Emmanuelle Petelet-Giraud

Nicole Baran

V Vergnaud-Ayraud

Angélie Portal

C Michel

Catherine Joulian

F Lucassou

Petelet-Giraud Emmanuelle

Vergnaud-Ayraud Virginie

Michel Caroline

Joulian Catherine

Lucassou Flora

Elucidating heterogeneous nitrate contamination in a small basement aquifer. A multidisciplinary approach: NO3 isotopes, CFCs-SF6, microbiological activity, geophysics and hydrogeology

Keywords: Nitrate contamination, basement aquifer, nitrate isotopes, groundwater residence time, microbiological activity, Electrical Resistivity Tomography geophysics, Plourhan catchment

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

Introduction

Nitrate contamination of groundwater remains a major concern despite all the efforts to protect waters aquifers against nitrate pollution, such as implementation of the European Council Directive 91/676/EEC adopted in 1991, and of the European Water Framework Directive 2000/60/EC and its daughter Directive 2006/118/EC. In addition to agronomic measures, a key issue is to arrive at better understanding the driving parameters of nitrate transfer towards, and distribution in, groundwater, i.e. the geological structures and the hydrogeological functioning.

The correlation between an increased use of (synthetic and organic) fertilizers and the increase of nitrate concentrations in groundwater has been known for a long time (e.g. [START_REF] Aquilina | Nitrate dynamics in agricultural catchments deduced from groundwater dating and long-term nitrate monitoring in surface-and groundwaters[END_REF] but uncertainties remain concerning the transit time between topsoil and aquifer. In particular, the immobilization/remobilization processes of N governing nitrogen cycling and nitrate transfer through soil are still poorly constrained in terms of "delay effect". [START_REF] Sebilo | Long-term fate of nitrate fertilizer in agricultural soils[END_REF], who investigated the fate of a single application of an isotopically labelled nitrogen fertilizers for three decades in an experimental field, concluded that attempts for reducing agricultural nitrate contamination of aquatic systems must consider the long-term legacy of past applications of synthetic fertilizer and the nitrogen-retention capacity of agricultural soils.

In addition to nitrogen-retention and -cycling processes in soil, the N stock -mainly nitratein the unsaturated aquifer zone must be considered [START_REF] Baran | Field data and modelling of water and nitrate movement through deep unsaturated loess[END_REF]. Nitrate being highly soluble, it is generally considered that it will transfer through the unsaturated zone towards groundwater at the same rate as infiltrating water. This implies that a good understanding, of the subsurface geology and its geometry as well as of the hydrogeological functioning of the aquifer system is essential, as they -at least partly -govern pollutant transfer time at the catchment scale. A proper understanding of the hydrogeological functioning can be obtained from groundwater residence-time assessment, using chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) gases for studying modern (< 60 yr) groundwater dynamics and mixing processes. These anthropogenic gases have been released into the atmosphere since the 1950s and are now found in natural waters, with atmospheric equilibria that allow groundwater dating based on their atmospheric concentrations chronicles (e.g. [START_REF] Busenberg | Use of chlorofluorocarbons (CCL3F and CCL2F2) as hydrologic tracers and age-dating tools -the alluvium and terrace system of Central Oklahoma[END_REF][START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF][START_REF] Aeschbach-Hertig | Noble Gas Thermometry in Groundwater Hydrology[END_REF][START_REF] Loke | Tutorial : 2-D and 3-D electrical imaging surveys[END_REF][START_REF] Chatton | Glacial recharge, salinization and anthropogenic contamination in the coastal aquifers of Recife (Brazil)[END_REF].

Furthermore, part of the N can leave the nitrogen cycle as gases (N2O and N2) resulting from denitrification, governed by heterotrophic or autotrophic denitrifying bacteria transforming NO3 to N2 [START_REF] Stein | The Nitrogen Cycle[END_REF]. This process affects the global N budget, especially the nitrate one, and must be considered as a major process when evaluating of the fate of N from soil to groundwater. Nitrogen and oxygen isotopes of nitrate (δ 15 N-NO3 and δ 18 O-NO3) are frequently used as tracers of nitrate sources in groundwater, as the various synthetic/organic N fertilizers have different isotopic compositions [START_REF] Kendall | Chapter 12: Tracing Anthropogenic Inputs of Nitrogen to Ecosystems[END_REF].

However, nitrate isotopes also are ideal tools for tracing N transformation processes, such as denitrification [START_REF] Böttcher | Using isotope fractionation of nitratenitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer[END_REF], as NO3 reduction to N2 is a highly fractionating process [START_REF] Kendall | Nitrate Isotopes in Groundwater Systems[END_REF] leading to a joint increase of δ 15 N-NO3 and δ 18 O-NO3, whereas simple dilution, also leading to a decrease in NO3 concentration, does not modify the isotopic signatures.

We designed and implemented an innovative interdisciplinary study, coupling complementary and robust approaches for defining the spatio-temporal nitrate distribution in a small catchment of a basement-rock aquifer. For that purpose, we had to answer the following questions.

(1) How is groundwater quality-especially for nitrate-controlled spatially and temporarily, by natural water-rock interactions and by anthropogenic input related to agricultural practises and its evolution over the past decades (hydrogeological, geophysical and chemical approaches)? (2) What are the nitrate sources and their potential attenuation, as shown by N and O isotopes of NO3 fingerprinting, and through assessing the denitrification potential by monitoring nitrate-reducing bacteria genes (isotopic and microbiological approaches)? (3) What is the groundwater residence-time distribution, in order to evaluate the hydrodynamics and the potential spatial variability of the aquifer, and as a supplementary constraint to support the hypothesis regarding the chemical and isotopic compositions (CFC-SF6 and rare dissolved gases approach)?

Site description

The Plourhan site we studied is a small (1.26 km 2 ) hydrogeological catchment. In 2006, a potentiometric map was drawn for resource-protection purposes, relying on different hydrogeological approaches such as the drilling of 12 piezometers and a deep borehole. It is located 10 km from the coast in Brittany (northwest France). Geologically, it is underlain by Brioverian basement composed of metasedimentary pelite and sandstone, and amphibolite, both overlain by an 8 to 20 m thick saprolite layer. The basement that can be highly fractured, especially the amphibolite. A major fault forms the boundary between metasediments and amphibolite, with a potentially wide fracture zone (Figure 1). This description, based on the 1:50,000-scale geological map was not precise enough in view of the small catchment size. For this reason, a detailed geological survey of the catchment was part of this study as was a geophysical survey, providing a better definition of the detailed geology and its geometry (see Supplementary material). In addition, a pedological study carried out during delineation of the area to be protected, showed relative soil uniformity of the site. The soil is brown and moderately deep (40 cm on average in the east of the site and about 80 cm in the southwest). Under the forest, to the northwest, a particular pedogenesis is observed, the soil being pseudo-gley.

