Clément Roques
email: clement.roques@univ-rennes1.fr

Olivier Bour

Luc Aquilina

Benoît Dewandel

Sarah Leray

Jean Michel Schroetter

Laurent Longuevergne

Tanguy Le Borgne

Rebecca Hochreutener

Thierry Labasque

Hydrological behavior of a deep sub-vertical fault in crystalline basement and relationships with surrounding reservoirs

Keywords: Hydrogeology, crystalline rocks, lineament, fault zone, large scale pumping test, sustainable groundwater resource

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

Crystalline rocks cover large areas throughout the world of about 35% of the continental surface (Amiotte [START_REF] Suchet | Worldwide distribution of continental rock lithology: Implications for the atmospheric/soil CO 2 uptake by continental weathering and alkalinity river transport to the oceans[END_REF][START_REF] Blatt | Proportions of exposed igneous, metamorphic, and sedimentary rocks[END_REF] and may constitute a crucial water resource for vast population. The porosity and permeability of primary crystalline rocks are extremely low, but their hydraulic properties can be greatly modified as a result of secondary physical processes (unloading, tectonic activities…) and/or geochemical processes such as weathering, fluid circulation [START_REF] Bahat | Mechanism of exfoliation joint formation in granitic rocks, Yosemite National Park[END_REF][START_REF] Caine | Fault zone architecture and permeability structure[END_REF][START_REF] Evans | Permeability of fault-related rocks, and implications for hydraulic structure of fault zones[END_REF][START_REF] Henriksen | Effects of fracture lineaments and in-situ rock stresses on groundwater flow in hard rocks: a case study from Sunnfjord, western Norway[END_REF]Taylor andHoward, 2002, 1999)… Various conceptual models of hydrogeological compartmentalization in crystalline rock aquifers have been proposed [START_REF] Chilton | Hydrogeological characterization and water-supply potential of basement aquifers in tropical Africa[END_REF][START_REF] Dewandel | A generalized 3-D geological and hydrogeological conceptual model of granite aquifers controlled by single or multiphase weathering[END_REF][START_REF] Maréchal | Vertical anisotropy of hydraulic conductivity in the fissured layer of hard-rock aquifers due to the geological structure of weathering profiles[END_REF][START_REF] Taylor | Lithological evidence for the evolution of weathered mantles in Uganda by tectonically controlled cycles of deep weathering and stripping[END_REF][START_REF] Wyns | Application of proton magnetic resonance soundings to groundwater reserve mapping in weathered basement rocks (Brittany, France)[END_REF]. They usually consist of two main reservoirs: 1) a layer of alterites (<15 meters bgs), identified as a specific reservoir with a relatively high porosity and storage, highly sensitive to rainfall recharge; 2) a superficial fractured zone, of various thickness and which may be characterized by relatively dense sub-horizontal and sub-vertical fracturing. This fractured reservoir has in general a higher permeability although well yields are typically limited to less than 10 m 3 /h. However, highly productive zones have been locally highlighted in regions exposed to past or current tectonic activity, such as Brittany. In practice, although controversial [START_REF] Gleeson | Identifying watershed-scale barriers to groundwater flow: Lineaments in the Canadian Shield[END_REF], hydrogeologists often use fracture outcropping analysis or "lineaments" mapping to determine drilling localization, especially when such lineaments result from extensive tectonic activity [START_REF] Gleeson | Identifying watershed-scale barriers to groundwater flow: Lineaments in the Canadian Shield[END_REF][START_REF] Henriksen | Effects of fracture lineaments and in-situ rock stresses on groundwater flow in hard rocks: a case study from Sunnfjord, western Norway[END_REF][START_REF] Sander | Lineaments in groundwater exploration: a review of applications and limitations[END_REF][START_REF] Singhal | Applied Hydrogeology of Fractured Rocks: Second Edition[END_REF]. Many factors must be considered to ensure the viability of the resource such as rock lithology affected by tectonic activity, stress fields and intensity of deformation. Such factors and fluid flow processes determine fault zone permeability [START_REF] Caine | Fault zone architecture and permeability structure[END_REF][START_REF] Evans | Permeability of fault-related rocks, and implications for hydraulic structure of fault zones[END_REF][START_REF] Gleeson | Identifying watershed-scale barriers to groundwater flow: Lineaments in the Canadian Shield[END_REF][START_REF] Goddard | Chemical changes and fluid-rock interaction in faults of crystalline thrust sheets, northwestern Wyoming[END_REF]. Few hydrogeological studies examined relationships between lineaments structures, hydrogeological flow organization and productivity wells [START_REF] Fernandes | The influence of Cenozoic tectonics on the groundwaterproduction capacity of fractured zones: a case study in Sao Paulo, Brazil[END_REF][START_REF] Henriksen | Effects of fracture lineaments and in-situ rock stresses on groundwater flow in hard rocks: a case study from Sunnfjord, western Norway[END_REF][START_REF] Holland | Evaluation of geologic and geomorphologic influences on borehole productivity in crystalline bedrock aquifers of Limpopo Province, South Africa[END_REF][START_REF] Richard | Factors Related to Well Yield in the Fractured-Bedrock Aquifer of New Hampshire[END_REF][START_REF] Sander | Lineaments in groundwater exploration: a review of applications and limitations[END_REF]. Fault zones may act as conduits, barriers, or as combined conduit-barrier systems that enhance or impede fluid flow [START_REF] Bense | Faults as conduit-barrier systems to fluid flow in siliciclastic sedimentary aquifers[END_REF][START_REF] Caine | Fault zone architecture and permeability structure[END_REF] but can also significantly influence groundwater flow, spring discharge, and water-table elevations [START_REF] Apaydin | Relation of tectonic structure to groundwater flow in the Beypazari region, NW Anatolia, Turkey[END_REF][START_REF] Gleeson | Identifying watershed-scale barriers to groundwater flow: Lineaments in the Canadian Shield[END_REF][START_REF] Melchiorre | Relationship between seismicity and subsurface fluids, central Coast Ranges, California[END_REF].

In some cases, aquifers near highly conductive fault zones and with relatively high production rates for crystalline rocks (from 20 to 100 m 3 /h) have been described: Ploemeur aquifer, Brittany (Le Borgne et al., 2006a;[START_REF] Leray | Numerical modeling of the productivity of vertical to shallowly dipping fractured zones in crystalline rocks[END_REF][START_REF] Ruelleu | Impact of gently dipping discontinuities on basement aquifer recharge: An example from Ploemeur (Brittany, France)[END_REF], the New Hampshire Bedrock Aquifer [START_REF] Richard | Factors Related to Well Yield in the Fractured-Bedrock Aquifer of New Hampshire[END_REF], Crystalline Rock Aquifers in New England [START_REF] Harte | Framework for Evaluating Water Quality of the New England Crystalline Rock Aquifers[END_REF] and in the Limpopo Province of South Africa [START_REF] Holland | Evaluation of geologic and geomorphologic influences on borehole productivity in crystalline bedrock aquifers of Limpopo Province, South Africa[END_REF].

The hydrodynamic functioning of fault structures in various geological contexts has been investigated in several studies. In sedimentary media, [START_REF] Bense | Faults as conduit-barrier systems to fluid flow in siliciclastic sedimentary aquifers[END_REF][START_REF] Burbey | Use of time-subsidence data during pumping to characterize specific storage and hydraulic conductivity of semi-confining units[END_REF] showed the important effects of the geometry and anisotropy of a fault zone on its hydraulic properties. Numerical studies, such as those by [START_REF] Anderson | Groundwater flow through anisotropic fault zones in multiaquifer systems[END_REF], also highlighted the influence of a vertical fault on groundwater flow. In the crystalline context, some studies have described the permeability architecture and hydrogeological functioning of fault zones for groundwater resources [START_REF] Boutt | A field study (Massachusetts, USA) of the factors controlling the depth of groundwater flow systems in crystalline fractured-rock terrain[END_REF][START_REF] Caine | Brittle structures and their role in controlling porosity and permeability in a complex Precambrian crystalline-rock aquifer system in the Colorado Rocky Mountain Front Range[END_REF][START_REF] Evans | Permeability of fault-related rocks, and implications for hydraulic structure of fault zones[END_REF][START_REF] Ganerod | Predictive permeability model of extensional faults in crystalline and metamorphic rocks; verification by pre-grouting in two sub-sea tunnels, Norway[END_REF][START_REF] Stober | Deep groundwater in the crystalline basement of the Black Forest region[END_REF] or hydrocarbon entrapment [START_REF] Aydin | Fractures, faults, and hydrocarbon entrapment, migration and flow[END_REF]. However, very few studies have analyzed the hydrological functioning of faults in a water abstraction context. In this context, aquifer yields will mainly depend on the ability of interactions between the fault and the surrounding reservoirs to allow recharge and water availability. On the other hand, groundwater abstraction from a deep resource will undoubtedly modify the hydrodynamic gradients and lead to mixing between the different reservoirs. The hydrogeological influence of deep fault zones on overlaying reservoirs is poorly known and is apparently difficult to characterize by field studies [START_REF] Banwart | Fracture zone in crystalline bedrock: Initial hydrochemical perturbation during tunnel construction at the Äspö Hard Rock Laboratory, southeastern Sweden[END_REF][START_REF] Carucci | Interaction between shallow and deep aquifers in the Tivoli Plain (Central Italy) enhanced by groundwater extraction: A multi-isotope approach and geochemical modeling[END_REF][START_REF] Folch | Hydrogeological interactions between fault zones and alluvial aquifers in regional flow systems[END_REF][START_REF] Gannon | Geophysical and geochemical characterization of the groundwater system and the role of Chatham Fault in groundwater movement at the Coles Hill uranium deposit[END_REF][START_REF] Sophocleous | Interactions between groundwater and surface water: the state of the science[END_REF]. This question has been tackled out only through few numerical studies, which have reported the hydrological efficiency of fault zones to act as preferential flow zone that enhances recharge processes from surrounding reservoirs [START_REF] Folch | Hydrogeological interactions between fault zones and alluvial aquifers in regional flow systems[END_REF][START_REF] Leray | Numerical modeling of the productivity of vertical to shallowly dipping fractured zones in crystalline rocks[END_REF].

