Charles Mottes
email: charles.mottes@cirad.fr

Landry Deffontaines

Jean-Baptiste Baptiste Charlier

Irina Comte

Pauline Della Rossa

Magalie Lesueur-Jannoyer

Thierry Woignier

Georges Adèle

Anne-Lise Tailame

Luc Arnaud

Georges Adele

Joanne Plet

Luc Rangon

Jean-Pierre Bricquet

Philippe Cattan

Spatio-temporal variability of water pollution by chlordecone at the watershed scale: what insights for the management of polluted territories?

Keywords: Chlordecone, pollution, observatory, hydrology, environment, management, practices

Introduction

The protection of surface and groundwater resources from pesticide pollutions is a remaining challenge. Such question is a live issue in tropical volcanic island and especially in the French West Indies (FWI) where organochlorine pesticide, namely chlordecone (CLD; C10Cl10O; CAS number 143-50-0; 491 g.mol -1 ) was used in agriculture in Guadeloupe and Martinique islands. CLD was applied from 1972 to 1993 to control the black banana weevils (Cosmopolites sordidus). Since a few decades, it pollutes soils, surface waters and groundwaters in a chronic manner. CLD is a very stable pesticide that shows a very slow degradation rate that is not evenly quantifiable [START_REF] Cabidoche | Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue[END_REF][START_REF] Clostre | Comparative fate of an organochlorine, chlordecone, and a related compound, chlordecone-5b-hydro, in soils and plants[END_REF][START_REF] Devault | Natural transformation of chlordecone into 5b-hydrochlordecone in French West Indies soils: statistical evidence for investigating long-term persistence of organic pollutants[END_REF]. Because of the very high affinity of chlordecone with soil organic matter -Koc ranging from 1200 to 17500 L.kg -1 [START_REF] Cabidoche | Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue[END_REF]Fernandez-Bayo et al. 2013a)-, only small fractions of the soil CLD amounts are drained by infiltrated water. However, despite its low mobility, the drained fraction of CLD was enough to highly contaminate other environmental compartments, such as aquifers and rivers [START_REF] Cabidoche | Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue[END_REF][START_REF] Cattan | A conceptual model of organochlorine fate from a combined analysis of spatial and mid-to long-term trends of surface and ground water contamination in tropical areas (FWI)[END_REF][START_REF] Charlier | CHLOR-EAU-SOL -Water -Caracterization of water and soil contamination by chlordecone of watersheds in Guadeloupe and Martinique[END_REF][START_REF] Crabit | Soil and river contamination patterns of chlordecone in a tropical volcanic catchment in the French West Indies (Guadeloupe)[END_REF][START_REF] Rossa | Linking current river pollution to historical pesticide use: Insights for territorial management?[END_REF][START_REF] Gourcy | Improving the knowledge of pesticide and nitrate transfer processes using age-dating tools (CFC, SF6, 3H) in a volcanic island (Martinique, French West Indies)[END_REF][START_REF] Mottes | Hydrological and pesticide transfer modeling in a tropical volcanic watershed with the WATPPASS model[END_REF]. Several crops grown on polluted soils might as well be contaminated depending on their nature [START_REF] Clostre | Soil thresholds and a decision tool to manage food safety of crops grown in chlordecone polluted soil in the French West Indies[END_REF]. Such pollution of food and water exposed population to sanitary issues [START_REF] Multigner | Chlordecone exposure and adverse effects in French West Indies populations[END_REF]. In order to face the problem, strategies were developed to limit the exposure of the population: drinking water is treated using active carbon [START_REF] Ranguin | Study of chlordecone desorption from activated carbons and subsequent dechlorination by reduced cobalamin[END_REF]) and decision rules were developed for cropping crops depending on their sensitivity to CLD pollution from soil [START_REF] Clostre | Soil thresholds and a decision tool to manage food safety of crops grown in chlordecone polluted soil in the French West Indies[END_REF]. The development of such management strategies were helpful to reduce the exposure of the population to the pesticide.

Chlordecone was found into aquatic coastal fauna [START_REF] Dromard | Contamination of marine fauna by chlordecone in Guadeloupe: evidence of a seaward decreasing gradient[END_REF]) and until pelagic cetacean [START_REF] Méndez-Fernandez | From banana fields to the deep blue: Assessment of chlordecone contamination of oceanic cetaceans in the eastern[END_REF]. Such results suggest subsequent amount of CLD transferred into the sea and raises the question of the sources, questioning the past or the present provenance of such contamination. It is known that soils are still the main CLD storage [START_REF] Lesueur-Jannoyer | Chlordecone aux Antilles : evolution des systemes de culture et leur incidence sur la dispersion de la pollution Agronomie[END_REF][START_REF] Levillain | Analysis of environmental and farming factors of soil contamination by a persistent organic pollutant, chlordecone, in a banana production area of French West Indies[END_REF]) and that CLD transfers from soils to the other environmental compartments is governed by water flows [START_REF] Mottes | From fields to rivers chlordecone transfer in water[END_REF]. To that point, CLD behavior in soils has been investigated [START_REF] Cabidoche | Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue[END_REF][START_REF] Clostre | Comparative fate of an organochlorine, chlordecone, and a related compound, chlordecone-5b-hydro, in soils and plants[END_REF]Fernandez-Bayo et al. 2013a;Fernandez-Bayo et al. 2013b), especially, the effect of the allophanic clay of andosols favoring CLD retention in soil (Woignier et al. 2012a), or the effect of added organic matter on CLD transfers (Woignier et al. 2012b).

All of these characteristics draw a complex system whose evolution is difficult to assess. On that topic, [START_REF] Cattan | A conceptual model of organochlorine fate from a combined analysis of spatial and mid-to long-term trends of surface and ground water contamination in tropical areas (FWI)[END_REF] analyzed the spatio-temporal variability of CLD and its main metabolite (CLD-5bhydro) in FWI. Authors showed significant decreasing trends of CLD concentrations in waters on some large watersheds. This is consistent with general literature on pesticides: because CLD is not applied anymore, one may expect slowly decreasing concentration in the terrestrial environment. However, because of the very low degradability of CLD, authors highlighted that the pluri-annual CLD decrease in leaching water was likely mostly generated by CLD transfer to another compartment rather than its degradation. This forces us analyzing jointly CLD concentration in the environmental compartments that are related to each other in order to understand the CLD exportation from the watershed system. According to the literature, several factors modify pesticides transfers at the watershed scale: climate, hydrogeological context, cropping practice, soil properties as well as the interaction between these factors [START_REF] Lewis | Pesticide Behavior, Fate, and Effects in the Tropics: An Overview of the Current State of Knowledge[END_REF][START_REF] Mottes | Pesticide transfer models in crop and watershed systems: a review Agron[END_REF][START_REF] Mottes | Relationships between past and present pesticide applications and pollution at a watershed outlet: The case of a horticultural catchment in Martinique[END_REF]. Evolution of CLD concentrations in rivers may also show different variabilities depending on the time and space scales considered. Previous studies showed that CLD seasonnal dynamics varied with the strong contributions of unpolluted water from a forested upstream sub-watershed [START_REF] Crabit | Soil and river contamination patterns of chlordecone in a tropical volcanic catchment in the French West Indies (Guadeloupe)[END_REF]. [START_REF] Gourcy | Improving the knowledge of pesticide and nitrate transfer processes using age-dating tools (CFC, SF6, 3H) in a volcanic island (Martinique, French West Indies)[END_REF] showed that groundwater residence times in aquifers last from years to decades in FWI. We expect that such long residence times may result in long-term evolution of CLD in rivers.

In this study, we aimed to characterized factors that determine CLD transfer outside the watershed system. Given that CLD is mainly exported through rivers, we focused on the conjoint analysis of agricultural practices and the evolution of CLD into surface and groundwater. We considered the case of the Galion watershed in Martinique (FWI). This watershed is monitored to survey CLD pollutions in surface and underground water compartments. First, we identified trends and variabilities of CLD concentration in the different environmental compartments monitored. Second, we discussed factors that may have influenced the modification of CLD concentrations in both, rivers and aquifers. Third, we assess the contribution of different sub-watersheds to CLD transfers into the sea (Galion bay). Finally, we propose strategies to prioritize actions and to help reduce CLD transfers into the environment.

Materials and Methods

Study site

The Galion river watershed (61°4.4004′W/14°36.5352′ N) is located on the East coast of the Martinique, a volcanic island in the French West Indies. This watershed is part of the OPALE observatory (Observatoire des Pollutions Agricoles aux Antilles -Observatory for agricultural Pollutions in Antilles). It was selected for its representativeness of the various agropedoclimatic conditions of Martinique.

The watershed covers 44.5 km 2 ranging from altitudes 0 to 694 m above sea level. The climate is humid tropical with oceanic influence. Rainfall range from 1500 mm.y -1 downstream up to 4000 mm.y -1 upstream. The hydrosystem is composed of 4 major permanent watercourses and a very dense network of intermittent gullies (Figure 1).

Steep slopes (>80%) with tropical forest and agricultural land for livestock and traditional food production characterize the watershed upstream. The soils are volcanic ash soils (andosol) characterized by high infiltration rates [START_REF] Cattan | Effects of tillage and mulching on runoff under banana (Musa spp.) on a tropical Andosol[END_REF]) and high organic matter concentration [START_REF] Dorel | Porosity and soil water properties of Caribbean volcanic ash soils[END_REF]. Soft slopes (~35%) cropped with banana and mixed farming on intergrade soils between andosol and compact ferralsol characterize the intermediate zone. A flood plain characterize the downstream zone where industrial crop productions, including banana and sugar cane, take place on ferralsols (Figure 1).

