Leroux Mk

Graillon N Md

Emeric Carbonnel
email: emeric.carbo@gmail.com

TIPS & TRICKS FOR TRANSORAL APPROACH FOR SUBMANDIBULAR LITHIASIS (TASL)

Keywords: submandibular gland, salivary lithiasis, minimal invasive surgery

Introduction : Sialolithiases mainly affect the submandibular gland. More often, the lithiasis is large and located at the junction of the middle and the posterior third of the duct, in the hilum region. In such situation, transoral approach is recommended to avoid sialadenectomy because of its lower morbidity. Tips and Tricks : Because of our experience with over 300 cases operated with this transoral approach, we have decided to describe the tips and tricks that can help the surgeon who operates large stones impacted in the hilum of the submandibulary gland. Discussion : the benefits of these tips and tricks are exposed. Conclusion : These keypoints can help to overcome intraoperative issues and save time.

INTRODUCTION

The salivary glands can be affected by multiple pathologies such as autoimmune and inflammatory diseases, neoplastic diseases, and sialolithiasis. Among the main salivary gland pathologies, sialolithiasis is the most frequent. In most cases, the submandibular glands are affected [START_REF] Sigismund | Nearly 3,000 salivary stones: some clinical and epidemiologic aspects[END_REF].

The rise of minimally invasive surgery for salivary lithiases in the 1990s was possible with the development of the sialendoscope, primarily the lithotripter. Transoral techniques were similarly modified and codified, such as in the combined approach [START_REF] Foletti | Salivary Calculi Removal by Minimally Invasive Techniques: A Decision Tree Based on the Diameter of the Calculi and Their Position in the Excretory Duct[END_REF][START_REF] Capaccio | Conservative transoral removal of hilar submandibular salivary calculi[END_REF][6]. According to the GTD classification, the management of lithiases of the submandibular glands is based on three criteria: the gland involved, topography of the lithiasis, and diameter of the lithiasis, as Foletti et al. published in the GTD classification [START_REF] Mcgurk | Surgical release of a stone from the hilum of the submandibular gland: a technique note[END_REF]. .

The transoral approach is recommended for palpable impacted large lithiasis (diameter > 8 mm) situated in the posterior third of Wharton's duct. Using the GTD classification, we observed that many lithiases are S-P-LI lithiases, which are submandibular lithiases over 8 mm in diameter (large and impacted) situated in the posterior third of Wharton's duct. Such lithiases are better operated with a transoral combined removal, also called the transoral approach for submandibular lithiases (TASL) [START_REF] Mcgurk | E :Intra-oral removal of stones from the hilum of the submandibular gland: report of technique and morbidity[END_REF] S-P-LI are always quite intra-orally palpable.

TASL is now performed by multiple teams [START_REF] Mcgurk | E :Intra-oral removal of stones from the hilum of the submandibular gland: report of technique and morbidity[END_REF][START_REF] Foletti | Salivary Calculi Removal by Minimally Invasive Techniques: A Decision Tree Based on the Diameter of the Calculi and Their Position in the Excretory Duct[END_REF][START_REF] Capaccio | Conservative transoral removal of hilar submandibular salivary calculi[END_REF][6]. TASL is minimally invasive, repeatable, allows functional recovery of the gland after the obstruction's removal, and minimizes scarring [START_REF] Benazzou | Transoral removal of submandibular hilar calculi[END_REF][START_REF] Park | Factors influencing intraoral removal of submandibular calculi[END_REF], even for large lithiases [START_REF] Makdissi | Glandular function after intraoral removal of salivary calculi from the hilum of the submandibular gland[END_REF].

We have performed this intervention in our department for approximately 10 years and more than 300 patients have undergone TASL. This surgery can be challenging for the practitioner.

The goal of this article is to detail certain tips and tricks of this intervention enabling the facilitation and shortening of the procedure for the oral and maxillofacial or ENT surgeon who performs it.

TECHNICAL NOTES: TIPS AND TRICKS

1-Preoperative assessment

Clinical examination is essential. The indication for this surgery is based on the fact that the lithiasis must be palpable. We also recommend the patient undergo an unenhanced CT or a CBCT scan before the procedure.

2-Installation in the operating room (Fig 1)

This step is very important.

The patient is placed in the dorsal decubitus position. Unlike other intra-oral operations, the head must be fixed so the chin can rest on the patient's chest. The head will also be fastened onto the table with medical adhesive. This position improves the exposure and facilitates access to posterior third lithiases.

3-Anesthesia

Nasotracheal intubation allows the oral area to be free, facilitating the intra-oral procedure.

In patients with precarious health, the procedure can be performed under local anesthesia combined with sedation.

4-Surgical technique

The surgical technique is performed as described by McGurk [START_REF] Foletti | Salivary Calculi Removal by Minimally Invasive Techniques: A Decision Tree Based on the Diameter of the Calculi and Their Position in the Excretory Duct[END_REF], with few improvements.

The buccal floor is infiltrated under the mucosa with saline solution with epinephrine 1% (0.25 mg in 20 cc).

We The assistant surgical aid must push up the submandibular gland to largely facilitate the exposure of the hilum in the mouth.

An incision is made on the lingual side of the salivary crest, medially to the sublingual gland.

It must run to the level of the wisdom tooth, posteriorly enough.

Then the mucosal plane is incised and some lingual veins can be coagulated in the anterior portion of the mouth floor. The medial portion of the sublingual gland is followed outside the sublingual aponeurosis. Then Wharton's duct is easily visible, especially in its anterior third because at this level, Wharton's duct is superficial. Once Wharton's duct is isolated, a silicon loop (Ethicon Ethiloop Ref EH384) is placed around it to apply a slight traction to the natural axis of the duct (Fig 3). Wharton's duct is then followed to the lithiasis level. The lithiasis can be palpated again to check its level between the first and third molar in the posterior duct third. Then Wharton's duct is opened on its top (at 12 o'clock) with the help of a n°12 blade from back to forth while maintaining contact with the lithiasis (Fig 4). At this step, the lingual nerve is in contact only with the hilum of the gland crossing it.

The lithiasis is gently removed from the duct using scissors while maintaining contact with the lithiasis. Brugirard curettes sometimes can assist.

A post-removal hilum check is carefully performed with angulated curettes to control the hilum vacuity. Then the two anterior thirds of the duct must be checked to control their permeability. This can be easily accomplished via retrograde irrigation using a bent ear cannula (Fig 5). If this is not feasible, retrograde irrigation can be replaced by anterograde irrigation through Wharton's duct papilla with the plastic part of a 20-gauge cathlon inserted on a 0000 probe passed through the papilla. When feasible, the Wharton's duct hilar incision is sutured with 1 or 2 4/0 silk thread stitches. The mouth floor is then sutured with 3 or 4 absorbable stitches.

5-Follow-up

A postoperative check-up is scheduled at 1 month and then 1 year postoperatively due to the risk of recurrence of submandibular lithiases (5% to 10% at 5 years).

However, we prefer to follow the patient and prescribe computed tomography only in rare cases with symptoms of salivary gland obstruction. If the patient is impaired and if the CT shows new lithiasis, a second surgery can be performed. After 3 of more TASL procedures, a submandibulectomy is recommended, but this is rare (2/300 patients). To prevent lithiasis recurrences, we recommend the patient undergo postprandial salivary gland pressure or massages from back to forth, particularly patients with submandibular gland meal-time tenderness. These massages can last for 30 seconds only on the concerned gland.

DISCUSSION

Preoperative assessment. We recommend the patient undergo a computed tomography (CT) scan because this is the most efficient exam for the preoperative diagnosis of sialolithasis (100% sensitivity, 75% specificity, 99% positive predictive value, and 100% negative predictive value [START_REF] Gardon | CT scan assessment in salivary gland lithiasis diagnosis[END_REF]). CBCT, which is interesting in terms of X-ray dose reduction and reduced metallic artefacts, can also be done, even if it provides less information regarding the soft tissues 10 . Multi-planar reconstructions allow the precise assessment of the topography of the lithiasis (extra-glandular, which is frequent, or intra-glandular, which is much rarer).

CT also assesses the opposite side: multiple sialolithiases is a condition that requires more aggressive management, that is, the removal of all of the lithiases in one session, which is the gold standard in salivary gland surgery.

Installation in the operating room. The exposition is essential to obtain the best visibility of the posterior lithiases. This is why we use this installation and instrumentation that minimally hinders the surgeon's view. It avoids the lower incisor teeth that could obstruct the view of the mouth floor. This position also provides soft tissue laxity that is needed to push the submandibular gland upward into the oral cavity.

Anesthesia. We advise performing this intervention under general anesthesia, both for the patient and practitioner's ease, especially for untrained surgeons or for frequent posterior lithiasis topography. The surgeon must differentiate between easily accessible anterior lithiases and posterior lithiases that require a posterior dissection, which is unacceptable for patients under local anesthesia due to the gag reflex and because this surgery is sometimes painful (when the assistant hand pushes the gland upward).

Surgical technique. We recommend infiltrating the buccal floor with serum solution/epinephrine and not lidocaine, as it causes bleeding.

The traction with the silicon loop facilitates the duct incision in the mouth floor to the hilum.