A natural spring emerges near the major fault and has been exploited since 1968 via a 4.4 m deep borehole that taps the water coming from the saprolite layer. This spring, hereafter referred to as Spring, feeds a small stream that exits the basin to the east. In 2006, 12 piezometers (Pz1 to Pz12), between 10 and 22 m depth, were drilled mainly for delineating the catchment. In these piezometers, the groundwater level is few metres below the soil level (bsl). They are screened all along the weathered layer down to the top of the "fresh" basement. The saturated zone is only a few metres thick. A few measurements before our study showed strong heterogeneity of the groundwater quality. Monthly monitored water levels between October 2006 and March 2008 were used for drawing potentiometric maps, thus defining the hydrogeological contours of the catchment and identifying flow directions (Fig. 1). Not surprisingly, the hydrological and topographic catchments have different boundaries. Thus, for the piezometers accessible during this study, Pz3, 6 and 8 are located upstream (respectively south and north of the Spring), while Pz5 and Pz9 are located in the middle of the catchment. The others (Pz10 and Pz7) lie outside the catchment.

In addition to these 12 piezometers tapping water in the saprolite, a deep borehole (SR1), drilled at about 30 m from the natural Spring within the fault zone, has a total depth of 133 m.

It is screened below 50 m (bsl) and intersected paragneiss, and metamorphic sandstone and pelite.

The water supply for the 2 000 inhabitants is based on the water pumped from the Spring (Figure 1) which must be diluted in order to fulfil the drinking-water quality standards for nitrate. Indeed, the Spring has nitrate concentrations exceeding the 50 mg/L drinking water standard and the dilution takes place at the treatment plant with water from another borehole. A protection area for the groundwater catchment was administratively defined in 2010 along parcel boundaries. It specified the regulation of (agricultural) land use, maintaining plots in forest or meadow areas, and plots where annual crops were allowed with specific agronomic recommendations. As of 2013, annual crops were no longer allowed and were replaced by meadows (Figure 2).

Historically, the studied site has been subject to strong agricultural pressure. From 1980 to 1990, vegetable production dominated. In the mid-1990s, land use was mainly dedicated to market gardening, with a significant surface for peas, potatoes and cauliflower. From the mid-1990s to the present, in an effort to improve groundwater quality, farmers significantly reduced the vegetable areas (Figure 2), compensated by an increase in wheat, maize and especially grass. Despite these efforts, nitrate concentrations in Spring water only decreased slowly and still remain above the 50 mg/L standard (Figure 2). Both studied periods (2009-2011 and 2016-2018) are shown as blue dots. Historical data are from the French national groundwater database (ades: https://ades.eaufrance.fr). History of the measures for improving water quality is indicated.

In the area, historically and until recently, nitrogen was applied in both mineral and organic form, depending on the cultivated plot. Detailed information is not available, but it is known that in 2014, in the protection area, four of the seven farms produced animal excrements (cattle, pigs, poultry) and five had plots on which animal excrements, green-waste compost and/or household waste were spread (Côtes d'Armor Chamber of Agriculture, 2014).

A detailed knowledge of land use allows addressing the issue of why groundwater quality is spatially heterogeneous. Such groundwater catchment is representative of lithology (basement), soil and climate of France and Western Europe.

Material and methods

3.1 Sampling strategy The study site was monitored for two distinct periods. The Spring was sampled on a monthly basis from August 2009 to June 2010 (10 campaigns = 1 st period) and from October 2016 to July 2019 (28 campaigns = 2 nd period), whereas piezometers were only sampled twice and four times during the 1 st and 2 nd periods, respectively. It should be noted that some piezometers were no longer accessible during the 2 nd period. Prior to sampling, several field parameters were measured, including electrical conductivity, pH, redox and oxygen content (Table S1 Supplementary material). Major ions were analysed for all samples.

Microbiological parameters were measured in each samples from the 2 nd period, as was groundwater dating. Isotopic investigations only concerned samples from April 2017.

Chemical and isotopic investigations

Analytical methods

For both sampling periods, the groundwater samples were collected in polyethylene bottles and filtered through 0.45 µm PVDF filters for chemical and nitrate isotopes analysis. Bottles dedicated to cation analysis were acidified with 15N ultrapure HNO3 to pH<2.

Physicochemical parameters of each sample were measured on site, including the electrical conductivity (EC), standardized to 25 °C, water temperature, pH, dissolved oxygen (O2 diss.) and redox potential (Eh). Samples were stored at 4 °C in the dark prior to analysis; this took place in the BRGM Laboratories by ICP-AES (Ca, Na, K, Mg; uncertainty <10%), ion chromatography (Cl, SO4, NO3, uncertainty <10%) and titration method according to N EN ISO 9963-1(HCO3 -; CO3 2-, uncertainty <5%).

Both δ 15 N and δ 18 O of NO3 were measured by the AgNO3 method at BRGM according to the following procedure: The filtered water is chemically purified through cationic and anionic ionexchange columns. Nitrate is recovered as HNO3 by HCl elution. The eluate is converted to AgNO3 by addition of silver oxide (Ag2O): HCl + HNO3 + Ag2O => AgCl(s) + AgNO3 + H2O.

The filtrate containing the nitrate, once freed from the other oxygenated compounds by adding BaCl2, is lyophilized to obtain solid AgNO3 [START_REF] Chang | A method for nitrate collection for δ 15 N and δ 18 O analysis for water with low nitrate concentrations[END_REF][START_REF] Silva | A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios[END_REF]. The solid AgNO3 precipitate is then weighed into silver capsules that are inserted separately into a furnace composed of a glassy carbon reactor heated to 1450 °C. The N2 and CO gases resulting from high-temperature combustion are driven by a helium flow to a chromatographic column. This separates the gas peaks before their introduction into the source of the mass spectrometer for determining the nitrate δ 18 O and δ 15 N isotopic ratios. The analytical accuracy is ± 0.5 ‰ for δ 15 N and δ 18 O.

Water samples for chlorofluorocarbons and sulphur hexafluoride (CFCs and SF6) analyses were collected in stainless-steel ampoules (40 mL and 300 mL, respectively) after rinsing them with at least three times the sample volume, without any contact with atmospheric air during sampling. The CFCs and SF6 concentrations, used for calculating of groundwater residence times, were measured using a gas chromatograph with an electron capture detector (GC-ECD) (Labasque et al., 2006;[START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF], at the Plateforme CONDATE Eau (OSUR, University of Rennes 1, France) for the 2 nd period. The concentrations of were measured with an analytical uncertainty of 1%, and the SF6 with an uncertainty of 5% (Labasque et al., 2006;[START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF]. Gas concentrations were used for calculating atmospheric mixing ratios (pptv), and compared to the atmospheric evolution curve (NOAA HATS program), to determine the apparent groundwater piston age. For each sampling point, three hypothetical lump parametric models, a piston-flow model (PFM), an exponential model (EM), and a binary mixing model (BMM), were systematically tested to determine the apparent groundwater residence time [START_REF] Matoszewski | Lumped parameter models for the interpretation of environmental tracer data[END_REF]. The overall uncertainty of derived groundwater-age estimates, including sampling biases and analytical errors, as well as the errors resulting from uncertainty of the recharge temperature and considering dispersionadsorption effects, is estimated to be ±3 years [START_REF] Labasque | Inter-laboratory comparison of the analyses of sulphur hexafluoride (SF6) and three chlorofluorocarbons (CFC-11, -12 and -113) in groundwater and an air standard[END_REF]. For the first sampling period (2009-2011) a simplified sample collection method was used (IAEA, 2006). The analyses were made by Spurenstofflabor at Wachenheim (Germany), using gas chromatography with an electron-capture detector after pre-concentration using the purgeand-trap technique. The detection limit was 10 -4 pmol, allowing the measurement of CFC concentrations down to 0.01 pmol.L -1 and of SF6 concentrations to 0.1 fmol. L -1 ; reproducibility was about ± 5% for water samples.