The first aim of this study is to characterize the hydrodynamic functioning of a sub-vertical permeable fault zone in crystalline basement from a large-scale field experiment. The main objectives are to i)describe the architecture of the aquifer system, ii) define the flow organization between the permeable zones and recharge processes towards the deep fault zone under natural and pumping conditions and iii) characterize the origin of groundwater and mixing processes due to groundwater abstraction. The experiment is carried out on a specific field site in Brittany (Western France), identified by lineament observation [START_REF] Carn-Dheilly | RAPSODI -Recherche d'aquifères profonds dans le socle du département de l'Ille et Vilaine[END_REF], where a permeable fault zone at more than 100 meters depth is able to provide about 100 m 3 /h according to air-lift flow measurements.

We first describe the characteristics of the groundwater system under ambient conditions to highlight the hydrologic functioning of deep structures at the catchment scale. We then describe a 9week large-scale pumping test that was carried out to identify the hydraulic properties of the aquifer system. During this test, various complementary experiments and measurements were conducted to investigate flow interactions between the different reservoirs and to identify mixing processes.

Finally, the combined analyses from this multidisciplinary experiment are used to develop a hydrogeological conceptual model of a sub-vertical fault zone in crystalline context.

GEOLOGICAL AND HYDROGEOLOGICAL SETTING

The experimental site of Saint-Brice en Coglès is located in the Mancellian Domain of the "Massif Armoricain", in Brittany (France), where it constitutes the north western French part of the Cadomian and Variscan orogenies ((Figure 1a). This formation outcrops from western to central Europe and is mainly composed of low to high grade (migmatite) metamorphic rocks, with regional-scale magmatic intrusions (Cadomian granodiorites). The Mancellian Domain is limited in the north by the English Channel, and to the south by the North Armorican Shear Zone (NASZ). The main lithologies encountered are Precambrian (Brioverian) sedimentary rocks ((Figure 1a), composed of alternating series of argillites/siltites and sandstones. These sedimentary rocks have been affected by low grade metamorphism which has formed a metamorphic aureole around the Cadomian granodiorites on a regional-scale [START_REF] Ballèvre | Palaeozoic history of the Armorican Massif: Models for the tectonic evolution of the suture zones[END_REF][START_REF] Brun | Cadomian tectonics in northern Brittany: a contribution of 3-D crustal-scale modelling[END_REF][START_REF] Chantraine | The Cadomian active margin (North Armorican Massif, France): a segment of the North Atlantic Panafrican belt[END_REF][START_REF] Cogné | L'orogène Cadomien -Geology of Europe from Precambrian to the post-Hercynian sedimentary basins[END_REF]. The most important deformation zone corresponds to a major ESE-WNW dextral strike-slip shear zone, the NASZ attributed to hercynian orogeny. Secondary SW-NE reverse faults are also found around this main shear zone. These terrains have been affected by Pyrenean and Alpine orogenies that have generated a diffuse deformation characterized by half-graben basins (Rennes basin, Landéan basin…) and NW-SE sub-vertical faults [START_REF] Grellet | Sismotectonique de la France métropolitaine dans son cadre géologique et géophysique[END_REF][START_REF] Vliet-Lanoë | Neotectonic and seismic activity in the Armorican and Cornubian Massifs: Regional stress field with glacioisostatic influence?[END_REF].

The Saint-Brice en Coglès site is located in the epimetamorphism zone of a large Cadomian pluton (Fougère granite). The rocks consist of Brioverian sediments metamorphosed into hornfels schists.

The vicinity of the site is characterized by parallel extensional N-S normal faults that form localized extensional basins (Geological map, BRGM [START_REF] Dadet | Notice explicative de la feuille Saint-Hilaire-Du-Harcouët à 1/50 000[END_REF], (Figure 1b).

(Figure 1)

Local geological structures were imaged by carrying out a structural analysis including landscape, geological mapping and geophysical (electric and seismic tomographies) analysis. Sub-vertical faults and extensional graben structures were clearly identified ((Figure 2a). In addition, three deep boreholes (MFT20, MFT80 and, F3) were drilled using "Down Hole Air Hammer" drilling method.

Another deep borehole (FC4, 250 m), which has been entirely cored, has been drilled along a profile perpendicular to the orientation of the main accident lineament (Figure 2b). The borehole characteristics are detailed in (Table 1. Several shallow boreholes (labeled T in (Figure 2b) were also drilled to characterize the shallow weathered compartment. The first 10 meters contained highly weathered material with high clay and sand contents. Examination of the cores revealed that the Brioverian rocks were highly deformed and fractured, and intruded by metric aplites, pegmatites and quartz veins, oriented N 50° to N 70°. Particularly, in the vicinity of the fault outcropping, the Brioverian rocks are affected by horizontal fractures, characterized by a density decrease with depth, to an average of about 40-50 meters. The main fault zone was identified in each borehole as a highly fractured and productive zone, ranging from a few meters thick (see core sampling in (Figure 2c) to 15 m in the outcropping part of the borehole ((Table 1 and(Table 2). The fault typically dips by 70° to the NNW ((Figure 2c). Wells were completed with a slotted casing in the deep productive zones.

(Figure 2) (Table 1)

MATERIAL AND METHODS

FIELD EXPERIMENT

The interactions between the fault zone and the other hydrologic domains were described and quantified by investigating the site under natural conditions and during a large-scale pumping test. A packer was placed 80m below the ground surface in borehole F3 to ensure that pumping was carried out deep within the fault zone. As a result, water abstraction occurred principally from the fault zone at 110 meters below ground surface ((Table 1). Pumping lasted for two months (63 days) at a flow rate of about 45 m 3 /h. During the pumping test, total rainfall was limited to 40 mm, and potential evapotranspiration was 90 mm (information available from Saint Mars-sur-la-Futaie weather station-48°26'N 0°58'W, 30km north-east of the Saint-Brice site). Complementary experiments and analyses carried out are described in the following.

WATER LEVEL MONITORING AND ANALYSIS

Water levels were recorded using a pressure sensor with barometric compensation (STS© and Schlumberger© pressure transmitters). The altitude of the well was determined by differential GPS.

The estimated uncertainty for the absolute water level measurements was about a centimeter. The mean hydraulic parameters of the system under pumping conditions were determined from classic analytical models. In addition, a logarithmic derivative method of drawdown measurement was used to analyze flow regimes [START_REF] Bourdet | Use of pressure derivative in well test interpretation[END_REF][START_REF] Renard | Understanding diagnostic plots for well-test interpretation[END_REF][START_REF] Spane | DERIV: A computer program for calculating pressure derivatives for use in hydraulic test analysis[END_REF]. This method also permitted identification of flow behaviors and an estimation of the system's transmissivity based on Cooper and Jacob's approximation [START_REF] Renard | Understanding diagnostic plots for well-test interpretation[END_REF].

HIGH RESOLUTION FLOW AND TEMPERATURE LOGGING

Structural heterogeneity of fractured rocks induces flow partitioning within the aquifer system. To examine preferential flow path and fracture connectivity we used high-resolution flow logging (Heat Pulse Flowmeter, Geovista©) under ambient and pumping conditions (Le Borgne et al., 2006b;[START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF]. Single-borehole flowmeter tests provide information on the main inflow or outflow zones in a well and measure the prevailing ambient flow conditions in the absence of pumping (Le Borgne et al., 2006b;[START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF]. During a pumping test, cross-borehole flowmeter tests (pumping in a given well and measurement in another) are based on the observation that transient flow during pumping in a given aquifer modifies the hydraulic head distribution with time in the different permeable domains and produces measurable changes in the vertical flow profiles. The connectivity and hydraulic properties of the main flow paths can be identified by transitory flow measurements (Le Borgne et al., 2006b;[START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF].

To define flow paths and origin of the water, temperature logging was also used with a multiparameter Hydrolab probe (OTT©) and Ocean seven 303 CTD (Idronaut©). Differences in hydraulic heads between the fractures in a borehole, under ambient conditions or during pumping, will modify the temperature profile with respect to the geothermic gradient. Therefore these temperature profiles are very useful to identify hydraulic connections between compartments and their evolution through time [START_REF] Anderson | Heat as a ground water tracer[END_REF][START_REF] Bense | Thermal anomalies indicate preferential flow along faults in unconsolidated sedimentary aquifers[END_REF][START_REF] Chatelier | Combined fluid temperature and flow logging for the characterization of hydraulic structure in a fractured karst aquifer[END_REF].

TRACER TESTS

To identify transfer time distribution and to characterize groundwater flows within the main transmissive fault zone, tracer tests were performed between observation wells and the pumped borehole. Tracer breakthrough concentrations were monitored in the pumped well (F3) within the upper casing and below the pump with a flow-through field fluorometer developed by the University of Neuchâtel [START_REF] Schnegg | Tracer tests made easier with field fluorometers[END_REF][START_REF] Schnegg | Online field fluorometers for hydrogeological tracer tests[END_REF]. The fluorometers had been previously calibrated with tracer and local fresh water in the laboratory. To optimize injection, we identified the main fractured part of the fault zone connected to the pumped well (F3) in each observation wells (MFT80 and FC4). Uranine tracer solution was injected in front of the main inflow zone and we checked that no tracer solution remained in the borehole after the injection by logging concentration through borehole depth.

GEOCHEMISTRY AND GROUNDWATER DATING

To identify water origin and estimate mixing processes, we analyzed geochemistry and groundwater dating data under ambient and pumping conditions. The apparent ages of water samples were determined by measuring Chlorofluorocarbons (CFCs) concentrations. These anthropogenic gases provide information on groundwater resident times up to 50 years and can give information on mixing processes between different water sources during flow changes [START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF][START_REF] Busenberg | Use of chlorofluorocarbons (CCl3F and CCl F2) as hydrologic tracers and age-dating tools: The alluvium and terrace system of central Oklahoma[END_REF][START_REF] Cook | Recent advances in dating young groundwater: chlorofluorocarbons, 3H/3He and 85Kr[END_REF][START_REF] Cook | Groundwater ages in fractured rock aquifers[END_REF][START_REF] Jaunat | Hydrochemical data and groundwater dating to infer differential flowpaths through weathered profiles of a fractured aquifer[END_REF].

Waters were sampled in stainless-steel ampoules after discarding at least three ampoule volumes, and ensuring that there was no contact of the water with atmospheric gases.