The hand of the second aid that pushes the submandibular gland into the oral cavity is called the "hand of god" because without it, the posterior lithiasis cannot be removed. The second surgical aid can put a wrist on the clavicular bone for strong support that helps to push hard onto the gland. An incision is made on the lingual side of the salivary crest, medial to the sublingual gland to avoid damage to the sublingual gland that can lead to iatrogenic ranulas.

Wharton's duct incision is made on the top (at 12 o'clock) of the duct so that the saliva can stay inside the duct as much as possible. The calculi are gently removed from the duct to avoid fragmentation and residual calculi. The retrograde irrigation ensures ductal vacuity and permeability after lithiasis removal. The closure is made with silk thread stitches because their non-inflammatory nature allows the avoidance of any risk of ductal postoperative stenosis.

Follow-up. We do not recommend postoperative systematic ultrasound because the results can stress the patient and postoperative US is sometimes difficult to interpret. There is no proof of massages' efficiency or candy sucking to avoid submandibular lithiasis recurrence but these simple techniques should logically prevent salivary stasis and the neoformation of lithiasis.

While TASL is now a codified and reliable technique for palpable lithiasis of the hilum of Wharton's duct, it can be challenging for practitioners who are not used to it.

Following the aforementioned tips and tricks, the procedure can be easier and faster for the ENT or maxillofacial surgeon.

Caption to illustrations :

  use a Doyen retractor (COLLIN Ref AT3209) to open the mouth because it leaves more

Fig 1 .

 1 Fig 1. Installation: nasotracheal intubation inclined upward, head fastened, and chin on the

Fig 2 .Fig 3 .

 23 Fig 2. The Doyen retractor is used to open the mouth.

Fig 4 .

 4 Fig 4. Wharton's duct incision on the top (at 12 o'clock) of the duct with an n°12 blade (SML

Fig 5 .

 5 Fig 5. Control of Wharton's duct permeability (emptiness). Saline solution injected into the

This work did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.


S Garel

J Ngo Boum-Nkot

Etame

Bertil Nlend
email: nlendbertil@yahoo.fr

H Celle-Jeanton

F. Huneau b.c , E. Garel b.c S Ngo Boum-Nkot

J Etame

Shallow urban aquifers under hyper-recharge equatorial conditions and strong anthropogenic constrains. Implications in terms of groundwater resources potential and integrated water resources management strategies

Keywords: urban hydrogeology, groundwater recharge, stable isotopes, geochemistry, integrated water resources management, threshold values

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

Introduction

While many works have been published on recharge, functioning and management of aquifers in arid or semi-arid areas (e.g., [START_REF] Sami | Recharge mechanisms and geochemical processes in a semi-arid sedimentary basin, Eastern Cape, South Africa[END_REF][START_REF] Allison | Vadose-Zone Techniques for Estimating Groundwater Recharge in Arid and Semiarid Region[END_REF][START_REF] Scanlon | Global synthesis of groundwater recharge in semiarid and arid regions[END_REF][START_REF] Rahmani | Groundwater recharge estimation in semi-arid zone: a study case from the region of Djelfa (Algeria)[END_REF], there is still very little knowledge about groundwater functioning in humid tropical regions and papers on humid tropics hydrogeology remain rare; mainly focusing on water quality and on the economic and environmental significance of this resource [START_REF] Foster | Groundwater conditions and problems characteristic of the humid tropics Hydrology of Warm Humid Regions[END_REF][START_REF] Foster | Urban groundwater use in tropical Africa -a key factor in enhancing water security?[END_REF][START_REF] Nlend | The impact of urban development on aquifers in large coastal cities of West Africa: Present status and future challenges[END_REF]. [START_REF] Fosberg | Delimitation of the Humid Tropics[END_REF] physically identify these regions by: (i) a monthly temperature equaling or exceeding 20°C for at least eight months of the year; (ii) a vapor pressure and a relative humidity average of ~20 millibars and 65% respectively, for at least 6 months of the year; (iii) an annual rainfall exceeding 1500 mm and;

(iv) the rain falling all year with at least six months having precipitation ≥75mm/month. They represent approximately 22% of the earth's surface (UNESCO 1990(UNESCO , 2000; State of the tropics 2014) and house a lot of developing countries which present the highest demographic growth rates and whose population is expected to represent 50% of humanity by 2050 (State of the tropics 2014). [START_REF] Wohl | The hydrology of the humid tropics[END_REF] reported then that humid tropics are dynamic environments where freshwater is under pressure from population growth, land use and climate change. In addition to their importance for the water supply, these hydro-systems are a key factor in the preservation of humid tropical ecosystems (animal-plant-soil-water), because of the intimate relationship between surface and groundwater and the frequently shallow groundwater table with abundant humid equatorial vegetation in such environments [START_REF] Foster | Groundwater systems in the humid tropics[END_REF]. Regarding all these aspects, there is an urgent need to increase the knowledge in aquifers functioning in humid tropical regions and peculiarly in sub-Saharan African areas which represent 30% of the humid tropics (45% in latin America and 25% in Asia) -according to State of the tropics (2014) -; and is often regarded as the world's fastest urbanizing region [START_REF] Saghir | Urbanization in Sub-Saharan Africa[END_REF].

In this objective, the Douala megacity (Cameroon, Western Central Africa) is a perfect example of a tropical area with approximately 4000 mm/yr, 230 rainy days/yr, a mean air temperature of 27°C [START_REF] Tchiadeu | La ville de douala : entre baisse des précipitations et hausse des températures. 25ème Colloque de l[END_REF]) and a rapid urbanization process due to a demographic boom that has begun in the early 80's [START_REF] Nlend | The impact of urban development on aquifers in large coastal cities of West Africa: Present status and future challenges[END_REF]. Douala is the economic capital of Cameroon and the most dynamic town in the Central Africa region by ensuring, for example, the transit of goods from the rest of the world to landlocked countries like the Central African Republic and Chad). The demographic dynamism of Douala is expressed by a large movement of people from the countryside to the coastal cities due to economic reasons. Indeed, this metropolis, the largest city in Cameroon with a big harbour, is highly developed with varied industries (textiles, agro-food, chemicals, glassworks, cement, wood, fertilizers, etc.). In this large city, 49% of the drinking water is produced from subsurface by the Cameroon Water utilities (Camwater) and the Mio-Pliocene (MP) sands aquifer represents the most exploited resource. This latter is tapped both formally by Camwater and informally by many industries and people for domestic uses and drinking water supply purposes [START_REF] Mafany | Impact of the geology and seawater intrusion on groundwater quality in Douala[END_REF]. However, the anthropogenic fingerprint at Douala has already showed harmful effects on local water resources as it experienced cholera epidemics in 2004 (Guevart et al. 2006) and 2012 (Ketchemen-Tandia & Banton 2012), making access to clean water a major priority for populations.

Regarding these important issues, this article follows the previous studies which have mainly dwell long on groundwater quality [START_REF] Takem | Acidification of shallow groundwater in the unconfined sandy aquifer of the city of Douala. Cameroon. Western Africa: implications for groundwater quality and use[END_REF][START_REF] Kopa | Hydrodynamique et qualité des eaux souterraines dans le bassin sédimentaire de Douala (Cameroun): Cas des aquifères sur formations quaternaires et tertiaires[END_REF][START_REF] Ngo Boum | Origin of mineralization of groungwater in the Bassa watershed (Douala-Cameroon)[END_REF][START_REF] Fantong | Variation of hydrogeochemical characteristics of water in surface flows, shallow wells and boreholes in the coastal city of Douala (Cameroon)[END_REF][START_REF] Tatou | Multivariate Statistical Analysis for the Assessment of Hydrogeochemistry of Groundwater in Upper Kambo Watershed (Douala Cameroon)[END_REF][START_REF] Ketchemen-Tandia | Factors Influencing the Shallow Groundwater Quality in Four Districts with Different Characteristics in Urban Area (Douala, Cameroon)[END_REF][START_REF] Wirmvem | Effect of diffuse recharge and wastewater on groundwater contamination in Douala Cameroon[END_REF] but the hydrogeological functioning of the MP aquifer is still very little known.

Therefore, this research is a first attempt to conceptualize the functioning (recharge, infiltration, groundwater flow organization and acquisition of mineralization) of the main groundwater resource of the city which is the MP aquifer in a context of high rainfall regime by using an innovating combination of hydrogeological, isotopic and chemical methods coming both from historical data collection and new investigations over the study area. Beside the hydrogeological and water quality analysis, the paper aims at determining the natural background levels (NBLs) and proposes threshold values (TVs) of groundwater chemical components of the region in order to propose management tools able to provide guidance for the future exploitation of groundwater resources. Such type of investigation can be used as a reference for urban porous aquifers in humid tropical areas and in the establishment of improved integrated water management strategies in Africa.

2.

Framework of the study region

Location and topography

The Douala region (03°45'N -04°20'N, 09°34'E -10°00'E) is a coastal site (~ 33 km to the ocean) located approximately 70 km southwest to the Cameroon Mount (~ 4100 masl), covering an area of more than 300 km² on the Gulf of Guinea, in the Republic of Cameroon (Fig. 1).

Figure 1 The relief is little contrasted with altitudes varying between ~3 masl on the estuary zone to about 70 masl in the eastern side of the region (Figure 2).