Major and noble gases were measured throughout the sampling campaign using a GC-TCD (CG 3000, SRA instrument) after headspace extraction with a He gas phase following the Sugisaki and Taki method (1987). The analytical error for Ne and Ar/N2 measurements was 3% and 2%, respectively. The analysis of Ne/Ar ratios allows calculating excess air and recharge temperatures [START_REF] Heaton | Excess air in groundwater[END_REF].

N and O isotopes of nitrate theoretical background

Since the 2000s, nitrogen and oxygen isotopes of nitrates (δ 15 N-NO3 and δ 18 O-NO3) are used for tracing nitrate sources in surface and groundwaters [START_REF] Widory | Tracking the sources of nitrate in groundwater using coupled nitrogen and boron isotopes: a synthesis[END_REF][START_REF] Widory | Improving the management of nitrate pollution in water by the use of isotope monitoring: the δ 15 N, δ 18 O & δ 11 B triptych[END_REF][START_REF] Kendall | Chapter 12: Tracing Anthropogenic Inputs of Nitrogen to Ecosystems[END_REF][START_REF] Fenech | The potential for a suite of isotope and chemical markers to differentiate sources of nitrate contamination: A review[END_REF][START_REF] Puig | Characterizing sources and natural attenuation of nitrate contamination in the Baix Ter aquifer system (NE Spain) using a multi-isotope approach[END_REF]. Nevertheless, N-sources discrimination is often not straightforward as nitrate is not a conservative molecule, and thus the N transformation process has to be considered for properly tracing N sources. N and O isotopes are also valuable tracers for better constraining the N cycle from soil to groundwater, i.e. immobilization, mineralization, nitrification and denitrification. These different processes may thus lead to a loss of the original δ 15 N-NO3 and δ 18 O-NO3 isotopic signatures through isotopic fractionation [START_REF] Zhang | Application of nitrogen and oxygen isotopes for source and fate identification of nitrate pollution in surface water: A review[END_REF]. However, such isotopic fractionations is an excellent tool for identifying these processes and thus for better constraining N mobility.

Denitrification by oxidation of pyrite (FeS2), i.e. during chemoautotrophic respiration of sulphur-oxidizing Thiobacillus denitrificans, has been identified as an important denitrification pathway in basement aquifers where pyrite is abundant [START_REF] Pauwels | Denitrification and mixing in a schist aquifer: influence on water chemistry and isotopes[END_REF]. Note that [START_REF] Aquilina | Autotrophic denitrification supported by biotite dissolution in crystalline aquifers (1): New insights from short-term batch experiments[END_REF] suggested another type of autotrophic denitrification: biotite dissolution with the oxidation of Fe(II) contained in biotite. Denitrification is a highly fractionating process [START_REF] Böttcher | Using isotope fractionation of nitratenitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer[END_REF][START_REF] Kendall | Nitrate Isotopes in Groundwater Systems[END_REF], following a Rayleigh distillation model, leading to a joint increase of the isotope ratios δ 15 N-NO3 and δ 18 O-NO3 of the residual NO3.

The N cycle in soil further modifies the isotopic signature of the nitrate leached out and transported toward groundwater [START_REF] Mengis | Limitations of using delta 18 O for the source identification of nitrate in agricultural soils[END_REF][START_REF] Sebilo | Long-term fate of nitrate fertilizer in agricultural soils[END_REF]. The newly formed nitrate through ammonium nitrification is generally assumed to take nitrogen from NH4 + (and/or NO2), while one oxygen comes from atmospheric O2 and two from water (H2O) considering a simple stoichiometric relationship (Equation 1; [START_REF] Kumar | Definitive 15 N NMR evidence that water serves as a source of 'O' during nitrite oxidation by Nitrobacter agilis[END_REF]Kendall, 1998 and references therein); Note that [START_REF] Buchwald | Oxygen isotopic fractionation and exchange during bacterial nitrite oxidation[END_REF] added constraints to this simple model considering oxygen isotope exchanges of intermediate species with ambient water, during nitrite oxidation corresponding to the final step of nitrification process. 1)

δ 18 O-NO3 = ⅔ (δ 18 O-H2O) + ⅓ (δ 18 O-O2) (Equation

Microbiological and biomolecular approaches.

The spatial and temporal evolution of the abundance of bacteria involved in nitrate reduction was assessed through measurements of nitrate reductase encoding genes narG and napA copies. In November 2016, April and October 2017, and May 2018, water (6 L) was collected from each piezometer and filtered (0.22 µm) to collect biomass. Microbial DNA was extracted from the filters using a FastDNA™ Spin Kit for Soil (MP Biomedicals, USA) according to manufacturer's recommendations, with a FastPrep®-24 at a speed of 5 m.s -1 during 30 s, and quantified with a Quantifluor dsDNA sample kit and the Quantus fluorimeter, according to the manufacturer's instructions (Promega, USA). The abundances of narG and napA gene copies was determined by real-time quantitative PCR (qPCR) according to [START_REF] Bru | Relative abundances of proteobacterial membrane-bound and periplasmic nitrate reductases in selected environments[END_REF].

In order to assess potential denitrification activity, batch experiments were run on waters sampled in April 2017 in the Pz3, Pz6, Pz8, Pz9 and Pz10 piezometers. Briefly, duplicate 10 300 mL flasks were filled in with 150 mL of water under anaerobic conditions (nitrogen atmosphere). Acetate (C2H3NaO2) was added as a carbon source in each flask at a final concentration of 100 mg/L. Considering nitrate concentration in each well, nitrate was added to the flasks to reach a final NO3 -concentration of 100 mg/L. Abiotic tests in the presence of sodium azide [START_REF] Cabrol | Inhibitory effects of sodium azide on microbial growth in experimental resuspension of marine sediment[END_REF] were also performed. The flasks were incubated for 20 days in the dark at 25 °C under stirring (100 rpm). Every 4 to 5 days, 8 mL of water was sampled for nitrate, nitrite and acetate quantification, and 5 mL of the gas phase was sampled for N2O analysis. Nitrate and acetate were quantified by ionic chromatography (Dionex IC3000-SP-EG-DC system equipped with an AS50 autosampler and a conductimetric detector) according to the NF EN ISO 10304-1 (2009) method. Nitrite was analysed by colorimetry according to the NF ISO 15923-1 (2014) method. N2O was analysed by gas chromatography (Varian CP-3800 GC equipped with a gas injection valve and an electron capture detector).