The bedrock is composed of volcanic formations of andesitic type [START_REF] Germa | The volcanic evolution of Martinique Island: Insights from K-Ar dating into the Lesser Antilles arc migration since the Oligocene[END_REF]. Geological formations are thus composed by ash flows, lava flows, and reworked formations as well as atmospheric fallout on a larger scale. Such geology generates a high spatial variability of lithology strata and contrasting weathering levels between geological units. Consequently, volcanic aquifer's units have small dimensions of a few square kilometres at most, generating complex groundwater flows according to more or less permeable materials [START_REF] Charlier | Structure and hydrogeological functioning of an insular tropical humid andesitic volcanic watershed: A multidisciplinary experimental approach[END_REF][START_REF] Vittecoq | Heterogeneity of hydrodynamic properties and groundwater circulation of a coastal andesitic volcanic aquifer controlled by tectonic induced faults and rock fracturing -Martinique island (Lesser Antilles -FWI)[END_REF]. We monitored the two sub-watersheds BDL (Base de Loisirs -8.5 km 2 ) and LD (La Digue -1.9 km 2 ) in addition to the whole GG (Grand Galion) sub-watershed that covers 35.9 km 2 (Figure 1). The monitoring of the GG sub-watershed aims at representing the functioning of the whole Galion watershed with widely integrated flow from upstream to downstream. The BDL sub-watershed corresponds to the upstream watershed and its monitoring aims at representing the functioning of the upstream zone. The LD watershed is a small sub-watershed located on extremely polluted soils and its confluence with the Galion River is located downstream the GG outlet. The monitoring of LD aims at representing the functioning of small tributaries. LD has no direct hydrological connection with BDL or GG.

Spatial variability of pollution

The 3 monitored sub-watershed (GG, BDL and LD) have different characteristics in terms of risk of pollution (section 2.4.2). Two sub-watersheds have a majority of surface with low risk of pollution, with BDL being the less polluted: 86% with no risk, followed by GG (75%). LD with 29% of low risks area has a majority of its area polluted (Erreur ! Source du renvoi introuvable.). Figure 1 shows that on GG, the polluted areas are mainly located in the intermediate area of the sub-watershed. They are located downstream on BDL and all over the watershed on the small LD sub-watershed because of its very large rate of polluted area.

Very high risk 1970+1980+1992

High risk (1970or1980)+1992 France. « Bois Lézard » is located upstream and « Trinité Réservoir » midstream (see Figure 1 for locations). This combination of stations provided an overview of the rainfall gradient from up to downstream.

Discharges at the watershed outlets of BDL and LD were measured at a 3 minutes time step using a MS5 sensor (HYDROLAB ® ) and recorded on DuoSens stations (OTT ® ). We gauged BDL and LD using current meter and Salinomadd (MADD ® ). We used the Manning-Strickler relationship based on measurement of sections of the stream to estimate discharge for high water levels.

The direction of the environment, management and housing (DEAL) from Martinique performed the GG watershed gauging and hydrological monitoring. Data are accessible through the HYDRO EauFrance website (http://www.hydro.eaufrance.fr/).

Groundwater level was recorded in 5 wells (see Figure 1 for location), equipped with automatic piezometric level recorders (OTT ® ): Malgré Tout (FMT)depth = 45 m, Mont-Vert (FMV) -15 m, Bassignac (FBASS) -50 m, Grand Galion (FGG) -50 m and Petit Galion (FPG) -43 m.

Water sampling in rivers and wells

We sampled water in rivers using two different methods. A first campaign was performed in 2013 by sampling all river confluences within 5 days with punctual (one off) sampling. For more detail, the protocol is described in Della [START_REF] Rossa | Linking current river pollution to historical pesticide use: Insights for territorial management?[END_REF].

The second campaign started in February 2016. We sampled water at the 3 sub-watershed outlets (BDL, LD and GG) with a time dependent sampling protocol. To that purpose, each pollution measurement station was equipped with an automatic sampler: ISCO 6712 (ISCO Incorporation ® ) sampler on BDL; Sigma SD900 (Sigma ® ) sampler on GG and LD. Throughout each week, that lasted from Monday to the next Monday unless exception, the sampling frequency of each sampler was set to sample twice 100 mL every 1h15. One sample was stored in a plastic container while the other in a glass container. Both containers were collected once a week meaning that each week the composite sample in each bottle integrated on average 135 samples. At the end of each week, we collected the two containers containing the composite samples and filled the bottles provided by the laboratory (3 glass bottles: 2 x 1 L +100 mL and 2 plastic bottles : 2 x 150 mL totaling 2.4 L) with the composite samples stored in the plastic and glass containers. Samples were collected every weeks from 01/02/2016 to 01/02/2018 and sent in ice coolers.

From October 2016 to March 2017, GG station was destroyed by a large storm (Matthew). During that period, we performed punctual sampling each week.

During our sampling Campaign, French Water Office ODE also sampled water monthly at the GG outlet. Punctual sampling was performed once a month and analyzed by LDA26 for pesticide concentration.

For groundwater, sampling was based on standard NF EN ISO 5667-3, and the FD T 90-523-3 and FD X31-615 guidelines. Before sampling in wells, at least three purge volumes were pumped with a submersible pump until stabilization of the chemical groundwater parameters. Samples were stored at 5°C and shipped in ice coolers before expedition for analysis.

Chlordecone analysis

We send the aliquots to laboratory for pesticide analysis. The selected LDA26 laboratory has been accredited by Cofrac, the French Accreditation Committee for pesticide analyzes providing guarantees for their technical skills and reliability as well as good management practices. LDA26 complies with ISO 17025 standards for testing and calibration.

Laboratory performed liquid/liquid extractions on samples. One liter of water sample was submitted to 3 extractions by using 50 mL of a mix of dichloromethane (80%) and ethyl acetate (20%). One cycle was performed at pH 7 and two cycles at pH 2. Extract was concentrated to 1mL in ethyl acetate. The extract was then split into twice 500 µL. 500 µL were use for GC MS and 500 µL for HPLC MS MS.

The HPLCC MS-MS was done on two transitions: 506.7/424.5 and 506.7/426.5 with a Sciex API 4000 QTRAP. Internal standards used as tracers were hexabromobenzene, tributylphosphate (added at extraction step), 2.4D3, Atrazine D5 (added at injection step) in multiple residual analysis.

The validation was done according to NFT 90210 standards. The limit of quantification was determined using six times two repetitions, the reproducibility using at least eight additions of CLD at LOQ at different dates. The trueness, recovery were estimated by using six times two additions of CLD at 3 concentrations. Uncertainty was estimated using six times two additions of CLD.

A GC quality control is performed every 20 samples at LOQ. Laboratory performed an internal calibration on 5 points before each analysis sequence. CLD addition on matrix and participation to inter laboratory comparison were also performed to control the quality of results.

CLD concentrations are given by the laboratory with a ±35% error interval, a limit of quantification (LOQ) of 0.01 µg.L -1 , a limit of detection (LOD) of 0.0033 µg.L -1 . The extraction efficiency is 89%.

Agricultural practices and risks of soil pollution 2.4.1 Actual agricultural practices

Surveys on farms located on the watersheds were performed between November 2016 and January 2016.

During surveys, decision rules for practices application and plantation dates were inquired. We analyzed the evolution of fallow and crop plantations at the watershed scale using the field geographical register (RPG) from 2016 and 2017 (Direction of Agriculture And Forest of Martinique -DAAF 972). These georeferenced data describe agricultural land uses in 2016 and 2017 as declared by farmers.

Mapping the risk of soil pollution

The risk of soil pollution by CLD was estimated following the method described in [START_REF] Desprats | Mapping of the risk of soil pollution by organochlorine pesticides in Martinique[END_REF] and applied by Della [START_REF] Rossa | Linking current river pollution to historical pesticide use: Insights for territorial management?[END_REF] on the Galion watershed. The method rely on a retrospective analysis of banana land use during the period of CLD application in Martinique (1972Martinique ( -1993)). Soil pollution risk was classified according following rule: i) Very high risk for banana plantations present in 1970, 1980 and 1992, ii) High risk for banana plantations present for two out of the three years, iii) Medium + for banana plantations present in 1992 only, iv) Medium -for banana plantations present in 1970 only, and v) No historical banana.

Data analysis

We compared distributions of CLD concentrations between the two years using the non-parametric Wilcoxon Mann-Whitney test. In order to visualize CLD concentration trends, we applied a Hodrick-Prescott filter on the data. This filter was designed to separate long-term trends to short-term variations, suiting our need for pollutants dynamic analysis as previously applied by [START_REF] Sun | Trend of dissolved inorganic nitrogen at stations downstream from the Three-Gorges Dam of Yangtze River[END_REF]. The best captures of trend were obtained with lambda parameter: 1600 × 3 4 . The baseflow component of the discharge time series was calculated using the one-parameter recursive digital filter proposed by [START_REF] Lyne | Stochastic time-variable rainfall-runoff modelling[END_REF] implemented in the HydRun package [START_REF] Tang | Hyd R un: A MATLAB toolbox for rainfall-runoff analysis[END_REF]. The best capture of baseflow component of the discharge time series were acquired with parameter value of 0.8.