Figure 2

Climate

The climate of Douala is characterized by strong marine influences [START_REF] Olivry | Fleuves et rivières du Cameroun. Paris: Collection "Monographies hydrologiques ORSTOM[END_REF]). The annual rainfall regime is unimodal with a long rainy season from March to November and a very short "dry season" in December-February (Fig. 3). Considering the 65 years of rainfall recorded at Douala meteorological station (04°01'N, 09°42'E), the annual average is 3854.3 mm, the maximum and minimum monthly values are respectively measured in August (742.4 mm) and December (34 mm). These very rainy conditions throughout the year make Douala to be considered as a singular hyper-rainy city.

Figure 3 The peculiar climate of Douala is also linked to the relative uniformity of air temperature and very high atmospheric humidity conditions. The mean annual temperature is 27°C; lowest and highest temperatures are observed respectively in August (25.4 °C) and February (28.6 °C), giving low annual thermal amplitude of 3.2 °C (Fig. 3). The relative humidity (records of 1971-2009) ranges between 79% and 90% during the rainy season (March to November) and between 77% and 81% during the dry season (December to February), with an annual mean of 85%.

Hydrography

The wet conditions described above are reflected at the surface by a very dense hydrographic network (Fig. 2) confirming the observations of [START_REF] Braune | The Role of Ground Water in Sub-Saharan Africa Ground Water[END_REF] in this part of Africa.

Apart from the swamps that cover lowland areas, two main watercourses drain the study area:

the Wouri and the Dibamba Rivers which present many tributaries (Fig. 1 & 2). Both the Wouri and the Dibamba dendritic drainage basins discharge into the Atlantic Ocean where tides entering the estuaries make them vulnerable to seawater intrusion [START_REF] Onguene | Overview of Tide Characteristics in Cameroon Coastal Areas Using Recent Observations[END_REF].

The hydrological regime of these rivers is similar to that of rainfall. Low flow are registered from November to December (discharge <200 m 3 /s) and then increase slowly up to June and abruptly up to September when it reaches a maximum value of 900 m 3 /s [START_REF] Olivry | Fleuves et rivières du Cameroun. Paris: Collection "Monographies hydrologiques ORSTOM[END_REF]. At this moment, the soil often becomes saturated and the streams overflow their channels causing floods favoured by the low altitude of the area. Then, from October discharge is decreasing.

Soils and vegetation

Douala has two types of soils: ferralitic and hydromorphic [START_REF] Segalen | Les sols et géomorphologie du Cameroun[END_REF][START_REF] Zogning | Les formations superficielles latéritiques dans la région de Douala : morphologie générale et sensibilité aux activités humaines[END_REF]Ndome 2010). In both cases, the authors identified a large proportion of fine and coarse sands (45 to 80%), and clays (10 to 50%). The proportions of kaolinite (50-60%) are the most important while goethite (35-42%) is significant and gibbsite (2-10%) is negligible (Ngeutchoua 1996). Ferralitic and hydromorphic soils occupy respectively 80% and 20% of the study region. The latter is mainly observed in alluvial plains and valley bottoms.

The vegetation is that of low altitude rainforest with large mangroves along the coast, evergreen forest and some plantations (banana, corn, sugar cane, fruits, etc). Riparian vegetation is observed near the valleys and depression areas (Din Ndongo 2001).

Geology and hydrogeology: emphasis on Mio-Pliocene formations

The Douala region is constructed on a 5 km-thick sequence of unconsolidated to semiconsolidated sedimentary rocks going from Pleistocene to Aptian (Fig. 4; [START_REF] Regnoult | Synthèse Géologique du Cameroun DMG[END_REF]).

These sediments are associated to tectono-sedimentary dynamics which prevailed during the opening of the South Atlantic and the formation of West African passive margin [START_REF] Moulin | Etude géologique et géophysique dees marges continentales passives : exemples du Zaïre et de l'Angola[END_REF][START_REF] Helm | Quantification des flux sédimentaires anciens à l'échelle d'un continent : le cas de l'Afrique au Méso-Cénozoïque[END_REF]. Continental in the eastern part of the basin, sediments progressively move westward to marginal sea, continental shelf and deep seabed environments. They can be divided into seven formations from the bottom to the top (Fig. 4): i) Mundeck's conglomerates and sandstones (Apto-Albian), ii) Logbadjeck's conglomerates, sandstones and limestones (Turonian-Santonian), iii) Logbaba's sands, sandstones and clays (Campanian-Maastrichtian), iv) Nkapa's limestone, sandstone, marl and shales (Paleocene-Eocene), v)

Souellaba's clays and sands (Oligocene-Lower Miocene), vi) Matanda's clays, sands and gravels (Upper Miocene-Pliocene) and vii) Wouri layers (Quaternary) consisting of clays, sands and gravels [START_REF] Dumort | Notice explicative sur la feuille Douala-Ouest carte géologique de reconnaissance (1/500000)[END_REF][START_REF] Ngaha | Contribution à l'étude géologique, stratigraphique et structurale de la bordure du basin atlantique du Cameroun[END_REF][START_REF] Regnoult | Synthèse Géologique du Cameroun DMG[END_REF][START_REF] Nguene | Paleoenvironment of the Douala and Kribi / Campo sub basins in Cameroon West Africa[END_REF]SNH/UD 2005;[START_REF] Manga | Stratigraphy, structure and prospectivity of the Southern onshore Douala Basin, Cameroon-Central Africa[END_REF]Njoh et al. 2014;[START_REF] Njoh | Preliminary investigation of Late Turonian-Early Campanian shallow marine foraminifera of the Mungo River/Logbadjeck Formation. NW Douala Basin. Cameroon[END_REF][START_REF] Ngaha | Paleogeographic Evolution of the Eastern Edge of the Douala Basin from Early Cenomanian to Turonian[END_REF].

Figure 4 The Mio-Pliocene aquifer is represented by Souellaba and Matanda formations, overlying the Eo-Oligocene and covered in some places by the Quaternary sediments 20 to 70 m thick [START_REF] Feumba | Hydrogéologie et évaluation de la vulnérabilité des nappes dans le bassin versant de Bessekè (Douala-Cameroun)[END_REF]. It consists in a multi-layered aquifer separated from more superficial and deeper aquifers by discontinuous clay layers. Its thickness globally increases NE to SW (Fig. 4). Although many lithological lateral variations, the global structure of the MP aquifer from the top to the bottom can be described as follows (Table 1) by using well logs and previous works on hydrogeology [START_REF] Bourgeois | Etude hydrogéologique d'orientation pour le captage d'eau minérale au Cameroun[END_REF] and Ketchemen-Tandia 2011):

(i) the Pliocene is clayey, clayey-sandy and sandy-clayey. Its depth generally does not exceed 40 m;

(ii) the Upper Miocene consisting of coarse grained sands with gravel intercalations.

Sandy sequences, slightly silty and calcareous, appear well developed at the NW of Douala where their thickness exceeds 200 m [START_REF] Bourgeois | Etude hydrogéologique d'orientation pour le captage d'eau minérale au Cameroun[END_REF];

(iii) the Lower Miocene formation made up of cross-bedded sands and marked by some passages of lignite and limestone.

Table 1 Separated by a clay layer, the difference between Mio-Pliocene and Quaternary formations lies in the lithology; Quaternary sands are finer than those of the Mio-Pliocene.

The lateral variations observed in lithology of the MP aquifer are also reflected in hydrodynamic settings (Table 2). High values of standard deviation (Table 2) in transmissivity and hydraulic conductivity suggest an important anisotropy of the MP aquifer.

Table 2 3. Methods of investigation

Inventory of sampling points

Wells logs and results from hydraulic testing, when available, have been compiled from previous projects in the study area. Indeed, priority has been given to wells having this basic information and which are still used currently by local populations. In addition to this inventory, a field campaign was conducted to target springs in the study area. Finally, a total of 58 sampling points, including 43 wells (26 boreholes and 17 hand-dug wells) and 15 springs on the MP formation have been selected according to their distribution over the Douala region. In general, hand-dug wells tap water at shallow levels (<18 m) while boreholes will correspond to water from more important depth (maximum = 90 m). Boreholes are often equipped with foot or hand pumps or directly connected to a faucet. The only disadvantage of this engineering structure in Douala is that they are most of the time screened at several depths. For some wells, this can complicate the understanding of groundwater flow and hydrochemical processes that take place within the different levels. Springs encountered in the study area are located at the bottom of steep slopes, bordering important marshy areas.

Field measurements, sampling and analytical procedures

Groundwater sampling and in situ measurements (water levels and physicochemical parameters) were achieved in February 2017. Springs have been sampled directly at their outflows. Wells and boreholes have been sampled by pumping, using the in-situ equipment.

