Feifei Cao
email: feifei.cao@etudiant.univ-reims.fr

Neil C Sturchio

Patrick Ollivier

Nicolas Devau

Linnea J Heraty

Jessy Jaunat

Sources and behavior of perchlorate in a shallow Chalk aquifer under military (World War I) and agricultural influences

Keywords: Perchlorate, Isotope, Groundwater, World War I, Chilean nitrate fertilizer Highlights

Perchlorate (ClO4 -) has been detected at concentrations of concern for human health on a large scale in groundwater used for drinking water supplies in NE France. Two sources are suspected: a military source related to World War I (WWI) and an agricultural source related to past use of Chilean nitrate fertilizers. The sources and behavior of ClO4 -have been studied in groundwater and rivers near the Reims city, by monitoring monthly the major ions and ClO4 -concentrations for two years (2017 -2019), and by measuring the isotopic composition of ClO4 -and NO3 -in water samples. ClO4 -was detected throughout the study area with high concentrations (> 4 µg⋅L -1 ) detected mainly downgradient of the Champagne Mounts, where large quantities of ammunition were used, stored and destroyed during and after WWI. A WWI military origin of ClO4 -is inferred from isotopic analysis and groundwater ages. Different tendencies of ClO4 -variation are observed and interpreted by a combination of ClO4 -concentrations, aquifer functioning and historical investigations, revealing major sources of ClO4 -(e.g., unexploded ordnance, ammunition destruction sites) and its transfer mechanisms in the aquifer. Finally, we show that concentrations of ClO4 -in groundwater seems unlikely to decrease in the short-to medium-term.

Introduction

Perchlorate (ClO4 -) is an environmental pollutant of growing concern due to its widespread occurrence in water and its adverse health effects by interfering with thyroid uptake of iodine and production of hormones (e.g., [START_REF] Brabant | The effect of perchlorate, thiocyanate, and nitrate on thyroid function in workers exposed to perchlorate long-term[END_REF][START_REF] Brabant | The effect of perchlorate, thiocyanate, and nitrate on thyroid function in workers exposed to perchlorate long-term[END_REF][START_REF] Greer | Health effects assessment for environmental perchlorate contamination: the dose response for inhibition of thyroidal radioiodine uptake in humans[END_REF]. The high solubility and nonreactivity of ClO4 -under typical geochemical conditions make it highly mobile in the aquatic environment [START_REF] Brown | The Chemistry of Perchlorate in the Environment[END_REF][START_REF] Urbansky | Perchlorate Chemistry: Implications for Analysis and Remediation[END_REF]. The origin of ClO4 -in the environment can be synthetic or natural [START_REF] Cao | Worldwide occurrence and origin of perchlorate ion in waters: A review[END_REF]. ClO4 -is widely used as an oxidant in solid rocket fuels and explosives [START_REF] Urbansky | Perchlorate Chemistry: Implications for Analysis and Remediation[END_REF]. Therefore, the use and manufacture of these products may constitute an important source of ClO4 -contamination. Other synthetic sources of ClO4 -include fireworks, air bags, matches, road flares, chlorate herbicides and bleach products [START_REF] Aziz | Alternative Causes of Wide-Spread, Low Concentration Perchlorate Impacts to Groundwater[END_REF][START_REF] Trumpolt | Perchlorate: Sources, Uses, and Occurrences in the Environment[END_REF].

In natural environments, ClO4 -can form atmospherically and accumulate by dry and wet deposition in vadose soils in arid and semi-arid environments such as the Atacama Desert (Chile), the southwestern United States, northwestern China, and Antarctica [START_REF] Jackson | Global patterns and environmental controls of perchlorate and nitrate cooccurrence in arid and semi-arid environments[END_REF][START_REF] Jackson | Deposition, accumulation, and alteration of Cl -, NO3 -, ClO4and ClO3salts in a hyper-arid polar environment: Mass balance and isotopic constraints[END_REF][START_REF] Lybrand | Nitrate, perchlorate, and iodate co-occur in coastal and inland deserts on Earth[END_REF][START_REF] Rao | Widespread natural perchlorate in unsaturated zones of the southwest United States[END_REF]. The natural NO3 --rich salt deposits in the Atacama Desert (Chilean nitrate), with ClO4 -as a minor component (0.1-0.5 wt.%), have been refined and distributed worldwide for use as NO3 -fertilizer, especially during the first half of the 19 th century, thus representing a potential widespread source of ClO4 -contamination in waters [START_REF] Ericksen | The Chilean nitrate deposits[END_REF][START_REF] Rajagopalan | Widespread presence of naturally occurring perchlorate in high plains of Texas and New Mexico[END_REF]. Over the last two decades, ClO4 -contamination has been reported from many countries including the USA, Canada, Chile, China, India, UK and France, related to various origins (e.g., [START_REF] Cao | Worldwide occurrence and origin of perchlorate ion in waters: A review[END_REF][START_REF] Cao | Sources and behavior of perchlorate ions (ClO4 -) in chalk aquifer of Champagne-Ardenne, France: preliminary results[END_REF][START_REF] Furdui | Anthropogenic Perchlorate Increases since 1980 in the Canadian High Arctic[END_REF][START_REF] Jackson | Perchlorate occurrence in the Texas Southern High Plains Aquifer System[END_REF][START_REF] Kannan | Occurrence of perchlorate in drinking water, groundwater, surface water and human saliva from India[END_REF][START_REF] Mclaughlin | Perchlorate in raw and drinking water sources in England and Wales[END_REF][START_REF] Qin | Spatial distribution of perchlorate, iodide and thiocyanate in the aquatic environment of Tianjin, China: environmental source analysis[END_REF][START_REF] Sturchio | Isotopic tracing of perchlorate sources in groundwater from Pomona, California[END_REF][START_REF] Urbansky | Perchlorate as an environmental contaminant[END_REF][START_REF] Vega | Perchlorate contamination in Chile: Legacy, challenges, and potential solutions[END_REF].

In France, recommended levels of ClO4 -in drinking water were first issued in 2011 by the French Agency for Food, Environmental and Occupational Health & Safety (ANSES): 15 µg⋅L -1 for adults and 4 µg⋅L -1 for children under 6 months. The level was subsequently reduced from 15 to 5 µg⋅L -1 for adults (ANSES, 2018). According to the national measurement campaign conducted by ANSES from 4 2011 to 2012, most of the sites having high concentrations of ClO4 -(> 4 µg•L -1 ) were located in NE France [START_REF] Anses | Campagne nationale d'occurrence de polluants émergents dans les eaux destinées à la consommation humaine[END_REF].

The large extent of ClO4 -contamination in NE France is unlikely to have been caused by point sources related to industrial activities (Figure 1). However, a potential link between the spatial distribution of high ClO4 -concentrations and the position of the trench areas of WWI (1914WWI ( -1918) ) was observed.