For statistical analysis, boxplots were calculated using R4.0.1 and RStudio (n=2). Data were analysed by the non-parametric Kruskal-Wallis test (p>0.05).

Results and discussions

4.1 Improving of the hydrogeological model Geological investigations consisted in accurately identifying the rock types, tectonic structures and degree of weathering, in order to refine the geological map at the scale of this small catchment and to better understand the hydrogeological functioning. This field work confirmed that the catchment is located on a Brioverian anticlinal axis oriented N100/110, intersected by faults oriented N150, sub N-S and N050. The weathering profile forms a paleosurface at an altitude of about 80 m above mean sealevel, and seems to be affected by faulting. Geophysical results showed major horizontal resistivity contrasts, with two distinct domains separated by a significant vertical discontinuity (F5; FIG. S1 Supplementary material). The western resistive domain corresponds to amphibolite, while the conductive eastern unit corresponds to metamorphic sandstone and pelite [START_REF] Egal | Carte géol[END_REF]; Figures 1 and3A). To the west, a vertical resistivity gradient is interpreted as a signature of the amphibolite weathering profile consisting of alterite (C0, Fig. S1) at the top, followed by a fractured layer (R1) and fresh basement rock (R2). These results are coherent with those observed in other basement contexts [START_REF] Dewandel | A generalized 3-D geological and hydrogeological conceptual model of granite aquifers controlled by single or multiphase weathering[END_REF][START_REF] Belle | Characterization and location of the laminated layer within hard rock weathering profiles from electrical resistivity tomography: implications for water well siting[END_REF][START_REF] Comte | The typology of Irish hard-rock aquifers based on an integrated hydrogeological and geophysical approach[END_REF]). In the sandstone and pelite domain, despite the complexity of the electrical pattern, the SR1 geological log distinguishes the alterite from the sandstone and pelite (C1, C2, Fig. S1) that overlie a highly conductive C3 paragneiss derived from the sandstone and pelite, forming a metamorphic halo [START_REF] Egal | Notice explicative, carte géologique de la France (1/50 000[END_REF] and being an aquifer formation. This suggests that the high conductivity values observed are caused by strong and connected fracturing of the rock, and thus that the paragneiss is the main aquifer formation of the catchment. The amphibolite seems more impermeable than the sandstone and pelite, and the alterite forms a surface aquifer (Figure 3). Further details are presented in the Supplementary material.

To complete the geological and geophysical investigations and improve preliminary conclusions, chemical data were examined (Figure 4). The total mineralization of the water can be assessed through its electrical conductivity (EC, µS.cm -1 ) and the Total Dissolved Solids (TDS, mg.L -1 ), both parameters being correlated. The TDS values of the Spring samples (natural outlet of the hydrogeological system) show small variations over the two sampling periods (205-257 mg.L -1 ), as do most groundwater samples within the catchment (182-235 mg.L -1 , Pz1, Pz2, Pz3, Pz4, Pz7, Pz9 and Pz10). Nevertheless, four groundwater samples (Pz6, Pz8, Pz12 and SR1) have clearly higher TDS values, ranging between 274 and 675 mg.L -1 , varying both throughout the sampling periods and according to the sampling depth (Figure 4B). Groundwater presents very heterogeneous chemical types at this small catchment scale, from Ca-HCO3 to Ca-SO4. Cation proportions vary between a sodic endmember (>50% Na+K, in Pz10) and a calcic end-member (up to 60% Ca), in Pz6 and Pz8 (Figure 4A). When comparing the revised structural scheme (Figure 3) and the geochemical data, it appears that samples with the higher calcium concentrations (i.e. Pz6, Pz8, Pz12 and SR1), which also have the highest TDS, reflect water-rock interactions with the weathered sandstone and pelite compartment. The other samples with low TDS values and lower calcium concentrations seems to results from water-rock interaction with the weathered amphibolite compartment. This chemical variability also reflects the complex geological settings of this small area (only 0.6 km²), in terms of both weathering profile and geological entities.

It is worth noting that water in the deep borehole SR1 has an increasing Ca load with depth, probably related to water rock-interaction with fractured sandstone. In addition, Pz4 and Pz7 in the weathered sandstone and pelite compartment, present geochemical signatures very similar to that of groundwater in the amphibolite compartment. The geological and geophysical investigations allowed refining the contours of each geological formation on the 1:50,000-scale geological map, these contours being coherent with the geochemical groundwater signatures. Howerver, interrogations remain for some piezometers that are too far from the two geophysical profiles for making sufficiently reliable projections.

4.2 Temporal and spatial anthropogenic pressure: a focus on nitrate As mentioned above, despite all efforts made to reduce nitrogen transfer to groundwater, nitrate concentrations only decrease slowly at the Spring and remain above the 50 mg.L -1 standard (Figure 2). The increase in NO3 concentrations has stopped in the early 2000s, reaching a plateau over five years before starting a decrease that continues today. This type of evolution is typical of what is commonly observed in Europe since the implementation of various programmes of measures for recovering a good chemical status of the groundwater bodies, even though the response time can be quite different (European Commission, 2018). This anthropogenic pressure is clearly shown in the small Plourhan catchment, where nitrate concentrations in groundwater can be as high as 88 mg.L -1 (Pz1, Oct-2009). However, the nitrate concentrations in the piezometers sampled during the two sampling periods (2009-2010 and 2016-2018), showed quite different values from one piezometer to the next. For instance, the Spring and PZ7 showed a NO3 decrease between the 1 st and 2 nd periods, from 80 to 61 mg.L -1 (Spring) and from 77 to 62 mg.L -1 (Pz7) (Figure 2; Figure 5B; table S1 Supplementary material). Pz8 has the largest decrease in nitrate concentrations, from 39 to 10 mg.L -1 on average between the two sampling periods. The temporal variations between 1 st and 2 nd periods are less pronounced for Pz3 and Pz6. Most of the groundwater samples from piezometers PZ5 and Pz9, and borehole SR1 now have nitrate concentrations below 10 mg.L -1 .