Assuming the fact that wells in Douala are daily used for domestic supply, it was considered during the campaign that there is no stagnant water within wells and that their purge is already achieved. The depth of groundwater in the well relative to the soil surface i.e water table (h) was measured by using a potentiometric probe. Knowing the altitude (Z) of the sampling point, the water table was converted in potentiometric level (H) through the formula H = Z -h and by considering the height of the well edge. Physicochemical parameters (pH, electrical conductivity, water temperature and dissolved oxygen) were measured using a WTW 2FD46G multi-parameter. Measurements have been validated only after all of the in situ parameters reached the stability criteria [START_REF] Rey | Using water stable isotopes for tracing surface and groundwater flow systems in the Barlow-Ojibway Clay Belt, Quebec. Canada[END_REF] for three consecutive measurements separated by 5-minutes intervals. Alkalinity measurements were performed using a digital titrator (HACH). Samples for major ions analysis (Cl -, NO3 -, SO4 2-, HCO3 -, Na + , K + , Mg 2+ and Ca 2+ ) were collected in two 30 mL polyethylene bottles after filtration through 0.45 µm nitrocellulose membranes. Samples for cations analysis were acidified with ultrapure HNO3 - at 70%. Bottles were filled to the top with no headspace and kept at 4°C until analyses.

Chemical analyses were conducted at the Hydrogeology Department of the University of Corsica, France by ionic chromatography using a Dionex ICS 1100 chromatograph. The quality of the analysis was checked by calculating the ionic balance error: analyses were rejected if the ionic balance error was greater than 5%.

Samples for stable isotopes of the water molecule were stored in 20 mL amber glasses bottles.

All care was taken to avoid evaporation by taking water directly in the well. As for chemistry, bottles were filled to the top with no headspace and samples were kept cool until being transferred to the laboratory where measurements were performed at the Hydrogeology Department of the University of Corsica, France using a liquid-water stable isotope analyzer DLT-100 (Los Gatos Research) according to the analytical scheme recommended by the IAEA [START_REF] Penna | On the reproducibility and repeatability of laser absorption spectroscopy measurements for δ 2 H and δ 18 O isotopic analysis[END_REF]. Values are reported in per mil units (‰) compared to Vienna Standard Mean Ocean Water standard (VSMOW). The quality of the isotopic analysis was checked using a standard deviation up to 1‰ for δ 2 H and up to 0.1‰ for δ 18 O.

Results and discussion

Estimation of Mio-Pliocene aquifer recharge

In this paper, two methods were applied: a water balance and a physical method on saturated zone (Water Table Fluctuations, WTF). These approaches were chosen given the availability of required data. The choice of these methods was driven by the goal to obtain an average rate of recharge for the aquifer. Moreover, WTF and water balance methods are known to be well applied both in humid and semi-arid contexts requiring low data and at relative low costs [START_REF] Scanlon | Choosing appropriate techniques for quantifying groundwater recharge[END_REF]. The first one relying on groundwater data and the second one on climate data.

Water balance calculation

Recharge is usually calculated as the remainder when losses identified in the form of runoff and evaporation, have been deducted from precipitation (see equations 1 and 2). The term "evaporation" here envelops transpiration.

P = R + Ea + ∆SWS + Ieff

(1)

Ieff = P -(R + Ea + ∆SWS) (2)
P is the monthly rainfall, Ea the real evapotranspiration, R the surface and sub-surface drainage, and ΔSWS the storage variation in the unsaturated zone. The rainfall used in the balance is given by the measurements at the Douala meteorological station. Real evapotranspiration (Ea), described in detail by Allen et al (1991), was estimated from the corrected potential evapotranspiration (PETcor) and average Kc values per crop (Allen et al.

1998). PETcor was estimated by correction of the potential evapotranspiration calculated by

Thornthwaite's equation, as proposed by [START_REF] Medeiros | Error autocorrelation and linear regression for temperature-based evapotranspiration estimates improvement[END_REF]. All these annual average parameters can be considered as homogeneous spatially regarding the topography of the study area.

Therefore, taking into account the potential evapotranspiration, actual evapotranspiration and rainfall amount recorded month by month the yearlong at Douala; the Figure 5 shows that the MP aquifer is potentially recharged from June to November when P > PET and when the soil water deficit is filled. From December to February, P<PET and PET > Ea, then the recharge of soil reserve is null and the groundwater table is down. Indeed, the potential recharge follows the rainfall regime.

Figure 5

This potential recharge of 2556.5 mm corresponds to effective infiltration plus runoff.

However, for the study area, no experimentally measured surface drainage data were available for the analyzed period. The data used in this paper are from Emvoutou (2018) who used remote sensing and GIS approach by considering: land use/land cover, drainage density, topography and slopes for the 2000-2004 period. The average value obtained is estimated at 46.4% of annual rainfall amount. Thus, the infiltration can be estimated at 767 mm/year i.e approximately 20 % of rainfall amount.

WTF approach

Based on the premise that rises in groundwater levels in shallow aquifers are due to recharge water arriving at the water table and going immediately into storage (Healy & Cook 2002), the water table fluctuation method can be applied here for estimating groundwater recharge.

The recharge rate can be obtained by applying the following equation:

Ieff = Sy ∆ ∆ (3)
Eq. ( 3) assumes that water that reaches the saturated zone is immediately stored and the other components of the subsoil water balance (underground evapotranspiration, base flow, inflow and outflow) are zero during the period of recharge.

"Sy" is the specific yield, approximately equal to the storage coefficient. It is the volume of water released from a unit volume of saturated aquifer material (Maréchal et al. 2006). "h" is the water table height and "t" is time.

In this paper, empirical value of Sy provided by [START_REF] Johnson | Specifc yield: compilation of specific yields for various materials[END_REF] has been used, while values of "h" come from the monthly monitoring carried out by [START_REF] Feumba | Relationship between climate and groundwater recharge in the Besseke Watershed (Douala-Cameroon)[END_REF] in Douala. They reported that the lowest depth to water (2 m) is observed in August while the highest one is in December (9.8 m). Therefore, the fluctuation of water table is ranging between 2.00 and 9.80 m thus giving ∆h = 7.8 m.

By considering the nature of soils (and the upper levels of MP) mainly made of sandy clay materials, and if it supposed that the studied aquifer material is homogenous vertically and laterally, the specific yield of 7% defined by [START_REF] Johnson | Specifc yield: compilation of specific yields for various materials[END_REF] can be used here to determine the aquifer recharge according to the equation 3.

By applying the formula presented in equation 3, recharge is then calculated from January 2010 to December 2010 thanks to the monitoring carried out by [START_REF] Feumba | Relationship between climate and groundwater recharge in the Besseke Watershed (Douala-Cameroon)[END_REF]. An average value of 546 mm has been obtained. Since the annual rainfall amount recorded in 2010 is 3585.2 mm, the recharge coefficient (Ieff/P) is approximately equal to 15%.

The small gap between the two calculated values from water balance and WTF can be explained by:

(i) the evaluation of water level at the period of one year which is not sufficient. (ii) the application of WTF method at a monthly scale can underestimate the recharge. For instance, if there is a small amount of recharge that is less than drainage from the water Benin where there is also ferralitic soils [START_REF] Obuobie | Groundwater level monitoring and recharge estimation in the White Volta River basin of Ghana[END_REF][START_REF] Kotchoni | Relationships between rainfall and groundwater recharge in seasonally humid Benin: a comparative analysis of long-term hydrographs in sedimentary and crystalline aquifers[END_REF]). In the next section we will identify the recharge areas and discuss about the infiltration process and groundwater flow within the aquifer.

Location of recharge areas, infiltration process and analysis of groundwater flow

Location of recharge areas

The potentiometric map of the MP aquifer is presented on Fig. 6. We identify on this map, two piezometric domes in Ndogpassi -Nyalla (SE) and Kotto (NE) corresponding to most elevated points (66 and 50 m.a.s.l respectively) which can be considered as focused recharge areas of the MP aquifer at Douala. Indeed, the piezometric structure of the aquifer is in good agreement with the topography (Fig. 2) of the region as described in section 3.1. This feature is a characteristic of unconfined aquifer.

Figure 6

Infiltration process

Water stable isotopes (Fig. 7) show that rainwater infiltrates the soil without undergoing evaporation phenomenon. Following [START_REF] Barnes | Tracing of water movement in the unsaturated zone using stable isotopes of hydrogen and oxygen[END_REF] and [START_REF] Lu | Partitioning of evapotranspiration using a stable isotope technique in an arid and high temperature agricultural production system[END_REF] observations in wet soils, the zone in which significant evaporation occurs is narrow or inexistent. This is in agreement with the hyper-humidity conditions characterizing the study region. However, MP groundwater are ranging from -5.1‰ to -3.1‰ δ 18 O and -25.7‰ to -9‰ δ 2 H with mean values of -3.5 δ 18 O and -13 δ 2 H while the weighted mean value of rainfall corresponds to -2.8‰ δ 18 O and -12.2‰ δ 2 H. This is particularity well displayed on Fig. 6 where groundwaters plot above the Douala and Global MWL (Meteoric Water Line). Such result suggests a complexity in infiltration mechanisms. [START_REF] Craig | Isotopic variation in meteoric waters[END_REF]. In aquifer, d-excessmean = 14.8‰. Approximatively, the same value is observed both in shallower and deep water. D-excess value of MP groundwater is significantly higher than the one recorded in the recharge water (Fig. 8). However, it is similar to that recorded in rainfall at Bangui (13.5‰) and Kisangani (15‰; IAEA/WMO 2019) in the Congo basin (Fig. 8) where there is an influence of continental recycling (Ketchemen et al. 2011;[START_REF] Risi | Role of continental recycling in intraseasonal variations of continental moisture as deduced from model simulations and water vapor isotopic measurements[END_REF][START_REF] Nlend | Identifcation of processes that control the stable isotope composition of rainwater in the humid tropical West-Central Africa[END_REF]. Could this process occur within the vadose zone through the transpiration of plants roots? According to [START_REF] Blavoux | Etude du cycle de l'eau au moyen de l'oxygène-18 et du tritium[END_REF] and [START_REF] Barnes | Tracing of water movement in the unsaturated zone using stable isotopes of hydrogen and oxygen[END_REF], transpiration both removes some enriched water and causes a water flux at depth which is greater than the evaporative flux at the soil surface. Indeed plants transpiration favors root water transport from deeper to shallower soil layers, leading to isotopic depletion [START_REF] Lu | Partitioning of evapotranspiration using a stable isotope technique in an arid and high temperature agricultural production system[END_REF]. Therefore, the d-excess-shift observed between recharge water and groundwater in Douala can be attributed to plants transpiration within the unsaturated zone.

