Li that fits with an influence of smelter effluents and/or wastewater treatment plants (WWTP), however weakly marked by this contribution for δ 7 Li. These observations are confirmed by the gadolinium anomaly (Gd*) regarding the influence of WWTP. A binary mixing model based on δ 7 Li, [Li] and [Cl] shows that less than 3% of smelter effluents is enough to explain the Li isotopic values in the Dommel River samples.

-The second group consists of the upstream Dommel River, Keersop, upstream Eindergatloop and Run samples, and the Run groundwater. Here, the variation in atmospheric-corrected δ 7 Li reflects an influence of different increasing stages of water/substratum interaction.

Our study provides novel insights into the variations of Li-isotopic composition in surface-and groundwaters, which may reflect natural processes, like water/substratum interactions, and the anthropogenic history of a catchment area, as well as Li dynamics in modern environments.

Figure 2 Figure 1 :

 21 Figure2shows the flow chart of the basin, with long-term average discharge figures

Figure 2 .

 2 Stations 240013, 240013AV, 240013Run and 240013DBR were sampled once a month from December 2004 to September 2006. Stations 241015, 241012, 240011, 240013Run, 240013AV and 240013

  watersheds, from the Dommel-Up and the sample taken from the Lake Veldhoven only fed by rainwater in the Run watershed, have the lowest dissolved Li concentrations from 1.55 µg/L (Run watershed groundwater in April) to 2.15 µg/(L (surface water from Eindergatloop Up in July; Fig. 3). The range of values in the surface waters from the Run watershed is 2.11 to 5.05 µg/L (SD=1.04, n=11) and the range of the groundwater in the same watershed is 1.55 to 4.16 µg/L (SD=0.85, n=11), both being of the same order of magnitude. The Eindergatloop-Down(stream) sample, with Li concentrations ranging between 17 and 39.20 µg/L can be impacted by smelter effluents with Li concentrations ranging from 91 to 526 µg/L (mean 288.36 µg/L, n=4) entering after the Eindergatloop-Up water (Li = 2.15 to 17.20 µg/L). The Dommel River samples, collected downstream from the confluence with the Eindergatloop (Dommel-DBR) and the Dommel-Down samples (after the Keersop and Run confluences) have the lowest and relatively close Li concentrations (3.29 µg/L for Dommel-DBR and 2.80 µg/L for Dommel-Down, both in April). Both Dommel-DBR and Dommel-Down have similar ranges of Li concentrations (3.29 to 20.24 µg/L and 2.80 to 19.17 µg/L, respectively). The highest mean Li concentrations are in the Dommel watershed: 9.33 µg/L (SD=9.33, n=9) for the Dommel-DBR and 7.96 µg/L (SD=1.04, n=11) for the Dommel-Down, reflecting a high smelter contribution.

Figure 3

 3 Figure 3 illustrates the isotopic signature versus the Li concentration in the dissolved

Figure 3

 3 Figure 3 Plot of δ 7 Li (‰) versus Li concentration (μg/L) in the dissolved load for the main stations of the Dommel Basin monitored during the survey. Rainwater samples are from Millot et al. 2010, Brest sand Dax are coastal rainwaters, Orleans an inland rainwater.

Figure 4 :

 4 Figure 4: Plot of δ 7 Li (‰) versus Li concentration (μg/g) in the suspended load for some stations of the Dommel Basin monitored during the survey.

Na

  an R-squared of around 0.83. Most data plot close to the dilution line of seawater, possibly indicating that both Na and Cl in samples originate mostly from sea salt and are not influenced by vegetation uptake, water/substratum interaction (inducing Na enrichment), or major anthropogenic input (inducing Cl enrichment).However, the superposition of seawater and the smelter input implies a significantly marked signal for the Eindergatloop-Down samples (direct influence of smelter effluents) and the Dommel-DBR (after the Eindergatloop confluence) and the Dommel-Down samples (after the Keersop and Run confluences) through the influence of the Eindergatloop[START_REF] Petelet-Giraud | Natural versus anthropogenic sources in the surfaceand groundwater dissolved load of the Dommel River (Meuse basin): constraints by boron and strontium isotopes and gadolinium anomaly[END_REF].