Groundwater CFCs concentrations were measured by the Geosciences Rennes Laboratory [START_REF] Labasque | Analyse des CFC dans les eaux souterraines[END_REF]. Water samples were degassed by N2 stripping and then injected into a gas chromatograph equipped with an electron capture detector (GC-ECD). The CFC concentration obtained by chromatographic analysis was converted to the atmospheric mixing ratio (pptv -Parts per Trillion by Volume) by applying a solubility function (modified Henry's Law for trace gases [START_REF] Ayraud | Compartmentalization of physical and chemical properties in hard-rock aquifers deduced from chemical and groundwater age analyses[END_REF][START_REF] Bu | Solubility of chlorofluorocarbon 113 in water and seawater[END_REF][START_REF] Warner | Solubilities of chlorofluorocarbons 11 and 12 in water and seawater[END_REF][START_REF] Weiss | Nitrous oxide solubility in water and seawater[END_REF]). The CFC atmospheric mixing ratios were then compared with the atmospheric evolution curve [START_REF] Walker | Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC-11 CFC-12, CFC-113, and carbon tetrachloride[END_REF] to determine the apparent recharge year, and thus the apparent age of the groundwater. Analytical uncertainty for the CFC concentrations was estimated [START_REF] Labasque | Analyse des CFC dans les eaux souterraines[END_REF] at around 3 % for recent water (<20 years) and 5 % for old water (between 55 to 30 years).

Major and trace elements were analyzed by ion chromatography (Dionex DX-100) and ICP-MS HP 4500 at the Geosciences Rennes Laboratory following standard laboratory methods. The analytical uncertainty for major anions and cations was about 2%.

RESULTS

AMBIENT FLOW CONDITION

(Figure 3 shows the evolution of hydraulic head in the main boreholes representative of the fractured domains (FC4, F3, MFT80 and MFT20) and in the upper weathered reservoir (T i ). The seasonal evolution of hydraulic head is similar in all the aquifer system with a fast response to rainfall during the recharge periods and a classic discharge regime observed during summer.

The hydraulic head gradient mostly follows the topography and indicates groundwater flow towards the outcropping of the fault zone and discharge into the river (FC4-F3-MFT80 in (Figure 2b&c).

Measurements of the hydraulic gradient with depth revealed a higher hydraulic head in the deeper boreholes ((Figure 3). This suggests a definite vertical compartmentalization of the aquifer system.

The three boreholes intersecting the permeable fault zone show similar hydraulic responses, with a potential upward flow, according to the hydraulic gradient analysis. The MFT20 well, located at the edge of the weathered and the fractured bed rock, remains under the influence of the fault zone.

The sub-surface aquifer situated within the upper weathered part (T i boreholes), appears to be much less influenced by the deep structure.

The fault domain responds quickly to rainfall even though it is locally covered by the sub-surface reservoir with lower hydraulic head. This suggests that the bed rock domain can respond rapidly to recharge. The recharge area is certainly along strike and up topographic gradient through favorable deep fractured zones.

(Figure 3)

A typical variation of flow rate with depth is shown in (Figure 4. All flow profiles obtained under ambient conditions reveal upward flow, implying that hydraulic head is higher in the deep fractures (235m for FC4) than in the upper fractures (45m for FC4). FC4 profile in ambient conditions show a major inflow zone at 245 m deep and a major outflow zone at 49 m deep (Figure 4). No other contributing fractures can be detected, either because of the limit of detection of the tool, or simply because of the lack of any hydraulic head gradient between the head in the fractures and the head in the borehole. In pumping conditions, and for a much lower hydraulic head in the borehole, few others fracture may be detected between 160 and 215 meters deep. In any case, the flow rate profiles obtained under pumping conditions confirms that the deep conductive fault zone provides about 50 to 75% of the total productivity of each borehole ((Table 2). For all profiles, the fracture transmissivity was determined by using the model developed by [START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF]. The transmissivity of the deep productive sections in each well appeared to be relatively high, ranging between 6 and 9 x 10 -4 m²/s ((Table 2).

(Table 2)

Groundwater dating by CFCs analyses also highlighted the influence of an old deep water origin, with tracer concentrations close to the detection limit in all deep boreholes ((Table 3), including the 20m depth well. The apparent water age is older than 55 years. Conversely, CFCs analyses in the first 10 m of the weathered domain revealed recent waters and contamination with nitrate. Globally, the apparent age determined in the upper weathered reservoir and based on a piston flow model, is 20 years. Moreover, the water age in this upper part of the aquifer system decrease uphill. This suggests that the subsurface aquifer is partially influenced by deep groundwater discharge (more or less 25% of old deep water).

(Table 3)

To summarize, three main hydraulic domains were highlighted at the Saint-Brice site: (1) the fault zone which has a higher transmissivity, (2) the surrounding domain affected by secondary fractures less transmissive and (3) the upper weathered part of the aquifer system, restricted to the first 10 meters. Under ambient conditions, the hydrogeological system is clearly driven by the discharge from a deep groundwater system towards the upper part of the aquifer system and the local stream.

(Figure 4)

PUMPING CONDITION

EVOLUTION OF HYDRAULIC HEADS AND DIAGNOSTIC PLOT

During the 60-days pumping experiment, the hydraulic heads in the different boreholes were recorded ((Figure 5). All wells located in or near the sub-vertical fault zone, including the shallow piezometers in the sub-surface weathered domain, reacted quickly to pumping. During the first 10 days, drawdown was limited to a few meters and was similar in all boreholes. This suggests a good connection between the upper and lower domains of the aquifer system. After 10 days of pumping, drawdown has exceeded the depth of the sub-surface wells (<10 m deep), due to desaturation of the upper weathered reservoir around the pumping well. As observed in all deep boreholes, drawdown increased significantly during the last 50 days of pumping ((Figure 5).

(Figure 5) Figure 6 displays the normalized drawdown (s/Q) of pumping well F3 and its derivative through time in a log-log diagram. The evolution of the derivative curve provides information on flow behaviors and on boreholes effects such as wellbore storage and skyn effects. Such diagnostic curves can be used to interpret different flow regimes [START_REF] Bourdet | A new set of type curves simplifies well test analysis[END_REF][START_REF] Renard | Understanding diagnostic plots for well-test interpretation[END_REF], e.g. double porosity, leakage effect, no-flows boundaries… Here, considering the specific geometry of the aquifer system, we assumed that flow regimes can be described through a simple model developed for fracture or strip aquifer embedded in lower permeable block [START_REF] Butler | Pumping tests in non-uniform aquifers: the linear strip case[END_REF][START_REF] Escobar | Pressure and pressure derivative analysis for long naturally fractured reservoirs using the TDS technique[END_REF]Gringarten, 1996;[START_REF] Moench | Double-Porosity Models for a Fissured Groundwater Reservoir With Fracture Skin[END_REF][START_REF] Rafini | Insights from numerical modeling on the hydrodynamics of non-radial flow in faulted media[END_REF][START_REF] Tiab | Analysis of pressure derivative data of hydraulically fractured wells by the Tiab's Direct Synthesis technique[END_REF]. Such fracture models involve generally three characteristic flow regimes: the early local response of the fault zone, a period of transition controlled by the geometrical fault zone properties at larger scale and finally the global hydraulic response of the system.

Flow behaviors can be detailed as follow through derivative description: ((Figure 6). 1) After the first early-time where the drawdown response is influenced by wellbore storage and skin effects, a short radial flow response is observed. This reflects the behavior of the local permeable fault zone, where water was principally extracted from the local fracture storage and eventually fed by proximal matrix storage and/or local leakage from overlaying aquifers. The constant derivative value suggests a transmissivity around . The derivative decrease between 0.1 and 1 day can be the consequence of preferential leaky flows from shallower reservoir. 2) Between 1 to 30 days of pumping, the drawdown derivative curve is characterized by a transitory period with a strong increase (derivative slope of ½). This derivative increase can be interpreted as double porosity behavior for which lateral inflows from surrounding reservoirs supply the permeable fault at larger scale. The fault geometry can also influence the drawdown during this transitory period which induces linear flow regime. 3) Finally, the system reached a radial flow condition during the last month of pumping, where the whole system was solicited indicating that the major flow comes from surrounding low-permeability domains [START_REF] Butler | Pumping tests in non-uniform aquifers: the linear strip case[END_REF]. The normalized constant derivative of 0.18 m at the end of the test implies a global transmissivity of the aquifer system about .

(Figure 6)

FLOWMETER TEST AND TEMPERATURE LOGGING

Flow and temperature logs under natural and pumping conditions are presented for MFT80 in (Figure 7. Other observation wells yielded similar results. Under ambient conditions, upward flows were identified ((Figure 7 Temperature logs ((Figure 7 2 ), carried out under ambient conditions and during pumping in all observation wells, showed a clear temperature decrease of about 0.2 to 1°C with pumping. The homogeneity of the profiles, with lower temperatures than under ambient conditions, suggests that the deep aquifer domain was fed by colder water coming from the sub-surface domain ((Figure 7 b 2 and c 2 ). This confirms that pumping is initiated within the fault zone at depth and that the upper aquifer domain is supplying the deeper structure.

(Figure 7)

PUMPING TEST MODELING

The hydraulic parameters of the aquifer system were evaluated by fitting the drawdown data with classic analytical models. Different solutions were applied to interpret the dataset at different times ((Figure 8). in view to characterize the two main flow behaviors involved during the test, previously characterized by the derivative interpretation: 1) the fault storage response, supported by leaky flow from the weathered part of the aquifer system (<10 days of pumping) and 2) the global double porosity response of the aquifer system during long term pumping.

For the first 10 days of pumping, the double porosity analytical solution according to Moench's model [START_REF] Moench | Double-Porosity Models for a Fissured Groundwater Reservoir With Fracture Skin[END_REF] was used to model the local fault zone response. This model is based on the concept of a fracture embedded within blocks of lower permeability, characterized by matrix storage. Solutions are presented in equations 19 and 20 from the Moench paper of 1984 and plotted for the early time in MFT80 for both drawdown and its derivative in (Figure 8a. Quadratic head losses were estimated by a variable rate pumping test at 2.5 10 4 m -5 s 2 . Hydraulic parameters deduced from all wells are listed in (Table 4. These parameters confirm that, during the first days of pumping, flow is controlled by a relatively high transmissive ( fault zone embedded in lower permeability blocks ( . A strong fracture skin value (set to 1) was required to model the first times of both drawdown and derivative, which suggest a strong delay contribution between surrounding blocks to the fractured zone [START_REF] Moench | Double-Porosity Models for a Fissured Groundwater Reservoir With Fracture Skin[END_REF].