Thus, a military source of ClO4 -related to WWI seems likely [START_REF] Hubé | Sur les traces d'un secret enfoui: Enquête sur l'héritage toxique de la Grande Guerre -Préface de Jean-Yves Le Naour[END_REF][START_REF] Hubé | Marquage pyrotechnique : analyse de la problématique environnementale[END_REF][START_REF] Ricour | Un exemple d'altération du fond géochimique naturel des sols et des eaux souterraines : les séquelles environnementales des grands conflits mondiaux en France[END_REF] but has never been clearly demonstrated. Indeed, synthetic ClO4 -produced industrially by electrolysis was largely used in explosives manufacturing during WWI. According to [START_REF] Hubé | Perchlorates : éléments historiques et d'expertise pour une évaluation de l'impact environnemental[END_REF], ~131,000 T of (per)chlorate explosives (primarily composed of NH4ClO4 , KClO4, or NaClO3) were used during WWI, mainly in grenade and trench artillery. In addition, ClO4 -could also be present in other explosives including black powder and nitro group explosives (e.g., TNT, nitroglycerine and nitrocellulose), as Chilean nitrate (with ClO4 -impurity) was intensively used in the manufacturing of these explosives. [START_REF] Jaunat | Les impacts de La Grande Guerre sur la qualité des eaux souterraines: les cas des perchlorate[END_REF] High levels of ClO4 -have also been detected in some regions outside the trench areas (e.g., Oise, Pasde-Calais; Figure 1). An agricultural source was suspected, as large quantities of Chilean nitrate was used as fertilizer in France between 1880 and 1950 [START_REF] Lopez | Recherche des origines de la pollution en perchlorate impactant des captages au sein des AAC de la région de Nemours et Bourron-Marlotte[END_REF], especially for sugar beet and wheat cultivation [START_REF] Zimmermann | Le nitrate du Chili[END_REF]. Around the year 1928, the estimated amount of Chilean nitrate applied annually for wheat cultivation in France was between 150 and 400 kg⋅ha -1 and could exceed 800 kg⋅ha -1 for beet cultivation [START_REF] Lopez | Vulnérabilité des eaux souterraines de France métropolitaine vis-à-vis des ions perchlorates[END_REF]. For a better management of water resources in NE France, it is now necessary to clarify the source of ClO4 -contamination (military and/or agricultural). In addition to hydrogeological and historical investigations, isotopic analysis of ClO4 -can provide a direct approach for ClO4 -source apportionment.

Measurements of stable isotope ratios of chlorine ( 37 Cl/ 35 Cl) and oxygen ( 18 O/ 16 O, 17 O/ 16 O) and the fractional abundance of the radioactive isotope 36 Cl in ClO4 -ions have shown that three primary source types of ClO4 -(synthetic; "Atacama" from nitrate salts mined in the Atacama Desert of Chile; and indigenous natural atmospheric deposition) can be clearly distinguished isotopically [START_REF] Bao | Natural perchlorate has a unique oxygen isotope signature[END_REF][START_REF] Böhlke | Stable isotope analyses of oxygen (18O:17O:16O) and chlorine (37Cl:35Cl) in perchlorate: reference materials, calibrations, methods, and interferences[END_REF][START_REF] Böhlke | Atacama perchlorate as an agricultural contaminant in groundwater: isotopic and chronologic evidence from Long Island, New York[END_REF][START_REF] Böhlke | Perchlorate Isotope Forensics[END_REF][START_REF] Hatzinger | Guidance Document: Validation of chlorine and oxygen isotope ratio analysis to differentiate perchlorate sources and to document perchlorate biodegradation U.S. Department of Defense[END_REF][START_REF] Jackson | Isotopic composition and origin of indigenous natural perchlorate and co-occurring nitrate in the southwestern United States[END_REF][START_REF] Poghosyan | Perchlorate in the Great Lakes: isotopic composition and origin[END_REF][START_REF] Sturchio | Isotopic tracing of perchlorate sources in groundwater from Pomona, California[END_REF][START_REF] Sturchio | Isotopic Tracing of Perchlorate in the Environment[END_REF][START_REF] Sturchio | Chlorine-36 as a tracer of perchlorate origin[END_REF][START_REF] Sturchio | Stable isotopic composition of chlorine and oxygen in synthetic and natural perchlorate[END_REF]. Additional background information about the ranges of isotopic composition of ClO4 -is detailed in the references above and summarized in [START_REF] Cao | Worldwide occurrence and origin of perchlorate ion in waters: A review[END_REF].

In this study, a representative study area with two potential sources of ClO4 -contamination (military and/or agricultural) was selected east of the Reims city in NE France (Figure 1). In this agricultural and military (WWI) context, the primary objectives of this study are to 1) assess the extent of ClO4 - contamination and its spatio-temporal evolution in the Chalk aquifer, 2) clarify the sources of ClO4 - contamination (military and/or agricultural) and 3) understand the mechanism of transport and predict the evolution of ClO4 -in groundwater in the short-to medium-term. To achieve these goals, we used an approach combining continuous monitoring of ClO4 -concentrations, isotopic composition measurements, historical and hydrogeological investigations, which could be further applied in other ClO4 -contaminated areas.

Materials and methods

Description of the study area

Location and land use

The study area is situated east of the city of Reims, in the Champagne region in NE France (Figure 1 and Figure 2). It covers about 500 km 2 between the Vesle River (as the southern boundary) and the Suippe River (as the northern and eastern boundaries; Figure 2). Land use in this area is largely agricultural (> 80%) with wheat, barley, sugar beet, and alfalfa as major crops. Forest covers about 15% of the study area, mainly on the Berru and Champagne Mounts and the riparian areas. Urban lands including towns, villages, and industrial sites represent only about 3% of the study area (CORINE Land Cover geographic database).

Geological and hydrogeological context

A detailed geological and hydrogeological description of the study area is given by [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF].

The Chalk formation (Upper Cretaceous, 66 -100 Ma) constitutes almost the entire surface of the study area, with only a limited exposure of Tertiary formation at Berru Mount (Figure 2). The Tertiary formation includes a succession of permeable (sand and coarse limestone) and impermeable deposits (clay and marl) where only a few thin aquifers of limited extent are developed with some springs flowing from the sand layers [START_REF] Laurain | Notice explicative, Carte géol[END_REF]. These aquifers are insufficiently productive to be used as drinking water supplies. The Chalk formation of the study area includes Coniacian (C3), Santonian (C4) and Campanian (C5) Chalks (Figure 2), which have similar lithology and hydrodynamic properties despite their different ages of formation. It is a pure, fine-grained carbonate rock, characterized by dual porosity (matrix and fracture) providing both slow and rapid flowpaths for groundwater (e.g., [START_REF] Foster | The Chalk groundwater tritium anomaly -A possible explanation[END_REF][START_REF] Headworth | Evidence for a shallow highly-permeable zone in the Chalk of Hampshire[END_REF][START_REF] Price | Fluid flow in the Chalk of England[END_REF]. The total porosity of the Chalk is about 40% [START_REF] Crampon | The Hydrogeology of the Chalk of North-West Europe[END_REF] with only 1% effective porosity [START_REF] Vachier | Mouvements de l'eau dans la zone non sature´e et alimentation de la nappe de la craie de champagne (France)[END_REF]. The first 10-20 m of the Chalk is significantly fractured; the density of fractures decreases with depth and distance to river valleys [START_REF] Allen | The physical properties of major aquifers in England and Wales[END_REF][START_REF] Mangeret | Reactive transport modelling of groundwater chemistry in a chalk aquifer at the watershed scale[END_REF][START_REF] Vachier | Mouvements de l'eau dans la zone non sature´e et alimentation de la nappe de la craie de champagne (France)[END_REF]. In addition, the Chalk formation is partially covered by superficial (Quaternary) formations including graveluche (a periglacial formation up to 10 m thick), colluvium (1-3 m thick) and alluvium (up to 10 m thick) with relatively high content of clay and silt [START_REF] Allouc | Avis de l'Anses relatif à la « Pertinence de la ré-évaluation de la valeur guide pour les ions perchlorate dans l'eau destinée à la consommation humaine[END_REF][START_REF] Vernhet | Carte geologique harmonisée du département de la Marne Rapport BRGM RP-55732-FR[END_REF] (Figure 2). [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. Source of geological map: [START_REF] Laurain | Notice explicative, Carte géol[END_REF], Allouc and Le Roux. (1995); source of water level data: Rouxel-David et al. (2002a).

