Groundwater exploration and drilling problems encountered in basaltic and granitic terrain of Nanded district, Maharashtra

Abstract

The Central Ground Water Board (CGWB) constructed 72 bore wells down to a targeted depth of 208 m in Nanded district, Maharashtra which is mainly underlain by hard rocks, namely basalts and granites. The yield of these bore wells varies from 0.10 litres/second (l/s) to 25 l/s and depth of weathering varies from less than 1 m to 24 meters below ground level (m bgl). The shallowest and the deepest aquifer zones are encountered at 7 and 172 m bgl respectively and majority of the productive aquifer zones are encountered within 100 meter (m) depth. Most productive zones are fractured basalts, fractured and weathered granites and 31 % of the zones are encountered within 25 m, 36 % within 25–50 m, 24 % within 50–100 m, 8 % within 100–150 m and less than 1 % within 150–173 m depth ranges. Out of the three major sets of lineaments, the NE-SW trending lineaments are more productive, and a maximum of five fracture zones are encountered in all. Surface geophysical studies (profiling) reveal that wherever the ratio between high resistivity and low resistivity is high, the discharge is high, and where the ratio is low the discharge is low. The depth to water level in these bore wells are in the range of 1.5–47.5 m bgl (below ground level). The transmissivity (T) and Storativity (S) values of 21 wells range from 3 to 593 m²/day and 6.5 × 10⁻⁶ to 7.32 × 10⁻² respectively. The groundwater from the area is mostly of Ca-Na-HCO₃-Cl, Ca-Na-HCO₃, Ca-HCO₃-Cl, Na-HCO₃, Na-HCO₃-Cl, Na-Cl, Ca-HCO₃ types in the descending order of dominance and a few are found to be unsuitable for drinking. At a few sites, drilling down to the targeted depth of 200 m couldn’t be completed due to highly fractured nature of formations, hydraulic backpressure, occurrences of saturated intertrappean beds and high magnetic nature of formations.