(Table 4)

We also used the "Two Aquifer-System" model developed by [START_REF] Hunt | Flow to a Well in a Two-Aquifer System[END_REF] to characterize drainage flows from weathered reservoir to the fractured domain. This model assumes a screened well in an aquifer overlain by an aquitard and an unconfined aquifer which may represent here the upper weathered reservoir. A mean transmissivity of for the deeper aquifer domain, and transmissivity and porosity estimates of and 2% respectively, for the superficial aquifer was required to fit both drawdowns and derivatives (plot in (Figure 8a, results in (Table 4).

For modeling the hydraulic behavior of the hydrological system at global scale after the first 8 hours of pumping, Moench's model [START_REF] Moench | Double-Porosity Models for a Fissured Groundwater Reservoir With Fracture Skin[END_REF] proved to be the simplest and most efficient model to reproduce the data ((Figure 8b). The estimated transmissivity of the fractured system is , embedded within a reservoir (matrix) with a lower permeability of and a relatively high specific storage of . A high value of fracture skin is still necessary to reproduce the transitory flow regime observed during transitory pumping times (Figure 6). The transmissivity deduced for long times pumping tends to the transmissivity of the global permeable system [START_REF] Butler | Pumping tests in non-uniform aquifers: the linear strip case[END_REF].

Note that the estimation of storativity in adjacent blocks based on Moench's model is relatively large.

However, storativity is in general difficult to estimate in heterogeneous crystalline rocks [START_REF] Burbey | Use of time-subsidence data during pumping to characterize specific storage and hydraulic conductivity of semi-confining units[END_REF]Le Borgne et al., 2006a;[START_REF] Meier | An evaluation of Jacob's Method for the interpretation of pumping tests in heterogeneous formations[END_REF][START_REF] Neuman | Perspective on "Delayed yield[END_REF]. This higher value may also reflect the fact that only two main compartments (fracture and matrix) were used in the model, whereas several capacitive domains, including a superficial one in the weathered rocks, may actually be involved in the response to pumping.

(Figure 8)

GEOCHEMISTRY AND GROUNDWATER DATING

Chemical analyses of major ions revealed Ca-Mg-CO 3 in water from the fault zone and Ca-Mg-CO 3 SO 4 -Cl, characteristic of anthropogenic activities, in water from the shallower weathered domain. A clear evolution of groundwater chemistry was observed during pumping, due to the mixing of water of different origins. In particular, mixing with the upper weathered reservoir was clearly apparent during the first ten days of pumping. The geochemical response after a longer period was dependent both on shallower and "matrix" contributions and modified by reactivity processes. Description of geochemical evolution will require a much more detailed analysis and will be published in another article (Roques et al., "In Preparation"). Our main focus here is to highlight changes in the apparent age of groundwater, using conservative anthropogenic tracers such as CFCs to highlight the mixing processes.

The evolution of groundwater apparent age was deduced from the CFC-12 and CFC-113 concentrations in samples taken at the pumping wellhead ( (Figure 9). An asymptotic increase in CFCs concentrations was observed, implying a decrease in apparent age, given the assumption of a piston flow model. Under ambient conditions, the CFCs concentrations in the F3 well were negligible, close to the measurement uncertainty, and characteristic of an apparent age exceeding 55 years. After 60 days of pumping, the mean apparent age of water sampled at the outflow was about 40 years. This clearly confirms the influence of the recent water solicited during pumping. Since old water was found in most of the boreholes, this recent water must come from the upper weathered aquifer (<10m). The result also confirms the relatively fast transfer time between compartments.

(Figure 9)

The contribution of the weathered aquifer may easily be estimated by considering a simple binary mixing assumption. We first assumed that mixing occurred between i) water of high apparent age i.e., 55 years (CFC12 concentrations of 0 to 50 pptv) the representative signature of deep fault water, and ii) water from the upper weathered compartment with a mean apparent age of 20 years (CFC12 concentrations of 550 pptv measured in the superficial boreholes before pumping). Based on these assumptions, the estimated contribution of the upper weathered aquifer was around 15-20% at the end of pumping (last 30 days, CFC concentration = 105 pptv).

These observations clearly demonstrate that the superficial compartment contributes to the global mixing process when the deep fault zone aquifer is pumped. It is difficult to clearly define the contribution of each compartment without a detailed analysis of the mixing process (Roques et al., in preparation), which is beyond the scope of the present work. However, both the geochemistry evolution and groundwater dating suggest that after one month of pumping about 20% of the pumped water comes from the upper part of the aquifer system.

TRACER TESTS

Two tracer tests were performed during pumping. Uranine tracer was injected in the main productive fault zone in two observation wells: MFT80 and FC4 at 55 and 235 meters depth respectively. (Figure 10 displays the evolution of normalized concentrations with time for the two breakthrough curves. In the first tracer test (MFT80 towards F3), the tracer was detected 4 hours after injection while the maximum concentration occurred at about 16 hours. The estimated travel distance between the injection point in MFT80 and the main inflow zone in F3 was around 53 meters, implying an average flow velocity of about 3.3 m/h. In the second test (FC4 to F3), the tracer appeared 20 hours after injection and the peak concentration was apparent after about 220h. A slight double peak was observed, certainly due to different flow paths between the two wells. Taking into account a travel distance of about 135m, the estimated average flow velocity in the deep structure was 0.6 m/h, i.e. 5.5 times slower than in the upper part of the fault zone. This implies either that the Darcy velocities were much lower in the deeper compartment or that porosity in the deeper compartment was much higher.

The [START_REF] Becker | First-passage-time transfer functions for groundwater tracer tests conducted in radially convergent flow[END_REF] solution for converging radial flow was used to compute the present breackthrough curves (Figure 10). The use of this model is justified for highly dispersive system like this one (Peclet number are around 5, where Pe = R/α, with R is the travel distance and α the dispersivity). The modeling objectives are to derive first order transport parameters and to estimate roughly flow partitioning within the permeable fault zone ((Figure 10). The modeling was achieved by setting the injected mass to the restitution rate (around 85% for MFT80 and 45% for FC4). It was not possible to use the same set of parameters to fit the two breakthrough curves ((Figure 10). Firstly, dispersivity was apparently two times higher during the deeper tracer test (FC4).

This increase in dispersivity can be explained by a scaling effect that is classically observed in large scale tracer tests [START_REF] Gelhar | A critical review of data on field-scale dispersion in aquifers[END_REF] and due to the integration of heterogeneities along pathways.

Secondly, fitting both curves would require either constant flow rates or constant parameters for the fault zone (thickness and porosity). Assuming an average fault thickness of 6 meters and a constant porosity of around 1 to 2% we were able to obtain a relatively good match for both breackthrough curves ((Figure 10). Porosity values obtained in this case appears relatively high for crystalline rocks, but still reasonable, considering FC4 cores samples (Figure 2), which show that the productive zones are highly fractured and open. The difference in restitution time between the two tracer tests could be honored in this case only by using different effective flowrates for the two tracer tests. This interpretation would imply that the flow-rate from the upper of the aquifer system represented about 2/3 of the total pumping rate while the flow rate from the deeper part was limited to the other 1/3.

Note that the difference in restitution time could be similarly modeled by assuming a permeability decrease with depth or a porosity increase with depth. However, an increase of porosity with depth would be difficult to explain, and permeability, as deduced from previous single borehole tests, appears to remain relatively constant with fault deepening ((Table 2). Such differences may be also partly the results of fault zone heterogeneities. Nevertheless, in both cases tracer injection were done in fractured zone of similar local permeability values. It therefore seems more reasonable to model both tracer tests by assuming flow partitioning through depth.

(Figure 10)

DISCUSSION: CONCEPTUAL MODEL

The results obtained in the previous sections were synthesized to derive a global conceptual model of the Saint-Brice en Coglès aquifer system. Our aim was to describe the hydraulic functioning of this steep fault zone under ambient and pumping conditions, and to estimate the groundwater resources actually available.

SUB-VERTICAL STRUCTURE PROPERTIES AND IMPACT ON GROUNDWATER FLOW

The main permeable structure has developed along an extensive normal fault zone. Boreholes at different depth (MFT80, F3 and FC4) were used to investigate the fault zone properties. The fault permeability appears relatively constant with depth and constitutes a sub-vertical strip fractured zone embedded in low-permeability domains and overlain by a weathered horizontal unconfined domain which does not exceed 10 meters depth (T i piezometers).

Various complementary approaches were used to characterize the discharge of deep groundwater through the sub-vertical permeable fault zone. Geochemistry and groundwater dating indicated the presence of an old water signature in the fractured domain from 250 to 20m. As the hydraulic heads in the deeper part are higher, the deep water finds a preferential flow path along the permeable fault zone and is discharged towards the subsurface aquifer domains in the watershed outlet ((Figure 11a). Fluxes from the deeper part of the aquifer system can be quantified from the fault zone permeability deduced from hydraulic tests and the mean hydraulic gradient between the deep and subsurface domain. Assuming that the flowrate is constant along the length of the fault (about 1000m deduced from the estimated lineament length), the discharge ranges between 170 and 200 m 3 /day. Although this amount of natural flux along fault zones is significant, it is generally poorly known, and is not usually considered in the local hydrogeological balance.

(Figure 11)

FLOW BEHAVIOR AND RELATIONSHIPS BETWEEN COMPARTMENTS UNDER PUMPING

CONDITIONS

A multi-disciplinary approach was adopted to investigate flow behavior and water exchanges between the different aquifer domains which enabled us to derive a conceptual model of the hydraulic behavior of this deep permeable fault zone.