Figure 5A shows the sodium versus chloride concentrations in groundwater of the study area. In the absence of evaporites in the local geological formations, chloride may derive from atmospheric [START_REF] Meybeck | Atmospheric inputs and river transport of dissolved substances[END_REF] or anthropogenic (agriculture, wastewater effluents, etc.) inputs and thus may have the same origin as nitrate. Significant amounts of chloride (> 80 mg.L -1 ) were measured in many groundwaters of the Brittany basement (> 80m deep) that were shown to be related to the last marine Mio-Pliocene transgressions, while shallow and modern groundwater has lower Cl concentrations (< 50 mg.L -1 ) [START_REF] Aquilina | Impact of climate changes during the last 5 million years on groundwater in basement aquifers[END_REF]. In this study, groundwater samples were taken at 10 to 22 m depth in the weathered layer and at the top of the basement, except in the SR1 borehole that is 133 m deep. The Cl concentrations are generally less than 60 mg.L -1 including in borehole SR1. Only Pz12 presents a clearly higher Cl concentration of ~140 mg.L -1 (two samples in 2009), as does Pz5 with Cl concentrations of 72 mg.L -1 (the specific case of Pz5 is further discussed in section 4.3). For most groundwater samples, Figure 5A shows an excess of sodium relative to the Na/Cl marine ratio, reflecting Na release in solution through interaction of groundwater with silicate minerals of the aquifer matrix. Only Pz12 has an excess of chloride compared to the Na/Cl ratio of the seawater dilution line, which may indicate anthropogenic input of Cl (both Na and Cl concentrations being higher than in the other samples). Manure and sewage effluents are known to be rich in Cl [START_REF] Kloppmann | Etude isotopique de la pollution azotée de la nappe d'Alsace entre[END_REF], and KCl is used as fertilizers or road salts. The NO3/Cl ratios of manure and sewage are generally lower than in mineral fertilizers, and can thus help distinguishing various anthropogenic sources [START_REF] Lu | Nitrate behaviors and source apportionment in an aquatic system from a watershed with intensive agricultural activities[END_REF]. Moreover, the NO3/Cl ratio may indicate a dilution effect versus nitrate removal through denitrification, as Cl behaves conservatively [START_REF] Chen | Nitrate Sources and Watershed Denitrification Inferred from Nitrate Dual Isotopes in the Beijiang River, South China[END_REF][START_REF] Widory | Improving the management of nitrate pollution in water by the use of isotope monitoring: the δ 15 N, δ 18 O & δ 11 B triptych[END_REF]. Figure 5B that at least part of the Cl is anthropogenic. For the Spring and Pz7 that were sampled during both sampling campaigns, NO3 and Cl concentrations decrease jointly between the two sampling periods, confirming this hypothesis. Pz12 with high NO3 concentrations (75-80 mg.L -1 ) has a different NO3/Cl ratio, with a higher Cl proportion. This may reflect a different anthropogenic source with a lower NO3/Cl than previous samples, which could be related to manure input (trend 2, Figure 5B). Note that, as Pz12 is no longer accessible; it was not possible to resample this well during the 2 nd period. To summarize, the joint decrease of NO3 and Cl concentrations between the two sampling periods could result from a positive impact of the mitigation measures applied on the catchment since about two decades, while a net decrease of nitrate alone should result from a chemical/microbiological process like denitrification.

Evidence of denitrification processes based on N and O isotopes of NO3, microbiology and molecular approaches

Denitrification leads to a decrease, or even 'disappearance' (below the analytical quantification limit), of nitrate concentrations in groundwater. Denitrification by oxidation of pyrite has been identified as an important denitrification pathway in the Brittany basementrock aquifers, where pyrite is abundant [START_REF] Pauwels | Denitrification and mixing in a schist aquifer: influence on water chemistry and isotopes[END_REF]. Pyrite oxidation releases dissolved sulphate in solution. Except for samples Pz5 and Pz9, discussed later, Figure 6 shows an overall trend of increasing sulphate content when the nitrate content decreases, which is typically the case for SR1 samples; Pz8 samples were sampled at various depths and, over both sampling periods, they had the highest SO4 concentrations and medium to low NO3 concentrations. Even Pz6 samples could indicate denitrification through pyrite oxidation. Note that pyrite was observed in fractures of the sandstone and pelite during drilling of Pz6 and Pz8. In the Spring and Pz7 samples, while nitrate concentrations dropped significantly between the two sampling periods, there was no significant increase in sulphate contents. The dual approach of N and O isotopes of nitrate was applied to a selection of samples from each major group defined above (Figure 5B). Among these samples from April 2017, ten were analysed to determine their NO3 isotopic compositions (Table S2, Supplementary material). Note that the three samples from borehole SR1 could not be isotopically analysed because nitrate concentrations were below the quantification limit. The δ 15 N-NO3 isotopic ratios of all samples ranged from +7 to +17‰, with nitrate concentrations ranging from 1 to 74 mg.L -1 in Pz5 and Pz3, respectively. For the samples with the highest nitrate concentrations, the Spring and Pz7 had a δ 15 N-NO3 of ~7-7.5 ‰ that could reflect a mixed influence of mineral and organic fertilizers, while Pz3 with a δ 15 N-NO3 of ~10.8 ‰ appears to be more influenced by organic fertilizer nitrate input (Figure 7), both types of fertilizer being used in the catchment. [START_REF] Kloppmann | Etude isotopique de la pollution azotée de la nappe d'Alsace entre[END_REF][START_REF] Widory | Tracking the sources of nitrate in groundwater using coupled nitrogen and boron isotopes: a synthesis[END_REF]. to have been lost due to immobilization and mineralization processes of nitrogen in the soil [START_REF] Mengis | Limitations of using delta 18 O for the source identification of nitrate in agricultural soils[END_REF][START_REF] Sebilo | Long-term fate of nitrate fertilizer in agricultural soils[END_REF]. Among the three oxygen atoms, associated with nitrogen of the newly formed NO3, two come from local water and one from atmospheric O2 (δ 18 O-O2 ~ +23‰; Equation 1). The isotopic signature of the local groundwater can be approximated using the mean isotopic signature of the local rainfall (weighted average of the The dual δ 15 N-NO3 and δ 18 O-NO3 isotopic approach confirms a denitrification process for several samples, with a joint increase in the δ 15 N and δ 18 O isotopic ratios of the residual nitrate (Figure 8). The slope of the relation between δ 18 O-NO3 and δ 15 N-NO3 for the samples undergoing denitrification is 0.78 (R²=0.85), indicative of denitrification activity [START_REF] Aravena | Use of multiple isotope tracers to evaluate denitrification in groundwater: study of nitrate from a large-flux septic system plume[END_REF]. In more details, samples can plot along a 0.5 slope (e.g., Pz6) and along a 1 slope (e.g., Pz8-14). Compilation of a large number of environmental studies showed that denitrification increases δ 18 O and δ 15 N in a ratio of about 0.5; which corresponds to a 15N ε about twice as large as 18O ε [START_REF] Mengis | Multiple geochemical and isotopic approaches for assessing ground water NO3 -elimination in a riparian zone[END_REF]. A lower slope between δ 18 O-NO3 and δ 15 N-NO3 of nearly 0.5 was also reported from field studies, and another of nearly 1.0 from laboratory studies [START_REF] Otero | Monitoring groundwater nitrate attenuation in a regional system coupling hydrogeology with multi-isotopic methods: the case of Plana de Vic (Osona, Spain)[END_REF][START_REF] Critchley | Stimulating in situ denitrification in an aerobic, highly permeable municipal drinking water aquifer[END_REF][START_REF] Margalef-Marti | Use of nitrogen and oxygen isotopes of dissolved nitrate to trace field-scale induced denitrification efficiency throughout an in-situ groundwater remediation strategy[END_REF][START_REF] Ceballos | Characterisation of the natural attenuation of chromium contamination in the presence of nitrate using isotopic methods. A case study from the Matanza-Riachuelo River basin, Argentina[END_REF]. The main hypothesis now recognized for this flatter slope in field-data sets compared to that in laboratory data, is the oxidation of the intermediates products (NO2 and/or NH4) to NO3 [START_REF] Granger | Isotopic overprinting of nitrification on denitrification as a ubiquitous and unifying feature of environmental nitrogen cycling[END_REF][START_REF] Margalef-Marti | Use of nitrogen and oxygen isotopes of dissolved nitrate to trace field-scale induced denitrification efficiency throughout an in-situ groundwater remediation strategy[END_REF], while a direct NO2 reduction to gaseous N products might lead to values close to the theoretical 1:1 line.

