Mapping Lithology and Assessing Recharge Characteristics in a Granitic Hard Rock Aquifer: Inference from 2D Resistivity, Induced Polarization, Tracer and Moisture Measurements

Abstract

Abstract: Two dimensional Electrical Resistivity Tomography (ERT) investigation along with Time Domain Induced Polarization (TDIP) investigation covering 1.6 km line were carried out at 3 natural recharge sites in a overexploited groundwater granite watershed, situated in a semi arid region in the state of Telangana, India. At these sites, shallow and/or deep moisture influx measurements were also carried out using injected tritium tracer and neutron moisture probe. The watershed is covered by sandy loam to silt loam soil, receives an average annual rainfall of 620 mm with the pre monsoon groundwater level ranging from 8m to 29m bgl. The spot investigations were done to assess and understand the recharge process and groundwater potential in terms of resistivity/conductivity and moisture characteristics of the subsurface rock formation. The measured and 2D inverted resistivity models shows soil, highly weathered and moderately weathered zones up to a depth of 22m followed by semi-weathered and massive granites up to the maximum depth of 78m. The resistivity values of the soil and shallow weathered zones varying between ~5-50 Ohm.m up to a depth of 12m. The large variation in resistivity values of the soil and weathered zone is probably indicative of large variation in the moisture potential. The resistivity models clearly mapped the granitic hard rock structure in the depth range of 22 – 78m, which has a significant resistivity contrast (~1500-11000 Ohm.m) with respect to the overlying semi weathered layers. The modeled 2D resistivity data at a site in two perpendicular directions shows similar geological structure and geoelectric layers. The TDIP dataset shows insignificant chargeability variations (1-7 mV/V) up to the investigated depth of 78m, which probably indicates low to moderate moisture and groundwater potential in the vadoze and saturated zones of the aquifer. The experimental investigations provide scope for assessing dynamic recharge and groundwater potential at selected sites in an overexploited granite watershed in the present geological context.