Transfers estimation

Chlordecone mass transferred at the outlet of each instrumented watersheds was computed for each day using the formula.

𝑄 𝑐𝑙𝑑 = 𝑊𝑒𝑒𝑘𝑙𝑦𝐴𝑉𝐺𝐶𝐿𝐷 * 𝑑𝑎𝑖𝑙𝑦𝐷𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 * 3600 ×24 1000

(1)

With 𝑄 𝑐𝑙𝑑 the daily mass of CLD transferred at outlet (g), 𝑊𝑒𝑒𝑘𝑙𝑦𝐴𝑉𝐺𝐶𝐿𝐷, the weekly average CLD concentration in water samples (µg.L -1 ) and 𝑑𝑎𝑖𝑙𝑦𝐷𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒 the average daily discharge at sampling point (m 3 .s -1 ). The other term is a unit conversion factor.

We calculated the daily mass of CLD transferred at outlet per unit of polluted area (classified in low, medium-, medium+, high or very high risks) by dividing 𝑄 𝑐𝑙𝑑 by the polluted surface of each subwatershed estimated with the methodology described in 2.4.2.

Results

Evolutions of CLD concentration in rivers 3.1.1 Infra-annual evolution

Rainfall, discharge and CLD concentrations recorded on the 3 monitoring stations are presented in Figure 2. The proximity of rainfall and flood peaks for the 3 stations on a daily basis, illustrated that the watershed quickly reacted to rainfall events at the sub-watershed as well as the watershed scales. Reaction time of each watershed or sub-watershed was less than one day. Figure 2d and 2f shows that the trend of the CLD concentrations were correctly assessed by the Hodrick-Prescott filter at BDL and LD stations because most measurements fell within the ±35% interval around the filtered values which correspond to the laboratory analytical uncertainty on CLD concentration measurements. Deviations from the trend may also results from shorter wavelength processes, such as flooding, sampling uncertainty, organic matter or sediments in samples. The dynamics highlighted by the filter showed patterns with period of increasing and decreasing concentrations. The up and down trends highlighted by the filter were within the same periods on BDL and LD stations. According to this result, it is likely that the dynamics we observed had a common causality. We observed that short-term increase of baseflow is associated with CLD concentration decrease on all stations, while short-term baseflow decrease are associated with CLD increase (see between 02/2016 and 09/2016 -Figure 2). During that period, CLD concentrations decrease on all station between 02/2016 and 04/2016, then it increased between 04/2016 and 05/2016, and decreased again between 05/2016 and 07/2016. This behavior corresponds to the hydrological functioning of the watershed and subwatersheds subjected to small hydrological variations where the baseflow in not subjected to changes in regimes that last more than few months. A finer analysis on the trends showed that an uptrend started few time after the "Matthew" storm (29/09/2016) when baseflow decreased and ended in March 2017. The trends were more difficult to assess on the GG station (Figure 2b) due to the lack of integrated data between 10/2016 and 03/2017 after debris transported in water few time after the Matthew storm destroyed our station.

In fact, punctual sampling at that time showed high values: from 12/2016 to 04/2017 in particular, 12 punctual samples out of 15 had CLD concentrations higher than 1.1 µg.L -1 while the average concentration from 02/2016 to 02/2018 was 0.75 µg.L -1 . The ODE follow up also suggested high CLD concentrations during that period. Starting from June, ODE data followed a bell shaped dynamic while our integrated sampling did not. In fact, concentrations measured by ODE decreased while the values of our integrated measurements increased (see 10/2017 in Figure 2b). Punctual sampling results diverged from the ones we obtained with integrated sampling indicating that CLD trends should not be easily deduced from punctual sampling at the infra-annual temporal scale.

To summarize, we identified general trends in the dynamics of CLD according to our integrated sampling. It showed a good consistency between stations LD and BDL that have no direct hydrological connection. However, we also identified that punctual weekly samples led to record high variations of concentrations. It prevents from assessing trends of CLD concentration on punctual sampling and justify assessing CLD concentrations on a weekly basis.

Inter-annual evolution

The inter-annual dynamics on LD (Figure 2f), highlighted an increase of CLD concentration in early 2017. Then, until late 2017, these concentrations remained higher than in 2016 (Table 2). Comparing average concentration between 2016 and 2017, we observed i) a slight increase of CLD average concentration on GG watershed (+15%), ii) average concentrations on BDL that remained stable over the two years, and iii) that CLD average concentration almost doubled (+82%) between 2016 and 2017 on the LD watershed. However, average concentrations did not reflect distribution of pollution between the two years The distributions of concentrations recorded in 2016 and 2017 were significantly different for all sub-watersheds, being highly significant for GG and LD sub-watersheds (p-value<0.001), but in a lower extent for BDL sub-watershed (p-value<0.05). This result shows that, on BDL, similar average concentration (0.46 µg.L -1 ) resulted from different river behavior in 2016 and in 2017 (significantly different distributions). Indeed, the trend analysis of BDL station showed that 2016 was characterized by a stable CLD concentration surrounding 0.46 µg.L -1 , while concentrations recorded during 2017 were more variable with a CLD peak (>2 µg.L -1 -03/2017) followed by an overall decrease of the concentration (<0.46 µg.L -1 ) before returning to the initial value in 10/2017 (0.46 µg.L -1 ) (Figure 2d).

Since we noticed in section 3.1.1 that LD and BDL have CLD trends showing similarities, the high increase in mean CLD concentration in 2017 for LD stemmed, on the one hand, from two CLD peaks instead of one for BDL and, on the other hand, from higher CLD contents after the first peak relative to Matthew storm. This may be explained by the fact that the stabilized increase of CLD concentration in LD in 2017 is related to a higher baseflow level than in 2016 (Figure 2e andf). The fact that this small watershed is mainly drained by subsurface flows make it more responsive to modifications of land uses for instance. Such relationship with baseflow was not observed on BDL or GG. However, it might be that the increase of CLD concentration we observed on GG between 10/2017 and 01/2018 has the same origin but delayed in time because it is 19 times larger than LD. Monitoring CLD concentrations on the 3 stations showed that CLD is a relatively stable and chronic pollution in rivers. We showed that the CLD dynamics observed in river has two components: a trend of inter-annual evolution that may be driven by baseflow as well as infra-annual variations surrounding that trended evolution whose intensities vary according to stations and may be driven by specific event like storms in the present case.

Sampling

Mass transfers

The average daily CLD mass transferred at the outlet of each sub-watershed, as well as the average daily CLD mass transferred per unit of area polluted by CLD are presented in Table 3. According to Erreur ! Source du renvoi introuvable., we deduced potentially polluted area on the 3 sub-watersheds: 116 ha for BDL, 901 ha for GG and 136 ha for LD, meaning that the area between GG and BDL holds 785 potentially polluted hectares. The highest values of daily average CLD transferred at outlet were recorded on GG, while lowest values were recorded on BDL (Table 3).

2017

Daily average (g.day -1 ) Daily average per unit of polluted area (g.day -1 .ha -1 ) Specific discharge (m 3 .s -1 .km -2 ) Daily average (g.day -1 ) Daily average per unit of polluted area (g.day -1 .ha -1 ) Specific discharge (m 3 .s -1 .km Table 3 shows that sub-watersheds with larger polluted area (GG and GG-BDL) are not necessarily the one that transfers the most CLD toward our monitored outlet. BDL and LD having transfers rates by polluted area 4 times higher that GG or GG-BDL (Table 3). It means that in terms of mass, the area between BDL and GG, in spite of large potentially polluted area had a lower transfer's rate to outlet. The upstream zone (represented by BDL) is humid with large rainfall and characterized by a larger specific discharge than the other sub-watersheds (Table 3). Large water volumes at relatively low CLD concentrations generated stabilized transfers at outlet as shown by the lower standard deviation than on GG or LD (Table 3). This also means that GG and LD are more subject to large variations of daily CLD mass transferred. The small specific discharge of GG-BDL also indicate that the area between GG and BDL has a small contribution to the discharge. Our results also shows that LD, in spite of its small size (191 ha) and low discharge compared to GG or BDL, transfers unneglectable mass of CLD because of very high water CLD concentrations.

Daily mass transferred by GG gave annual values from 33 kg.y -1 to 50 kg.y -1 of CLD transferred to the Galion Bay. For LD watershed, annual value transferred to the Galion bay ranged from 19 kg.y -1 to 25 kg.y -1 . This means that the total yearly contribution of that two outlets to the Galion bay may be estimated from 50 kgCLD.y -1 to 75 kgCLD.y -1 . Using a simple assumption of constant transfer rate, and without considering years of applications, we estimated total transfers to the bay from 1993 to 2018 to be between 2500 kg and 3750 kg. This might be even more because we did not took into account transfers from 1970 to 1993. These amounts represent between 2.4 and 3.6 kgCLD per ha of polluted area. This is low compared to the amount of CLD applied ≈3 kg.ha -1 at each application (Cabidoche et al. 2009;[START_REF] Levillain | Analysis of environmental and farming factors of soil contamination by a persistent organic pollutant, chlordecone, in a banana production area of French West Indies[END_REF]) during more than 20 years. Calculations of CLD stocks in soils according to [START_REF] Levillain | Analysis of environmental and farming factors of soil contamination by a persistent organic pollutant, chlordecone, in a banana production area of French West Indies[END_REF] gave values of few kg of CLD stocked in soils in the 0-60 cm depth. This suggests that, it is very likely that large amounts of CLD already transferred to other environmental compartments, meaning that substantial amount of CLD could be in the marine environment and in aquifers. Because CLD was applied around banana stems and banana fields were mainly cropped on bare soils with stemflow at that period [START_REF] Charlier | Modelling runoff at the plot scale taking into account rainfall partitioning by vegetation: application to stemflow of banana (Musa spp.) plant[END_REF]) that conjointly favors surfaces transfers. It is likely that the larger fraction of CLD that lacks in our CLD balance went mainly to the marine environment during storms towards fast surface transfer processes such as runoff or subsurface flows, both concentrated in surface drains that were a very common on banana field at that time.