Thus, it is generally accepted that when isotopic nitrate data plot along a slope from 0.5 to 1, the trend can be interpreted as a denitrification process.

Samples Pz6, Pz10 and the three Pz8 samples (14, 16 and 18m deep) are clearly marked by a denitrification, with an alignment describing a slope between 0.5 and 1 (Figure 8). In addition, Pz6 and Pz10 samples show dissolved N2O (intermediate denitrification product), depending on the sampling campaigns, at concentrations that can reach 200 µg.L -1 . It should be noted that Pz3, although in the same alignment, is not affected by denitrification, but rather by a different source of nitrate (more "organic" in this case, Figure 7 and Figure 8) with the highest NO3 concentration measured during the study. Pz5 and Pz9, with low nitrate contents (and low sulphate contents) and have δ 15 N-NO3 isotope signatures that do not indicate any denitrification.

Isotopic analyses thus suggested a microbial nitrate reduction activity in some piezometers, i.e. Pz6, Pz8 and Pz10, but not all the piezometers in the catchment. In water samples of 2017, the relative abundance of copies of narG and napA genes, encoding nitrate reductases involved in nitrate reduction by denitrifying bacteria, was higher in Pz6, Pz8 and Pz10 than in other piezometers, such as Pz3, Pz5 and Pz7, for which isotopic investigation suggested no nitrate reduction (Figure 9 A and B). Results obtained from isotopic investigation and molecular analyses agree, showing higher nitrate-reductase gene abundances in piezometers where nitrate reduction was highlighted by isotopic signatures. Abundance of gene copies was also assessed over a period of two years (through the four sampling campaigns including April 2017), revealing the presence of both narG and napA genes in all piezometers of the catchment, thus showing a potential for microbial nitrate reduction in all studied piezometers of the Plourhan site (data not shown). Spatio-temporal variations in gene abundance were highlighted according to the sampling time and location. This agrees with earlier results that showed a temporal variation in the abundance of denitrification genes in groundwater, and a wide-ranging spatial distribution of these genes in a single catchment (Hernandez-del Amo et al., 2018).

To complete the isotopic and molecular approaches for nitrate reduction characterization in samples collected in April 2017, the potential nitrate reduction activity of microbial communities sampled from several piezometers was measured in batch experiments, run under standardized conditions for denitrification. Nitrate reduction activity was found in all the tested wells (Figure 9C). This agrees with the quantification of functional genes that were also detected, albeit at different relative abundances, in all piezometers (Figure 9 A and B),

showing a spatially widespread potential of the groundwater microbial communities for reducing nitrate. These results agree with earlier work, showing the wide distribution of microbial communities involved in denitrification in groundwater (Hernandez-del Amo et al., 2018). Batch experiments showed a high heterogeneity among the piezometers, as nitrate reduction rates can be high (Pz7 and Pz10) or low (Pz6 and Pz9). They also showed that the complete denitrification pathway occurred, as nitrite (NO2 -) and N2O were detected as well (data not shown). This strongly suggested that a complete denitrification can occur in the different groundwater of the catchment provided that in situ conditions are favourable. Intermediate and high potential nitrate reduction rates found in batch experiments for Pz8 (at two depths) and Pz10, respectively, agree with the denitrification activity showed by isotopic data and the narG and napA genes abundance results found for these piezometers. The low potential nitrate reductase activities found for Pz9 and SR1 also agree with the isotopic results, suggesting no denitrification for Pz9. The results were inconclusive for SR1 as the nitrate concentration was too low, and because of the low abundance of genes encoding the nitrate reductase enzymes. The results from the batch experiments for these four waters thus agreed with the isotopic and molecular approaches.

However, no relation was found for the other piezometers. For Pz3 and Pz7, intermediate to high activity was measured in batch experiments, but isotopic and molecular approaches both suggested a low nitrate reduction potential. For Pz6, whereas the isotopic and molecular approaches suggested nitrate reduction activity and a potential for such activity, respectively, batch experiments only showed a weak nitrate reduction potential. These differences could be due to unsuitable in situ conditions for denitrification compared to batch experiments, in the case where batch experiments showed a high potential, but no nitrate reduction was detected using isotopes. Another explanation might be that the in situ nitrate reduction, shown by isotopic work is actually ancient, explaining why only a weak activity was detected in batch experiments.

Therefore, according to geochemistry, isotopic tools and/or biomolecular characterization, the Pz5, PZ9 and SR1 wells highlight different water origins and/or mechanisms than in other piezometers. This is further investigated in the following section.

4.4 Groundwater residence time assessment: CFCs, SF6 and noble dissolved gases

Dissolved gases (CFCs and SF6) were analysed during the two sampling periods (2009-2011 and 2016-2018), to assess the groundwater residence time and the local aquifer dynamics (Table S3, Supplementary material). In this type of aquifer, the exponential model-reflecting a diffuse recharge at the catchment scale resulting in a mixture of old and young groundwater components at the outlet-generally is the most appropriate one. Here, however, the binary mixing model, that mixes different reservoirs, could be also suitable for reflecting the structure of the basement aquifer, consisting of fractured bedrock overlain by a weathered layer.

The Spring samples appear to be systematically contaminated with CFC-11 and CFC-12.

Howerever, as CFCs have been used worldwide in a wide range of activities, this type of contamination is common near urban areas or landfills [START_REF] Höhener | Worldwide occurrence and fate of chlorofluorocarbons in groundwater[END_REF] A first group of samples is marked by lower recharge temperatures than the present-day one of 11-12°C in Brittany [START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF]. The noble-gas concentrations (Ar and Ne, Figure 10) measured in the SR1 borehole showed a recharge temperature of around 3-4 °C, i.e. 7 to 9 °C lower than the present-day one, indicating the presence of old groundwater in the system that could be related to the last glaciation period of -19 to -17 ky [START_REF] Aquilina | Impact of climate changes during the last 5 million years on groundwater in basement aquifers[END_REF]. In addition, the CFC and SF6 measurements also indicate a long residence time (mean 230±40 yr using an exponential model), with a possible contribution of a recent component not exceeding 10 % that may reflect water inflow resulting from the pumping for groundwater sampling (Figure 11).

