Jean-Christophe Maréchal

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

Computing the Water Inflows Discharge and Assessing the Impacts of Tunnels Drilled in Hard Rocks: The A89 (France) Motorway Case Study

Most Hard Rocks were exposed to deep weathering processes, from which their hydraulic conductivity is mostly inherited, within their stratiform fissured layer located below the low hydraulic conductivity unconsolidated saprolite, and within the permeable vertical fissured layer at the periphery of preexisting geological discontinuities. From this conceptual model, first the water inflows discharge into the tunnels and second the surface hydrogeological and hydrological impacts due to shallow motorway tunnels have been forecasted. These three tunnels, up to 4 km long and whose depth ranges between 0 and 300 m below ground level (bgl), are located on the A89 motorway in France. The method is based on: 1. the location of the tunnel within or below the various layers constituting the weathering profile. Three different weathering profiles with ages from Triassic to post Miocene were identified, mapped, and characterized (fissuration) in the various lithologies of the area; 2. steady state groundwater discharge measurements in existing tunnels (railway) of the area where weathering profiles were similarly mapped; 3. application of the Goodman et al. (1965) analytical solution to: (i) inverse steady state railway tunnels groundwater discharge into the hydraulic conductivity of the various layers of the weathering profiles, and (ii) compute the discharge of the future motorway tunnels, on the basis of the hydraulic conductivity of these weathering profiles. The actual discharge of the now completed motorway tunnels validates the accuracy of the methodology. This method proves to be very efficient for forecasting the water inflows into shallow tunnels (a few 100 m deep) in hard rock areas.

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.

Insights about transport mechanisms and fracture flow channeling from multi-scale observations of tracer dispersion in shallow fractured crystalline rock

In fractured media, solute transport is controlled by advection in open and connected fractures and by matrix diffusion that may be enhanced by chemical weathering of the fracture walls. These phenomena may lead to non-Fickian dispersion characterized by early tracer arrival time, late-time tailing on the breakthrough curves and potential scale effect on transport processes. Here we investigate the scale dependency of these processes by analyzing a series of convergent and push-pull tracer experiments with distance of investigation ranging from 4m to 41m in shallow fractured granite. The small and intermediate distances convergent experiments display a non-Fickian tailing, characterized by a -2 power law slope. However, the largest distance experiment does not display a clear power law behavior and indicates possibly two main pathways. The push-pull experiments show breakthrough curve tailing decreases as the volume of investigation increases, with a power law slope ranging from -3 to -2.3 from the smallest to the largest volume. The multipath model developed by Becker and Shapiro (2003) is used here to evaluate the hypothesis of the independence of flow pathways. The multipath model is found to explain the convergent data, when increasing local dispersivity and reducing the number of pathways with distance which suggest a transition from non-Fickian to Fickian transport at fracture scale. However, this model predicts an increase of tailing with push-pull distance, while the experiments show the opposite trend. This inconsistency may suggest the activation of cross channel mass transfer at larger volume of investigation, which leads to non-reversible heterogeneous advection with scale. This transition from independent channels to connected channels when the volume of investigation increases suggest that both convergent and push-pull breakthrough curves can inform the existence of characteristic length scales.