The most relevant observation is that the sub-vertical fault is clearly connected with the sub-surface reservoir and the two compartments are therefore highly dependent on each other. Globally, pumping in the deep structure induces downward groundwater flow from the upper aquifer domain ((Figure 11b). Due to the limited storage in the fault zone, water fluxes during pumping are mainly supported by the overlaying reservoir and/or adjacent domains. After two months of water extraction, it is clear that pumping involves contributions from the surrounding compartments, which implies sub-surface and lateral inflows (matrix storage) towards the sub-vertical structure. Tracer test analyses showed that about 2/3 of the water flow originates from the upper part of the aquifer system. This partition of fluxes seems to be governed mainly by hydraulic boundary conditions which limit the deep supply during pumping. As a result, the superficial part of the aquifer seems to constitute a key reservoir supplying the faulted bedrock domain during pumping. The lateral influence of pumping seems to be limited to 300 meters from the fault, as observed in some subsurface boreholes. Thus, in the case of a fault with a large dip (approximately 70°) and a high permeability contrast with surrounding compartments, the radial expansion of the drawdown and pressure variation is preferentially transferred along the fault zone. In the present case, pressure variations diffuse mainly along the graben structure, in the lower topographic zone where the wetland has developed ((Figure 11b). This behavior implies that fault zones, when they are permeable, may allow pressure diffusion along the fault plane that can act almost as a boundary condition to drain groundwater from the surrounding reservoirs. Similar behavior has been described through numerical modeling of groundwater flow in fault zones [START_REF] Leray | Numerical modeling of the productivity of vertical to shallowly dipping fractured zones in crystalline rocks[END_REF].

CONCLUSIONS

In this study, we used a multidisciplinary approach to investigate the hydrogeologic behavior of a sub-vertical permeable fault and quantify its interactions with surrounding reservoirs, under ambient and pumping conditions. This study provides a good example of the functioning of a sub-vertical fault determined from lineament mapping.

Our results suggest that sub-vertical fault and sub-surface reservoirs are highly dependent on each other. Under ambient conditions, the fault allows the discharge of regional old water into superficial aquifer domains. The natural discharge rate of the fault zone is estimated to be around 170 to 200 m 3 /day. Although relatively low, this value should be taken into account when estimating water fluxes, hydrologic budget and solute transport at the watershed scale. During the early stages of pumping, the hydrological system is strongly dependent on storage from the sub-surface weathered reservoir. Once the upper weathered reservoir has become mainly unsaturated, the system acts as a classic dual porosity medium with a highly transmissive structure embedded in lower permeable compartments. Thus, this high permeability fault zone appears to be an efficient thin permeable domain that permits rapid diffusion of pressure but is strongly dependent on sub-surface and adjacent domains of higher storativity. All field measurements suggest that under pumping conditions, most of the flow comes from superficial domains and from the vicinity of the fault zone, with a recharge area located at the surface mainly along the fault zone. Moreover, such steep fault zones, although of relatively high transmissivity, remain relatively limited in terms of groundwater yield. In this study, the pumping test was carried out for 2 months at a flow-rate of 45 m 3 /h, but the piezometric analysis indicates that this rate was certainly too high to be sustainable. A better estimate of sustainable flowrate would be around 20 to 30 m 3 /h. This is a much lower value than some other fault zone aquifers, such as gently dipping fault zones which may provide higher groundwater resources (Le Borgne et al., 2006a;[START_REF] Ruelleu | Impact of gently dipping discontinuities on basement aquifer recharge: An example from Ploemeur (Brittany, France)[END_REF]. This difference confirms the Parameters used in the model are presented in the table (Welty and Gelhar, 1994). [START_REF] Paillet | Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations[END_REF].

Table 3: CFC and SF6 concentrations converted into atmospheric concentration (PPTV, (Bu and Mark J Warner, 1995;[START_REF] Warner | Solubilities of chlorofluorocarbons 11 and 12 in water and seawater[END_REF]).

Table 4: Pumping test results for pumping well F3 and observation wells MFT80, FC4 and MFT20.

RMSE is calculated over the analyzed part of the pumping test.

a.

b.

FIG1.pdf

  a 1 ), implying that the hydraulic head in the deeper part of the fault zone was higher than in the upper part of the aquifer domain. During pumping, downward flows were measured, indicating a greater decrease of the hydraulic head in the fault zone. Downward flow could be even 3 times greater than the previous upward flow, which implied a clear inversion of flow between the hydraulic permeable domains ((Figure7b 1 ). Nevertheless, the downward borehole flows slowed down after a few days, in conjunction with the decrease in head gradient between the two compartments ((Figure7c 1 ).

Figure 1 :

 1 Figure 1: a) Location map and regional geological map. Star symbol indicates the St Brice Field site, ° 'N ° 'W, alt. a.s.l. odified fro Cha trai e, (Chantraine et al., 2001)). b) Local

Figure 2 :

 2 Figure 2: Geological conceptual model of the St Brice field site. a) Conceptual geological structure in a

Figure 3 :

 3 Figure 3 : Typical hydraulic head variations measured in the different boreholes at Saint-Brice and

Figure 4 :

 4 Figure 4: Flowrate profiles in FC4 borehole under (a) ambient conditions (before pumping), measured

Figure 5 :Figure 6 :

 56 Figure 5 : Drawdown in the pumping well (F3, blue triangle dots), in observation wells intersecting the

Figure captions

  Figure captions

Figure 7 :

 7 Figure 7: Flow and temperature profiles under ambient (grey lines) and pumping conditions (after 9

Figure 8 :

 8 Figure 8 : Drawdown (s) evolution with time and its derivative s' duri g pu pi g. O ser atio s

Figure 9 :

 9 Figure 9 : Apparent age of water sampled at the outflow and deduced from CFC analysis.

Figure 10 :

 10 Figure 10 : Tracer test data between the shallow aquifer (MFT80, gray point symbol) and the deep

Figure 11 :

 11 Figure 11 : Conceptual model of groundwater flow at the St-Brice site under a) ambient conditions

  FIG9.tif

Table 1 :

 1 Borehole descriptions

Table 2 :

 2 Transmissivity results obtained by modeling borehole flow profiles using Paillet solutions

role of the dip of the fault-zone that may greatly increase borehole yield in some circumstances[START_REF] Leray | Numerical modeling of the productivity of vertical to shallowly dipping fractured zones in crystalline rocks[END_REF].

Acknowledgments

Most funding came from the CASPAR project in collaboration with OSUR and BRGM and co-funded by the French Water Agency of Loire-Brittany (AELB), the Regional of Council of French Brittany, the Department of Ile-et-Vilaine and the French Ministry for Education and Research. We also wish to thank the European Interreg IV project Climawat, which partly funded some of the experiments. We want to thank Matthew Becker and anonymous reviewer for their very constructive and encouraging comments. We would also like to thank Nicolas Guihéneuf for his help to interpret our tracer test results. We thank all the people who were involved in the project: Odile Hénin, Martine Bouhnik-Le Coz, Patrice Petitjean, Patricia Madec, Hélène Pauwels, Flora Lucassou, Jean-Pierre Jegou, Eric Palvadeau, Antoine Armandine Les-Landes, Joaquin Jimenez-Martinez, Pascal Goderniaux, Thomas Stieglitz, Stéphane Durand, Ghislain Ferre, Olivier Dauteil, Christian Camerlynck, Pierre-Yves Galibert, Fayçal Rejiba, Philippe Bardy, Antoine Rivera…

We propose hydrogeological conceptual model of fault aquifer in crystalline basement.

We characterize interactions between a fault aquifer and surrounding compartments.

Sub-vertical fault resource is strongly dependent on superficial aquifer storage.

We estimate groundwater resource availability.

Highlights.doc


M.D Loïc Sentilhes
email: loicsentilhes@hotmail.com

Ph.D François Goffinet

Ph.D Gilles Kayem

Management of placenta accreta

Keywords: ICU, intensive care unit NICU, neonatal intensive care unit PPH, postpartum hemorrhage abnormally invasive placenta, placenta accrete or percreta, management, antenatal diagnosis, cesarean hysterectomy, conservative treatment

Cesarean hysterectomy is the gold standard treatment for placenta accreta. For placenta percreta with bladder invasion or for young women who want the option of future pregnancy and who agree to close follow-up monitoring, conservative treatment is a valid option. 4

Introduction

Abnormally invasive placentas are a life-threatening condition characterized by placental villi being abnormally adherent to the myometrium due to the absence of, or defects in, the normal decidual basalis and the fibrinous Nitabuch´s layer [START_REF] Tseng | Differential expression of vascular endothelial growth factor, placenta growth factor and their receptors in placentae from pregnancies complicated by placenta accreta[END_REF]. They are commonly classified into three distinct grades according the degree of the placental villi invasion into the myometrium: placenta accreta (placental villi are attached to the decidual surface of the myometrium), placenta increta (placental villi more deeply invading into the myometrium) and placenta percreta (placental villi invade through the myometrium and the uterine serosa and sometimes into adjacent organs, such as the bladder) [START_REF] Oyelese | Placenta previa, placenta accreta, and vasa previa[END_REF]. In a clinical setting and daily practice, these histopathological differences are probably non-existent. Therefore, in this article, the term "placenta accreta" is used as a general term to describe all of these three conditions whereas the term percreta refers to placenta percreta [START_REF] Oyelese | Placenta previa, placenta accreta, and vasa previa[END_REF].

Placenta accreta has become an important cause of maternal morbidity and mortality.

For example, more than half of women are transfused or admitted to an intensive care unit (ICU) [START_REF] Eller | Optimal management strategies for placenta accreta[END_REF][START_REF] Warshak | Effect of predelivery diagnosis in 99 consecutive cases of placenta accreta[END_REF] and mortality rates as high as 7% have been reported to be associated with placenta percreta [START_REF] O'brien | The management of placenta percreta: conservative and operative strategies[END_REF]. Moreover, placenta accreta is the leading cause of peripartum hysterectomy [START_REF] Daskalakis | Emergency obstetric hysterectomy[END_REF], failed vessel ligation [START_REF] Sentilhes | Fertility and pregnancy outcomes following uterine devascularization for severe postpartum haemorrhage[END_REF] and pelvic arterial embolization [START_REF] Sentilhes | Predictors of failed pelvic arterial embolization for severe postpartum hemorrhage[END_REF]. Nevertheless, the optimal management of placenta accreta remains a topic of debate. The aim of our review was to assess the available useful procedures in an attempt to contribute to decreasing the morbidity of this condition.