This recent component contained nitrate in some SR1 samples from the first sampling period (2009), with NO3 reaching 38 mg.L -1 at 60 m depth versus 3.7 mg.L -1 at 110 m depth. In addition, during the second sampling period (2017-2018), an excess of 50 to 100% dissolved N2 was determined compared to Ne and Ar concentrations, with concentrations reaching 1.2.10 -3 to 1.4.10 -3 mol.L -1 compared to 7.10 -4 to 8.10 -4 mol.L -1 in the Spring samples This may be a result from the denitrification of the incoming nitrate, no NO3 being detected (<LQ) during the second sampling period. In a second location, Pz5 tapping the saprolite and the upper part of the fractured amphibolite, noble-gas analyses (n=2, Figure 10) also showed a low recharge temperature of around 6 °C. While no nitrate was detected during the first sampling period, about 1 mg.L -1 was measured during the second sampling period, which could corroborate the CFC-SF6 analyses indicating between 70 and 85 % of old water (i.e.

CFC-SF6 free), and thus a small contribution of recent water possibly enriched in nitrate (Figure 11).

Above, we also pointed out a relatively high Cl concentration of up to 72 mg.L -1 , i.e. the highest concentration in the studied area (except Pz12 with both high Cl and NO3 concentrations). An anthropogenic Cl contribution is unlikely as there is no nitrate and no denitrification process was detected. Pz5 samples are the only ones with a Na/Cl ratio close to that of seawater (Figure 5A). Combining the low recharge temperature, the absence of A second group, represented by Pz9, has a long mean residence time of over 60 years (Figure 11) , with a large proportion (80-90%) of old water and no CFC-SF6, but no evidence of low recharge temperatures. These waters thus appear to be younger than the ones of Pz5 and SR1, but old enough not to have received anthropogenic nitrate.

The other groundwater samples have apparent ages of about 30 years, such as Pz10 and Pz3. Pz6 shows a variability between the two samples analysed for dating purposes. This can reflect mixing in various proportions between old and present-day waters (Figure 11)

according to the period of the hydrological cycle, the apparent age being slightly older in May 2018 than in October 2017. This variability was also observed for the major elements, which is typical of the sandstone and pelite aquifer that has different aquifer dynamics than the amphibolite aquifer. Pz7 has a high SF6 contamination; based on the other tracers, the waters could correspond to a mixing between a large proportion of recent water (70-80%) and a 30-40-years old water. Such different groundwater residence times are known from other parts of Brittany.

Evidence of past climatic and geological event was found in groundwater from Lanleff monitoring station < 10km west of Plourhan (Armandine Les Landes et al., 2015) and in various other parts of Brittany by [START_REF] Aquilina | Impact of climate changes during the last 5 million years on groundwater in basement aquifers[END_REF]. Most shallow groundwaters at a regional scale, however, is strongly impacted by anthropogenic activity and has mean apparent residence times of 10 to 30 years [START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF]. All earlier studies studied different regions, which raises questions about the functioning and connectivity of the aquifer systems studied, questions that might be answered by detailed geological studies of the sites.

Conclusions and perspectives

The Plourhan catchment (~100 ha) contains a main spring that still exceeds the limit of 50 mg/L NO3 despite the numerous remedial measures implemented since the 1990s. We investigated the catchment with complementary and robust geological and hydrogeological approaches, better to constrain the driving parameters of nitrate transfer and distribution in groundwater, and thus to improve our understanding of how the hydrosystem functions.

First, geochemical water sampling over about 10 years showed a quite high variability in groundwater quality: (1) a temporal variability especially regarding nitrate and (2) a spatial variability despite the small size of the catchment. Though a link with agricultural practices and land use is one of the key factors controlling nitrate fate in groundwater, other factors must be considered as well, such as the hydro-biochemical functioning of the aquifer system, which can also largely control water quality.

Secondly, the syrudy of nitrate isotopes (N and O) clearly demonstrated denitrification in some piezometers of the catchment area. Such processes were generally demonstrated by microbiology and molecular biology studies, highlighting the presence of functional genes involved in denitrification as well as the capacity of the groundwater microbial community for denitrifying when in situ anoxic and/or nutritive conditions are favourable. Other groundwater samples had low nitrate contents, even below the quantification limit, without any denitrification process occurring. Study of the average groundwater residence time, based on dissolved CFCs and SF6 and on rare gases; locally discovered old waters, particularly in nitrate-free piezometers, demonstrating that such waters never received any anthropogenic nitrate. One piezometer contained old water, with a recharge temperature well below the current average temperature in the area, reflecting water from the last glaciation period (-19 to -17 ky).

Third, the presence of such very old groundwater in the catchment area, raises questions about the structure of the aquifer system at the scale of the recharge area. The detailed geological and geophysical work, especially with electric tomography profiles, led to improvement of the local structural model. A large number of compartments delimited by faults was shown to exist, as was a highly variable thickness of soil and weathered rock in the study area. This better undestanding of the geological structure has shown the heterogeneity of the hydrosystem, where some compartments appear to be disconnected from the general groundwater flow. This may explain the presence of old residual groundwater not affected by the agricultural activity in the catchment.

The Plourhan site has shown the relevance and necessity of using an integrated approach for significantly improving our knowledge in terms of the functioning of complex hydrosystems-such as basement ones-and its impact on groundwater quality. This approach can be advantageously applied to other sites, especially in basement domains where denitrification processes can deeply modify the local water quality, and where the geological structure mau be quite complex. The various methods of such an approach can be implemented independently, and successively, steered by gradually improving knowledge, or in a coupled manner as done here. The understanding of hydrodynamic functioning sensu lato is as important as that of nitrate reduction. Both are paramount for implementing the most relevant programmes of measures (as required by the European Water Framework Directive; Fifth WFD and FD Implementation Reports, 2019), for recovering a good water quality and assessing the effectiveness of such measures and methods, notably by considering the timeframe between implementation of the measures and their impact on groundwater quality.

Acknowledgments

The French Loire-Bretagne Water Agency (AELB) together with the BRGM financially

Figure 1 :

 1 Figure 1: (A) Location of sampling sites and geophysical investigations in the Plourhan site (NW France; background 1:25,000-scale IGN map). Piezometers with grey symbols are no longer accessible. (B) Geological map of the studied area (@BRGM Pontrieux sheet, 1:50,000 scale).

Figure 2 :

 2 Figure 2: Temporal evolution of the nitrate concentrations in the Spring since the early 1990s (green crosses).

Figure 3 :

 3 Figure 3: (A) Revised geological map of the study area. (B) Geological cross-sections derived from the electrical resistivity distribution profiles (FIG S1 in supplementary material).

Figure 4 :

 4 Figure 4 : (A) Piper diagram of the Plourhan groundwater samples. Both studied periods are shown, empty symbols are samples with data only available during the first period 2009-2011. (B) Total Dissolved Solids (TDS) versus Ca concentration.

  shows NO3 vs. Cl concentrations, part of the groundwater samples with the highest nitrate concentrations (Spring, Pz7, Pz3, Pz2 and Pz1) defining a straight line correlating NO3 and Cl concentrations (trend 1, Figure 5B), suggesting a common origin of both elements, i.e.