Figure 5 :

 5 Figure 5: Na vs. Cl concentrations for stations of the Dommel Basin monitored during the survey. Data from Petelet-Giraud et al. (2009). The dashed blue line is the seawater dilution line, the solid black line is the linear relationship with a 95% confidence interval shaded. Rainwater samples are from[START_REF] Verney-Carron | Négrel and Millot[END_REF].

Figure 6 :

 6 Figure 6: Cl vs. Li concentrations for the Dommel Basin monitored during the survey. Cl-content data are from Petelet-Giraud et al. (2009). The dashed blue line represents seawater dilution line, the solid black line is the linear relationship with a 95% confidence interval shaded.

Figure 7 :

 7 Figure 7: (a) Relationship between dissolved δ 7 Li* and Li* concentration, all corrected for atmospheric inputs. (b) Relationship between dissolved δ 7 Li* and Li/Cl* ratio, all corrected for atmospheric inputs.

δ 7

 7 Li values, it serves to detect a flux of potentially contaminated water into surface-or groundwaters. Petelet-Giraud et al. (2009) found the highest Gd* values in the Eindergatloop. The smelter discharging in this tributary has no Gd anomaly, but samples taken in the Eindergatloop before and after the smelter have distinct Gd anomalies (Fig. 9a) caused by the Lommel WTTP. The Dommel-Up, just after the discharge of the Overpelt WWTP, also shows a high Gd* in the sample analysed for Li isotopes (Gd*=4.51) but lower values were found as well (Gd*=1.47, March 2005), reflecting variations in WWTP effluents. The Keersop and Run rivers have no Gd anomalies and their contribution is enough to compensate for the Gd anomaly in the Dommel River samples DBR and the Dommel-Down, except for three samples clearly affected by a larger Gd* (July 20 th and September 30 th ).

Figure 9 :

 9 Figure 9: (a) Relationship between dissolved δ 7 Li and the gadolinium anomaly (Gd*). See Petelet-Giraud et al., (2009) for REE data and Gd* calculation. (b) Relationship between dissolved δ 7 Li* and Cl* concentration, all corrected for atmospheric inputs.

Figure 10 :

 10 Figure9bshows the relationship between dissolved δ 7 Li* and Cl* concentration,

Figure 11 :

 11 Figure 11: Plot of the δ 7 Li* values of the dissolved load (corrected for atmospheric input) and the δ 7 Li of suspended sediments in the Run and Dommel rivers. The solid line represents equal values.

Figure 12 :

 12 Figure 12: Relationship between dissolved δ 7 Li* and molar Li/Na* ratio, corrected for atmospheric input.

Table 1 .

 1 Li concentrations (µg/L) and δ 7 Li (‰) of the main stations in the Dommel Basin monitored for this study.The dissolved Li-isotope composition (δ 7 Li) shows a large range of variation from +5.39‰ to +27.8‰. The surface-and groundwaters from the Run and the Keersop watersheds and the Dommel-Up have the lowest δ 7 Li values between +5.4‰ (Run in April) and +11.7‰(groundwater from the Run watershed in January). Contrary to the Li concentrations, the 4‰ to +8.9‰ (SD=1.10, n=10) and the range in groundwater from the same watershed is narrow and comprised between +9.70‰ to +11.7‰ (SD=0.69, n=10), both being of the same order of magnitude. The Eindergatloop-Down sample shows a higher δ 7 Li value (20.8‰) than

