SIMULATION OF FLOW IN MULTILAYERED AQUIFER SYSTEM WITH AND WITHOUT DISCONTINUITY

Abstract

Ever-increasing dependence on groundwater for various uses has not only triggered environmental hazards but depletion of many aquifers too. This necessitates rejuvenation of the aquifer systems through artificial recharge methods. Recharging is influenced by hydrogeology of the aquifer, the hydraulic properties, flow parameters of the porous medium and flux boundaries of / ^ the aquifer system. Therefore, efficiencyof recharging schemes vis-a-vis hydraulic response of the aquifer system needs to be investigated. Nevertheless, quantitative results on flow regimes and hydraulicresponsesofaquifersystems recharged by surfacewater sources are still evasive especially in complex aquifer systems. In this context, analytical as well as numerical groundwater models can be of greats***- A;u-^^> /^ f<*~ ^jr^^t' A^ ,j vifi~> A critical review ofthe literature relevant to surface water - groundwater interaction, artificial recharge practices, analytical as well as numerical groundwater flow modelling and geophysical aspects of porous media has been carried out. Based on the review, it has been perceived that there is a possibility of developing{ney)r analytical approaches for aquifer simulation in multilayered aquifer systems by considering the analogy between groundwater flow and electrical flow in the context,ofadvances made in exploration geophysics. Unlike numerical models, analytical models •h render faster techniques'to simulate flow/ hydraulic potentials in aquifer systems with fewer data sets. Further, they offer qualitative insights that may not berevealed through the quantitative results obtained by numerical simulations. However, analytical models developed for idealised conditions are not effective for detailed investigations on the flow characteristics in complex aquifer systems with discontinuities (like distributed clay lenses in alluvial aquifer systems). Alternately, numerical modelling is desirable in such cases. Thus, analytical and numerical methods can be complementary to each other for conducting effective flow simulation studies in isotropic/ anisotropic layered aquifer systems with or without discontinuities. In view ofthe above, the major objectives ofthe present thesis are set as: (i) development ofanalytical models for simulation ofhydraulic potentials and streamlines due to recharging by surface water sources in multilayered aquifer systems, (ii) development ofanalytical models for aquifer simulation in ahomogeneous anisotropic aquifer system with recharging sources kept on the earth surface, (iii) validation ofthe generated analytical models in several numerical experiments and (iv) numerical simulation studies in an aquifer-aquitard-aquifer system with continuous as well as discontinuous aquitard in the medium. The analytical solutions have been derived by invoking the advances made in the realm ofexploration geophysics. The numerical simulation studies in an aquifer-aquitard-aquifer system brought forth many results on the recharging characteristics ofsuch a system. Steady state analytical expressions for hydraulic potentials and streamlines due to recharging by apoint source have been derived for the case of alayered aquifer system with amaximum of three layers. By applying appropriate convolution techniques the solutions have been modified to suit recharge by finite-length line sources and areal sources. Subsequently, by devising an alternate methodology, analytical expressions have been developed for the case of multilayered aquifer systems with large ofnumber of layers (say, nlayers). Arecurrence relationship has been developed to achieve this in an iterative manner. Several computational algorithms using these analytical expressions have been coded in FORTRAN??. For athree-layered aquifer system the computational algorithms developed are: (i) 3LPNT- for apoint source, (li) 3LLIN- for afinite length line source and (iii) 3LARL- for an areal source. Whereas, NLPNT is the computational algorithm for the case ofamultilayered aquifer system. Anumber ofnumerical examples pertaining to the recharge ofthe multilayered aquifer systems have been presented and discussed with illustrations. Also, the comparison of corresponding results obtained from analytical models and that obtained from the numerical flow model, MODFLOW is undertaken. The results match extremely well, confirming the validity ofthe developed analytical models. Besides, aquifer simulations using these analytical models are found to be highly efficient compared to an identical simulation by the numerical model MODFLOW in terms of data requirement and computational speed. Further, by making use ofanalogies from geophysical vertical electric sounding methods and applying relevant domain transformation techniques, analytical expressions have been developed for simulating hydraulic potentials due to recharging by a point source in a homogeneous (transverse) anisotropic aquifer system with dipping beds. Based on the analytical solutions, an effective computational algorithm (HANI-P) has been devised to enable simulation ofthe anisotropic aquifer system. Illustrations with numerical examples have been provided for several coefficients of anisotropy and angles of dip of the soil strata in the anisotropic medium. Also, analytical solutions for hydraulic potentials have been developed for the case ofrecharging ofa homogeneous anisotropic aquifer system due to a finite-length line source. The corresponding computational algorithm, HANI-L is used to demonstrate numerical examples. Also, the validity ofthese analytical expressions have been verified for homogeneous isotropic conditions. In order to gain further insight into the recharging characteristics of a three-layered aquiferaquitard- aquifer system, elaborate numerical simulation studies have been carried out. An aquiferaquitard- aquifer system having a surface water recharging source has been conceptualised with a provision to keep the aquitard continuous or discontinuous (as the case may be) in the model discretisation. The analyses performed pertain to: (i) Effect of the dimension as well as the positioning of a continuous aquitard on the recharging of the aquifer system; (ii) Effect of the dimension of the aquitard discontinuity in a centrally positioned aquitard on the recharging of the aquifer system; (iii) Effect of the location of the discontinuity in the aquitard with respect to the source on the recharging of the aquifer system; (iv) Effect of depth-wise positioning of the discontinuous aquitard in the aquifer system on the recharging of the aquifer system. Using several dimensionless parameters, analyses of the simulation results have been earned out to infer useful conclusions that are presented with illustrations.