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Hydrogeological studies
Well Inventory
Under the hydrogeological component of the programme, wells are inventoried. Different types of wells are studied for recording their yielding capacities, main aquifers contributing to yield, etc. The nature and period of their use and sustainability are also recorded. The hydrostatic heads of the aquifers are monitored on a monthly basis through shallow dugwells (monitoring stations), piezometers, deep wells, etc, in the areas. Water samples are collected from selected wells and analysed to determine the variation of water quality over time and space.
Aquifer Tests
Hydrogeological studies include determination of aquifer parameters by conducting pumping tests on dug / bore / tube wells and analysis of pumping test data.
Important aquifer parameters are:
Porosity : measure of void space in the rock formations. It is defined in percentage as the ratio of the void pore space to the total volume of the rock formation sampled.
Hydraulic conductivity : rate of flow under a unit hydraulic gradient through a unit cross-sectional area of aquifer. The unit is in m/ day.
Transmissivity : rate of flow of groundwater under a unit hydraulic gradient through an aquifer of unit width and unit thickness. That is, transmissivity is the product of hydraulic conductivity and thickness of the aquifer. The unit is in m2/day.
Storativity or storage coefficient, applicable for confined aquifers : volume of water released from storage per unit surface area of a confined aquifer per unit decline in hydraulic head. It is dimensionless.
Specific yield, applicable for unconfined aquifers : volume of water released from storage under gravity by an unconfined aquifer per unit surface area of aquifer per unit decline of the water table. Specific yield is dimensionless or can be given in %.
Specific capacity of a well : ratio of discharge of the well to the drawdown, in m3/hour/m.
Hydrogeological input also comprises the study of unsaturated zone. This includes micro-level studies at selected sites and areas as well as utilizing the information on unsaturated zone obtained from remote sensing study and geophysical measurements.
Ground Water Monitoring
Monitoring of depth to water in phreatic aquifer and piezometric head in the deeper confined aquifers, in time and space, is a very important activity in the estimation of ground water resources and their change with time. Study of pressure heads in each individual aquifer of the multi-layered aquifer system is very important to understand the ground water movement across the system. From the well inventory, representative wells tapping the phreatic aquifer are selected at a close interval for monitoring. Piezometric heads are measured from existing / purpose-built piezometers. Some of the monitoring wells are fitted with pressure transducers to record continuous change in the pressure heads. The differences in the phreatic and piezometric heads and their variation are analyzed to understand the ground water flow pattern along and across the aquifer units.
Based on the processing of data from well inventory, monitoring and the field observations, ground water high stress areas and vulnerable areas are identified.
Example of a tentative hydrogeological map, Tumkur district.
Source: CGWB