The chalk aquifer is an important groundwater resource of the region. Precipitation is the only recharge source of the aquifer, mainly from November to March, due to the excess of rainfall compared to evapotranspiration [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF][START_REF] Chiesi | Transfert et epuration dans la zone non saturee de la craie en champagne : etude de quelques cas concernant les nitrates et l'atrazine[END_REF]. The surface runoff is usually considered as very low or absent for the Champagne Chalk aquifer [START_REF] Chiesi | Transfert et epuration dans la zone non saturee de la craie en champagne : etude de quelques cas concernant les nitrates et l'atrazine[END_REF][START_REF] Foster | The Chalk groundwater tritium anomaly -A possible explanation[END_REF][START_REF] Mathias | Transient simulations of flow and transport in the Chalk unsaturated zone[END_REF]. The study area is divided into two parts by the groundwater divide line across the summit of Berru Mount and the Champagne Mounts, which delimits the Vesle River watershed in the south and the Suippe River watershed in the north (Figure 2).

Suspected sources of ClO4 -contamination

Groundwater contamination by ClO4 -in the study area is potentially caused by both the WWI-related military source and the agricultural source related to the past use of Chilean nitrate fertilizer. The study area was intensively marked by events of WWI [START_REF] Facon | Les batailles des monts de Champagne 1914-1918[END_REF][START_REF] Laurent | La Grande Guerre en Champagne et la deuxième victoire de la Marne[END_REF]. As shown in Figure 3, almost the entire study area was crossed by trenches, particularly in the center of the area [START_REF] Taborelli | Les conditions géographiques et l'organisation spatiale du front de la Grande Guerre : application à l'évaluation environnementale post-conflit en Champagne-Ardenne[END_REF]. From 1914 to 1918, these trench areas were the scenes of several intensive battles of WWI (e.g., the Battles of Champagne), especially on the Champagne Mounts. During WWI, military tunnels were dug by the German army, such as the tunnels of Mont Cornillet (N1), Mont Perthois (N2) and Mont sans Nom (N3) (Figure 3). These tunnels, usually equipped with quantities of ammunition, connected the German front positions with the rear and allowed the German army to fire until the last moment. It is highly probable that quantities of ammunition still remain underground at these sites, representing potential sources of groundwater contamination of ClO4 -and other pyrotechnic compounds. Furthermore, it is likely that the precise locations of some of these underground sites are unknown.

After WWI, large quantities of unused ammunition were still present on the French land and it was necessary to clear the battlefield in order to make these lands habitable and cultivable on a large scale. Thus, unused ammunitions were either dismantled to recycle valuable materials or destroyed by explosion. The ammunition explosion sites were recognizable in historical aerial photographs by anthropogenic forms such as alignments of shell-holes, an access road, and a storage area. Several explosion sites were identified in the study area from inspection of historical aerial photographs (IGN Remonter le temps database; Figure 3). While traces of N5 and N6 could no longer be recognized, the destruction activities on site N1 lasted until the 2000s and some military wastes are still present at this site (BASOL database). The sites and activities listed are not exhaustive since the compilation is based on existing data from historical archives and/or former aerial photographs that did not cover the entire study area. In addition, traces of military activities could have been quickly erased, making it difficult to identify all related sites. The ammunitions exploded on battlefields during WWI or destroyed after the war could have released ClO4 -into the environment. In addition, large quantities of unexploded ammunitions (about 30% of the total quantity of ammunition used) could still persist in the subsoil [START_REF] Desailloud | Faut-il craindre les ions perchlorate dans l'environnement ? La Presse Médicale[END_REF], representing a continuous and diffuse source of ClO4 -contamination. As a traditional agricultural area for wheat and sugar beet, the study area was extensively cultivated with the use of Chilean nitrate fertilizer from 1880 to 1950. It was estimated that the average annual consumption of nitrogen in Marne (the department where the study area is located) was between 7,000 and 10,000 T in the 1950s [START_REF] Lopez | Vulnérabilité des eaux souterraines de France métropolitaine vis-à-vis des ions perchlorates[END_REF]. The use by spreading and the storage of these Chilean nitrates could represent diffuse or point sources of ClO4 -contamination in groundwater.

Sampling and analysis methods

An intensive sampling network was established in the study area with a total of 36 sampling points including 26 groundwater (19 boreholes, 3 pumping stations and 4 springs) and 10 surface water (2 in the Aisne-Marne Canal, 2 in the Vesle River, 3 in the Suippe River and 3 in their tributaries) (Figure 2), as surface water is here closely related to groundwater of the Chalk aquifer. The boreholes and pumping stations have long screen depths which cover at least half of the well depths (Table 1), thus providing a weighted average of groundwater. A first screening campaign was carried out in June 2017, which yielded a chemical map of the study area and the extent of ClO4 -contamination. Fourteen sampling points (primarily points with high concentrations of ClO4 -, well distributed in the study area;

Figure 2) were then selected for monthly monitoring for 2 years to observe the spatio-temporal evolution of groundwater geochemistry and ClO4 -concentrations. In addition, groundwater ages, water flow rate in rivers, explosive concentrations, and isotopic compositions of ClO4 -and NO3 -were measured.

The measurement methods and results of groundwater level, physico-chemical parameters, major ions and groundwater dating using CFCs and SF6 have been fully described and interpreted previously in [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. These results are directly used in this study.

Water discharge of rivers was measured by the cross-section method using an OTT C2 Small Current Meter. ClO4 -was analyzed by ion chromatography at BRGM (Orléans, France) with a quantification limit of 0.5 µg•L -1 . Thirty-nine explosives (see SI.1) were analyzed in waters with an RPHPLC-DAD system by the company Envilytix GmbH (Wiesbaden, Germany), as described by [START_REF] Bausinger | Exposure assessment of a burning ground for chemical ammunition on the Great War battlefields of Verdun[END_REF].

Isotopic analyses of ClO4 -were carried out in water samples collected from 3 sites in the study area: FBN4 (at high and low water levels), FVDV (at high water level) and PY (at high water level; Figure 2). In order to obtain a pure military isotopic signature of ClO4 -, a water sample was also collected at the limit of the Suippes Military camp (outside the study area; Figure 1), as the only source of ClO4 - here that is related to WWI without any potential agricultural influence. Water samples were collected using PVC columns containing ClO4 -specific ion exchange resin (IX resin) and then purified and analyzed as described previously by [START_REF] Gu | Removal, Recovery and Fingerprinting of Perchlorate by Ion Exchange Processes in Ion Exchange And Solvent Extraction: a Series of Advances[END_REF]), Hatzinger et al. (2011, 2018), and [START_REF] Böhlke | Stable isotope analyses of oxygen (18O:17O:16O) and chlorine (37Cl:35Cl) in perchlorate: reference materials, calibrations, methods, and interferences[END_REF]. The extraction and purification of ClO4 -was done in the Department of Civil and Environmental Engineering of Texas Tech University (USA) and the key steps can be summarized as follows: 1) the resin was washed by deionized water and flushed with 4M HCl to remove NO3 -, SO4 2-, HCO3 -and organics; 2) the absorbed ClO4 -was eluted from the IX resin using a solution of 1M FeCl3 and 4M HCl [START_REF] Gu | Treatment of Perchlorate-Contaminated Groundwater Using Highly Selective, Regenerable Ion-Exchange Technologies[END_REF][START_REF] Gu | Regeneration of Perchlorate (ClO4-)-Loaded Anion Exchange Resins by a Novel Tetrachloroferrate (FeCl4-) Displacement Technique[END_REF][START_REF] Gu | Recent Advances in Ion Exchange for Perchlorate Treatment, Recovery and Destruction[END_REF]; 3) eluted ClO4 -was purified by a series of precipitation, liquid-liquid extraction, evaporation, and cation exchange processes, then crystallized as a ClO4 -salt for isotopic analysis. The relative abundances of stable isotopes of chlorine ( 37 Cl and 35 Cl) and oxygen ( 18 O, 17 O and 16 O) in ClO4 -were measured using isotope-ratio mass spectrometry (IRMS) at the Environmental Isotope Geochemistry Laboratory of the University of Delaware (USA).

