Influence of various factors on the fluctuation of groundwater level in hard rock terrain and its importance in the assessment of groundwater

Abstract

A vast terrain, nearly 65% of the total area of India is occupied by hard rocks, more predominantly in the area of peninsular shield covering southern part of India. The peninsular shield is mostly constituted of granites, gneisses, basaltic rocks of volcanic origin and metamorphic complexes. In basaltic rock, the main aquifer horizons are in vesicular, fractured and weathered zone. These basaltic rocks are devoid of primary porosity. The groundwater occurs largely in the secondary porosity of weathered mantle and developed at a shallow depth. These aquifers are mostly phreatic, occurring up to the depth of 10 to 15 m. This weathered mantle is underlain by massive rocks which form the bottom of the phreatic aquifer, limiting its downward extent. The groundwater in the shallow aquifers gets replenished annually, and therefore, the status of water levels and its fluctuation play a key role in the assessment of groundwater. The pre-monsoon and post-monsoon groundwater levels indicate the degree of saturation and extent of recharge in the hard rock aquifers. A network of 3920 hydrograph stations (observation wells) has been established in Maharashtra by Groundwater Surveys and Development Agency (GSDA) and water levels are monitored since last three decades. On the basis of analysis of these water level data, it has been observed that the decline in water levels has influences of various factors. The instances of lowering of water level have been noticed in many of the less exploited watersheds also, due to various reasons. The present paper discusses various factors controlling water levels and probes that manifestation of decline in water levels cannot be conclusive of overexploitation. Key words: Maharashtra, secondary porosity, groundwater level fluctuation, hard rock terrain, assessment, dynamic recharge, shallow unconfined aquifer.