Figure 5 :

 5 Figure 5 : (A) Sodium versus chloride concentrations in Plourhan groundwater samples. 'Coastal rain' refers to the mean rainfall at Brest (100 km to the west; Negrel et al., 2007). The Seawater dilution line (Berner-Kay and Berner, 1987) is presented as reference. Both studied periods are plotted, each sampling point has the same colour with different symbols: empty ones are samples from the first period (2009-2010). (B) Nitrate versus Chloride concentrations in Plourhan groundwater. The red dashed line represents the 50 mg.L -1 nitrate standard. The black dashed lines represent the two main NO3 vs. Cl trends.

Figure 6 :

 6 Figure 6 : Sulphate versus nitrate concentrations in mg.L -1 in Plourhan groundwater samples for both sampling periods.

Figure 7 :

 7 Figure 7 : δ 15 N-NO3 vs. NO3 concentrations for the April 2017 samples. The purple dashed line represents the denitrification model according to the Raleigh equation, starting from the Spring sample characteristics. Bar charts on the right side represent the δ 15 N frequencies of mineral fertilizers and animal waste. The main nitrate sources measured in France are also shown[START_REF] Kloppmann | Etude isotopique de la pollution azotée de la nappe d'Alsace entre[END_REF][START_REF] Widory | Tracking the sources of nitrate in groundwater using coupled nitrogen and boron isotopes: a synthesis[END_REF].

Figure 7

 7 Figure 7 also shows that, with the exception of Pz5 and Pz9, the samples present an increasing δ 15 N-NO3 when NO3 concentrations decrease, which is a typical trend of denitrification according to the Rayleigh model. Here, we chose to apply a denitrification model starting from the sample signatures with the highest nitrate concentration, and assumed not to be impacted by denitrification as shown by their chemical composition. The denitrification model is thus based on the signature of Spring and Pz7 samples (δ 15 N-NO3 = -7‰, [NO3] = 62 mg.L -1 ) with an enrichment factor ε = -5‰. It explains a partial denitrificationfor samples Pz6, Pz10 and Pz8 at different depths as these samples plot along the calculated denitrification curve (Figure7). The relatively low ε value (-5‰) is similar to the one found by[START_REF] Pauwels | Denitrification and mixing in a schist aquifer: influence on water chemistry and isotopes[END_REF] in the deep aquifer of the pyrite-bearing Brioverian schist at the Naizin site (SE Brittany), and suggests a relatively rapid denitrification process[START_REF] Mariotti | 15 N isotope biogeochemistry and natural denitrification process in groundwater: application to the chalk aquifer of northern France[END_REF] in full agreement with the results of[START_REF] Pauwels | Field tracer test for denitrification in a pyrite-bearing schist aquifer[END_REF] in the same area. Note that a low ε value (-4‰) was further confirmed in the same site during an artificial tracer test along a fracture in the pyrite-bearing Brioverian schist basement[START_REF] Petelet-Giraud | Denitrification processes in a schist aquifer: multi-isotope constraint applied to a field tracer test[END_REF].

Figure 8 :

 8 Figure 8: δ 18 O-NO3 versus δ 15 N-NO3 with typical δ 15 N and δ 18 O values of nitrates derived or nitrified from various nitrogen sources (after Kendall, 1998). The two red "denitrification" arrows indicate typical slopes expected for data resulting from nitrate denitrification with initial δ 15 N = +7‰ and δ 18 O = +4‰. Typical ranges of values for δ 18 O-NO3 produced by nitrification of ammonium and organic matter are referred to as "Nitrification". The size of the circles for each sample of April 2017 is proportional to the measured nitrate concentration. All isotopic data of the April 2017 sampling campaign are shown in Figure 8, coupling δ 15 N-NO3 and δ 18 O-NO3. Firstly, it should be noted that the original δ 18 O-NO3 isotope signature of synthetic mineral fertilizers (δ 18 O-NO3 ~ +20‰) is not found in groundwater samples, even before any denitrification (Spring and Pz7 samples). The initial 18 O isotopic signature seems

  precipitation amounts). According to the French national map by Millot et al. (2010), the local groundwater signature is estimated to be δ 18 O-H2O = -6‰. According to Equation 1, the theoretical δ 18 O value of the newly formed nitrate in the soil is thus δ 18 O-NO3 ~ +4 ‰, which agrees with the measured values of the Spring and Pz7 samples (+4.1 and +4.6 ‰ respectively, and considering an analytical accuracy of ± 0.5 ‰).

Figure 9 :

 9 Figure 9 : Microbiological and biomolecular analyses of water collected in piezometers at the Plourhan site in April 2017. A and B: Relative abundance of narG gene copies (A) and napA gene copies (B) involved in bacterial nitrate reduction, determined with quantitative PCR. (C) Potential nitrate reduction activity measured in batch experiments under standardized conditions for denitrification (for all piezometers, no nitrate reduction was observed in abiotic flasks). Significant differences between conditions were searched for by means of the Kruskal-Wallis non-parametric test (p>0.05) and are mentioned as letters (a, b… f).

Figure 10 :

 10 Figure 10: Mean Neon versus Argon concentrations (mol/L) for each groundwater sample monitored during the period 2016-2018 (n=2 for Pz10 to n=16 for the Spring). Blue and red lines represent the theoretical values from solubility equations for an altitude of 100 m, a temperature between 0 to 20 °C and an excess air between 0 and 8 mL/L. Samples plotting in around the same red line have the same recharge temperature, but different excess air.

  nitrate and the relatively high Cl concentration considered to be natural, this sampling point could reflect the legacy of marine transgressions over the past 5 millions years. Armandine LesLandes et al. (2015) considered that Cl of marine origin, not supplied by precipitation (> 40 mg L -1) is likely related to marine transgressions that flooded the Armorican Massif since the Messinian. The large quantities of Cl injected during each transgression were then diluted by meteoric recharge after the end of the transgression period. This includes recharge during the last glaciation, shown by a low recharge temperature indicated by noble gases. In Pz5 at 18 m depth, the water comes from the upper part of the amphibolite basement and from the lower part of the overlying weathered material. This old water, preserved from anthropogenic impact, is disconnected from the groundwater feeding the Spring about 500 m distant, relatively disconnected from present day recharge, and only remobilized by pumping for sampling.

Figure 11 :

 11 Figure 11: Mean NO3 concentrations versus the mean percentage of the young-water component in each groundwater sample of the study area.

  supported this study through the POLDIF Research project led by N. Baran. The authors thanks Florian Koch, Benjamin Maurice and Mickael Charron for their precious asssitance in the field, Marine Gremont for her synthesis of the measures for recovering water quality. All colleagues from the BRGM Labs involved in the data production are thanked for their valuable work. Jean-Michel Schroetter is warmly thanked for the many fruitful discussions on the local geological settings and how they can govern water circulations and water quality. E. Petelet-Giraud addresses special thanks to Gilles L. and Christophe G. for their fruitful comments on figure 8. We are grateful to Dr. H.M. Kluijver for proofreading and editing the English text.