For isotopic analysis of NO3 -, water samples were collected in 100-mL polyethylene bottles after filtration through 0.45 µm membranes. Nitrogen and oxygen isotope ratios were measured using an automated denitrifier method as described in [START_REF] Morin | Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer from 65°S to 79[END_REF] and [START_REF] Savarino | Isotopic composition of atmospheric nitrate in a tropical marine boundary layer[END_REF]. This technique uses Pseudomonas aureofaciens bacteria to convert NO3 -to N2O, which is then analyzed for its isotopic composition after thermal decomposition to O2 and N2. Isotopic analysis was performed on a Thermo Finnigan MAT253 equipped with a gas-bench interface at the Laboratoire de Glaciologie et Géophysique de l'Environnement at the University of Joseph Fourier Grenoble (France).

Statistical analysis methods

Interactions between ClO4 -concentrations and groundwater level as well as major ions were calculated by using an approach based on a semi-parametric regression model. More precisely, a generalized additive model (GAM) with cubic splines accounting for autocorrelation of data through a first order autoregressive model (AR1) was used. Each GAM was carried out using the Akaike information criterion (AIC) score and the likelihood ratio. The corresponding adjustment coefficient of determination (Adj. R 2 ) associated with a significance test at a P-value < 0.05 was used to characterize correlations between response variable and the explicative one.

The GAM was also adopted to decipher time-dependent changes in perchlorate concentrations. A model was built for each of the 14 sampling points, which were monitored monthly. Each of these GAM was built considering a Gamma distribution of perchlorate concentration and the log link function was used as it was the most appropriate for this kind of distribution. Two time-dependent smooth functions were used to build each model. Seasonal changes in perchlorate concentrations are represented by the first term taking into account days of year. The penalized cubic regression spline was used for this smooth function to allow a nonlinear response of perchlorate to time during a calendar year. The second term was constructed to filter out the long-term trends (inter-annual variations). For convenient output, we have redefined the time-scale for this term as a continuous variable. For this second term, the thin plate regression spline was used.

For the two smooth functions, the smoothness was controlled by the number of knots and the associated effective number of degrees of freedom. The optimal number of knots was estimated through cross-validation. In consequence, the basis dimension for the two smooth functions was adapted to have enough degrees of freedom to fit the data while these values remained small enough to maintain reasonable computational efficiency and avoided over-fitting the data. For the two smooth functions, the method of the finite difference was used to calculate the first derivative of the function and the associated confidence interval. This first derivative was used to estimate the rate of change to identify periods of statistically significant change (either increase or decrease in perchlorate concentration).

The underlying assumption of homogeneity for model residuals has been checked by plotting deviance residuals against fitted values. QQ plots (sample quantiles against theoretical quantiles) and Shapiro tests were used to assess normality of the model residuals. We have examined autocorrelation of the model residuals; the autocorrelation function values showed that the model residuals were not correlated as they dropped to small values within a couple of days. Validity of the model was also assessed through AIC score and Adj.R 2 . Analysis of deviance was used to assess the significance of the null hypothesis for the two smooth functions. All statistical tests were carried out using R-software 252 [START_REF] Core | R: A Language and Environment for Statistical Computing[END_REF]. 253

Results and discussion

Occurrence of ClO4 -and explosives

Perchlorate was detected at almost all sampling sites (33 out of 36) (Table 1). Mean ClO4 - concentrations measured during the two years monitoring were > 4 µg•L -1 at 17 sites, including two sites with mean concentrations > 15 µg•L -1 , representing 49% and 6% of the sampling sites, respectively. Low concentrations (< 4 µg•L -1 ) were measured at 19 sites, representing 51% of the sampling points. An analysis of the geographic distribution of ClO4 -is presented in Figure 4, revealing some major trends and potential sources of ClO4 -. Lower concentrations of ClO4 -(< 4 µg•L -1 ) were mainly found on the Berru Mount, in the Vesle River and in the Aisne-Marne Canal. As mentioned above, the Tertiary formation on the Berru Mount is represented by a succession of permeable and impermeable layers, which contains several small aquifers in which water could be renewed quickly by precipitation.

As a result, low levels of ClO4 -were detected in this area. In the Vesle River, ClO4 -concentrations ranged from 0.9 ± 0.4 µg•L -1 to 1.2 ± 0.5 µg•L -1 . The Vesle River originates far away upstream, receiving groundwater discharge from outside the study area that is little affected by ClO4 - contamination. In the Aisne-Marne Canal, ClO4 -was not detected (< 0.5 µg•L -1 ), indicating that the canal has little or no input from the contaminated groundwater or river water (Vesle River) of the study area.

Most of the sampling sites with ClO4 -concentrations exceeding 4 µg•L -1 were located downgradient of the Champagne Mounts (Figure 4), where large quantities of ammunitions were used, stored, and destroyed during and after WWI. The highest concentrations of ClO4 -were found at borehole FVDV, with a maximum of 62.5 µg•L -1 (in September 2018) and an average of 44.4 ± 4.9 µg•L -1 . Plume-like patterns of ClO4 -were observed along the sections A -A' and B -B' (the same direction as the groundwater flow line; Figure 4). At FBN4, FEP1 and SEP (A -A'), mean ClO4 -concentrations over the two years monitoring were 20.8 ± 3.2 µg•L -1 , 12.9 ± 2.4 µg•L -1 and 5.5 ± 3.0 µg•L -1 respectively, indicating a progressive decrease with distance downgradient. A similar pattern was observed at FPM1, FPM2 and FPM3 (B -B') with concentrations of 14.1 ± 2.0 µg•L -1 , 6.1 ± 0.6 µg•L -1 and 3.5 ± 0.8 µg•L -1 respectively. The high ClO4 -levels and the observed plume-like patterns indicated that potential point-sources of ClO4 -could be present upstream at FVDV, FBN4 and FPM1. Specifically, the military tunnel N1 and the ammunition destruction sites (N4 and N5) were likely responsible for the high concentrations of ClO4 -measured at these sites (Figure 4).

Although ClO4 -concentrations in river waters were generally lower than those of groundwater, some river sites had relatively high ClO4 -concentrations (>10 µg•L -1 ), such as RPY in the Py River (Figure 4 and Table 1). The Py River is downstream from the Suippes military camp that represents a potential source of ClO4 -contamination to the Py river watershed (Figure 1). In the Suippe River upstream of the confluence with the Py River (RS1), low ClO4 -concentrations (3.4 ± 0.5 µg•L -1 ) were measured while, downstream of this confluence (RS2), higher concentrations (6.5 ± 2.1 µg•L -1 ) were found. Unlike the widespread contamination of ClO4 -on the study area, organic explosives have not been detected in surface and groundwater samples, which could be explained by their low persistence and mobility in soil and water [START_REF] Clausen | Conceptual Model for the Transport of Energetic Residues from Surface Soil to Groundwater by Range Activities. Cold regions research and engineering lab[END_REF]. During WWI, nitro group explosives such as TNT, nitroglycerine and nitrocellulose were largely used. TNT can be rapidly degraded in most soil and aquifer systems; therefore, its presence is typically restricted to areas near its introduction to the environment. At most sites, TNT can be completely attenuated in the surface soil, thereby preventing contamination of the unsaturated zone (UZ) or groundwater [START_REF] Clausen | Conceptual Model for the Transport of Energetic Residues from Surface Soil to Groundwater by Range Activities. Cold regions research and engineering lab[END_REF]. Nitroglycerin is soluble when present alone and is subject to rapid biodegradation, but when present with nitrocellulose it is insoluble. Nitrocellulose is also insoluble, resulting in its low mobility in the environment [START_REF] Quinn | Chapter 11 -Wildlife Toxicity Assessment for Nitrocellulose[END_REF].