	NAME/DATE	Li µg/L δ 7 Li ‰	NAME/DATE	Li µg/L δ 7 Li ‰
	DOMMEL			TRIBUTARIES
	Dommel Up (241015)			Eindergatloop Up (241011)
	17-Jul.-07	3.32	5.5	30-Jan.-06	17.21	-
	Dommel DBR (240013DBR)			17-Jul.-2007	2.15	5.5
	9-Sep.-05	20.24	27.8	Eindergatloop Down (241012)
	30-Sep.-05	17.05	23.4	30-Jan.-06	39.20	-
	25-Nov.-05	8.70	21.9	17-Jul-2007	17.41	20.8
	20-Dec.-05	5.24	22.5	Keersop (240036)
	23-Jan.-06	6.37	19.3	30-Jan.-06	2.13	-
	27-Feb.-06	8.34	20.9	17-Jul.-07	2.62	9.4
	6-Apr.-06	3.29	18.3	Run (240013RUN)
	1-Jun-06	7.65	24.5	20-Jul.-05	5.05	7.2
	17-Jul.-07	7.12	-	9-Sep.-05	4.64	8.9
	Dommel Down (240013)			30-Sep.-05	4.14	7.6
	20-Jul.-05	9.37	22.6	25-Nov.-05	2.68	6.8
	9-Sep.-05	19.17	27.2	20-Dec.-05	2.11	6.3
	30-Sep.-05	15.99	23.0	23-Jan.-06	2.44	6.2
	25-Nov.-05	7.49	21.5	27-Feb.-06	3.86	6.5
	20-Dec.-05	5.61	20.9	8-Mar-06	2.41	-
	23-Jan.-06	4.81	18.7	6-Apr.-06	2.37	5.4
	27-Feb.-06	7.62	20.1	3-May-06	2.34	5.5
	8-Mar-06	5.68	-	1-Jun-06	3.16	5.5
	6-Apr.-06	2.80	17.9
	3-May-06	4.37	20.1
	1-Jun-06	6.16	24.1
	17-Jul.-07	6.51	-
	OTHER WATER SAMPLES
	Run-GW (240013AV)			Smelter
	20-Jul.-05	4.16	9.7	20-Jul.-05	526.12	26.5
	9-Sep.-05	3.69	9.8	30-Jan.-06	185.82	-
	30-Sep.-05	3.75	10.0	1-Jun-06	350.94	-
	25-Nov.-05	3.22	11.4	17-Jul.-07	90.55	26.7
	20-Dec.-05	2.37	10.2
	23-Jan.-06	2.01	11.7	Lake Veldhoven
	27-Feb.-06	2.63	10.9	20-Jul.-05	2.07	18
	8-Mar-06	2.32	-
	6-Apr.-06	1.55	10.4
	3-May-06	1.86	11.1
	1-Jun-06	2.54	10.3

sample taken from the Lake Veldhoven only fed by rainwater in the Run watershed has a higher δ

7

Li value (+18.0‰). The range of δ 7 Li values in surface waters from the Run watershed is +5.

7

Li value for modern river water of about 23‰

[START_REF] Huh | Lithium and its isotopes in major world rivers: implications for weathering and the oceanic budget[END_REF][START_REF] Misra | Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering[END_REF]

, varying from negative values around -4.0‰ in Himalayan rivers

[START_REF] Kisakurek | Li and δ 7 Li in Himalayan rivers: proxies for silicate weathering? Earth Planet[END_REF]

) to a positive range as in the Amazon River basin (+1.2‰ to +32‰,

[START_REF] Dellinger | [END_REF]

or in the Mackenzie River basin (Canada; +9.3‰ to +29.0‰,

Millot et al., 2010a)

.

  The highest δ 7 Li of around +24.3‰ was observed in the suspended matter from the smelter effluent. For the Run and Dommel suspended-matter samples, it should be noted that all data display negative δ 7 Li values, in agreement with literature data.

	The Run samples have the lowest Li
	concentrations in the suspended matter, between 5.16 and 7.46 µg/g (mean concentration of
	6.22 µg/g, SD=1.04, n=5), and the Dommel-Down samples have the highest suspended matter
	Li with concentrations between 12 and 18 µg/g (mean of 14.33 µg/g, SD=2.50, n=5). Only
	one sample of suspended matter from the smelter effluents was analysed with a Li
	concentrations of around 11.50 µg/g. The Li isotope composition (δ 7 Li) of the suspended
	matter in the Run samples varies between -6.4 and +0.6‰ (mean δ 7 Li=-4.2‰, SD=2.80,

n=5). A more restricted range is observed for the Dommel-Down with δ 7 Li values ranging between -2.8 and -3.8‰ (mean δ 7 Li=-3.4‰, SD=0.38, n=5).

Acknowledgements

This work was supported by the European Union FP6 Integrated Project AquaTerra (Project no. GOCE 505428) under the thematic priority "Sustainable Development, Global Change and Ecosystems". The views expressed are purely those of the writers and may not under any circumstances be regarded as stating an official position of the European Commission. This work benefited from the collaboration of M. Robert in the Neptune laboratory. We are grateful to Dr. H.M. Kluijver for proofreading and editing the English text. Financial support from the Région Centre is also acknowledged for the acquisition of the Neptune MC-ICP-MS. This paper is dedicated to the memory of our wonderful colleague and friend, Dr. Thomas (Tom) Bullen, 1951-2018(Shouakar-Stash et al., 2018).