We showed that small watershed can nowadays have significant chlordecone transfers contributions to the ocean. In spite of the potentially large amounts of CLD that transferred in the past, we suggest that such areas with fast hydrological response should be managed with care and prioritized for pesticides reduction strategies as well as decontamination. We expect that the results of mitigation measures could be observable in the river in a couple of years for these areas.

Evolutions of CLD in groundwaters

Piezometric levels on the 5 wells located on the watershed and CLD concentrations recorded in 4 of these aquifers are presented in Figure 3: FPG (Fig. 3b), FMT (Fig. 3c), FBASS (Fig3d), FMV (Fig3e). FBASS an FMT are deep aquifers while FGG, FPG, and FMW have piezometric levels around few meters. CLD concentrations in groundwaters located in contaminated areas were 10 to 50-fold higher than concentrations measured in rivers (Figure 2 and 3). No chlordecone was found in FPG waters that is consistent with its location upstream the polluted areas (Figure 1). Based on the punctual data set, we can say that the CLD concentrations in aquifers varied along the year, but we could not define a clear relationship between CLD concentrations and piezometric levels due to the scarcity of sampling. All aquifers sampled on the 03-11-2016, at the beginning of the high water level period, had particularly high CLD concentrations. Then, CLD concentrations decreased in the following samples (Figure 3b,d,e), but remained always in a high range of values. Besides, each aquifer had its own piezometric dynamic as shown in Figure 3. For instance, FMT had more inertial dynamic than FBASS or FMV, and even more than FGG or FPG. According to [START_REF] Charlier | CHLOR-EAU-SOL -Water -Caracterization of water and soil contamination by chlordecone of watersheds in Guadeloupe and Martinique[END_REF], the dynamic between recharge and discharge to rivers can be very long for deep aquifers with residence times from 4 years (FMV), 6 to 8 years (FMT) to more than 20 years (FBASS).

Evolutions of agricultural practices

CLD polluted soil of the watershed are still mainly cropped with banana tree. This is because CLD is not quantified in banana fruit even when cropped on highly polluted soils [START_REF] Clostre | Soil thresholds and a decision tool to manage food safety of crops grown in chlordecone polluted soil in the French West Indies[END_REF]. CLD being stored into soil, several grower practices might affect CLD transfers. According to our banana grower survey, we could draw a general cropping system practiced by banana growers. Banana cropping system usually last around 7 years. During those 7 years, 5 or 6 years are indeed cropped with banana and the remaining 1 or 2 years consist in a fallow period. The fallow period allows for the soil to rest and eliminates diseases that developed in soil (nematodes and weevils) during the repeated banana cropping cycles. The fallow is established using herbicide injected into banana trees to kill them. Then, soil is tilled (Figure 4b). A fallow cover crop may either be planted, for instance with C. spectabilis or B. decumbens, or results from spontaneous species (such as C. ruditosperma). Depending on farms, at the end of the fallow period, either soil is tilled again before plantation, or banana tree are planted in a mulch of plants killed with herbicide. On the farms of the Galion watershed, tillage is usually carried on before or during the fallow period.

Figure 4a shows the field of one farmer of the LD watershed right after the Matthew storm. This farmer cut all developed banana tree in its farm (25 ha out of the 79 ha of cropped area by all farmers on the watershed) because most developed trees were laid down by the storm. Figure 5 shows fallows surface evolution between 2016 and 2017. We can see that 2017 was characterized by large increase of fallow area on the watershed that almost doubled: in 2016 fallow covered 79 ha whereas it covered 139 ha in 2017. This means that larger area than usual were tilled on the watershed between 2016 and 2017. This practice of large fallow integration after a cyclone or a violent tropical storm is a common practice among banana growers in the FWI resulting in large-scale modification of agricultural practices.

Discussion

CLD evolutions in rivers and aquifers

Our results showed that river and aquifer pollutions by CLD was roughly stable and chronic. Several authors have reported this [START_REF] Crabit | Soil and river contamination patterns of chlordecone in a tropical volcanic catchment in the French West Indies (Guadeloupe)[END_REF][START_REF] Mottes | Hydrological and pesticide transfer modeling in a tropical volcanic watershed with the WATPPASS model[END_REF][START_REF] Mottes | Relationships between past and present pesticide applications and pollution at a watershed outlet: The case of a horticultural catchment in Martinique[END_REF]). However, CLD concentrations in rivers vary according to sites. On the small LD subwatershed, we showed that CLD interannual variability observed in river may be driven by baseflow. In the case of watersheds draining the highest reliefs of volcanic islands, [START_REF] Crabit | Soil and river contamination patterns of chlordecone in a tropical volcanic catchment in the French West Indies (Guadeloupe)[END_REF] found correlation between CLD concentration and discharge because the main process of CLD evolution was the dilution of water drained from lateral cultivated areas by high amounts of unpolluted waters from the forested areas upstream. In the case of watersheds draining mostly cultivated zones, [START_REF] Mottes | From fields to rivers chlordecone transfer in water[END_REF] did not find such relationship on the Ravine watershed, highlighting complex conditions of CLD transfers. At the watershed scale, Della [START_REF] Rossa | Linking current river pollution to historical pesticide use: Insights for territorial management?[END_REF] showed that the spatial variability of CLD concentration in water during low flow periods was explained by the potential pollution of the fields located on the watershed of the sampled points. At the regional scale, comparing numerous watersheds in Martinique Island, [START_REF] Cattan | A conceptual model of organochlorine fate from a combined analysis of spatial and mid-to long-term trends of surface and ground water contamination in tropical areas (FWI)[END_REF] showed that the spatial variability of CLD concentration may be explained by a combination of the location of banana cropping areas, soil and geological factors; these conditions defining a high diversity of situations in terms of the mid-to long-term duration of pollution.

Unlike previous studies, our results highlights that, at a weekly time step, CLD concentrations in rivers showed annual evolution trends (Figure 2). Aquifers located in polluted area (Figure 1) showed higher CLD concentrations during recharge (Figure 3) then followed by CLD uptrends in rivers (Figure 2). Figure 2 and Figure 3 analyzed conjointly showed that the uptrends we observed in rivers followed the generalized CLD peak in aquifer observed in 11/2016. Here, we are in the opinion that CLD concentration increased in aquifers and rivers as the consequence of, either translatory flow [START_REF] Lischeid | Apparent translatory flow in groundwater recharge and runoff generation[END_REF], or recent leaching of water highly concentrated with CLD. This suggests that surrounding that time, factors generated an increase of CLD leaching to aquifers and subsurface flows.

Factors explaining increased CLD leaching

Increased CLD leaching from soils, that is still a large reservoir of CLD (Lesueur-Jannoyer et al. 2012), to aquifers and subsurface flows is likely to have generated the increases of CLD concentrations in aquifers and rivers we observed. Several factors that increase the availability of CLD into soil to water flows can explain an increase of CLD leaching.

Effect of hydroclimatic conditions

Our results showed higher CLD concentrations on all wells situated in polluted area on the watershed at a specific date: 03-11-2016 but we could not conclude on a relationship with piezometric levels. This pattern is coherent with previous works carried out in similar hydrogeological context in Martinique Island by [START_REF] Arnaud | Groundwater Quality Assessment[END_REF]. Based on a higher resolution of sampling (monthly), these authors showed that the intensity of piezometric fluctuations was not correlated with the amplitude of variations in CLD concentration, but that the CLD higher concentrations were often observed during periods of rising groundwater levels. Our results suggest that period of soil drying may increase the availability of CLD to water flow during the first leaching events. This is in accordance with the observations of [START_REF] Arnaud | Groundwater Quality Assessment[END_REF], showing that the main CLD peak in groundwaters from the last decade occurred during the first major recharge event after a several year period of deficient recharge that resulted in a very low water table. This was true for 8 wells located all over the contaminated zones of the Martinique Island, meaning that such regional pattern is likely controlled by hydroclimatic conditions. Following this explanation, we expect that the main CLD higher concentrations observed in rivers of the Galion watershed after the storm of October 2016 is related to the higher groundwater concentrations observed at that time.