3.2 Sources and fate of ClO4 -in the Chalk aquifer

Isotopic composition of ClO4 -and NO3 -

The results of Cl and O stable isotope analysis for ClO4 -in water samples are presented by dual isotope plots in comparison to published data for synthetic, Atacama and selected US indigenous natural ClO4 -occurrences (Table 3 and Figure 5). There is no evidence of ClO4 -biodegradation, which is consistent with the typical oxic condition of the unconfined Chalk aquifer [START_REF] Barhoum | Spatial heterogeneity of high-resolution Chalk groundwater geochemistry -Underground quarry at Saint Martin-le-Noeud, France[END_REF][START_REF] Edmunds | Baseline geochemical conditions in the Chalk aquifer, Berkshire, U.K.: a basis for groundwater quality management[END_REF]; the isotopic composition of ClO4 -could therefore reflect initial values of the sources. The δ 37 Cl, δ 18 O and Δ 17 O values of ClO4 -in water samples collected at the Suippe military camp (RM; pure military source), FVDV and FBN4 (at high water level) plotted exactly within the synthetic ClO4 - range, proving similar military sources of ClO4 -at these sites (Figure 5). However, the results for FBN4 (at low water level) was different, with a lower δ 18 O value (-29.6‰) falling outside the published synthetic ClO4 -ranges. Nevertheless, the δ 37 Cl and Δ 17 O values (-3.3‰ and -0.3‰, respectively) were typical of synthetic ClO4 -. Therefore, ClO4 -at FBN4 (at low water level) was interpreted as of synthetic origin, but different from other samples and currently reported synthetic ClO4 -products. Indeed, the manufacturing processes of synthetic ClO4 -and the materials used more than 100 years ago during WWI may be different from those of today. Even during WWI, different ClO4 -salts (NH4ClO4 and/or KClO4) were used in explosives; they were produced in different facilities and the method could also have evolved during the WWI conflict. This could possibly explain why this "unusual" synthetic end-member was observed at FBN4 (at low water level). The different ClO4 - isotopic compositions observed at FBN4 at high and low water levels suggest that there could be different sources of ClO4 -at different depths. The estimated groundwater ages in the study area were less than 50 years [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. Moreover, because the thickness of UZ is less than 30 m (Table 1)

and considering that the flow rate is about 1 m•year -1 through the Chalk matrix, the time of transfer in the UZ were less than 30 years (e.g., [START_REF] Barraclough | A tracer investigation into the importance of fissure flow in the unsaturated zone of the British Upper Chalk[END_REF][START_REF] Brouyère | Migration of contaminants through the unsaturated zone overlying the Hesbaye chalky aquifer in Belgium: a field investigation[END_REF][START_REF] Chen | Water, nitrate and atrazine transfer through the unsaturated zone of the Chalk aquifer in northern France[END_REF][START_REF] Wellings | Recharge of the Upper Chalk aquifer at a site in Hampshire, England: 2. Solute movement[END_REF]. Therefore, military sources of ClO4 -contamination are most likely related to military activities after WWI (destruction of ammunitions) rather than during the conflict, or to the release of ClO4 -from unexploded ordnance persisting in the subsoil (or unused ammunitions stored on the surface).

In the RPY sample, a substantial fraction of ClO4 -with an Atacama-type isotopic composition is evident. The lower value of δ 37 Cl (-6.0‰) and the higher value of Δ 17 O (+1.3‰) are both consistent with a mixture of a synthetic end-member with typical Atacama ClO4 -. This could probably be explained by the nitrogen explosives of WWI (black powder and nitro group explosives) made with Chilean nitrate and/or the past use of Chilean nitrate as fertilizer. Indeed, the PY River water consists of aquifer discharge from the entire watershed. Although synthetic (military) ClO4 -is indicated by isotopic analysis at two sites with the highest ClO4 -concentrations (FVDV and FBN4), Atacama ClO4 - from Chilean nitrate fertilizer might be present, especially at sites with low ClO4 -concentrations related to diffuse sources.

The NO3 -isotope data at all the sampling points did not show any evidence of an Atacama source for the NO3 -(Figure 5). However, this cannot rule out the possibility of the existence of Atacama NO3 -in water samples, as Atacama NO3 -could have been replaced and/or assimilated with the biogenic NO3 - in the soil [START_REF] Böhlke | Atacama perchlorate as an agricultural contaminant in groundwater: isotopic and chronologic evidence from Long Island, New York[END_REF]. Indeed, the distinctive isotopic composition of oxygen in atmospheric NO3 -is preserved only in hyper-arid environments and is lost in moist soils where higher biological activity occurs [START_REF] Böhlke | Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California[END_REF][START_REF] Michalski | Oxygen and nitrogen isotopic composition of nitrate in commercial fertilizers, nitric acid, and reagent salts[END_REF]. Therefore, more information is needed to better evaluate the regional extent of ClO4 -sources related to the past use of Chilean nitrate in the study area. [START_REF] Gu | Regeneration of Perchlorate (ClO4-)-Loaded Anion Exchange Resins by a Novel Tetrachloroferrate (FeCl4-) Displacement Technique[END_REF][START_REF] Bao | Natural perchlorate has a unique oxygen isotope signature[END_REF][START_REF] Böhlke | Perchlorate Isotope Forensics[END_REF][START_REF] Böhlke | Atacama perchlorate as an agricultural contaminant in groundwater: isotopic and chronologic evidence from Long Island, New York[END_REF]Jackson et al., 2005b[START_REF] Jackson | Isotopic composition and origin of indigenous natural perchlorate and co-occurring nitrate in the southwestern United States[END_REF]Parker et al., 2008 ;Plummer et al., 2006 ;[START_REF] Rajagopalan | Widespread presence of naturally occurring perchlorate in high plains of Texas and New Mexico[END_REF][START_REF] Rao | Widespread natural perchlorate in unsaturated zones of the southwest United States[END_REF]Sturchio et al., 2007[START_REF] Sturchio | Isotopic Tracing of Perchlorate in the Environment[END_REF]

Temporal variability of ClO4 -compared with groundwater levels and major ions

The temporal variation of ClO4 -concentrations measured from June 2017 to June 2019 was compared with groundwater level fluctuation and the temporal variation of NO3 -and Cl -concentrations (two major agriculture-derived ions, [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF], in order to explore the potential sources of ClO4 - (point and/or diffuse source), and the possible future evolution of ClO4 -in the Chalk aquifer.

At most sites, inter-annual variations of ClO4 -concentrations were observed and also revealed by the statistical trend analysis (Figure 6 and Table 4). The periods when ClO4 -concentrations changed significantly under the seasonal and annual effects were estimated with the statistical methods 20 presented in section 2.3 (results detailed in SI.2). Higher ClO4 -concentrations were observed in 2018 (Figure 6 and SI.2), corresponding with the higher groundwater level in 2018. At sites having mean ClO4 -concenrations > 10 µg•L -1 (FVDV, FBN4, FEP1 and FBN5; Figure 6), several peaks of ClO4 -concentration were observed and the temporal variation of ClO4 -was poorly correlated with the groundwater level fluctuation (Table 5). The peaks could possibly be explained by localized flushing of ClO4 -from the UZ by natural (rainfall) or artificial (irrigation) recharge processes, indicating the presence of point sources of ClO4 -contamination upstream of these sites. At FVDV and FBN4, the two most contaminated sites, the correlation coefficients between ClO4 -and groundwater level were the lowest (Table 4 and Figure 6), as ClO4 -concentrations here were mainly controlled by the ClO4 -transfer waves following flushing rather than the groundwater level fluctuation.

