TR(BR)-3/96-97 : Effect of discontinuous aquitard on the seepage from a static water body

Abstract

In semi-arid areas of India percolation tanks are in use for artificial recharge of the aquifer. However, the efficacy of the percolation process has not yet been studied in terms of the parameters which influence the recharge. The seepage from a surface water body is dependent on the hydrogeological setup of the region and aquifer characteristics. Present study investigates the seepage pattern beneath a recharging source. For the purpose of investigation, a case study with recharge/discharge sources in a two layer aquifer system separated by a barrier with possible discontinuities is considered. A three-dimensional groundwater flow model is employed for simulating flow in the system. The recharging source, a pond/lake, is positioned at the centre of the aquifer system in plan. A number of cases have been studied by making openings of varying widths in the aquitard, symmetrically located below the recharge-source to analyse the impact of a discontinuity on seepage distribution. This report deals with the influence of the dimensions of the opening in the aquitard and the aquifer characteristics on the discharge as well as on the hydraulic potentials. Distribution of hydraulic potentials in the entire system for different diffusivity values and for various aquitard openings are analysed. A number of cases were studied and results are presented graphically.

The analyses revealed the following: (i) So long as the hydraulic diffusivity (SIT) is kept constant the fractional seepage, Fsb to the aquifer system is invariant. (ii) The zone of influence of the recharge source decreases as the hydraulic diffusivity increases. (iii) Even for a very minor discontinuity the contribution of seepage to the second aquifer is significantly high. (iv) Further, the seepage is predominantly to the bottom aquifer irrespective of the dimension of discontinuity in the aquitard for large ratios of hydraulic diffusivity. The study revealed that diffusivity has significant effect on the efficiency of a recharge pond rather than the individual aquifer parameters. Also, it is observed that even minor discontinuity in the barrier causes significant change in the seepage pattern.