Soil water use by plants can be dominant when the soil water reserves are null (December-February) and even during the replenishment from March to May (Fig. 5). [START_REF] Barbeta | Relative contribution of groundwater to plant transpiration estimated with stable isotopes[END_REF] reported that this plant groundwater uptake represents on average 49 % of transpired water in dry seasons and 28 % in wet seasons worldwide. This hypothesis is supported by the common occurrence of deep roots which can exceed respectively 3.5 m and 1 m for woody and herbaceous species of the study region. In addition, according to [START_REF] Barbeta | Relative contribution of groundwater to plant transpiration estimated with stable isotopes[END_REF], as Douala is a flat area with landscape mostly represented by plain and riparian zones, the contribution of groundwater to transpiration must be particularly significant. The absence of significant difference between isotopic compositions of shallower and deep water (respectively hand dug-wells and boreholes, Fig. 7) may illustrate the influence of transpiration on shallow groundwater stable isotopes. The changes from rainwater to aquifer are likely a result of the lateral roots extracting water from the subsoil and redistribution to the superficial layer, The same observation was done by [START_REF] Lu | Partitioning of evapotranspiration using a stable isotope technique in an arid and high temperature agricultural production system[END_REF] thanks to experimentation employing customized transparent chambers coupled with a laserbased isotope analyzer measuring continuously near surface variations in the stable isotopic composition of evaporation (E, δE), transpiration (T, δT) and ET (δET) to partition the total water flux.

Fig. 7 shows also the existence of paleo-groundwater within the MP multilayer aquifer system.

Indeed the sample of Yabea exhibits a strongly depleted signature (-5.1‰ δ 18 O) compared to the other points. It is a borehole of 95 m deep with a water table at 54 m. According to [START_REF] Ketchemen-Tandia | Déterminants hydrogéologiques de la complexité dusystème aquifère du bassin sédimentaire de Douala (Cameroun)[END_REF] this groundwater resource from the most confined part of the aquifer, exhibiting depleted isotope values, has zero tritium content; while tritium contents recorded in springs and hand-dug wells vary from 2.1 to 5.9 TU, in good agreement with the regional rainfall input (GNIP data at the Ndjamena station; IAEA/WMO 2018). This indicates that the groundwater of Yabea is, for sure, originating from a recharge having occurred prior to the atmospheric bomb tests (i.e. before 1953; Fontes 1983) and therefore is more than 70 years old and probably corresponding to a palaeorecharge episodes dating a few millennials ago considering the amplitude of the isotopic depletion (Jirakova et al., 2011) as already described in the Western Africa [START_REF] Huneau | Flow pattern and residence time of groundwater within the southeastern Taoudeni sedimentary basin (Burkina Faso Mali)[END_REF]. Therefore the studied aquifer gathers water from recent infiltration and paleo-recharge.

Figure 8

Analysis of groundwater flow

Water reaching the saturated zone globally moves from the eastern side of the study region toward the discharge area in the Wouri estuary zone (Fig. 6).

However, the variable groundwater flow directions and the variability of the distance between the potentiometric surface contours (Fig. 6) characterize a non-uniform flow regime.

Potentiometric surface contours become more and more distant from the recharge area, indicating a dominance of permeable formations towards the estuary in agreement with geological/hydrogeological description. Moreover, the general shape of the potentiometric surface is convex, showing a radial aquifer with divergent streamlines contrary to semi-arid and arid regions where convergent streamlines towards the water course axis are generally observed (e.g., [START_REF] Ouali | Present recharge of an aquifer in a semi-arid region: an example from the Turonian limestones of the Errachidia basin, Morocco[END_REF][START_REF] Lienou | Caractérisation des relations eaux de surface -eaux souterraines en milieu tropical sec : exemple du bassin de la nema (Sine Saloum, Senegal)[END_REF].

Hydraulic gradients (i) from upstream to downstream, calculated along the transect line C-D (Fig. 6) from hand-dug wells; vary between 3.5‰ and 2.1‰ with a mean of 2.3‰ characteristic of unconfined aquifer conditions. Thanks to this mean value of i, and assuming a porosity (n) of 0.07 for MP sands and an average hydraulic conductivity (K) of 1.5×10 -3 , a calculation of groundwater velocity (V = k * i/n) has been performed. V was estimated to 4.9x10 -5 m/s or 5 m/day. Therefore, it would take 70 years for water to travel from the upstream zone (at PK 13) to the estuary area (Fig. 6). This Darcy velocity is very important compared to that generally observed in porous media [START_REF] Piccinini | Point dilution tests to calculate groundwater velocity: an example in a porous aquifer in NE Italy[END_REF]). It may be attributed firstly to the high hydraulic conductivity of the aquifer and secondly to the high rainfall recorded at Douala which leads to shallow groundwater table and therefore contribute somehow to high values of hydraulic gradient as observed.

In the discharge area, the MP aquifer can develop interactions with the Wouri and Dibamba Rivers (Fig. 6). It can be suspected that when there are superimposed, a natural upward water flux from the MP to the Quaternary aquifers should occur. Moreover, in this discharge zone, groundwater can also be influenced by ocean tides. However, regarding the geographical position of the aquifers (Fig. 6), Quaternary sands seem to be more vulnerable to saline intrusions than the Mio-Pliocene formation. Finally, it is worthwhile noting that groundwater discharge also occurs at numerous springs in the study area due to very local topographic control.

In summary, the keys messages in this section are: (i) rainwater reaches the saturated zone without undergoing evaporation; (ii) the hyper-rainy conditions contribute directly or indirectly to water recycling within the vadose zone and to maintain the groundwater at a shallow level even during the dry season.

In the next section we will investigate which factor controls the water chemistry along the flow and especially if there is any influence of the regional hyper-recharge rate.

Water chemistry

Main features

As shown in 3). This can probably be associated to the geothermal gradient. The pH values show a wide range from strong acidic conditions to slightly alkaline (Table 3) thus reflecting various inputs into the aquifer system. However, when considering mean and/or median values (Table 3), the acidic character of MP water is obvious. The electrical conductivity (EC) in boreholes, hand-dug wells and springs range from 13.6 to 1632 μS/cm, 47 to 1048 μS/cm, and 111.8 to 375 μS/cm respectively (Table 3). As for pH values, this tends to reflect various influences on MP groundwater system. Nevertheless, the median EC in boreholes is significantly lower than those in hand-dug wells and springs. Therefore, the deeper the water flows, the more the groundwater mineralization decreases. It is already possible to hypothesize that the shallower aquifer system is the most affected by various inputs entering the system and peculiarly by anthropogenic factors such as: industrial effluents, water from pit latrines and leaking of solid waste.

Table 3 20 Dissolved oxygen (DO) concentrations show wide range of values from 2.3 mg/l to 7.8 mg/l, 2.2 mg/l to 7 mg/l and 2 mg/l to 7.8 mg/l for boreholes, hand-dug wells and springs respectively (Table 2). This heterogeneity in DO values can be related to microbial activities within the different flow paths of the Mio-Pliocene [START_REF] Keesari | Microbial Evaluation of Groundwater and its Implications on Redox Condition of a Multi-Layer Sedimentary Aquifer System[END_REF]. On the other hand, the fact that most boreholes are screened at different depths could affect the redox conditions within the system.

Concerning the major ions chemistry, NO3 -is the dominant anion in boreholes and springs accounting respectively for 74% and 64% of the total major ions; after which the concentrations of anions in groundwater types follow the order Cl -> SO4 2-> HCO3 -. In water from hand-dug wells there is no dominant anion; Cl -, SO4 2-and NO3 -present approximately the same percentage (around 33%). In contrast, the concentrations in major cations are dominated for each sample by Na + followed by Ca 2+ , K + and Mg 2+. This order of abundance of ions is in agreement with the hydrochemical types observed on the Piper diagram (Fig. 9) with a clear abundance of Na-K-Cl water types for the different flow paths. However, it is worthwhile noting that the borehole sample of Yabea (the most depleted in stable isotopes; Fig. 7) is the only one presenting a Ca-Mg-HCO3 water type. This water-type reflects a lack of anthropogenic influence on this water and a predominance of geological natural processes on groundwater chemistry.