In contrast, at sites showing ClO4 -concentrations < 10 µg•L -1 (FA, FBN1, FP1 and FPM3; Figure 6), the temporal variation of ClO4 -was significantly correlated to the groundwater level fluctuation (Table 5). The correlation relationship was stronger at FA and FBN1 (Adj. R 2 = 0.42 and 0.54, respectively;

Table 5) with larger groundwater level fluctuation; at FP1 and FPM3 where the groundwater level fluctuated less, the correlation relationship was weaker (Adj. R 2 = 0.35 and 0.20, respectively; Table 5). Diffuse sources of ClO4 -were presumed to be present at these sites. During low water level periods, most of the sources were apparently disconnected from the saturated zone. As water level rose, the contamination source was re-activated and more contaminants were released in water and flushed into the saturated zone, resulting in the increase of ClO4 -concentrations. Generally, a poor correlation was observed between ClO4 -and major ions (Table 5). The temporal evolution of ClO4 -was compared with the chronicles of NO3 -and Cl -, the two major agriculturederived ions in groundwater of the Champagne Chalk aquifer [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. At most sites, the temporal evolution of ClO4 -was different from that of NO3 -and Cl -(Figure 6), indicating different origins of ClO4 -versus NO3 -and Cl -. At FEP1, FPM1 and PP, despite the temporal heterogeneity of ClO4 -levels, the concentrations of NO3 -and Cl -were stable over time (Figure 6). As described in [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF], estimated groundwater ages at these points were > 30 years in a piston flow model, which is related to the superficial formations limiting rapid transport of water and solutes (NO3 -and Cl -) from surface (agriculture-derived ions are distributed mainly in the soil area) to the saturated zone.

Consequently, the aquifer receives recharge mainly from upstream of the superficial formation covered area. Water traveled laterally in the saturated zone during > 30 years and solute concentrations were greatly buffered, resulting in stable NO3 -and Cl -levels independent of water level fluctuation [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. The large variation of ClO4 -concentrations, in contrast to the temporal stability of NO3 -and Cl -, could be interpreted as an indication that the location of ClO4 -sources is much deeper than those of agriculture-derived ions. As groundwater levels rose, the contamination front of ClO4 - was reached, generating changes in ClO4 -levels, despite the fact that flushing from the soil was largely limited by superficial formations. At FPM3 (downstream from the borehole FPM1; Figure 2), a time lag was observed between groundwater levels and concentrations of NO3 -and Cl -, likely as a result of slow transport through the discontinuous graveluche formations. However, ClO4 -levels varied with groundwater levels without time lag, implying again that deeper ClO4 -sources were more quickly activated by the rise in water table.

At FA, unlike the positively correlated relationship between ClO4 -and groundwater level, NO3 -and Cl - were negatively correlated with groundwater level (Figure 6 and Table 5), indicating a dilution effect.

At this site, groundwater dating showed 75-80% of modern water by the binary mixing model, indicating a water table constituted mainly by freshly percolated rainwater that favors the process of dilution [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. During the rapid flow process following precipitation, the low mineralized rainwater entered the aquifer resulting in decrease of NO3 -and Cl -concentrations. For ClO4 -, it seemed that the inputs by the potential sources of ClO4 -in the UZ following the rise of groundwater level was more important than the dilution process, which led to an increase in ClO4 -with the groundwater level.

At FBN1 and FBN5, the temporal variabilities of ClO4 -, NO3 -and Cl -were well correlated (Figure 6 and Table 5). Indeed, located near the groundwater divide line (Figure 2), deep water table levels were observed (> 22 m, Table 1) and low permeability was suggested at these two sites [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF].

According to groundwater dating, the groundwater flow was well described by the exponential mixing model (mean residence time of < 20 years), indicating a spatially uniform recharge [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF].

ClO4 -, NO3 -and Cl -were thus flushed from the potential sources located in the UZ following recharge, showing a similar temporal variation.

At RPY (Py River) and RS2 (Suippe River after the confluence of Py River), a decrease in ClO4 - concentration was observed during high flow period (May 2018 and May 2019; Figure 6), implying a dilution effect on ClO4 -concentration. Indeed, the rivers are recharged by the Chalk aquifer with little surface runoff produced. In river valleys, the Chalk is usually highly fractured with shallow groundwater levels and preferential flow of rainwater is favored especially at high water level, resulting in dilution of the aquifer and the river water. A similar tendency was observed at PDO (pumping station near RPY), which is consistent with a mixture of groundwater and surface water as implied by groundwater dating (30% to 60% of modern water and an end-member of ~40-year old water; [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. The potential sources of ClO4 -located deeper than the sources of agriculture-derived ions in the UZ refer most probably to unexploded ordnance still present underground after WWI. Indeed, some shells fired during the war could reach > 10 meters underground without exploding [START_REF] Unmas | Mines terrestres, restes explosifs de guerre et engins explosifs improvisés -Manuel de sécurité, 3 e édition[END_REF].

Unused ammunitions could still be present underground in military tunnels of WWI. In addition, in the Champagne Mounts area, some unused ammunitions could have been cleaned up by burying in specified boreholes [START_REF] Debant | La lente dépollution du CEA de Moronvilliers. L'hebdo du vendredi[END_REF]. Over time, the release of the explosive charge occurs as a result of the general corrosion of the envelope and/or its perforation. The time required for perforation was estimated between 250 and 450 years, at a rate of 1 mm/year on average [START_REF] Parker | UXO Corrosion -Potential Contamination Source[END_REF]. These unexploded ordnances can be difficult to locate and clean up due to their number and their deep underground location. Contamination plumes observed at some sites indicate that the ammunition destruction sites may be the main point sources of ClO4 -, as repeated detonation causes an accumulation of residual ClO4 -at these sites. The residue of ammunition destroyed by detonation can persist for more than 100 years in soils [START_REF] Hubé | Perchlorates : éléments historiques et d'expertise pour une évaluation de l'impact environnemental[END_REF]. Despite the huge quantities of fully exploded ordnance, their residue is estimated at around 0.01% and can only persist for a short time underground, representing a relatively small contribution to the ClO4 -contamination of groundwater. Considering that most of the potential sources of ClO4 -have long persistence times, the ClO4 -contamination in groundwater of the study area may not decline in the short to medium term.

Mass flow rate of perchlorate

In the Suippe River and its tributaries (Figure 4), the mass flow rate of ClO4 -(M) has been estimated according to the measured water flow rate (Q) and ClO4 -concentrations (C):

= × (1)
The data of water flow rate and ClO4 -concentration used for calculation was measured on October 2017. The estimated mass flow rate of ClO4 -was 4.6 kg•month -1 upstream the Suippe River (RS1) and 13.5 kg•month -1 downstream of its confluence with the Py River (RS2) (Table 2). The increase of mass flow rate from RS1 to RS2 (8.9 kg•month -1 ) was approximately equal to the contribution from Py River (9.6 kg•month -1 ), implying that the contribution from the Chalk aquifer between RS1 and RS2 is negligible with low groundwater discharge and low ClO4 -concentrations in the riparian aquifer.

The estimated mass flow rate of ClO4 -downstream along the Suippe River (RS3) was 14.6 kg•month -1 .

The Arnes River, which joins the Suippe River between RS2 and RS3, has a ClO4 -flux less than 0.1 kg•month -1 . The increase of ClO4 -mass flow rate from RS2 to RS3 (1.1 kg•month -1 ) was due to the contribution of Chalk aquifer discharge (zero runoff is assumed on the Champagne Chalk). The groundwater discharge from RS2 to RS3 was 322 L•s -1 , estimated from the water flow rate at RS2, RS3 and RA. According to equation ( 1), a ClO4 -concentration of 1.3 µg•L -1 in aquifer discharge on October 2017 was estimated, which represents the average concentration from the Chalk aquifer of left and right bank of the Suippe River. The estimated monthly ClO4 -flow rate on October 2017 represents approximately the minimum level in the Suippe River and its tributaries, as the water flow rate was measured during low discharge period, whereas ClO4 -concentrations in rivers were relatively stable over time according to the low values of standard deviations presented in Table 1.