Effects of cropping practices

CLD applications were forbidden after 1993, so that theoretically no applications occurred from that date. As a result, the only agricultural practices that could have increased CLD leaching are the one that modified environmental characteristics that affect pesticide transfers [START_REF] Mottes | Pesticide transfer models in crop and watershed systems: a review Agron[END_REF]. Here, modifications of the soil organic matter content, allophane clay content, or both potentially decreased CLD retention by soil [START_REF] Woignier | Compost addition reduces porosity and chlordecone transfer in soil microstructure[END_REF]Woignier et al. 2012a;Woignier et al. 2012b;[START_REF] Woignier | Soil microstructure and organic matter: Keys for chlordecone sequestration[END_REF]. It has been shown that the peculiar hierarchical (fractal) microstructure of allophane clay traps CLD (Woignier et al. 2012a;[START_REF] Woignier | Soil microstructure and organic matter: Keys for chlordecone sequestration[END_REF]. It is known that tillage activate soil organic matter decomposition [START_REF] Haddaway | How does tillage intensity affect soil organic carbon? A systematic review[END_REF]) and expose soil to direct sunlight, hence, to strong drying at its surface resulting in destruction of allophane (Quantin 1972) and allophane microstructure. During drying, the fractal microstructure progressively disappears [START_REF] Woignier | Preservation of the allophanic soils structure by supercritical drying Microporous and[END_REF]). As a result, tillage would act on both degradation of soil organic matter and allophanes trapping properties in andosols. On top of that, tillage destroy the soil structure. The destruction of the soil structure imply a modification of the water paths into soil [START_REF] Alletto | Tillage management effects on pesticide fate in soils[END_REF]. Indeed, structured soils present preferential infiltration flow that may be washed from extractible CLD after several time. Destroying those paths induce water to access more porosity in soil and thus extract more CLD from soil. This may explain the difference of one order of magnitude found in CLD soil organic carbon coefficients (Koc) between modelling field studies [START_REF] Cabidoche | Long-term pollution by chlordecone of tropical volcanic soils in the French West Indies: A simple leaching model accounts for current residue[END_REF][START_REF] Mottes | Hydrological and pesticide transfer modeling in a tropical volcanic watershed with the WATPPASS model[END_REF] and laboratory data [START_REF] Baran | Mapping of groundwater risks of contamination by pesticides in Martinique[END_REF]Fernandez-Bayo et al. 2013a). In the last, soils are totally destroyed [START_REF] Oecd | Test No. 106: Adsorption-desorption using a batch equilibrium method. Organisation for Economic Co-operation and Development[END_REF] while in the first one structured soils are considered. Finally, tillage induces modifications of soil organic matter and clay characteristics are modified as well as the soil structure. All these evolutions favor the leaching of CLD.

The modification of water path in soil could results from tillage but mays also results from modifications of soil covers. According to [START_REF] Lewis | Pesticide Behavior, Fate, and Effects in the Tropics: An Overview of the Current State of Knowledge[END_REF] and to [START_REF] Mottes | Pesticide transfer models in crop and watershed systems: a review Agron[END_REF] practices that modify ground cover may modify pesticide leaching. This is particularly true for vegetative ground cover such as banana tree. According to several authors, we know that banana trees concentrate rainfall along their stem by a process called stemflow [START_REF] Cattan | Spatial and temporal variations in percolation fluxes in a tropical Andosol influenced by banana cropping patterns[END_REF][START_REF] Charlier | Modelling runoff at the plot scale taking into account rainfall partitioning by vegetation: application to stemflow of banana (Musa spp.) plant[END_REF][START_REF] Sansoulet | Spatially Distributed Water Fluxes in an Andisol under Banana Plants: Experiments and Three-Dimensional Modeling Vadose[END_REF]. This means that, in a cropped banana field, water will preferentially infiltrate right under banana tree [START_REF] Cattan | Spatial and temporal variations in percolation fluxes in a tropical Andosol influenced by banana cropping patterns[END_REF][START_REF] Sansoulet | Spatially Distributed Water Fluxes in an Andisol under Banana Plants: Experiments and Three-Dimensional Modeling Vadose[END_REF] generating preferential flow paths under banana tree. As for structured soil, we can expect that such preferential water path are "washed" from CLD after several time of semiperennial banana cropping. Stemflow was also shown to increase runoff [START_REF] Charlier | Modelling runoff at the plot scale taking into account rainfall partitioning by vegetation: application to stemflow of banana (Musa spp.) plant[END_REF], meaning that this process also acts on reducing the overall leaching on banana fields. Consequently, stemflow increases erosion of particulate CLD transfers as well as other surface pollutant transfers. To conclude, it means that any modification that reduce stemflow is likely to increase leaching on the field.

Our results showed that between 2016 and 2017 the surface of tilled polluted fields increased (Figure 5) on all monitored sub-watershed. Figure 1 shows that most soils of the watersheds contain allophane clay, meaning that either tillage, dry period or both could have resulted in CLD uptrend. This means that it increased the access of CLD to leaching water in 2017. Such processes could have generated increasing CLD concentration in subsurface flows thus generating uptrends in river CLD concentrations we observed on LD and BDL.

Integration of processes at the watershed scale

Our results showed that the different aquifers have contrasted piezometric inertia (Figure 3). In the Galion watershed, [START_REF] Charlier | CHLOR-EAU-SOL -Water -Caracterization of water and soil contamination by chlordecone of watersheds in Guadeloupe and Martinique[END_REF] showed variable mean residence times varying from 4 years (FMV), 6 to 8 years (FMT) to more than 20 years (FBASS). They also reported smaller residence times of one year for sources or during high water levels. Because, rivers in FWI are mainly governed by underground flows [START_REF] Charlier | Hydrological behaviour and modelling of a volcanic tropical cultivated catchment[END_REF][START_REF] Crabit | Soil and river contamination patterns of chlordecone in a tropical volcanic catchment in the French West Indies (Guadeloupe)[END_REF][START_REF] Mottes | Hydrological and pesticide transfer modeling in a tropical volcanic watershed with the WATPPASS model[END_REF], this means that, at the watershed scale the river is likely to be fed by different aquifer's units having different inertia. The combination of residence times and the multiplicity of aquifers contribute to the stability of CLD concentration at the weekly time step. According to our results along with the ones of [START_REF] Charlier | CHLOR-EAU-SOL -Water -Caracterization of water and soil contamination by chlordecone of watersheds in Guadeloupe and Martinique[END_REF], there is every likelihood that only underground major contributors changing conjointly may modify the trend of CLD concentration in rivers. The increased CLD concentration at LD confirms this. LD watershed is mainly governed by subsurface flow and the monitored aquifer (FMV) has a short mean residence time of 4 years, allowing it to be sensitive, among others, to strong cyclonic events. Our results showed that the mid-term CLD concentration increase in 2017 is related to the increase of the baseflow on this river (Figure 2e). According to our results, we are in the opinion that increasing CLD concentrations rely on the activation of new water paths on polluted areas on the sub-watershed.

Our results showed that in rivers, CLD concentration in punctual and integrated sampling diverged (Figure 2b). This behavior was unexpected, because we would expect to see both monitoring programs showing the same dynamics for a chronic pollution such as CLD. One explanations could be that CLD concentrations shows large variations with time at the event scale as it was shown for CLD [START_REF] Mottes | From fields to rivers chlordecone transfer in water[END_REF]) and for other pesticides [START_REF] Liger | Comparison of different active sampling strategies for monitoring pesticide pollutions in surface waters[END_REF]. The variability of CLD concentrations we obtained when we performed weekly punctual sampling instead of integrated sampling (between Oct 2016 and April 2017) supports that hypothesis because CLD concentration variation are very high at that period. This could mean that integrated sampling integrates the contributions of the different zones of the watersheds taking into account the delayed transfers' times according to positioning on the watershed.

The CLD concentration trend we measured corresponds to the evolution of average concentrations that integrates such variability. A variable transfers time according to positioning on the watershed was suggested by [START_REF] Mottes | Hydrological and pesticide transfer modeling in a tropical volcanic watershed with the WATPPASS model[END_REF] who included a delay in modelling CLD transfers into river according to the distance of contributing areas. On large watershed, this short-term effect might be buffered at the weekly time step or induce larger dynamics on CLD transfers. On that point, our results showed that the spatial organization of polluted fields differed in the 3 sub-watersheds, with LD being polluted on almost all its area, BDL downstream and GG mid-stream. It could be that the spatial organization of polluted area affected the CLD dynamics observed at the watershed scale (Figure 2): BDL showed uptrend followed by downtrend that could correspond to downstream contributions followed by upstream contributions. As a result, the variation of CLD concentration of punctual and integrated sampling might results from the contributions of the different transfer times of surface and subsurface flow generating a dynamics at the short to mid-time scale depending on the size of the watershed.

CLD short terms variation could also come from the fact that sediments interacts with CLD in the dissolved phase, especially in the sampling containers of our sampling stations. With automatic integrated samplers, erosion events are sampled along the week with water. As a result, sediments enters into the sampling container. They could increase CLD concentration in the sample because of CLD bound to sediments. According to ODE data, sediments analyzed at the GG station between 2008 and 2015 revealed CLD concentration varying between <10 µg.kg -1 and 30 µg.kg -1 . These values are low compared to soil CLD concentrations (0.1 -5 mg.kg -1 ), but could be sufficient to modify CLD concentrations in sampling containers according to the concentration value measured (Table 2) [START_REF] Katagi | Behavior of pesticides in water-sediment systems Reviews of Environmental Contamination and[END_REF]. Finally, higher variability of punctual sampling may be related to the difficulty to follow the same protocol at each sampling date. Notably sampling site may differ according to the possibilities of access to the river.

