Extended abstract: The Bongouanou Départment (administrative unit) is located in the humid tropical region of Ivory Coast (Plate 1), where agricultural and pastoral practices are very developed. In this region, the groundwater resource is much exploited. Although the situation is not alarming at this stage, it remains that the quantity and the quality of this resource should be carefully studied also in regards to the increase in diffused and point source pollution. To satisfy the water needs and to prevent the pollution risks, one approach is to improve our knowledge in the aquifers productivity and identify the vulnerable zones. It is for this purpose that this study has been initiated. The lithology is composed of volcano-sedimentary formations (80% of the territory), granitic intrusions (19%) and a small quaternary formation bordering the N'zi and Comoé rivers (Plate 2). Thus, the substratum consists almost entirely of schisteous and granitic basement, with, in its upper part, important indurated layers. There are therefore in this environment reservoirs of alterites and networks of cracks or fractures. To achieve groundwater availability assessment, we used two ENVISAT radar scenes of 12.5 m spatial resolution, VV polarization, from 26/02/2004 and 16/03/2004, taken by the Advanced Synthetic Aperture Radar (ASAR). Speckle filtering of radar images has been done using a Lee filter (Plate 3). Then image enhancement can show morphological discontinuities in the area (Plate 4). Drill parameter statistics provide insight into basement depth slices more productive in groundwater. Indicators designed for the study of spatial productivity are: accessibility and operability The criteria of definition of each indicator are recorded in table 1. The Arcview 3.3 software is a tool with full functionality for performing a multicriteria analysis thanks to its 'Interpolate grid' and 'Map calculator' functions of the 'Spatial Analyst' menu. Thus, at the end of the process, the four thematic classes of water productivity selected after aggregation are: bad, mediocre, good and excellent. From enhanced radar images, lineaments have been extracted and validated through comparison with photofractures derived from the photogeologic map of South-West region of Ivory Coast (Pate 5), and through ground checks. Finally, the fracturation map of Bongouanaou area has been obtained. This map (Plate 6) contains 2897 fractures, with 701 spacing measurements ranging from 0.234 m to 7.42 km for an area of 6670 km2. The fracture lengths range from 0.234 km to 21.23 km, with major fracture directions being: N-S and NW-SE. The cumulative length fracturing density map is shown in Plate 7. Calculations of average induced permeabilities show values ranging from 8.16-10-7 to 9.21-10-5 m.s-1, with an average of 0.91.10-5 m.s-1. A statistical study of average specific flow rates and average flow rates as a function of the total depth of the drilling was carried out and represented by the graphs in plate 8. On plate 8a it is found that the specific flow rates are low at depths less than 40 m and greater than 80 m, with values of less than 0.2 m2.h-1. At depth ranges between 40 and 80 m, the geometric mean specific flow rates are greater than 0.2 m2.h-1, with maxima of 0.57 m2.h-1 and 0.38 m2.h- 1 between 40 and 50 m and between 60 and 70 m. At the level of the average flow rates versus depths (Plate 8b), the depth range with high mean flows is between 40 and 80 m, with also maxima between 40 - 50 m and 70 - 80 m. Depth classes with high specific flows correspond to significant average flows. It can be remembered that in this basement the hydrodynamic characteristics (transmissivity, specific flow and permeability) are important between the depths between 40 and 80 m, with very sensitive peaks between 40 to 50 m and 60 to 70 m. Strong hydraulic properties at depths between 70 and 80 m are likely to be due to very deep fractures of tectonic origin. A study of the relationships between the specific flows and the exploitation rates is carried out and represented by the graph of plate 9. There is a strong correlation between the logarithmic values of these two parameters. The mathematical function describing this relationship is: Q / s = 0.089 Q 1.53 with Q / s : specific flow rate (m2.h-1) Q: operating rate (m3.h-1) From this relationship, it is obvious that areas with high flows correspond to high hydraulic characteristics or high porosities. The potentially productive depth range in groundwater in this basement is therefore between 40 and 80 m, with a very remarkable peak in the 40 and 50 m interval. What are the regions concerned by these depths? On the accessibility map, North-West, South-West, North and Central-East, precisely the regions of Tiémelékro, Anoumaba, M'batto, Assié-Koumassi and Arrah offer good and excellent accessibility (plate 10). Regions of good and excellent exploitability also coincide with areas with high fracturing densities. Areas of good groundwater productivity generally provide significant flows at relatively shallow depths. Fractures and alteration affect the upper part of the substratum more than the deep part. A very high thickness of alteration (> 60 m thickness) tends to reduce the accessibility and the exploitability of the groundwater of the underlying fissured horizon (Plate 12). The development of the vulnerability map was done by calculating the vulnerability index using the DRASTIC method which integrates seven: the depth of the aquifer (D), the recharge or the effective infiltration (R), the lithology of the aquifer (A), the nature of the soil (S) , the topography or slope (T), the nature of the unsaturated zone (I) and the induced permeability (C). As the study environment is relatively homogeneous, the classification, coding and weighting of the criteria defining the vulnerability indicator are shown in table 2. The DRASTIC (IDRASTIC) indices obtained oscillate between 43 and 154. An approach developed in 1995 by the Quebec Ministry of Agriculture, Fisheries and Food consists of dividing the hydrogeological vulnerability scale into three or four major classes (see table 3).The calculation of the extreme percentages of vulnerability made it possible to obtain the following classification according to the ranges of vulnerability index: - 29%

Le département Bongouanou est situé dans la région tropicale humide de l’Est de la Côte d’Ivoire où la pratique agricole et pastorale est très développée. De plus, la ressource en eau souterraine est très exploitée à cause de la surpopulation. Bien que la situation ne soit pas encore alarmante dans cette région de socle, il n’en demeure pas moins que la quantité et la qualité de cette ressource sont sans cesse mises à l’épreuve, compte tenu de la croissance des sources de pollution, tant diffuses que ponctuelles. Pour satisfaire les besoins en eau et prévenir les risques de pollution, une des approches est la connaissance de sa productivité et l’identification des zones vulnérables. C’est dans ce but que cette étude a été initiée, en croisant l’analyse du réseau de fracturation détecté par les capteurs radar Envisat avec les paramètres de forage, avec des approches statistiques et d’analyse multicritères. Les résultats statistiques montrent que cet aquifère de socle a une productivité en eau souterraine modeste, avec un débit maximum de 21 m3.h-1 et une moyenne de 4,34 m3.h-1. La tranche de profondeur très productive en eau est comprise entre 40 et 80 m. Les cartes d’accessibilité et d’exploitabilité établies par analyse multicritère des facteurs (probabilité de succès des forages, profondeur d’extraction, niveau piézométrique et débits d’exploitation) mettent en évidence les zones très productives. En effet, les secteurs très fracturés et de faibles pentes (zone de plaine) tels que le Sud, l’Est, l’Ouest et le Nord, ont des ressources facilement accessibles et fournissent des débits importants à faibles profondeurs. Toutefois, la méthode DRASTIC, utilisée pour réaliser la carte de vulnérabilité à la pollution, montre des indices très élevés dans ces régions.