Conclusions and perspectives

This study examined sources and evolution of ClO4 -contamination in groundwater of NE France. The NE region of France is suspected to have multiple sources of ClO4 -related to military activities of WWI and/or the past use of Chilean nitrate as fertilizer in agriculture. An intensive sampling network was established on a study area of a representative watershed, where ClO4 -concentrations were monitored monthly for two years (2017-2019). The measured concentrations and isotopic contents of ClO4 -and the historical investigations have been combined with previously published data on groundwater dating as well as hydrologic and geochemical characteristics of the Chalk aquifer [START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF], which allowed to clarify the sources of ClO4 -and to understand its evolution in groundwater.

This work produced the first precise ClO4 -contamination mapping in the study area east of Reims city with ClO4 -concentrations in ground-and surface water ranging from < 0.5 to 62.5 µg•L -1 . About half of the sampling sites showed ClO4 -concentrations > 4 µg•L -1 and most of these sites were located downgradient of the Champagne Mounts area, where huge quantities of ClO4 -were used, stored, or destroyed during and after WWI. Point sources of ClO4 -were presumed to exist in the study area, as indicated by the plume-like patterns of contamination observed at some sites. The isotopic signature of ClO4 -at the two most contaminated sites showed a synthetic origin, proving the military source of ClO4 -contamination in the study area. In addition, the estimated groundwater ages in the study area were < 50 years, implying that ClO4 -contamination is related to sources that may still persist in the subsoil long after the end of WWI (e.g., unexploded ammunition) or post-WWI military activities (e.g., destruction of ammunition). The isotopic analysis of ClO4 -in river water showed a minor but distinct component of Atacama ClO4 -, indicating the presence of some Chilean nitrate in the watershed.

Annual variations of ClO4 -concentration were observed, indicating the influence of recharge processes and groundwater levels on ClO4 -contamination. Two major temporal trends of ClO4 -concentration were observed: 1) ClO4 -concentrations poorly correlated to groundwater level with peaks of contamination due to flushing at sites having ClO4 -concentrations > 10 µg•L -1 , implying the presence of point sources; 2) ClO4 -concentrations highly correlated to groundwater level at sites showing ClO4 - concentrations < 10 µg•L -1 , where diffuse sources were suggested. In addition, the rapid response of ClO4 -concentration following the rise of groundwater level compared with relatively stable concentrations of agricultural ions at some sites indicated that the location of ClO4 -sources could be much deeper than those of agricultural ions (mainly in the soil area). Considering the long persistence time of the explosive residues related to unexploded ammunitions and ammunition destruction activities, the ClO4 -contamination in groundwater of the study area seems unlikely to decrease in the short-to medium-term.

The multi-tool methodology developed in the study area could furtherly be applied to other ClO4 - contaminated sites in NE France with suspected military and agricultural sources, with the aim of making appropriate recommendations for a long-term management of groundwater resources.

Moreover, this characterization methodology could more widely applicable elsewhere in Chalk aquifers or other multi-porosity mediums for the prediction of solute transport (natural or anthropogenic) and for the evaluation of aquifer vulnerability. However, this research can be improved and developed in several different aspects. The continuous monitoring of ClO4 -concentrations and geochemistry can be continued to obtain longer time series, with the aim to confirm and specify our conclusions and also to study the evolution of ClO4 -in the Chalk aquifer under different climate conditions. More measurements of water flow rate in rivers can be realized (during low, medium and high discharge period) in order to better estimate the mass flow rate of ClO4 -. In addition, groundwater sampling and analysis should also be realized at different depths of boreholes to obtain vertical profiles of ClO4 -concentrations as well as other chemical characteristics, which allows to explore their relationships and to further confirm the position of potential ClO4 -sources underground. In this study, isotopic analysis of ClO4 -has only been realized at sites with high levels of ClO4 -. This analysis should also be performed at sites having low concentrations of ClO4 -to further confirm whether traces of Atacama ClO4 -exist on these low contaminated sites related to diffuse sources. Finally, hydrogeochemical numerical modeling tools could be a relevant complement of this study to better understand the transfer mechanism of ClO4 -in the Chalk groundwater.

Figure 1 :

 1 Figure 1 : Distribution of ClO4 -contamination in groundwater and the positions of the WWI trench areas in NE France

Figure 2 :

 2 Figure 2 : Geological and hydrogeological map of the study area recorded at high water level with the location of sampling points (modified from[START_REF] Cao | Heterogeneous behavior of unconfined Chalk aquifers infer from combination of groundwater residence time, hydrochemistry and hydrodynamic tools[END_REF]. Source of geological map:[START_REF] Laurain | Notice explicative, Carte géol[END_REF],Allouc and Le Roux. (1995);

Figure 3 :

 3 Figure 3 : Trenches, military tunnels and ammunition destruction sites related to WWI on the study area (position of trenches from[START_REF] Taborelli | Les conditions géographiques et l'organisation spatiale du front de la Grande Guerre : application à l'évaluation environnementale post-conflit en Champagne-Ardenne[END_REF]

Figure 4 :

 4 Figure 4 : Spatial distribution of ClO4 -contamination on the study area and military sites related to the WWI

Figure 5 :

 5 Figure 5: Summary of isotope data for ClO4 -and NO3 -in water samples of the study area, compared with major known ClO4 - sources including synthetic, Atacama and indigenous natural ClO4 -from the Southern High Plains (Texas) (SHP) (Ader et al.,

  . Arrow in the δ 37 Cl vs δ 18 O graph represents the slope of biodegradation (δ 37 Cl = 0.4 δ 18 O) and arrow in the Δ 17 O vs δ 18 O graph represents the direction of mass dependent fractionation.

Figure 6 :

 6 Figure 6 : Temporal variation of perchlorate concentrations compared with groundwater level fluctuations from June 2017 to June 2019

Table 1 :

 1 Properties of sampling points, water table depth, concentrations of Cl -, NO3 -and ClO4 -and isotopic compositions of NO3 -in ground-and surface water samples (N: number of sampling; NA: not available. -: not applicable)