Figure 9

Evolution of major ions chemistry along a flow-path

The evolution of concentrations in major ions along the E-F transect (Fig. 6) is displayed on Following these two previous sections describing the main features and the evolution of groundwater chemistry along the flow, the next section will aim at determining the hydrochemical processes involved.

Processes controlling water chemistry

Generally, the soluble ions concentrations in groundwater are partly deriving from the precipitation input, evaporation, dilution, water rocks interactions, residence time and anthropogenic factors. All the natural processes can be investigated through Gibbs diagram which is helpful to discriminate geochemical evolutions in groundwater. A Gibbs diagram has been drawn for boreholes groundwater on Fig. 11. It shows that groundwater has been affected largely by water-rock interaction (Figure 11) even if three groups can be separated: (i) the group of precipitation dominance characterized by low mineralization (13.6 < EC < 19 and 3.2 < TDS < 11.5) suggesting a recent infiltration of rainwater in the aquifer system, (ii) the group of water-rock reactions with samples characterized by a freshening process due to important recharge conditions and (iii) the group of water-rocks reactions with rock dominance represented only by the sample of Yabea. This sample presents a high value of TDS (EC = 431 µS/cm) with low concentration in Na + . The Gibbs diagram confirms that water from this borehole may be an old groundwater in the deep part of the aquifer system.

Indeed, a significant time of water-rocks interactions would explain the mineralization as well as lower content in Na + may refer to a groundwater reservoir protected from anthropogenic influences.

Figure 11

In summary, it is clear that the water residence time is a control factor of groundwater chemistry. In this study, the long residence time of groundwater leads to a Ca -HCO3 water type with high TDS (see Fig. 9) and no evidence of human pollution.

However, details must be provided concerning the water-rock interaction process highlighted on the Gibbs diagram (Fig. 11). It is a relatively broad classification that may include many kinds of processes such as silicate weathering, carbonate dissolution, and evaporate dissolution. In the present case, the fact that 69% and 88% of samples have respectively a molar ratio higher than 2 and 1 -respectively for Ca 2+ /Mg 2+ and Ca 2+ /Na + ratios -indicates a predominance of silicate weathering process in groundwater chemistry [START_REF] Lee | Geochemistry of groundwater in Tertiary and Cretaceous sediments of the south-eastern coastal plain in eastern Georgia, South Carolina, and southeastern North Carolina[END_REF][START_REF] Lakshmanan | Major ion chemistry and identification of hydrogeochemical processes of groundwater in a part of Kancheepuram district, Tamil Nadu, India[END_REF][START_REF] Rajmohan | Identification and evolution of hydrogeochemical processes in the groundwater environment in an area of the Palar and Cheyyar River basins, Southern India[END_REF]. The lithology of the strata in the study area, consisting of sands and clays and the mineralogy of the aquifer, supports the above argument. This silicate weathering through hydrolysis reactions is favoured at Douala by the relative high temperatures and low thermal amplitude according to [START_REF] Segalen | Les produits alumineux dans les sols de la zone tropicale humide[END_REF] who

showed that hydrolysis reactions are more intense and faster in humid tropical regions.

Thanks to the constantly high temperature, the presence of free H + ions in the superficial water allows a chemical decomposition of the rock minerals.

It is worthwhile noting that the sample of Yabea identified on the Gibbs diagram in the field of rock dominance is affected by carbonate dissolution. This assertion is supported by the saturation indexes (SI) corresponding to -0.99, -1.1 and -1.5 respectively for calcite, aragonite and dolomite while for the rest of boreholes SIcalcite, SIaragonite and SIdolomite vary respectively from -2.3 to -4.9, -2.5 to -5.1 and -3.5 to -9.7. Indeed, since previous works on geology mentioned the presence of carbonate layers in the lower Pliocene, this reinforces the hypothesis that this sample corresponds to a borehole tapping a very deep part of the MP aquifer and therefore relatively old water. The dissolution phenomenon may also be enhanced by the hyper-rainy character of Douala which enables significant drainage in promoting the solubilisation of elements [START_REF] Helgeson | Thermodynamic and kinetic constraints on reactionrates among minerals and aqueous solutions, II. Rateconstants, effective surface area, and the hydrolysis of feldspar[END_REF]. Thus, the peculiar climate of Douala is a control factor of water-rocks interactions accelerating the reactions of hydrolysis and dissolution.

Besides, the water-rocks interaction reinforced by the water residence time and the hot and humid climate of Douala, another considerable control factor of groundwater chemistry is the anthropization phenomenon.

To check the influence of this phenomenon on the Mio-Pliocene aquifer of Douala, we compiled historical data on nitrates concentrations measured in 2011 [START_REF] Fantong | Variation of hydrogeochemical characteristics of water in surface flows, shallow wells and boreholes in the coastal city of Douala (Cameroon)[END_REF], 2014 [START_REF] Emvoutou | Fonctionnement hydrodynamique du système aquifère du bassin sédimentaire côtier dans la ville de Douala. Apports des outils géochimiques[END_REF][START_REF] Feumba | Hydrogéologie et évaluation de la vulnérabilité des nappes dans le bassin versant de Bessekè (Douala-Cameroun)[END_REF][START_REF] Wirmvem | Effect of diffuse recharge and wastewater on groundwater contamination in Douala Cameroon[END_REF]) and 2017 (this study) during the dry season at Douala. Nitrates generally well reflect the impact of human activities on the water resource. Thus, regarding the urbanization trend in Douala, the high increase in population and inhabitants density [START_REF] Nlend | The impact of urban development on aquifers in large coastal cities of West Africa: Present status and future challenges[END_REF], we expect that NO3 -content in groundwater would increase overtime. For the 2011-2017 period, maximum values show a linear variation, a positive trend: from 24.0 mg/l to 245.6 mg/l. Median values evolve from 6.0 to 29.3 mg/l (Fig. 12). These latter values are significantly above the global natural levels of NO3 -in groundwater [START_REF] Appelo | Geochemistry, Groundwater and Pollution[END_REF]. The progressive increase of nitrates in groundwater is a worrying feature revealing the strong anthropogenic constrains on groundwater resource and therefore the water quality decline.

Figure 12 Further, the influence of cation exchange was investigated. Like [START_REF] Emvoutou | Fonctionnement hydrodynamique du système aquifère du bassin sédimentaire côtier dans la ville de Douala. Apports des outils géochimiques[END_REF], we found a lack of cation exchange which is evidence that the MP aquifer is not impacted by seawater intrusion. Indeed, according to [START_REF] Martínez | Hydrogeochemistry and cation-exchange processes in the coastal aquifer of Mar Del Plata, Argentina[END_REF] when sea water intrudes a fresh water aquifer, cation exchange takes place and sodium is taken-up by the exchanger, while Ca 2+ is released into groundwater. Therefore, the increase in water mineralization, chloride and sodium near the estuary (Fig. 9) are not linked to saline intrusion.

The Cl -/HCO3 -ratio confirms this fact; with 1.8 molar ratio (Cl -= 94.4 mg/l) for the sample of Essengue while [START_REF] Wirmvem | Effect of diffuse recharge and wastewater on groundwater contamination in Douala Cameroon[END_REF] reported that surface water at the estuary has a Cl - /HCO3 -molar ratio of 100 with approximately 5000 mg/l of Cl -.

In summary, the water-rocks interactions processes can then be considered as amplified by the residence time and the typical climate conditions of Douala (hot and humid). In addition, the distribution of NO3 -in boreholes, hand dug-wells and springs (Table 3) and the historical evolution of this ion in groundwater suggest a strong impact of human activities on the aquifer system. Further, as it has been highlighted that seawater does not intrude freshwater, high levels of Cl -, SO4 2-and Na + would also correspond to consequences of human activities.

In the next section, we will evaluate the anthropogenic part in water chemistry by determining the natural background and threshold values for major elements.

Determination of natural background levels (NBL) and threshold values (TV)

Background level of a substance in groundwater is defined as the concentration of a given element, species or chemical substance present in solution, which is derived from natural geological, biological or atmospheric sources [START_REF] Dumort | Notice explicative sur la feuille Douala-Ouest carte géologique de reconnaissance (1/500000)[END_REF]. However, there are various methods to identify NBL: hydrochemical simulation of solution in aquifers [START_REF] Sellerino | Methods of determination ofnatural background levels in groundwater: a review. The definition of natural background levels (NBLs) in groundwater and soils -Case studies in Italy and Portugal. Implementation of eco-compatible protocols for agricultural soil remediation in Litorale Domizio-Agro Aversano NIPS[END_REF], pre-selection [START_REF] Coetsiers | Natural background levels and threshold values for groundwater in fluvial Pleistocene and Tertiary marine aquifers in Flanders, Belgium[END_REF], component separation, probability plot and other statistical methods (e.g., [START_REF] Sellerino | Methods of determination ofnatural background levels in groundwater: a review. The definition of natural background levels (NBLs) in groundwater and soils -Case studies in Italy and Portugal. Implementation of eco-compatible protocols for agricultural soil remediation in Litorale Domizio-Agro Aversano NIPS[END_REF][START_REF] Zhang | Natural background levels of chemical components in groundwater of Hutuo River catchment area, North China Plain[END_REF]. Among these set of methods, the preselection is the simplest to approach the natural groundwater composition of an aquifer when no national methodology exists for the derivation of NBL. In addition, this method is the more suitable for non-expert users and all the stakeholders involved in water management.

