Benoît Dewandel

Use of hydraulic tests at different scales to characterize fracture network properties in the weathered‐fractured layer of a hard rock aquifer

The hydrodynamic properties of the weathered-fractured layer of a hard-rock pilot watershed in a granitic terrain are characterized using hydraulic tests at different scales. The interpretation of numerous slug tests leads to characterize the statistical distribution of local permeabilities in the wells. The application of flowmeter profiles during injection tests determines the vertical distribution of conductive fracture zones and their permeabilities. It appears that the extension of the most conductive part of the weathered-fractured layer is limited down to 35 meters depth. The partition of drainage porosity between blocks (90%) and fractures (10%) is determined thanks to the interpretation of pumping tests using a double porosity model. The application of anisotropic and single fracture analytical solutions on pumping test data allows to determine, respectively, the degree of anisotropy of permeability ( ) and the radius (4 to 16 meters) of the horizontal conductive fractures crossed by the wells. Two different scales of fractures networks are identified: the primary fracture network (PFN), which affects the matrix on a decimeter scale by contributing to an increase in the permeability and storage capacity of the blocks, and the secondary fracture network (SFN), which affects the blocks at the borehole scale. SFN is composed of two sets of fractures. The main set of horizontal fractures is responsible for the sub-horizontal permeability of the weathered-fractured layer. A second set of less permeable sub-vertical fractures insures the connectivity of the aquifer at the borehole scale. The good connectivity of fractures networks is shown by fractional dimension flow solutions. The absence of scale effect in the study area suggests that the hydraulic conductivity at the borehole scale is laterally homogeneous. Finally, the analysis and synthesis of the hydrodynamic properties allow to propose a comprehensive hydrodynamic model of the fractured-weathered layer. Many geological and hydrogeological indicators suggest that a continuous and laterally homogeneous weathering process is responsible for the origin of the fractures and permeability encountered in the aquifer. These results confirm the major role played by weathering in the origin of fractures and on resulting hydrodynamic parameters in the shallow part of hard-rock aquifers.

Projected impacts of climate change on farmers' extraction of groundwater from crystalline aquifers in South India

Local groundwater levels in South India are falling alarmingly. In the semi-arid crystalline Deccan plateau area, agricultural production relies on groundwater resources. Downscaled Global Climate Model (GCM) data are used to force a spatially distributed agro-hydrological model in order to evaluate Climate Change (CC) effects on local groundwater extraction (GWE). The slight increase of precipitation may alleviate current groundwater depletion on average, despite the increased evaporation due to warming. Nevertheless, projected climatic extremes create worse GWE shortages than for present climate. Local conditions may lead to opposing impacts on GWE, from increases to decreases (+/−20 mm/year), for a given spatially homogeneous CC forcing. Areas vulnerable to CC in terms of irrigation apportionment are thus identified. Our results emphasize the importance of accounting for local characteristics (water harvesting systems and maximal aquifer capacity versus GWE) in developing measures to cope with CC impacts in the South Indian region.

A methodology for regionalizing 3-D effective porosity at watershed scale in crystalline aquifers

13 An innovative approach for regionalizing the 3-D effective-porosity field is presented and 14 applied to two large, overexploited and deeply weathered crystalline aquifers located in 15 southern India. The method derives from earlier work on regionalizing a 2-D effective-16 porosity field in that part of an aquifer where the water table fluctuates, which is now 17 extended over the entire aquifer using a 3-D approach. A method based on geological and 18 geophysical surveys has also been developed for mapping the weathering profile layers 19 (saprolite and fractured layers). The method for regionalizing 3-D effective porosity 20 combines: water-table fluctuation and groundwater budget techniques at various cell sizes 21 with the use of satellite based data (for groundwater abstraction), the structure of the 22 weathering profile and geostatistical techniques. The approach is presented in detail for the 23 Kudaliar watershed (983 km 2), and tested on the 730 km 2 Anantapur watershed. At watershed 24 scale, the effective porosity of the aquifer ranges from 0.5% to 2% in Kudaliar and between 25 0.3% and 1% in Anantapur, which agrees with earlier works. Results show that: i) depending 26 on the geology and on the structure of the weathering profile, the vertical distribution of 27 effective porosity can be very different, and that the fractured layers in crystalline aquifers are 28 not necessarily characterized by a rapid decrease in effective porosity; and ii) that the lateral 29 variations in effective porosity can be larger than the vertical ones. These variations suggest 30 that within a same weathering profile the density of open fractures and/or degree of 31 weathering in the fractured zone may significantly varies from a place to another. 32 The proposed method provides information on the spatial distribution of effective porosity 33 which is of prime interest in terms of flux and contaminant transport in crystalline aquifers. 34 Implications for mapping groundwater storage and scarcity are also discussed, which should 35 help in improving groundwater resource management strategies. 36 37

Diagnostic Plots Applied to Well-Tests in Karst Systems

Pumping tests conducted on wells intersecting karst heterogeneities such as the conduit network are difficult to interpret. Nevertheless, this case can be solved by assimilating the horizontal karst conduit to a finite-conductivity vertical fracture. In this case, several flow patterns corresponding to the respective contributions of karst subsystems (fractured matrix, small conduits and main karst drainage network) can be identified on the diagnostic plot of drawdown derivative. This is illustrated on two examples from Mediterranean karst systems. A pumping test on a well crossing the main karst drainage network of the Cent-Fonts karst system shows (i) a preliminary contribution of the karst conduit storage capacity followed by (ii) linear flows into the fractured matrix. A pumping test on a well intersecting a small karst conduit of the Bas-Agly karst system shows the existence of (i) bi-linear flows within both the karst conduit an the fractured matrix at early times, followed by (ii) radial flows within the fractured matrix and (iii) finally the contribution of a major karst cavity. The use of diagnostic plots allows identifying the various flow regimes during pumping tests, corresponding to the response of the individual karst aquifer subsystems. This is helpful in order to understand the structure of the karst aqui-fer and flow exchanges between subsystems.

Water Budgeting and Construction of Future Scenarios for Prediction and Management of Ground Water under Stressed Condition

The Indo-French Centre for Groundwater Research (IFCGR) has carried out intensive research for understanding the structure and functioning of the hard rock aquifers in granitic area by taking a small watershed at Maheshwaram, RR Dist of Andhra Pradesh and has developed a number of specific techniques during five years for aquifer characterization, groundwater balance both global and discretized including the technique of the “Double Water Table Fluctuation (DWTF)” (Marechal et al., 2005), well suited for arid and semi-arid areas to improve the groundwater resource management in hard rock aquifers. This method has been implemented in the small pilot watershed of 53 km2 area, a representative of the Southern Indian catchment in terms of overexploitation of the hard rock aquifers in semi-arid climate, cropping pattern, rural socio-economic context, etc. Between June 2001 and June 2004 three annual water budgets have been prepared. It is observed that both water balances for a “bad” and “good” monsoon could more or less be counterbalanced, the water balance is negative for a normal monsoon inducing a depletion of the Water Table at about 1 m/year due to consecutive deficit rainfall years. Due to this, in the long, medium, or short-term, drastic groundwater problems will inexorably occur if any realistic solution is not found. Therefore, it is now essential that policy makers be equipped with accurate predictive tools for simulating the groundwater table in the coming years in order to better guide their future actions in total coherence.