	Name Type	N	Depth (m)	Screen depth (m)	Water table depth (m)	Cl -(mg•L -1 )	NO3 -(mg•L -1 )	ClO4 -(µg•L -1 )	NO3 -δ 18 O (‰)	NO3 -δ 15 N (‰)
	FA	Borehole	21	35 3.5 -30	2 -6	35.4 ± 6.3 39.7 ± 6.0 3.1 ± 0.9	-1.4	-0.7
	FAP Borehole	21	15 6.5 -15	3 -12	30.7 ± 9.5 28.4 ± 6.7 5.4 ± 2.2	-1.9	1.7
	FBN1 Borehole	21	48 4 -48	26 -40.5	25 ± 8.8	28.9 ± 7.6 3.6 ± 1.7	2.1	3.9
	FBN4 Borehole	21	28 16 -28	2.5 -18	52.9 ± 2.6 54.3 ± 2.6 20.8 ± 3.2 -0.6	1.0
	FBN5 Borehole	21	47 24 -43	22.5 -35	15 ± 9.2	31.6 ± 11.3 7.9 ± 3.7	-0.6	1.1
	FEP1 Borehole	21	25 7 -25	7.5 -17	35.2 ± 1.0 36.7 ± 1.4 12.9 ± 2.4 -0.3	0.5
	FP1 Borehole	21	19 9 -15	4.5 -13	28 ± 2.9	29.9 ± 3.9 6.6 ± 1.9	-1.3	1.1
	FPM1 Borehole	21	24 NA	NA	7.6 ± 0.7	19 ± 0.8	14.1 ± 2.0 -1.4	0.4
	FPM3 Borehole	21	21 7 -21	9 -18	18.6 ± 5.0 24 ± 5.7	3.5 ± 0.8	-0.5	0.3
	FVDV Borehole	21	22 12.5 -21 7 -15	39 ± 1.6	40 ± 2.3	44.4 ± 6.9 -1.7	0.8
	PDO Pumping station	21	25 7 -25	NA	33.9 ± 1.2 42.1 ± 2.0 11.5 ± 1.7 -1.8	0.6
	PP	Pumping station	21	80 23 -80	NA	21.8 ± 1.1 25.8 ± 0.9 9.2 ± 4.0	-1.5	0.0
	PS	Pumping station	4	16 7 -16	NA	23.6 ± 1.4 32.4 ± 2.7 4.1 ± 1.4	-0.2	1.6
	FCE Borehole	1	85 NA	NA	7.2	3.1	0.5	NA	NA
	FNA Borehole	1	47 NA	NA	20.0	24.3	1.3	NA	NA
	FBN3 Borehole	3	56 12 -56	16 -22	13.4 ± 2.4 15.2 ± 2.2 1.3 ± 0.4	5.4	7.5
	FBN2 Borehole	4	32 10 -32	17 -26	31.6 ± 4.1 18.2 ± 12.5 2 ± 1.7	6.6	10.3
	FEP2 Borehole	6	23 NA	15 -18.5	8.8 ± 0.6	9.5 ± 0.8	2 ± 1.3	-0.3	-0.5
	FPM2 Borehole	5	35 NA	30 -31	16.2 ± 5.1 22.1 ± 5.2 6.1 ± 0.6	-1.3	0.6
	FP2 Borehole	4	21 NA	8 -13	165.8 ± 18.9 38 ± 3.3	1.2 ± 0.7	-0.9	3.6
	FP	Borehole	4	23 7 -22.5 12 -17	39.0 ± 3.3 45 ± 5.0	5.9 ± 0.9	-1.7	0.6
	FB	Borehole	1	33 NA	12.8	28,0	36.6	2.2	NA	NA
	SEP Spring	12	-	-	-	23.5 ± 1.9 33.1 ± 2.6 5.5 ± 3.0	-1.5	1.8
	SCG Spring	2	-	-	-	28.5 ± 0.2 40.7 ± 0.1 4.7 ± 2.1	NA	NA
	SB1 Spring	1	-	-	-	16.5	7.8	< 0.5	NA	NA
	SB2 Spring	1	-	-	-	18.4	2.3	2.1	NA	NA
	RS1 Suippe River	21	-	-	-	25.8 ± 1.2 29.3 ± 1.9 3.4 ± 0.8	1.0	3.7
	RPY Py River	21	-	-	-	20.7 ± 1.5 28.5 ± 1.6 11.6 ± 3.1	NA	NA
	RS2 Suippe River	21	-	-	-	24.3 ± 1.3 29.6 ± 1.3 6.5 ± 2.1	0.8	2.6
	RA	Arnes River	4	-	-	-	22.1 ± 1.0 26.1 ± 1.2 1.5 ± 1.0	0.4	3.1
	RS3 Suippe River	4	-	-	-	25.2 ± 0.4 28.8 ± 0.8 5.1 ± 1.5	0.4	3.1
	RV1 Vesle River	3	-	-	-	29.3 ± 1.1 25.9 ± 3.7 1.2 ± 0.5	0.7	5.9
	RV2 Vesle River	3	-	-	-	25 ± 1.9	28.5 ± 2.8 0.9 ± 0.4	0.4	4.7
	RP	Prosnes River	1	-	-	-	21.3	25.6	3.6	NA	NA
	C1	Canal	1	-	-	-	18.3	12.7	< 0.5	NA	NA
	C2	Canal	1	-	-	-	15.5	13.1	< 0.5	NA	NA

Table 3 :

 3 Isotopic compositions of ClO4

	Name	Sample date	δ 18 O (‰)	Δ 17 O (‰)	δ 37 Cl (‰)
	FVDV	04/05/18	-20,7	0,2	0,3
	FBN4 (HW)	04/06/18	-21,8	0,2	0,2
	FBN4 (LW)	01/18/19	-29,6	-0,3	-3,3
	RPY	04/10/19	-22,9	1,3	-6.0
	RM	02/22/19	-18,2	0,1	-0,2

-in ground-and surface water samples (HW: high water; LW: low water)

Table 4 :

 4 Seasonal and annual effects on ClO4 -concentrations (the effect is considered as statistically significant with Pvalue < 0.05, related values < 0.05 are marked in bold)

		Seasonal trend	Annual trend
	FVDV	0,079	> 0.001
	FBN4	0,097	0,209
	FEP1	0,052	0,291
	FBN5	0,003	0,007
	FA	0,176	0,015
	FBN1	0.028	0,374
	FP1	0,161	0,165
	FPM3	0,067	0,033
	RS1	0,053	0,767
	RPY	0,021	0,022
	RS2	0,011	0,240
	PDO	0,624	0,135
	FPM1	0,097	> 0.001
	PP	0,012	0,326

Table 5 :

 5 Correlation of ClO4 -with groundwater levels and major ion concentrations. Values correspond to Adj. R 2 issued from the generalized additive model (statistically significant as the P-value < 0.05, related Adj. R 2 values when P-value <

	0.05 are marked in bold; NA: not available)
	Name	Water level	Cl -	NO3 -	SO4 2-	Na +	K +	Mg 2+	Ca 2+
	FVDV	0,07	-0,04	0,21	0,37	0,15	0,53	0,41	0,08
	FBN4	0,07	0,03	0,32	0,24	0,03	0,18	0,31	0,01
	FEP1	0,08	0,00	0,08	0,05	0,32	-0,02	-0,03	0,03
	FBN5	0,22	0,71	0,64	0,02	0,28	0,67	0,41	0,61
	FA	0,42	0,54	0,56	0,68	0,04	0,29	0,24	0,64
	FBN1	0,54	0,53	0,67	0,55	-0,02	0,62	0,35	-0,02
	FP1	0,35	0,17	0,25	0,23	0,28	0,48	-0,05	0,05
	FPM3	0,20	-0,03	-0,04	-0,05	-0,04	0,17	0,46	0,18
	RS1	NA	0,61	0,13	0,53	0,49	0,37	-0,06	0,05
	RPY	NA	0,10	0,06	-0,01	-0,03	-0,06	-0,06	0,13
	RS2	NA	0,51	0,11	0,36	0,02	0,17	0,53	0,13
	PDO	NA	-0,05	0,06	0,01	-0,03	0,16	-0,03	0,19
	FPM1	NA	0,28	0,51	-0,02	0,10	0,33	0,22	0,11
	PP	NA	0,29	0,02	-0,05	0,16	0,03	0,08	-0,06

Table 2 :

 2 Mass flow rate of ClO4 -in the Suippe River and its tributaries (data measured on October 2017)

	Name	Location	Water flow rate (L•s -1 ) ClO4 -concentration (µg•L -1 ) ClO4 -mass flow rate (kg•month -1 )
	RS1	Suippe River	401	4.3	4.6
	RS2	Suippe River	752	6.7	13.5
	RS3	Suippe River	1184	4.6	14.6
	RPY	Py River	276	12.3	9.1
	RA	Arnes River	110	< 0.5	< 0.1

Acknowledgements

This work was co-funded by the BRGM, the Agence de l'Eau Seine-Normandie, the Region Grand-Est, the Grand-Reims Metropole and ARS Grand-Est. Authors want to thank Nicolas Caillon of University of Joseph Fourier Grenoble for the NO3 -isotopic analysis; Alexandre Conreux and Julien Hubert of Gegenaa laboratory in University of Reims Champagne-Ardenne for their contribution in sampling campaigns and major ions analysis; and Alain Devos, Alexis Carlu, Thibaud Damien and Sarah Bambara for their participation in river flow measurement. Authors would also like to thank the owners and operators for access to their boreholes.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.