TR(BR)-9/97-98 : Influence of fractured zone on seepage from a water body

Abstract

Artificial recharge ponds/tanks offer ample scope for replenishing dwindling aquifers. In a multilayered aquifer system the recharging may be influenced by hydrogeological aspects such as geology of various layers, fractures/ discontinuities, inhomogeneity and anisotropy in the aquifer. With the existence of a confining layer having very low permeability, recharging of an underlying aquifer from a surface water body can not be successful. However, through discontinuities/ fractures in the confining layer water may percolate down. Influence of the dimension of a discontinuity in the confining layer on the seepage of water down to a semi-confined aquifer has already been studied earlier. In continuation to the previous study, the effect of positioning of a fractured zone with respect to the recharge- source in a multilayered aquifer is investigated presently. The behaviour of hydraulic potential/ discharge from the aquifer as the discontinuity (a fractured zone) is located at points away from the source is studied. Further, the influence of positioning of the fractured aquitard at different depths in the aquifer system on the flow domain is also investigated. Normalised heads/potentials in the central section, distribution of potential in the aquifer, change in discharges for various fractured zone positions etc. have been used for the purpose of analysis. Evidently, variations in the flow/discharge characteristics of the unconfined top aquifer is not significant. However, location of the opening/fractured zone in the confining layer influences the potential distribution as well as discharge from the bottom semi-confined aquifer. A gradual decrease in hydraulic potential evidenced by reduced discharge from the bottom aquifer is observed as the opening/fractured zone being placed farther from the recharging source. It is found that for a given location the extent of influence of the location of a fractured zone (opening) is high for smaller openings. As the opening becomes large. the influence of its position in the aquitard tends to minimal. Further, the fractional seepage to the bottom aquifer is found to be decreasing as the position of the fractured aquitard in the system is at larger depths. For a given position of the fractured aquitard, the maximum seepage to the bottom aquifer occurs when the fractured zone irrespective of its dimension is located centrally below the recharging source. Various cases have been studied with different openings at different locations and with different positions of the fractured aquitard. The hydraulic potential in the aquifer is found to be higher when the fractured aquitard is closer to the impermeable lower boundary, that is when the aquitard is at its lowest position with reference to the datum of the aquifer system. However, the fractional seepage occurring down to the bottom aquifer is found to be decreasing as the aquitard is placed nearer to the lower boundary. The various cases have been repeated for a range of combination of aquifer parameters so as to examine sensitivity with respect to different aquifer systems.