The pre-selection methodology here consists in the removing samples from unknown depth, samples close to the estuary (where saline intrusion could be suspected or tide influence), groundwater analyses with nitrates concentrations larger than 10 mg/l, so as to remove polluted samples.

Then the NBL will be helpful to determine the threshold values for groundwater. According NBL and TV calculated are shown in Table 4, resulting from the application of the selected methodology to the dataset. The values for NBL are presented as minimum and maximum.

The TV was calculated by using the maximum NBL values.

Table 4 shows that the concentrations in chemical elements are mostly above the NBL in shallower water from hand-dug wells and springs than in deep water from boreholes. This can be explained by the fact that shallow groundwater is more sensible to anthropogenic influences. However, samples presenting HCO3 -contents above the NBL are mostly observed in boreholes. These samples correspond to the deepest groundwater. Since, this element is only related to natural processes it is therefore clear that the deeper the groundwater flows, the more the groundwater mineralization increases due to water-rocks interactions. Groundwater depth should be considered in identifying geochemical processes. Likewise, as expected, the majority of samples which show chemical contents above the TV are from hand-dug wells.

However, it is worthwhile noting, as it can be seen from Table 4, that 73% and 46% of samples from boreholes have a NO3 -above the defined NBL and TV respectively.

This observation implies that the aquifer pollution is probably not limited to shallower flow path. Thus water from boreholes is also vulnerable to nitrate pollution. This latter appears as a serious concern in term of qualitative potential of groundwater.

Finally, the pristine natural background value of nitrate obtained in this study for MP sands (6.6 mg/l) is very close to that obtained by [START_REF] Zhang | Natural background levels of chemical components in groundwater of Hutuo River catchment area, North China Plain[END_REF] in North China plain on gravel sediments. The nitrate TV (28.3 mg/l) is also similar to the one found by [START_REF] Coetsiers | Natural background levels and threshold values for groundwater in fluvial Pleistocene and Tertiary marine aquifers in Flanders, Belgium[END_REF] on fluvial deposits in Flanders, Belgium. NBL and TV values obtained in this study also correspond to that provided by [START_REF] Appelo | Geochemistry, Groundwater and Pollution[END_REF]. This tends to validate the accuracy of the results for the Douala region and the methodology used.

Table 4

Final discussion: proposition of a conceptual scheme of MP aquifer functioning

The present study brings out the detailed hydrogeological features characterizing an aquifer in hyper-humid region. The approach used involves hydro-climatic data, measures of potentiometric levels, stable isotopes and hydrochemistry. It is the first time that such kind of study is achieved in the Central Africa tropical humid region. The results obtained allows for the design of the hydrogeological conceptual model of the region including the structure and functioning of the aquifer system (Figure 13). Member states to correctly assess the chemical status of ground-water bodies and to implement the Groundwater Directive [START_REF] Wendland | Derivation of natural background levels and threshold values for groundwater bodies in the Upper Rhine Valley (France, Switzerland and Germany)[END_REF][START_REF] Preziosi | Natural background levels and threshold values derivationfor naturally As, V and F rich groundwater bodies: a methodological case study in Central Italy[END_REF]. Indeed NBL and thresholds limits constitute a basis for clean-up remediation and adequate water legislation.

From the conceptual scheme obtained, in term of groundwater potential, the important recharge of the Mio-Pliocene aquifer (linked to the hyper-humid climate) reflects the good quantitative potential of the resource. This hyper-recharge also affects the groundwater quality by maintaining the groundwater at a shallow level even during the dry season. Thus, groundwater resource would be easily affected by contaminants at the surface. High-intensity rainfall events pose a risk to shallow and poorly protected groundwater sources. This is particularly an issue for unimproved dug wells and springs. Shallow groundwater levels also pose a significant risk to shallow water sources in urban settings due to reduced attenuation in the unsaturated zone when groundwater intersects with the base of pit latrines and sewer networks [START_REF] Lapworth | Urban groundwater quality in sub-Saharan Africa: current status and implications for water security and public health[END_REF]. Another feature resulting indirectly from aquifer hyperrecharge is the high groundwater velocity which can foster the rapid transfer of pollutants within the aquifer. Thus in the light of this work, it is urgent to implement pragmatic water management strategies. The groundwater recharge zones identified on the potentiometric map (Fig 6) must be protected. Any activity presenting a risk of contamination of water should be prohibited and managers should promote the establishment of protection perimeters around the tapping points. The knowledge of lateral flow velocity could help to design security distances between pollution sources and groundwater pumping points. In addition, for security and sustainability of groundwater use, a legal framework must now be established for groundwater abstraction especially for boreholes construction to control more efficiently the exploitation of the deeper part of the aquifer since it gathers relatively old and good quality water. Indeed, the paleo-groundwater in the Mio-Pliocene must be considered as a strategic resource since it may be non-renewable. Therefore, the abstraction of this type of water must be planned and only reserved to public authorities. Furthermore, multilevel screened wells must be prohibited to avoid mixing between different flow paths. In addition, stakeholder participation through local committees for environment protection should be in charge of the maintenance of landfills in settlements and/or watersheds. This is essential to limit the influence of human activities on the aquifer system through a better land use management.

Furthermore, since surface waters are intimately related to groundwater, the continuous degradation of this latter would impact on the long-term the socio-ecosystem of the local river.

Finally, beyond the critical evaluation of NBL and TV determination method, this research aims at increasing consciousness in the water management authorities that groundwater status assessment should be conceived as a scientific evaluation procedure and not as a technical/administrative exercise to be performed using any set of available data. As such, it should be supported by adequate knowledge of hydrogeological setting, hydrochemical processes occurring in groundwater bodies, and human activities acting overland to be progressively verified.

Conclusion

The results of this work may be extended to many porous aquifers in hyper-humid tropics. Hydraulic conductivity (m/s) 1.5×10 -3 2.6 x 10 -5 m/s 0.2 m/s 0.03 Shallow urban aquifers under hyper-recharge equatorial conditions and strong anthropogenic constrains. Implications in terms of groundwater resources potential and integrated water resources management strategies.

Graphical abstract

Figure 7

 7 Figure 7

Figure 10 .

 10 Figure 10. Only boreholes samples are considered on this flow-path since they appear to be

Figure 10 Concentrations

 10 Figure10

to

  [START_REF] Last | Threshold Values for Identification of Contamination Predicted by Reduced-Order Models[END_REF] TV represent a groundwater quality standard that is based on interactions with aquatic and terrestrial ecosystems, interference with legitimate uses or functions of groundwater, and hydro-geological characteristics including background levels. In this study it was proposed to define the TV following two different cases based on the ratio between the natural background level and a relevant reference value[START_REF] Muller | Final proposal of a methodology to set up groundwater threshold values in Europe. Specific targeted EU research project BRIDGE (contract[END_REF]): Case 1: NBL < REF: TV = (NBL+REF)/2 Case 2: NBL > REF: TV = NBL REF is an appropriate reference value, defined by the World Health Organization (WHO) or by any other legislation.

Figure 13 It

 13 Figure13

Fig. 1 Fig. 2 Fig. 3

 123 Fig. 1 Maps of the study area showing the geology of the Douala basin and the location of the sampling points. The cut line A-B refers to geological cross section presented in Fig. 3.

Fig. 4

 4 Fig.4Geological cross section through the Douala sedimentary basin[START_REF] Regnoult | Synthèse Géologique du Cameroun DMG[END_REF]). The corresponding cut line is plotted on Figure1.

Fig. 5

 5 Fig. 5 Water balance in the region of Douala.

Fig. 6 Fig. 7 δ

 67 Fig. 6 Potentiometric map of the Mio-Pliocene aquifer in Douala (Cameroon) in February 2017.Fig. 7 δ 18 O vs δ 2 H diagram. Groundwater samples are reported as well as the Global Meteoric Water line (Craig 1961), the Local Meteoric Water Line and the Bangui and Kisangani water lines (IAEA/WMO 2019).

Fig. 8 Fig. 9

 89 Fig. 8 Deuterium excess (D-excess) in groundwater versus potentiometric level. The mean values of D-excess in rainwater at Douala are also plotted.

Fig. 10

 10 Fig. 10 Evolution of major ions concentrations along the groundwater flow path from Nkolbong (upstream, SE side of the Douala region), to Essengue (downstream, NW side of the Douala region) according to the E-F transect shown in Fig. 6.

Fig. 11

 11 Fig. 11 Gibbs diagram for deep Mio-Pliocene groundwater at Douala.

Fig. 12

 12 Fig. 12 Box whisker plot illustrating the temporal evolution of NO3-over the years on the Mio-Pliocene aquifer at Douala, Cameroon.

