Time lapse monitoring of the vadose zone response of a granitic aquifer in experimental hydrogeological park: a case study in south India

Introduction: The faults and fractures of the granite are, according to their position in relation to the plane of deformation, hypothetically interpreted as tension and shear faults. The faults in shear position are supposed to be tight and have very little groundwater. The tension faults, on the other hand, are supposed to be open and to be capable of a high yield of groundwate. The electrical conductivities of rocks and soils are highly dependent of the water saturation. Variations in electrical resistivity are monitored by time lapse electrical resistivity tomography (TLERT) during a long duration pumping test.Rocks such as granite and schist are generally poor aquifers because they have a very low porosity. However, if these rocks are highly fractured, they make good aquifers. A well is a hole drilled into the ground to penetrate an aquifer. Normally such water must be pumped to the surface.Material and methods: Climate of South India is mostly tropical. The study of climate is very important from many aspects. It is predominantly important for crops, tours, vegetation etc. Henceforth necessary to understand the working and eating habits, also. In fact the study of climate is correlated to Topography and Temperature of the region. In fact the region has a tropical climate and depends on monsoons for rainfall. This region includes Karnataka, inland Tamil Nadu and western Andhra Pradesh.Most importantly it gets between ۴۰۰ and ۷۵۰ millimetres (۱۵.۷ and ۲۹.۵ in) of rainfall annually . The summers are hot and dry . But the winters are cool with temperatures around ۲۰-۲۴°C (۶۸-۷۵°F).This experiment is carried out in the Experimental Hydrogeological Park (EHP) located in Choutuppal, ۴۵ km south-east of Hyderabad. Vadose zone of EHP comprises an uppermost thin layer of red soil (<۱m), sandy regolith (۱m-۳m), saprolite (۳m-۱۵m), and then the fissured granite. The pumping test lasts for ۵ days and the piezometric variations are between ۱۳ m and ۱۸ m during pumping in CH۰۳ borehole. This fissured granite is characterized by an important horizontal fracture density controlling the flow. An East West profile was laid with ۴۸ electrodes and ۳ m spacing interval. CH۰۳, pumping well, was in the center of the profile covering ۸ observation wells in both directions. ۲۷ time-lapse datasets were inverted using Res۲Dinv adopting least square inversions. The inverted resistivity datasets seem to be correlated with weathered profile and the variations of resistivity may be correlated with variation of hydraulic head. The variations of resistivity are more important close to CH۰۳ and decreases with distance away from it. This behavior is coherent with the depression cone created by the pumping. Moreover, resistivity variations in the vadose zone highlight an influence of the pumping on the water content evolution of this zone. The observed heterogeneous response seems to be correlated with the geological media heterogeneity. TLERT appears to be a powerful tool to follow dynamic behavior of both saturated level and vadose zone for a given event. Grounwater punping monitoring helps to the water content evolution and groundwater productivity.Results: The different distribution pattern of resistivity during and after the pumping are noticed. There is no occurrence of rainfall event during the experiment and the watershed. However, there is a temporary storage tank towards the eastern end of the profile, constructed for storing the water. This tank has no effect on the percolation, but the saturation effect is observed the surficial level. A very sharp breakup is seen after almost ۴۰hr of pumping. This sharp boundary or may be fracture is strongly observed upto ۲۰m depth.Conclusion: Subsurface hydrology of the granitic terrene is studied, monitored and analyzed by a simpler approach of TLERT. The inverted resistivity datasets successfully correlates with the hydraulic head data measured at the water table at the same timings. This research, intended to examine the validity of time-lapse electrical imaging has been extremely successful, showing that repeat measurements of resistivity recorded at the surface can accurately delineate changes in saturation in the subsurface. The observed heterogeneous response seems to be correlated with the geological media heterogeneity. The precise location of a fracture can be determined with this non-invasive and quick method in the presence of significant vertical flow. Vadose zone connectivity in terms of pathways in both horizontal as well as vertical directions. It helps in reducing the uncertainty in the model parameters.

Keywords:

Granitic Aquifer , Groundwater Productivity Vadose Zone , Time Lapse Electrical Resistivity Tomography