Methods

Data for this review were identified based on searches of Medline, Current Contents, and PubMed from 1965 to January 2012, and references from relevant articles using the following search terms: "placenta accreta", "placenta increta", "placenta percreta", "morbidly adherent placenta", "antenatal diagnosis", "extirpative approach", "cesarean-hysterectomy", "conservative treatment". Only articles published in English were included. Further articles were identified by cross-referencing. The majority of publications on placenta accreta are cohort or case-control studies, or case series/reports and descriptive studies. No randomized controlled trials were retrieved. Articles on more general topics were selected on the basis of their relevance or interest as regards this specific subject with particular attention given to articles that dealt with the management of placenta accreta.

Organization of the delivery

Role of prenatal identification of placenta accreta

Prenatal identification of placenta accreta is essential in order to manage optimal delivery circumstances for these patients [START_REF] Kayem | PACCRETA: clinical situations at high risk of placenta ACCRETA/percreta: impact of diagnostic methods and management on maternal morbidity[END_REF]. The key role of this in the management of patients with placenta accreta was highlighted by Warshak et al. [START_REF] Warshak | Effect of predelivery diagnosis in 99 consecutive cases of placenta accreta[END_REF]. These authors reported that patients with a pre-delivery diagnosis of placenta accreta for whom a cesarean hysterectomy with no attempted removal of the placenta was planned at 34-35 weeks gestation, required fewer units of packed red blood cells and tended to have a lower estimated blood loss than those with no pre-delivery diagnosis (4). This decrease in maternal morbidity for patients with a predelivery diagnosis of placenta accreta has also been reported by others [START_REF] Tikkanen | Antenatal diagnosis of placenta accreta leads to reduced blood loss[END_REF].

Another advantage of prenatal identification is the possibility to plan the delivery in a suitable center with a multidisciplinary team and adequate equipment and resources including a maternity-oriented intensive care unit (ICU), an embolization unit with interventional radiologists, a blood bank capable of managing massive transfusion requirements, and the availability of other technical skills (urologists, vascular surgeons). It makes intuitive sense that expertise and experience are useful when managing difficult and somewhat uncommon problems. This has been demonstrated by Eller et al. who found that maternal morbidity is reduced in women with placenta accreta who delivered in a tertiary care hospital with a multidisciplinary care team in comparison with those managed in standard obstetric care facilities [START_REF] Eller | Maternal morbidity in cases of placenta accreta managed by a multidisciplinary care team compared with standard obstetric care[END_REF].

Timing of delivery

The optimal timing of delivery for the patients with placenta accreta remains controversial.

Whatever the gestational age chosen, practitioners have to plan an organization to include the unexpected occurrence of possible cases of acute bleeding requiring an emergency cesarean section. In a large multicenter trial, 23.9% (27/113) of women with placenta accreta in whom cesarean section was planned had an emergency delivery [START_REF] Sentilhes | Predictors of failed pelvic arterial embolization for severe postpartum hemorrhage[END_REF]. O'Brien et al. reported that after 35 weeks, 93% of patients with placenta accreta experienced hemorrhage necessitating delivery [START_REF] O'brien | The management of placenta percreta: conservative and operative strategies[END_REF]. In order to decrease this risk of emergency delivery, Warshak et al. proposed that patients with a highly suspected placenta accrete should be delivered by cesarean hysterectomy at 34-35 weeks (4). Compared with patients with no pre-delivery diagnosis of placenta accreta (n=37; mean gestation at delivery 33.9±1.1 weeks), those with a pre-delivery diagnosis (n=62; mean gestation at delivery 34.7±1.2 weeks) demonstrated significantly higher rates of admission to the neonatal ICU (86% compared with 60%), and longer neonatal hospital stays (10.7±1.9 compared with 6.9±2.1 days). Despite the limited size of their patient cohort, this study suggested that higher neonatal morbidity may occur if the cesarean is planned before 36 weeks of gestation (4). Finally, an analytical decision model showed that the preferred gestational age at which to deliver these individuals at high risk is almost certainly at or beyond 34 weeks, but not later than 37 weeks [START_REF] Robinson | Effectiveness of timing strategies for delivery of individuals with placenta previa and accreta[END_REF]. Nevertheless, it has been emphasized that the current literature detailing the probability of events related to emergency delivery of women with suspected placenta accrete, is not definitive enough to allow the unequivocal establishment of a single ideal gestational age at which to deliver the women who are at high risk [START_REF] Robinson | Effectiveness of timing strategies for delivery of individuals with placenta previa and accreta[END_REF]. Therefore, in absence of any bleeding, it seems currently reasonable to plan delivery at about 35-36 weeks gestation in cases of suspected placenta accreta. This opinion seems to be shared by the majority of American practitioners as about 50% of respondents reported in two national surveys to have planned the cesarean section at 36 weeks gestation [START_REF] Esakoff | PAMUS: placenta accreta management across the United States[END_REF][START_REF] Jolley | Management of placenta accreta: a survey of Maternal-Fetal Medicine practitioners[END_REF].

Procedures

There are four basic options for management of placenta accreta: the extirpative method, the cesarean hysterectomy, conservative treatment and the one-step conservative surgery which is an alternative conservative technique. Whatever the option chosen, when placenta accreta is suspected before delivery in a woman with an anterior placenta previa, it is recommended to perform a vertical fundal uterine incision to avoid the placenta and reduce the risk of massive postpartum hemorrhage (PPH).

The extirpative method

This procedure consists of performing a forcible manual removal of the placenta delivery in an attempt to obtain an empty uterus. The technique is associated with a higher rate of massive PPH and subsequent peripartum hysterectomy than the conservative approach [START_REF] Kayem | Conservative versus extirpative management in cases of placenta accreta[END_REF][START_REF] Bretelle | Management of placenta accreta: morbidity and outcome[END_REF]. Therefore, this option should be abandoned when other procedures are available [START_REF] Sentilhes | Fertility and pregnancy outcomes following conservative treatment for placenta accreta[END_REF].

Unfortunately, this happens in most cases with undiagnosed placenta accrete, i.e. it is not a voluntary decision but rather the result of an unforeseen intraoperative complication when an accreta is not suspected. That concept underlines the importance of preoperative diagnosis.

Cesarean section hysterectomy

This procedure consists of performing a hysterectomy after the birth of the child without attempting removal of the placenta when placenta accreta is strongly suspected antenatally or after an attempted placental removal when the diagnosis of placenta accreta is not made until during delivery. This option is currently recommended by the American College of Obstetrics and Gynecology as well as various authors, and is considered the gold standard treatment for placenta accreta [START_REF] Tseng | Differential expression of vascular endothelial growth factor, placenta growth factor and their receptors in placentae from pregnancies complicated by placenta accreta[END_REF][START_REF] O'brien | The management of placenta percreta: conservative and operative strategies[END_REF][START_REF] Oyelese | Placenta previa, placenta accreta, and vasa previa[END_REF][START_REF] College | Postpartum hemorrhage[END_REF][START_REF] Medicine | Society for Maternal-Fetal[END_REF].The maternal mortality rate of cesarean hysterectomy seems to be relatively low. Among 95 deaths after 1 461 270 births between 2000 and 2006, only one death secondary to placenta accreta had been reported in the United States [START_REF] Clark | Maternal death in the 21st century: causes, prevention, and relationship to cesarean delivery[END_REF]. Nevertheless, mortality rates as high as 7% have been reported to be associated with placenta percreta [START_REF] O'brien | The management of placenta percreta: conservative and operative strategies[END_REF]. Surprisingly, there are very few case series which have assessed maternal morbidity after cesarean hysterectomy. The most methodologically sound studies have only recently been published [START_REF] Eller | Optimal management strategies for placenta accreta[END_REF][START_REF] Warshak | Effect of predelivery diagnosis in 99 consecutive cases of placenta accreta[END_REF][START_REF] Angstmann | Surgical management of placenta accreta: a cohort series and suggested approach[END_REF][START_REF] Hoffman | Morbidity associated with non emergent hysterectomy for placenta accreta[END_REF]. Table 1 summarizes maternal morbidity of the largest reported retrospective case series of a peripartum hysterectomy for placenta accreta. One of the most interesting is the study of Eller et al. [START_REF] Eller | Optimal management strategies for placenta accreta[END_REF], who found, in a series of 76 cases of cesarean hysterectomy for placenta accreta a 42.1% transfusion rate (≥ 4 red blood cells), 28.9% cystotomy rate, ureteral injury in 6.6% of the women, while infectious complications occurred in 33.3% and in all major morbidity among 59% of the women (Table 1). In spite of some methodological flaws ( 24), it appears that maternal morbidity is significantly reduced when no attempt is performed to remove the placenta (3). Warshak et al. ( 4) described morbidity of cesarean hysterectomy for placenta accreta at a similar level to what Eller et al.

(3) observed. In that series of 62 cases of cesarean-hysterectomy, performed without attempting to remove the placenta, the average estimated blood loss, number of red blood cells transfused, and operative time was 2344 mL, 4.7 and 194 minutes, respectively, while the rates of bladder injuries, ureteral injuries and neonatal ICU referrals were 23%, 8% and 72%, respectively (4) (Table 1).

The main advantage of cesarean hysterectomy (without attempted removal of the placenta) is probably to limit the risk of bleeding in order to reduce maternal morbidity, although this still remains high [START_REF] Eller | Optimal management strategies for placenta accreta[END_REF][START_REF] Sentilhes | Arteriovenous malformation following conservative treatment of placenta percreta with uterine artery embolization but no adjunctive therapy[END_REF]. Its major drawback is the loss of fertility, which may seem particularly unwarranted when placenta accreta is not confirmed histologically (this was the case in 28% of cases of prenatal suspicion of placenta accreta in the study of Eller et al. ( 3)). Moreover, there is a lack of consensus regarding several detailed key points of the cesarean hysterectomy strategy:

 Should preventive arterial embolization be performed after birth? This has been proposed to decrease maternal morbidity following cesarean hysterectomy.