Fig. 13

 13 Fig. 13 Synthesis of hydrological functioning of Mio-Pliocene aquifer at Douala.

Fig. 1 Fig. 2 Fig. 3

 123 Fig. 1 Maps of the study area showing the geology of the Douala basin and the location of the

Fig. 4

 4 Fig.4Geological cross section through the Douala sedimentary basin[START_REF] Regnoult | Synthèse Géologique du Cameroun DMG[END_REF]). The

Fig. 5 Fig. 6

 56 Fig. 5 Water balance in the region of Douala

Fig. 7

 7 Fig. 7 δ 18 O vs δ 2 H diagram. Groundwater samples are reported as well as the Global Meteoric

Fig. 8

 8 Fig. 8 Deuterium excess (D-excess) in groundwater versus potentiometric level. The mean

Fig. 9 Fig. 10

 910 Fig. 9 Piper diagram of Mio-Pliocene groundwater at Douala (Cameroon)

Fig. 11

 11 Fig. 11 Gibbs diagram for deep Mio-Pliocene groundwater at Douala.

Fig. 12

 12 Fig. 12 Box whisker plot illustrating the temporal evolution of NO3 -over the years on the

Fig. 13

 13 Fig. 13 Synthesis of hydrological functioning of Mio-Pliocene aquifer at Douala

  table, then the water table will continue to drop but at a slower rate.In summary, we observe that the aquifer recharge period corresponds to six successive months. By taking into account the replenishment of soils reserves, we can conclude that the

aquifer is recharged approximately all the yearlong. This indicates that the studied aquifer presents excellent quantitative potential, especially for water supply. The recharge rate of 15% -20% (578.2 -732.3 mm/year) of annual rainfall amount is lower than that highlighted by model analysis

[START_REF] Mohan | Predicting groundwater recharge for varying land coverand climate conditions -a global meta-study[END_REF]

-where the annual recharge rate is estimated at more than 1 m-, but similar to that obtained in other similar humid tropical areas such in Ghana and

  Table 3, groundwater temperatures (median values of 28.6°C, 28.2°C and 27.9°C respectively for boreholes, hand-dug wells and springs) are close to that of air temperatures in February (26.9°C to 30.5°C with a mean of 28.6°C) when the field measurements were carried out. It is clear that temperatures increase slightly with the depth as water from boreholes present the highest values in term of maximum, mean and median (Table

  Indeed, this study by highlighting specific behaviors (e.g., high recharge rate, water recycling within the vadose zone through plants transpiration, high groundwater velocity, acceleration of hydrolysis and dissolution reactions thanks to the hot and humid climate) of aquifers in hyper-humid areas, is a contribution to a better understanding of tropical hydrology. The water table fluctuation method has proven to be effective for estimating groundwater recharge in a shallow water table aquifer of humid area. The estimate is consistent with that found in the literature and not far to that obtained by the application of water balance method. In spite of low thermal amplitude characterizing the study region, the use of water stable isotopes appears as a very powerful tool to study processes affecting the transfer of rainwater to the vadose zone. Stable isotopes also shown that the studied aquifer submitted to hyper-recharge gathers both recent infiltrated water and paleogroundwater. While in the past they have been implemented only in arid and temperate climates, this study constitutes a new testimony of the possibilities offered by isotope techniques in hydrogeological studies in humid countries.

	B. Nlend: Conceptualization, Investigation, Methodology, Resources, Writing -original and
	revised manuscripts. H. Celle-Jeanton: Conceptualization, Methodology, Resources,
	Supervision, Writing -review & editing. F. Huneau: Conceptualization, Investigation,
	samples analyses, Supervision, Writing-review & editing. E. Garel: samples analyses,
	Methodology, Writing -review & editing. S. Boum-Nkot: Conceptualization, Writing -
	review & editing. J. Etame: Methodology & supervision.
	Substantial information on groundwater recharge, groundwater flow conditions, groundwater
	chemistry added to the determination of NBLs and TV will be helpful to elaborate future
	groundwater management strategies, especially in developing countries of Africa. Indeed, the
	limited waste management and high intrinsic vulnerability lead to urban aquifer (as Mio-
	Pliocene at Douala) highly impacted by chemical loading posing a clear health risk to users.
	NBLs defined are essential when considering deterioration of groundwater quality, and in
	addition these values can be a basis when restoring groundwater quality. Threshold values
	should be incorporated in formulating new water quality standards for groundwater in
	Cameroon, and more generally in sub-Saharan Africa. More generally, this approach provides
	useful information for the establishment of a new water supply scheme with respect to the
	climate specificity and the urban context.
	Authorship contribution statement

Table Hydro -

 Hydro stratigraphy of the Mio-Pliocene aquifer.

Table

  Hydrodynamics settings of the Mio-Pliocene aquifer in Douala. Data were collected from historical records of drillings (n= 68) for water wells.

Table 3

 3 Descriptive statistics of chemical parameters. Concentrations of major ions are expressed in mg/L.

Table

  Estimated NBLs* and TV** of parameters for the MP aquifer at Douala. Reference (Ref) used are those defined by WHO (2011).

Table 1

 1 Hydro-stratigraphy of the Mio-Pliocene aquifer.

				Hydrogeological
	Age	Lithology	Thickness
				characteristic
	Pliocene	Clay, sandy-clay, clayey-sands	0 -40 m	Aquiclude/aquitard/aquifer
	Lower	Cross-bedded sands with passages
	Miocene	of lignite and limestone
		Coarse grained sand (slightly silty		Aquifer
	Upper		>220 m
		and calcareous)
	Miocene
		Gravel

Table 2

 2 Hydrodynamics settings of the Mio-Pliocene aquifer in Douala. Data were collected from historical records of drillings (n= 68) for water wells.

		Mean	Minimum	Maximum Standard deviation
	Transmissivity (m²/s)	5.6 x 10 -3	4.6 x 10 -4	6.8 x 10 -²	6.1 x 10 -3

Table 3

 3 Descriptive statistics of chemical parameters. Concentrations of major ions are expressed in mg/L.

				Groundwater from boreholes	Groundwater from hand-dug wells	Groundwater from springs (n = 10)
					(n = 26)					(n = 18)
	Parameter	Unit	Min	Max	Mean Med	SD	Min Max	Mean Med	SD	Min	Max	Mean Med	SD
	pH		3	8.5	4.7	4.4	0.9	3.2	7.6	5.7	5.8	0.8	3.3	7.6	4.9	4.8	1.1
	Temp.	°C	27.4	31.5	28.9	28.6	1	27.8 29	28.3	28.2	0.4	27.2	29.8	28	27.9	0.7
	EC	µS/m 13.6	1632	200.7 105.8 318.9 47	1048	428.4 399.5 268.8 111.8 375	255.3 274	85
	DO	mg/l	2.3	7.8	6	6.5	1.4	2.2	7	4.5	4.4	1.4	2	7.8	4.3	4.3	1.8
	K +	mg/l	0.1	19.3	4.7	2.9	5.4	1	41	12.4	10.6	10.3	2.3	11.7	6.8	6.7	3.6
	Na +	mg/l	0.0	52.9	15.8	13.1	14.5	4.7	86.9	36.6	37.6	23.5	9.7	60.8	28	24.7	13.7
	Ca 2+	mg/l	0.2	32.9	5.4	3	7.4	1.8	81.1	24.7	22.9	21.4	1.4	21.3	7.7	6	6.4
	Mg 2+	mg/l	0.1	21.4	2.3	0.8	4.8	0.3	6.7	2	1.3	1.7	0.5	1.7	1	0.9	0.3
	Cl -	mg/l	1.6	94.4	17.1	7.4	20.9	4.3	131.1 50.4	45.9	38.8	6.8	72.1	40	30	19.4
	SO4 2-	mg/l	0.5	44.7	7.2	1.6	11.9	0.8	90.5	35	43.8	26	0.7	33.4	9.3	4.9	10.3
	HCO3 -	mg/l	0	137	9.4	0	28.8	0	222.2 28.9	0.4	68.9	0	0	0	0	0
	NO3 -	mg/l	1.1	145.5 35	26.1	36.1	7.6	245.6 56.7	43.6	56.1	28.2	93.4	61.6	61	21.6

Table 4

 4 Estimated NBLs* and TV** of parameters for the MP aquifer at Douala. Reference (Ref) used are those defined by WHO (2011).

	Parameters Ref	NBLs	TV	Number of samples above the NBL (%) Number of samples above the TV (%)
			Min Max		Boreholes Hand-dug wells Springs Boreholes Hand-dug wells Springs
	EC (µS/cm) 250 13.6 41.3 145.7	73.1	100	100	38.5	77.8	90
	K + (mg/l)	100 0.1	0.5	50.3	76.9	100	100	0	0	0
	Na + (mg/l)	200 0.04 6.7 103.4	65.4	88.9	100	0	0	0
	Ca 2+ (mg/l)	75	0.2	1.1	38.1	65.4	100	100	0	15.4	0
	Mg 2+ (mg/l)	30	0.1	0.8	15.4	61.5	83.3	80	0	0	0
	Cl -(mg/l)	250 1.6	4	127	69.2	100	100	0	5.6	0
	SO4 2-(mg/l) 250 0.5	1.5 125.8	30.8	88.9	80	0	0	0
	HCO3 -(mg/l) 200	0	9	104.5	15.4	11	0	3.9	5.6	0
	NO3 -(mg/l)	50	1.1	6.6	28.3	26.9	100	100	46.1	66.7	100

*NBLs = Natural background levels, **TV = Threshold values

Acknowledgments

This paper constitutes a part PhD study of the first author who was supported by a doctoral scholarship from the French Ministry of Foreign Affairs. The authors thank the French Embassy to the Republic of Cameroon for all mobility facilities provided during the study.