Exploration Of Groundwater Resources using Two Dimensional Electrical Resistivity Tomography (2D ERT) method in Hard Rock areas of Lesser Himalayan Zone, Nepal

Groundwater is widely distributed in nature, but its occurrence and distribution is confined to certain geologic formations and structures. Hydrogeology of the area depends on the local lithology, geological structure, geomorphology and climate. Surface geophysical method-Electrical methods are widely applicable for groundwater exploration because of the close relationship between electrical conductivity and the physical properties of aquifers. The main propose of the study was to assess groundwater potential at hardrock area of Nalan VDC Dhading District Nepal at an elevation of 4100 feet above mean sea level. Geological mapping, spring inventory and geophysical survey were carried out. Geophysical Survey (2D Electrical Resistivity Imaging) was carried out using Resistivity meter WDJD-4 using Wenner and Dipole-Dipole configuration. Data were analyzed using geophysical software Res2Dinv which is used for processing and interpretation of the resistivity data. A total of five 2D ERT profiles were carried out in the area to map the subsurface lithology of the area. Geologically the area lies on Kuncha Formation of Lesser Himalayan Zone. The results of the ERT shows that the subsurface has different geo-electric layers representing different geological condition: unsaturated colluviums, weathered and fractured phyllite, water bearing fractured rocks, meta-sandstone and quartzite. The resistivity value of around 4000 W-m represents unsaturated colluviums, 2000-4000 W-m represents fractured phyllite, 200-500 W-m represents highly saturated fractured rock and over 10000 W-m represents quartzite bedrock in the area. Water bearing fractured zone is encountered at a depth of 230-375 feet from the surface. Result of the study was verified by drilling deep tube wells at recommended location in the area. Water table was encountered at a depth from 245-375 feet during drilling for extraction of groundwater. Due to steep topography and small catchment area above the drilling location, the discharge of the tube well was 64.16 lit/minute. Results of this study are useful for technical groundwater management in the hardrock areas, as they clearly identified suitable borehole locations for groundwater prospecting.

Keywords: Groundwater, Electrical Resistivity methods, 2D ERT, Lithology, Drilling