Nevertheless, only one limited study including a small cohort (n=26) has addressed this issue. Angtsmann et al. observed a significant decrease in blood loss, number of transfused patients and number of red blood cell units transfused in a short ambispective cohort of women who underwent arterial embolization before hysterectomy compared with patients who had a hysterectomy just after the birth of the child [START_REF] Clark | Maternal death in the 21st century: causes, prevention, and relationship to cesarean delivery[END_REF]. In our practice, we do not use prophylactic embolization before performing an hysterectomy.

 Should temporal internal iliac occlusion balloon catheters be used?

The main potential advantage of these catheters is to reduce blood loss and its inherent morbidity by reducing the blood flow distal to the site of occlusion, through decreasing the blood flow to the uterine arteries after a balloon inflation in the internal iliac arteries, as well as improving the visualization of the surgical field until hemostasis or hysterectomy can be achieved. Nevertheless, some authors have advocated that routine inflation of the balloons immediately following delivery of the infant may exacerbate collateral blood flow [START_REF] Shrivastava | Case-control comparison of cesarean hysterectomy with and without prophylactic placement of intravascular balloon catheters for placenta accreta[END_REF]. It has been suggested that the extensive network of collateral vessels present in the gravid pelvis may explain some failures of occlusive balloons, whereas collateral circulation from cervical, ovarian, rectal, femoral, lumbar, and sacral arteries may also contribute to the overall blood loss [START_REF] Shrivastava | Case-control comparison of cesarean hysterectomy with and without prophylactic placement of intravascular balloon catheters for placenta accreta[END_REF].

Another advantage of the preoperative placement of hypogastric catheters is that they can be used for postoperative embolization should bleeding complications occur after an intended hysterectomy. However, owing to differences regarding the technique, protocols, heterogeneous sets of management and only small cases series with discordant results [START_REF] Shrivastava | Case-control comparison of cesarean hysterectomy with and without prophylactic placement of intravascular balloon catheters for placenta accreta[END_REF][START_REF] Levine | Placenta accreta: comparison of cases managed with and without pelvic artery balloon catheters[END_REF][START_REF] Bodner | Balloon-assisted occlusion of the internal iliac arteries in patients with placenta accreta/percreta[END_REF][START_REF] Dilauro | Prophylactic balloon occlusion of internal iliac arteries in women with placenta accreta: Literature review and analysis[END_REF], there is currently no consensus regarding their use. In particular, the Royal College of Obstetricians and Gynaecologists has underlined the need of further evaluation before its recommendation [START_REF]Green-top Guideline No. 27: Placenta praevia, placenta praevia accrete and vasa praevia: diagnosis and management[END_REF]. Among the only three retrospective comparative studies that have assessed maternal outcome following cesarean hysterectomy alone vs. placement of balloon catheters before performing cesarean hysterectomy [START_REF] Shrivastava | Case-control comparison of cesarean hysterectomy with and without prophylactic placement of intravascular balloon catheters for placenta accreta[END_REF][START_REF] Levine | Placenta accreta: comparison of cases managed with and without pelvic artery balloon catheters[END_REF][START_REF] Bodner | Balloon-assisted occlusion of the internal iliac arteries in patients with placenta accreta/percreta[END_REF], there were no differences for estimated blood loss, transfused blood products, operative time and length of hospitalization between patients who had balloon catheters plus hysterectomy (n=19) and those who had hysterectomy alone in the largest series (n=50) [START_REF] Shrivastava | Case-control comparison of cesarean hysterectomy with and without prophylactic placement of intravascular balloon catheters for placenta accreta[END_REF]. As this procedure is associated with a high rate of harmful effects, it seems difficult to recommend this controversial procedure requiring fetal radiation in the absence of a significant decrease of maternal morbidity. In fact, it has been estimated that surgical complications, such as puncture site hematomas, false aneurysms, dissection of femoral arteries, air in pressurized lines, symptomatic hypotension, and bowel or ureteral injury, and serious thromboembolic events, occur approximately in 6-16% and about 5% of women, respectively [START_REF] Dilauro | Prophylactic balloon occlusion of internal iliac arteries in women with placenta accreta: Literature review and analysis[END_REF].

 Should ureteral stents be used?

There is no study that has specifically assessed this issue. The most interesting findings are provided by the study of Eller et al. [START_REF] Eller | Optimal management strategies for placenta accreta[END_REF]. In this study, in cases of suspected placenta accreta, a preoperative ureteral stent placement was attempted in 44%of cases (25/57). The stent placement was achieved on the right and left sides in only 68% (17/25) of cases, on only one side in 16% (4/25) of cases, and neither side in 16% (4/25). Women with preoperative bilateral ureteral stents had a lower incidence of early morbidity compared with women without stents (18% (3/17) versus 55% (22/40), p= .018). Moreover, these women had a nonsignificant reduction in ureteral injury (0 vs. 7%). Therefore, this study suggests that ureteral stent placement may help to reduce the risk of ureteral injury (3). However, further evaluation is required before considering routine ureteral stents used for all women with placenta accreta.

In recent American surveys, 26% to 35% of respondents reported using ureteral stents in planning for a placenta accreta case [START_REF] Esakoff | PAMUS: placenta accreta management across the United States[END_REF][START_REF] Jolley | Management of placenta accreta: a survey of Maternal-Fetal Medicine practitioners[END_REF].

Conservative treatment

This option consists of delivering the child, tie and then cut the umbilical cord at its base to leave the placenta in place adhering either partially or totally to the myometrium, and to close the hysterotomy (Fig. 1). Conservative treatment can also be performed following a vaginal delivery when the placenta is not delivered and no plane of cleavage is found between the uterus and the placenta, but only if this attempt to remove the placenta was careful, without force or insistence. Conservative treatment avoids a hysterectomy in about 75-80% of cases but is associated with a risk of transfusion requirements, infection and severe maternal morbidity [START_REF] Kayem | Conservative versus extirpative management in cases of placenta accreta[END_REF][START_REF] Bretelle | Management of placenta accreta: morbidity and outcome[END_REF][START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF][START_REF] Alanis | Conservative management of placenta increta with selective arterial embolization preserves future fertility and results in a favorable outcome in subsequent pregnancies[END_REF]. In addition, it requires long-term monitoring. Until recently, only a very limited amount of data regarding maternal outcome after conservative management was available, based only on case reports and short case series from individual tertiary-care institutions [START_REF] Kayem | Conservative versus extirpative management in cases of placenta accreta[END_REF][START_REF] Bretelle | Management of placenta accreta: morbidity and outcome[END_REF][START_REF] Alanis | Conservative management of placenta increta with selective arterial embolization preserves future fertility and results in a favorable outcome in subsequent pregnancies[END_REF].

In order to obtain statistical power and satisfactory external validity, a French multicenter retrospective study was conducted to determine the maternal outcome after conservative treatment [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. The results of this study concerning 167 cases of placenta accreta (59% of placentas were partially left in place and 41% were totally left in place) are summarized in Table 2 [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. They confirm the overall success rate previously published [START_REF] Kayem | Conservative versus extirpative management in cases of placenta accreta[END_REF][START_REF] Bretelle | Management of placenta accreta: morbidity and outcome[END_REF] with uterine preservation (no hysterectomy) in 78.4% of cases, and severe maternal morbidity in 6% (10/167) [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. One maternal death related to multiorgan failure occurred in a patient with medullary aplasia, nephrotoxicity with acute renal failure, followed by peritonitis with septic shock, after the injection of methotrexate in the umbilical cord.

An empty uterus was obtained spontaneously in 75% of cases after a median of 13.5 weeks (min: 4 weeks, max: 60 weeks). Hysteroscopic resection and/or curettage were performed to obtain an empty uterus in 25% with a median of 20 weeks (min: 2 weeks, max: 45 weeks) [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. In a recent American survey, 32% of providers had attempted conservative management for placenta accreta [START_REF] Esakoff | PAMUS: placenta accreta management across the United States[END_REF].

Nevertheless, similarly to the cesarean hysterectomy, the precise modalities of conservative treatment remain to be determined:  Should a routine gentle attempted removal of the placenta be made in cases of suspected prenatal placenta accreta?

The main advantage is not only to avoid leaving an in situ placenta, in case of false-positive imaging, but also to be able to remove the "non-accreta" portion when the placenta adheres partially to the myometrium, in order to reduce the volume left in the uterus. However, this attempted removal of the placenta can cause severe bleeding with the risk of maternal complications and hysterectomy. Our current practice is to attempt gently to remove the placenta only in cases of unconvincing findings of placenta accreta. An example was a nulliparous woman with a history of curettage in whom the ultrasonography revealed only intraplacental lacunae in a low-lying anterior placenta, with inconclusive findings on magnetic resonance imaging, as well as no evidence of placenta accreta during the cesarean.

 Should methotrexate adjuvant treatment be administered? Some authors have proposed the use of methotrexate to hasten the placental resolution [START_REF] Mussalli | Placenta accreta and methotrexate therapy: three case reports[END_REF].

Its efficacy for this indication has never been demonstrated and only case reports and very short case series with no control group have been reported [START_REF] Timmermans | Conservative management of abnormally invasive placentation[END_REF]. The RCOG does not recommend its routine use [START_REF]Green-top Guideline No. 27: Placenta praevia, placenta praevia accrete and vasa praevia: diagnosis and management[END_REF]. The low rate of placental cell turnover compared to what is observed in early pregnancy suggests a much lower efficacy of methotrexate in late compared to early pregnancy. In addition, methotrexate exposes the patient to the risk of neutropenia or medullary aplasia, even with a single dose in a young patient for the treatment of ectopic pregnancy [START_REF] Isaacs | Life-threatening neutropenia following methotrexate treatment of ectopic pregnancy: a report of two cases[END_REF]. These types of side-effects can have a dramatic impact in a patient with an intrauterine placenta with a 30% risk of infectious complications [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. Finally, the only case of maternal death after conservative treatment was secondary to a cascade of complications (bone marrow suppression, sepsis, renal failure) attributed to an intra-umbilical cord administration of methotrexate [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. For these reasons, we do not advocate the use of methotrexate in cases of conservative treatment.

 Should a preventive uterine devascularization be carried out in the absence of bleeding?

