A Conceptual Numerical Model to Simulate Aquifer Parameters

While in constant head aquifer, groundwater level before pumping and after recovery attain a constant value, it is quite challenging to estimate the actual discharge value in an atoll aquifer where the head fluctuates with tides. Lens shaped atoll aquifers are fragile in nature and pumping causes the water table to fall which could further aggravate sea water intrusion. Thus the objective of the study is to propose an alternative method and demonstrate the importance of tide-induced head fluctuations in analysing the aquifer parameter in atoll environment. The solution to a one-dimensional unsteady groundwater flow model in an atoll aquifer with sinusoidal fluctuations of the tide shows that the tidal response method (TRM) is reliable and safe in comparison to conventional methods of aquifer characterization. The aquifer parameter is described by the ratio of transmissivity to storage coefficient, known as hydraulic diffusivity. The observations of the tide and groundwater levels using loggers at 16 wells over the Kalpeni Atoll, Lakshadweep, India show that with distance from the coast, tidal efficiency decreases roughly exponentially and time lag increases linearly. The calculated hydraulic diffusivity (T/S) values using TRM range from 764 to 1,090,000 m(2)/day. In parallel, pumping tests are conducted in six shallow dug wells to validate the TRM data. Draw down and recovery data during pumping test are influenced by the tide-induced head fluctuations. The effect complicates the interpretation of the drawdown data. After subtracting the net tidal effects from the draw down data, the calculated parameter values are observed to be closer to the estimated parameter values from TRM.