Our results suggest either that there are variability of CLD concentrations at the instant time scale or a strong interaction between dissolved and particulate phases. This means that the average concentration we obtain in weekly samples results from CLD trends, variations of CLD concentrations all over the week and from sediment dynamics in the river.

How to manage CLD pollution ?

Our results showed that causes and processes that modify CLD concentration in water, are interrelated, making it difficult to isolate single effects. A complexity exists at the watershed scale where different organization levels and processes interact. Agricultural practices triggered according to climatic windows stand for an example of such interaction. Banana is still mainly cropped on polluted soils because it is not sensitive to soil pollution by CLD. Banana farming practices follow decision rules based on climate (cyclonic season, dry season), market.... Banana plantations increase in July to reach good price marked at the first harvest. Fallow, are largely practiced post-cyclonic after banana trees are laid down by high speed winds. Tillage is easier and recommended at medium humidity conditions corresponding to conditions of the beginning or of the end of the dry season. This means that monitoring of CLD in river will always show systemic response to event such as storm of cyclones. After them, large recharge occurs, stemflow disappear, tillage and fallow are practiced during dryer periods, all these potential causes of CLD increase have a similar triggering event, which is the storm. In a counterpart, it means that we can acts on several leeways at the watershed scale. In a mid-term objective, we recommend to integrate knowledge on pesticides transfers at different scale into modelling tools such as WATPPASS [START_REF] Mottes | Hydrological and pesticide transfer modeling in a tropical volcanic watershed with the WATPPASS model[END_REF] and to adapt their formalism to help farmers along with extension workers and decision makers to test and assess the effect of different scenarios on CLD transfers. Test the collective response of farmers at the watershed scale to specific climatic events such as storm that actually induce practice response with deleterious effects on CLD concentrations.

Our analysis showed that processes at different scale interact. Because, we cannot act on the aquifersriver relationship, we recommend focusing on limiting transfers from polluted fields. To that point, we showed that hot spots, such as LD watershed are heavy polluted by CLD resulting in large CLD concentrations in water. Such high concentration results in large amount transferred at outlet and reaching the marine environment. At the watershed scale, we recommend to focus management of transfers, on that kind of area for 3 major reasons. First, in spite of their small surface, they are large contributor to pollution transfers. Second, according to our results, such small watershed react quicky to practices and hydroclimatic conditions so that monitoring will be able to assess the positive impact of practices change on water quality. Third, they are managed by less farmers, making it easier to consistently manage all fields from the zone.

Decontamination of soils remains the final objective [START_REF] Chaussonnerie | Microbial Degradation of a Recalcitrant Pesticide: Chlordecone Frontiers in[END_REF][START_REF] Mouvet | Remediation by chemical reduction in laboratory mesocosms of three chlordecone-contaminated tropical soils[END_REF]). In the meanwhile, in terms of actions, we showed that tillage that active different processes favoring CLD transfers should be avoided as much as possible on polluted soils, or at least positioned on specific area where required such as plantation lines. To avoid pollution peaks, we recommend to avoid long drying of soils, in particular soils with allophanic clays. Maintaining a ground cover and a minimal controlled irrigation to maintain minimal hydratation could help reduce CLD peak at the end of the dry season.

Sequestration method using organic matter could be applied [START_REF] Clostre | Field validation of chlordecone soil sequestration by organic matter addition[END_REF]. Using farm available organic matter, returning large amount of crop residues, the use of grass or covercrop to produce large amount of OM returned to soil, and bringing externals sources of OM from unpolluted fields could help increase OM concentration that stabilize CLD into soils.

Conclusion

We showed that CLD s concentrations vary at different spatial and time scale. The large variability of biophysical and decisional processes as well as their interactions at that scale require a systemic approach for their understanding and management. Such systemic analysis requires a local characterization of the hydrogeological functioning and of the agricultural practices to better understand losses in rivers. In the French West Indies, these involves historical CLD application, soil characteristics and the modifications by agricultural practices, and the reload conditions of aquifers defined by climatic events and the permeability of underground geological formations modulating transfers times.

Figure 1 :

 1 Figure 1: Map of the Galion watershed, positions of the 3 sub-watersheds, wells, rain gauges, risk of CLD pollution risk. Pollution risks following the method described in Desprats et al. (2004) and applied by Della Rossa et al. (2017).

Figure 2 :

 2 Figure 2 : Hydrology and chlordecone dynamics on the 3 sampling sites of the watershed. Red dots: ODE data. Grey lines on CLD concentration graphics: Hodrick-Prescot filter and ±35% error interval. Grey dashed lines on discharge graphics: baseflow. Orange vertical line: Matthew storm.

Figure 3 :

 3 Figure 3 : Piezometric levels and CLD concentrations in aquifers of the Galion watershed. Black dots: chlordecone concentrations. Grey lines: analytical uncertainty on chlordecone concentrations.

Figure 4 :

 4 Figure 4 : a) Cycloned field just after the Matthew storm on the La Digue (LD) sub-watershed (C. Mottes Oct-2016). b) Fallow (grass with dead banana tree on the left) been tilled with excavator (bare soil) on the La Digue (LD) sub-watershed (C. Mottes Apr-2017).

Figure 5 :

 5 Figure 5: Evolution of fallow and banana between 2016 and 2017 (data from RPG 2016 and 2017 DAAF).

Medium 1992 Medium 1970

						No
						historical
						banana
	BDL	0.5%	11.3%	0.9%	1.1%	86.2%
	GG	2.2%	14.6%	4.9%	3.4%	74.9%
	LD	29.7%	27.6%	3.5%	10.5%	28.7%
	Table 1 : Percentage of risk of contamination for each watershed (percentage of total area)
	2.3

Hydrological monitoring and water sampling 2.3.1 Rainfall, hydrological and hydrogeological monitoring

	Daily rainfall for station « Bois Lézard » (BL) and « Trinité Réservoir » (TR) were acquired from Météo

Table 2 :

 2 Average chlordecone concentration on the 3 sampling points for the 3 sampling campaigns(2013, 2016 and 2017).

		CLD	CLD
	site	Mean	Mean
		(2016)	(2017)
		(µg.L -1 )	(µg.L -1 )
	GG	0.70	0.81
	BDL	0.46	0.46
	LD	5.54	10.08

Table 3 :

 3 -2 ) Chlordecone mass transferred by each watershed monitored

	GG	91±171	0.10	0.047	137±303	0.15	0.065
	LD	52±169	0.38	0.052	68±124	0.5	0.042
	BDL	49±24	0.42	0.147	51±53	0.44	0.144
	GG-BDL	49±159	0.06	0.018	97±266	0.12	0.027

Acknowledgements

This work was made possible thanks to the financial support of the National Chlordecone Action Plan (PNAC), French Water Office of Martinique (ODE Martinique) and European Regional Development Fund (FEDER) with the RIVAGE project (MQ0003772-CIRAD). We also thank F. Massat and P. Rey from LDA26 for providing details on the protocols from the laboratory.


H Gouin
email: henri.gouin@univ-cezanne.fr

A MECHANICAL MODEL FOR LIQUID NANOLAYERS

Keywords: Nanolayers, disjoining pressure, thin flows, approximation of lubrication

Liquids in contact with solids are submitted to intermolecular forces making liquids heterogeneous and stress tensors are not any more spherical as in homogeneous bulks. The aim of this article is to show that a square-gradient functional representing liquid-vapor interface free energy corrected with a liquid density functional at solid surfaces is a well adapted model to study structures of very thin nanofilms near solid walls. This result makes it possible to study the motions of liquids in nanolayers and to generalize the approximation of lubrication in long wave hypothesis.

Introduction

At the end of the nineteenth century, the fluid inhomogeneity in liquidvapor interfaces was taken into account by considering a volume energy depending on space density derivative. 1 This van der Waals square-gradient functional is unable to model repulsive force contributions and misses the dominant damped oscillatory packing structure of liquid interlayers near a substrate wall. 2 Furthermore, the decay lengths are correct only close to the liquid-vapor critical point where the damped oscillatory structure is subdominant. 3 In mean field theory, weighted density-functional has been used to explicitly demonstrate the dominance of this structural contribution in van der Waals thin films and to take into account longwavelength capillary-wave fluctuations as in papers that renormalize the square-gradient functional to include capillary wave fluctuations. 4 In contrast, fluctuations strongly damp oscillatory structure and it is mainly for this reason that van der Waals' original prediction of a hyperbolic tangent is so close to simulations and experiments. [START_REF] Rowlinson | Molecular theory of capillarity[END_REF] The recent development of exper-imental technics allows us to observe physical phenomena at length scales of a few nanometers. [START_REF]Springer handbook of nanotechnology[END_REF] To get an analytic expression in density-functional theory for liquid film of a few nanometer thickness near a solid wall, we add a liquid density-functional at the solid surface to the square-gradient functional representing closely liquid-vapor interface free energy. This kind of functional is well-known in the literature. [START_REF] Nakanishi | [END_REF] It was used by Cahn in a phenomenological form, in a well-known paper studying wetting near a critical point. 8 An asymptotic expression is obtained in 9 with an approximation of hard sphere molecules and London potentials for liquid-liquid and solidliquid interactions: we took into account the power-law behavior which is dominant in a thin liquid film in contact with a solid. For fluids submitted to this density-functional, we recall the equation of motion and boundary conditions. We point out the definition of disjoining pressure and analyze the consequences of the model. Finally, we study the motions in liquid nanolayers; these motions are always object of many debates. Within lubrication and long wave approximations, a relation between disjoining pressure, viscosity of liquid, nanolayer thickness variations along the layer and tangential velocity of the liquid is deduced.

