Local Scale Heterogeneity Assessment for MAR in Karst Aquifer using Geophysics

English Abstract:

The focus of this work is to understand the structure of the limestone aquifer for management of water resources. Managed aquifer recharge is of great interest to replenish the heavy loses of groundwater in Raipur of Central India. The area is a water stressed city and further urbanization poses threats by altering the hydrological properties of the aquifer system. The hydrogeophysical approach was used to characterize the unsaturated zone of karst terrain to check its efficacy for MAR. Electrical resistivity methods were used at 12 sites to locate the feasible conductive zones along the local drainage. Vertical Electrical Soundings (VES) were used to capture the local resistivity variation. After the dense VES soundings, profiling by Electrical Resistivity Tomography (ERT) with 4-10 m electrode spacing was done by Wenner-Schlumberger and gradient method. The area of interest remains limited to 35-40 m below the surface, however many strong lateral and vertical anomalies were found in the data. The low resistivity in the unsaturated zone against the high values of limestone is the main target for infiltration. The low resistivity anomalies in the 2D inverted sections could be water filled conduits or solution channels with uncertain geometry. These were confirmed by the drilled data at 3 sites which correlated well with the resistivity data. Litholog information confirmed that the dissolution has modified the low primary porosity by widening fractures, bedding planes and developing solution voids. The local hydrogeological heterogeneity was mapped based on the resistivity variations thereby proposing the favorable sites for construction of MAR structures.