We have only very limited data to answer this question. This preventive devascularization can be achieved by the usual techniques of PPH management (embolization, bilateral uterine artery ligation, stepwise uterine devascularization, bilateral ligation of hypogastric arteries), although these uterine-sparing procedures seem to be less effective in cases of placenta accreta [START_REF] Sentilhes | Fertility and pregnancy outcomes following uterine devascularization for severe postpartum haemorrhage[END_REF][START_REF] Sentilhes | Predictors of failed pelvic arterial embolization for severe postpartum hemorrhage[END_REF]. As Angstmann et al. have shown that the implementation of a preventive embolization before performing hysterectomy may reduce the risk of blood loss [START_REF] Angstmann | Surgical management of placenta accreta: a cohort series and suggested approach[END_REF], it is possible to speculate that within the framework of conservative treatment a prophylactic devascularization could prevent the occurrence of secondary hemorrhage and reduce the risk of blood loss (36) (Fig. 2). It could also accelerate the placental resolution. In contrast, these devascularization techniques may have harmful effects [START_REF] Sentilhes | Predictors of failed pelvic arterial embolization for severe postpartum hemorrhage[END_REF][START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. In the French multicenter series of 167 placenta accrete cases treated conservatively, the only two cases of uterine necrosis occurred in two of 62 patients who underwent arterial embolization [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. However, further evaluation is required to answer this issue.

 How should patients with conservative management be monitored?

Unfortunately, there are no data available regarding this important issue. In our practice, we u administer prophylactic antibiotics during five days and discharge the patient from hospital on the 8 th postoperative day. Moreover, subsequent follow-up requires usually weekly visits until the complete resorption of the placenta. The visits include a clinical examination (bleeding, temperature, pelvic pain), pelvic ultrasound (volume of retained tissue) and laboratory tests for infection (hemoglobin and leukocytes count, C-reactive protein, vaginal sample for bacteriological analysis) [START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF].

The one step-conservative surgery

This alternative conservative procedure has been mainly described by one author [START_REF] Palacios-Jaraquemada | Anterior placenta percreta: surgical approach, hemostasis and uterine repair[END_REF][START_REF]Diagnosis and management of placenta accreta[END_REF][START_REF]Placental adhesive disorders[END_REF].

It consists of resecting the invaded area together with the placenta and performing the reconstruction as a one-step procedure [START_REF]Diagnosis and management of placenta accreta[END_REF]. The main stages of this alternative technique achieved through a median or transverse suprapubic incision are a) vascular disconnection of newly-formed vessels and the separation of invaded uterine from invaded vesical tissues; b) performing an upper-segmental hysterectomy; c) resection of all invaded tissue and the entire placenta in one piece with previous pervious local vascular control; d) use of surgical procedures for hemostasis; e) myometrial reconstruction in two planes and f) bladder repair if necessary [START_REF]Placental adhesive disorders[END_REF]. This procedure does not appear to alter the subsequent obstetrical outcome.

Palacios-Jaraquemada has reported 45 pregnancies following a one step-procedure for placenta accreta. Of these, 44 were uneventful and only one was complicated by a recurrence of placenta accreta [START_REF]Diagnosis and management of placenta accreta[END_REF].

As we have a limited experience with the one-step conservative surgery [START_REF] Kayem | Conservative management of a near-term cervico-isthmic pregnancy, followed by a successful subsequent pregnancy: a case report[END_REF], we find it difficult to reach a definite opinion regarding this technique. Nevertheless, we feel that this procedure may be less reproducible than a conservative treatment, in particular because achieving hemostasis may be very challenging for an inexperienced team.

In summary, the available useful procedures in an attempt to decrease the morbidity of placenta accreta remain to be clearly determined by further studies. The prospective PACCRETA study has been launched in order to answer some of the questions raised in this article [START_REF] Kayem | PACCRETA: clinical situations at high risk of placenta ACCRETA/percreta: impact of diagnostic methods and management on maternal morbidity[END_REF].

Finally, we would like to emphasize the adverse effects related to false positive or negative findings with regard to a prenatal suspicion of placenta accreta using Doppler ultrasound and/or magnetic resonance imaging. These two imaging examinations do unfortunately not achieve a sufficiently high level of accuracy to detect a placenta accreta [START_REF] Kayem | PACCRETA: clinical situations at high risk of placenta ACCRETA/percreta: impact of diagnostic methods and management on maternal morbidity[END_REF]. The consequences of a false negative result are obvious, i.e. increased maternal morbidity [START_REF] Warshak | Effect of predelivery diagnosis in 99 consecutive cases of placenta accreta[END_REF][START_REF] Kayem | PACCRETA: clinical situations at high risk of placenta ACCRETA/percreta: impact of diagnostic methods and management on maternal morbidity[END_REF][START_REF] Tikkanen | Antenatal diagnosis of placenta accreta leads to reduced blood loss[END_REF][START_REF] Eller | Maternal morbidity in cases of placenta accreta managed by a multidisciplinary care team compared with standard obstetric care[END_REF]) and have been mentioned above. Similarly, caregivers should be aware that false positive results, which may occur in 28% of cases (3), also increase maternal morbidity. They may lead caregivers to perform unnecessary radical or conservative-surgical procedures and expose the women to their inherent complications. This includes in all cases a high level of anxiety and for the cesarean hysterectomy, loss of fertility and surgical injury; while for the conservative treatment, infection/sepsis as well as bleeding that may lead to a hysterectomy can result, to which can be added the long period of follow-up and anxiety. Caregivers should therefore pay particular attention to both false negative and positive of prenatal imaging examinations.

Cesarean hysterectomy or conservative treatment?

There is only one small retrospective comparative study assessing the maternal outcome following cesarean hysterectomy (n=16) compared to conservative treatment (n=10) (41). No differences were observed between the two groups except for the estimated blood loss which was lower in the conservative treatment group (3625mL ± 2154 vs. 900mL ± 754) for a rate of uterine preservation of 60% (6/10) (41). Compared to cesarean hysterectomy, the main disadvantage of conservative treatment is the length of the management and the need for prolonged follow-up of several months, while the main benefit is the preservation of the uterus and thus fertility preservation. Fertility and subsequent obstetrical prognosis of patients who desire a pregnancy after conservative treatment does not appear to be altered [START_REF] Sentilhes | Fertility and pregnancy outcomes following conservative treatment for placenta accreta[END_REF]. The main risk in case of future pregnancies seems to be the recurrence of placenta accreta estimated at 28.6% (95% confidence interval 11.3-52.2%) [START_REF] Sentilhes | Fertility and pregnancy outcomes following conservative treatment for placenta accreta[END_REF]. Due to the small number of studies, a limited inclusion of patients in these studies, the inhomogeneity of patient characteristics and their management, and methodological flaws, it is very difficult to compare maternal morbidity after conservative treatment vs. cesarean hysterectomy. However, it is possible that severe maternal morbidity is increased in cases of conservative treatment because infectious complications, uterine necrosis and secondary hemorrhage associated to conservative treatment can be dramatic. Thus, it seems reasonable to propose in the present state of knowledge, a cesarean-hysterectomy if the patient has no desire for a future pregnancy, is at a relatively advanced reproductive age and is multiparous.

Nevertheless, we believe that conservative management is an option for patients who are properly counseled and motivated, in particular, for women who want the option of a future pregnancy and who agree to close follow-up monitoring in centers with adequate equipment and resources [START_REF] Kayem | Management of placenta accreta[END_REF][START_REF] Sentilhes | Arteriovenous malformation following conservative treatment of placenta percreta with uterine artery embolization but no adjunctive therapy[END_REF][START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF]. However, regardless of the chosen option (cesarean hysterectomy or conservative treatment), the patient and her partner should be warned of the high risk of maternal complications related to placental disease.

Placenta percreta with bladder invasion

The bladder is the most frequently invaded adjacent organ when there is a placenta percreta (Fig. 3). In this condition, maternal morbidity is severe and high, with mostly urological complications (72.2%), including partial cystectomy (44%), bladder lacerations (26%), urinary fistula (13%), gross hematuria (9%), ureteral transaction (6%) and later small capacity bladder (4%). A maternal death occurred in 5.6% of cases [START_REF] Washecka | Urologic complications of placenta percreta invading the urinary bladder: a case report and review of the literature[END_REF]. Pre-operative placement of a ureteral catheter is probably of great help to minimize ureteral complications [START_REF] Eller | Optimal management strategies for placenta accreta[END_REF]. In this condition, it is likely that conservative treatment has a lower morbidity compared to radical treatment [START_REF] Sentilhes | Arteriovenous malformation following conservative treatment of placenta percreta with uterine artery embolization but no adjunctive therapy[END_REF][START_REF] Sentilhes | Maternal outcome after conservative treatment of placenta accreta[END_REF][START_REF] Hayes | The morbidly adherent placenta: diagnosis and management options[END_REF]. This opinion was already shared by 69% of the members of the American Society of Perinatal Obstetricians in 1996 [START_REF] O'brien | The management of placenta percreta: conservative and operative strategies[END_REF].

Discussion

In the case of a prenatal suspected placenta accreta, the extirpative method must now be abandoned. It seems reasonable to plan delivery at approximately 35-36 weeks of gestation.

The advantages and disadvantages of cesarean hysterectomy and conservative management should be clearly exposed to the patient and the partner, who must be involved in deciding on the chosen option. Considering the present state of knowledge, it seems reasonable to propose a cesarean hysterectomy if the patient has no desire for a future pregnancy, is at a higher reproductive age and is parous. In these latter cases, preoperative ureteral stent placement as well as prophylactic embolization before hysterectomy may be considered in order to reduce maternal morbidity, whereas the benefit-harm balance is against the routine use of internal iliac occlusion balloon catheters. In contrast, if the patient wants the possibility of another pregnancy, is young and nulliparous, conservative treatment with no adjuvant methotrexate therapy should be proposed. In rare cases of placenta percreta with bladder invasion, it seems reasonable to focus on conservative treatment.

Legends for Figures

Fig. 1 .

 1 Fig. 1. Peri-operative view of a placenta percreta left in situ after the closing of the vertical

Fig. 2 .

 2 Fig. 2. Angiograms respectively before and after a prophylactic pelvic arterial embolization

Fig. 3 .

 3 Fig. 3. Hysteroscopic view of a placenta percreta with a bladder invasion.

Acknowledgement

The authors are grateful to Richard Medeiros, Rouen University Hospital Medical Editor, for his valuable editorial assistance.

Funding

The authors declare no source of funding