The density-functional

The free energy density-functional of an inhomogeneous fluid in a domain O of boundary ∂O is taken in the form

F = O ε dv + ∂O ϕ ds. (1)
The first integral is associated with square-gradient approximation when we introduce a specific free energy of the fluid at a given temperature θ, ε = ε(ρ, β) as a function of density ρ and β = (grad ρ) 2 . Specific free energy ε characterizes together fluid properties of compressibility and molecular capillarity of liquid-vapor interfaces. In accordance with gas kinetic theory, λ = 2ρ ε ′ β (ρ, β) is assumed to be constant at given temperature [START_REF] Rocard | Thermodynamique[END_REF] and

ρ ε = ρ α(ρ) + λ 2 (grad ρ) 2 , (2)
where term (λ/2) (grad ρ) 2 is added to the volume free energy ρ α(ρ) of a compressible fluid. Specific free energy α enables to connect continuously liquid and vapor bulks and pressure P (ρ) = ρ 2 α ′ ρ (ρ) is similar to van der Waals one. Near a solid wall, London potentials of liquid-liquid and liquid-solid interactions are    ϕ ll = -c ll r 6 , when r > σ l and ϕ ll = ∞ when r ≤ σ l , ϕ ls = -c ls r 6 , when r > δ and ϕ ls = ∞ when r ≤ δ , where c ll and c ls are two positive constants associated with Hamaker constants, σ l and σ s denote fluid and solid molecular diameters, δ = 1 2 ( σ l + σ s ) is the minimal distance between centers of fluid and solid molecules. [START_REF] Israelachvili | Intermolecular and surface forces[END_REF] Forces between liquid and solid have short range and can be described simply by adding a special energy at the surface. This is not the entire interfacial energy: another contribution comes from the distortions in the density profile near the wall. 9,[START_REF] De Gennes | [END_REF] For a plane solid wall (at a molecular scale), this surface free energy is

φ(ρ) = -γ 1 ρ + 1 2 γ 2 ρ 2 . ( 3
)
Here ρ denotes the fluid density value at the wall; constants γ 1 , γ 2 are positive and given by relations

γ 1 = πc ls 12δ 2 m l m s ρ sol , γ 2 = πc ll 12δ 2 m 2 l
, where m l and m s denote respectively masses of fluid and solid molecules, ρ sol is the solid density. 9 Moreover, we have λ = 2πc ll 3σ l m 2 l .

We consider a horizontal plane liquid interlayer contiguous to its vapor bulk and in contact with a plane solid wall (S); the z-axis is perpendicular to the solid surface. The liquid film thickness is denoted by h. Conditions in vapor bulk yield grad ρ = 0 and ∆ρ = 0. Another way to take into account the vapor bulk contiguous to the liquid interlayer is to compute a density-functional of the complete liquid-vapor interlayer by adding a supplementary surface energy ψ on a geometrical surface (Σ) at z = h to volume energy (2) in liquid interlayer (L) and surface energy (3) on solid wall (S). 13 This assumption corresponds to a liquid interlayer included between z = 0 and z = h, a liquid-vapor interface of a few Angström thickness assimilated to surface z = h and a vapor layer included between z = h and z = ∞. Due to small vapor density, let us denote by ψ the surface free energy of a liquid in contact with a vacuum,

ψ(ρ) = γ 4 2 ρ 2 (4)
where γ 4 ≃ γ 2 and ρ is the liquid density in a convenient point inside the liquid-vapor interface. 13 Density-functional (1) of the liquid-vapor layer gets the final form

F = (L) ε dv + (S) φ ds + (Σ)
ψ ds

Equation of motion and boundary conditions

In case of equilibrium, functional F is minimal and yields the equation of equilibrium and boundary conditions. In case of motions we simply add the inertial forces ρ Γ and the dissipative stresses to the results. 14-16

Equation of motion

The equation of motion is 14,15 ρ

Γ = div (σ + σ v ) -ρ grad Ω , ( 5
)
where Γ is the acceleration, Ω the body force potential and σ the stress tensor generalization

σ = -p 1 -λ grad ρ ⊗ grad ρ, with p = ρ 2 ε ′ ρ -ρ div (λ grad ρ). The viscous stress tensor is σ v = κ 1 ( tr D) 1 + 2 κ 2 D
where D denotes the velocity strain tensor; κ 1 and κ 2 are the coefficients of viscosity. For a horizontal layer, in an orthogonal system of coordinates such that the third coordinate is the vertical direction, the stress tensor σ of the thin film takes the form :

σ =   a 1 , 0, 0 0, a 2 , 0 0, 0, a 3   , with        a 1 = a 2 = -P + λ 2 dρ dz 2 + λ ρ d 2 ρ dz 2 a 3 = -P - λ 2 dρ dz 2 + λ ρ d 2 ρ dz 2
Let us consider a thin film of liquid at equilibrium (gravity forces are neglected but the variable of position is the ascendant vertical). The equation of equilibrium is :

div σ = 0 (6)
Eq. ( 6) yields a constant value for the eigenvalue a 3 ,

P + λ 2 dρ dz 2 -λρ d 2 ρ dz 2 = P v b .
where P v b denotes pressure P (ρ v b ) in the vapor bulk of density ρ v b bounding the liquid layer. Eigenvalues a 1 , a 2 are not constant but depend on the distance z to the solid wall. [START_REF] Derjaguin | Surfaces forces[END_REF] At equilibrium, Eq. ( 5) yields:

14 grad [ µ (ρ) -λ∆ρ ] = 0, ( 7
)
where µ is the chemical potential at temperature θ defined to an unknown additive constant. The chemical potential is a function of P (and θ) but it can be also expressed as a function of ρ (and θ). We choose as reference chemical potential µ o = µ o (ρ) null for bulks of densities ρ l and ρ v of phase equilibrium. Due to Maxwell rule, the volume free energy associated with µ o is g o (ρ) -P o where P o = P (ρ l ) = P (ρ v ) is the bulk pressure and g o (ρ) = ρ ρv µ o (ρ) dρ is null for liquid and vapor bulks of phase equilibrium. The pressure P is

P (ρ) = ρ µ o (ρ) -g o (ρ) + P o . (8)
Thanks to Eq. ( 7), we obtain in the fluid and not only in the fluid interlayer,

µ o (ρ) -λ∆ρ = µ o (ρ b ),
where

µ o (ρ b ) is the chemical potential value of a liquid mother bulk of density ρ b such that µ o (ρ b ) = µ o (ρ v b )
, where ρ v b is the density of the vapor mother bulk bounding the layer. We must emphasis that P (ρ b ) and P (ρ v b ) are unequal as for drop or bubble bulk pressures. Density ρ b is not a fluid density in the interlayer but density in the liquid bulk from which the interface layer can extend (this is the reason why Derjaguin used the term mother liquid , 17 page 32 ). In the interlayer

λ d 2 ρ dz 2 = µ b (ρ), with µ b (ρ) = µ o (ρ) -µ o (ρ b ) (9)

Boundary conditions

Condition at the solid wall (S) is associated with Eq. (3) 15 :

λ dρ dn |S + φ ′ (ρ) |S = 0, ( 10
)
where n is the external normal direction to the fluid; Eq. ( 10) yields

λ dρ dz |z=0 = -γ 1 + γ 2 ρ |z=0 .
Condition at the liquid-vapor interface (Σ) is associated with Eq. ( 4):

λ dρ dz | z=h = -γ 4 ρ | z=h . (11)
Eq. ( 11) defines the film thickness by introducing a reference point inside the liquid-vapor interface bordering the liquid interlayer with a convenient density at z = h. 13 We must also add the classical surface conditions on the stress vector associated with the total stress tensor σ + σ v to these conditions on density.

The disjoining pressure for horizontal liquid films

We consider fluids and solids at a given temperature θ. The hydrostatic pressure in a thin liquid interlayer included between a solid wall and a vapor bulk differs from the pressure in the contiguous liquid phase. At equilibrium, the additional pressure interlayer is called the disjoining pressure. [START_REF] Derjaguin | Surfaces forces[END_REF] The measure of a disjoining pressure is either the additional pressure on the surface or the drop in the pressure within the mother bulks that produce the interlayer. The disjoining pressure is equal to the difference between the pressure P v b on the interfacial surface (pressure of the vapor mother bulk of density ρ v b ) and the pressure P b in the liquid mother bulk (density ρ b ) from which the interlayer extends :

Π(h) = P v b -P b . If g b (ρ) = g o (ρ) -g o (ρ b ) -µ o (ρ b )(ρ -ρ b ) denotes the primitive of µ b (ρ) null
for ρ b , we get from Eq. ( 8)

Π(ρ b ) = -g b (ρ v b ), (12)
and an integration of Eq. ( 9) yields

λ 2 dρ dz 2 = g b (ρ) + Π(ρ b ). (13)
The reference chemical potential linearized near ρ l (respectively ρ v ) is

µ o (ρ) = c 2 l ρ l (ρ-ρ l ) (respectively µ o (ρ) = c 2 v ρ v (ρ-ρ v ) )
where c l (respectively c v ) is the isothermal sound velocity in liquid bulk ρ l (respectively vapor bulk ρ v ) at temperature θ. [START_REF] Gouin | [END_REF] In the liquid and vapor parts of the liquid-vapor film, Eq. ( 9) yields

λ d 2 ρ dz 2 = c 2 l ρ l (ρ -ρ b ) (liquid) and λ d 2 ρ dz 2 = c 2 v ρ v (ρ -ρ v b ) (vapor).
The values of µ o (ρ) are equal for the mother densities ρ v b and ρ b ,

c 2 l ρ l (ρ b -ρ l ) = µ o (ρ b ) = µ o (ρ v b ) = c 2 v ρ v (ρ v b -ρ v )
, and consequently,

ρ v b = ρ v 1 + c 2 l c 2 v (ρ b -ρ l ) ρ l .
In liquid and vapor parts of the liquid-vapor interlayer we have,

g o (ρ) = c 2 l 2ρ l (ρ -ρ l ) 2 (liquid) and g o (ρ) = c 2 v 2ρ v (ρ -ρ v ) 2 (vapor).
From definition of g b (ρ) and Eq. ( 12) we deduce the disjoining pressure

Π(ρ b ) = c 2 l 2ρ l (ρ l -ρ b ) ρ l + ρ b -ρ v 2 + c 2 l c 2 v (ρ b -ρ l ) ρ l . ( 14
) Due to ρ v 2 + c 2 l c 2 v (ρ b -ρ l ) ρ l ≪ ρ l + ρ b , we get Π(ρ b ) ≈ c 2 l 2ρ l (ρ 2 l -ρ 2 b
). Now, we consider a film of thickness h; the density profile in the liquid part of the liquid-vapor film is solution of system :

     λ d 2 ρ dz 2 = c 2 l ρ l (ρ -ρ b ) (S1) with λ dρ dz |z=0 = -γ 1 + γ 2 ρ |z=0 and λ dρ dz | z=h = -γ 4 ρ | z=h .
Quantity τ is defined such that τ = c l / λρ l , where 1/τ is a reference length and γ 3 = λτ . Solution of system (S1) is

ρ = ρ b + ρ 1 e -τ z + ρ 2 e τ z ( 15
)
where boundary conditions at z = 0 and h yield the values of ρ 1 and ρ 2 :

(γ 2 + γ 3 )ρ 1 + (γ 2 -γ 3 )ρ 2 = γ 1 -γ 2 ρ b (S2) -e -hτ (γ 3 -γ 4 )ρ 1 + e hτ (γ 3 + γ 4 )ρ 2 = -γ 4 ρ b .
The liquid density profile is a consequence of Eq. ( 15) when z ∈ [0, h]. Taking Eq. ( 15) into account in Eq. ( 13) and g b (ρ) = (c 2 l /2 ρ l )(ρ -ρ b ) 2 in linearized form for the liquid part of the interlayer, we get

Π(ρ b ) = - 2 c 2 l ρ l ρ 1 ρ 2 . ( 16
)
By identification of expressions ( 14), ( 16) and using (S2), we get a relation between h and ρ b . We denote finally the disjoining pressure by Π(h). Due to the fact that ρ b ≃ ρ l , 17 the disjoining pressure reduces to

Π(h) = 2 c 2 l ρ l (γ 1 -γ 2 ρ l )(γ 3 + γ 4 )e hτ + (γ 2 -γ 3 )γ 4 ρ l × (γ 2 + γ 3 )γ 4 ρ l -(γ 1 -γ 2 ρ l )(γ 3 -γ 4 )e -hτ [(γ 2 + γ 3 )(γ 3 + γ 4 )e hτ + (γ 3 -γ 4 )(γ 2 -γ 3 )e -hτ ] 2 .
Let us notice an important property of mixture of van der Waals fluid and perfect gas where the total pressure is the sum of partial pressures of components: 18 at equilibrium, the partial pressure of the perfect gas is constant through the liquid-vapor-gas interlayer -where the perfect gas is dissolved in the liquid. The disjoining pressure of the mixture is the same than for a single van der Waals fluid and calculations and results are identical to those previously obtained.

Motions along a liquid nanolayer

When the liquid layer thickness is small with respect to transverse dimensions of the wall, it is possible to simplify the Navier-Stokes equation which governs the flow of a classical viscous fluid in the approximation of lubrication. [START_REF] Batchelor | An Introduction to Fluid Dynamics[END_REF] When h ≪ L, where L is the wall transversal characteristic size, i) the velocity component along the wall is large with respect to the normal velocity component which can be neglected ; ii) the velocity vector varies mainly along the direction orthogonal to the wall and it is possible to neglect velocity derivatives with respect to coordinates along the wall compared to the normal derivative ; iii) the pressure is constant in the direction normal to the wall. It is possible to neglect the inertial term when Re ≪ L/h (Re is the Reynolds number of the flow). Equation of Navier-Stokes is not valid in a liquid nanolayer because the fluid is strongly inhomogeneous and the elastic stress tensor is not scalar. However, it is possible to adapt the approximation of lubrication for viscous flows in a liquid nanolayer. We are in the case of long wave approximation: ǫ = h/L ≪ 1. We denote the velocity by V = (u, v, w) where (u, v) are the tangential components. In the approximation of lubrication we have : e = sup (|w/u| , |w/v|) ≪ 1. The main parts of terms associated with second derivatives of liquid velocity components correspond to ∂ 2 u/∂z 2 and ∂ 2 v/∂z 2 . The density is constant along each stream line ( ρ = 0 ⇐⇒ divV = 0) and iso-density surfaces contain the trajectories. Then, ∂u/∂x, ∂v/∂y and ∂w/∂z have the same order of magnitude and ǫ ∼ e. As in Rocard model, we assume that the kinematic viscosity coefficient ν = κ 2 /ρ depends only on the temperature. [START_REF] Rocard | Thermodynamique[END_REF] In motion equation, the viscosity term is (1/ρ) div

σ v = 2ν [ div D + D grad { Ln (2 κ 2 )} ] ; D grad{Ln (2 κ 2 )
} is negligible with respect to div D. In both lubrication and long wave approximations the liquid nanolayer motion verifies

Γ + grad[ µ o (ρ) -λ ∆ρ ] = ν ∆V with ∆V ≃ ∂ 2 u ∂z 2 , ∂ 2 v ∂z 2 , 0 (17)
In approximation of lubrication, the inertial term is neglected and Eq. ( 17) separates into tangential and normal components to the solid wall. As in equilibrium, the normal component of Eq. ( 17) is

∂ ∂z [ µ o (ρ) -λ ∆ρ ] = 0 ⇒ µ o (ρ) -λ ∆ρ = µ o (ρ b ).
To each value ρ b (different of liquid bulk density value ρ l of the plane interface at equilibrium) is associated a liquid nanolayer thickness h. We can write µ o (ρ) -λ ∆ρ = η(h), where η is such that η(h) = µ o (ρ b ). For one-dimensional motions colinear to the solid wall (direction i o and velocity u i o ), the tangential component of Eq. ( 17) yields :

grad µ o (ρ b ) = ν ∂ 2 u ∂z 2 i o ⇐⇒ ∂µ o ∂ρ b ∂ρ b ∂x = ν ∂ 2 u ∂z 2 . (18)
A liquid can slip on a solid wall only at a molecular level. [START_REF] Churaev | [END_REF] The sizes of solid walls are several orders of magnitude higher than slipping distances which are negligible and kinematic condition at solid walls is the adherence condition (z = 0 ⇒ u = 0). From the continuity of fluid tangential stresses through a liquid-vapor interface of molecular size and assuming that vapor

) 19
where χ b = ρ b ν is the liquid kinetic viscosity. Eq. ( 19) yields the mean spatial velocity of the isothermal liquid nanolayer as a function of the disjoining pressure gradient. Like as the disjoining pressure depends on the nanolayer thickness, the mean flow velocity is a function of thickness variations along the flow. Taking into account that in the liquid nanolayer ρ ≃ ρ b , then Thanks to Eq. ( 19) an equation for h-perturbations is :

∂h ∂t + h 3 χ b ∂ ∂x h 2 ∂ ∂x Π(h) = 0. ( 20
)
Eq. ( 20) is an equation of diffusion in parabolic structure with a good sign of diffusion coefficient associated with stability when ∂Π(h) ∂h < 0. 17

∂ρ b ∂ρ b ∂x 1 2 z 2 -

 2 viscosity stresses are negligible, we obtain (z = h, ⇒ ∂u ∂z = 0). Consequently Eq. (18) implies ν u = ∂µ o h z . The mean spatial velocity u of the liquid in the nanolayer is u = 1 h h o u dz; previous computations yield ν u = -h 2 3 grad µ o (ρ b ) with u = u i o . Let us remark that ∂µ o (ρ b ) The pressure P v b in the vapor bulk is constant along flow motions and Π(h) = P v b -P (ρ b ); consequently, we get ∂µ o (ρ b )

  and the mean spatial velocity corresponds also to the mean velocity with respect to the mass density. In shallow water approximation, the equation of continuity yields Eq. (14) in ref. ( 21 ) associated with h-perturbations : ∂h ∂t + h divu = 0.

Acknowledgments

This paper supported by PRIN 2005 (Nonlinear Propagation and Stability in Thermodynamical Processes of Continuous Media) is dedicated to Prof. Tommaso Ruggeri.