Ghazali, Abdul Halim

Universiti Putra Malaysia Institutional Repository

UPM Research Report 1997-2000, Section 2-Extended AbstractsAbdul Halim GhazaliGroundwater Utilisation from Density-Stratified Non-Homogeneous Unconfined Aquifers*Facultyof EngineeringUniversitiPutraMalaysia43400UPM,Serdang,SelangorMalaysiaE-mailolCorrespondingAuthor:abdhalim@eng.upm.edu.myKey words: groundwater modeling, density-stratified aquifer, saltwater intrusion, piezometric head variation, wellpenetration.IntroductionMany basic studies have been con-ducted to formulate the flow of fluidsand mass transport equation, of saltpollutant in porous media. These stud-ies have resulted in analytical solutionsto simple flow problems with simpleboundary conditions. Because of theinadequate techniques in analyticalsolutions and the advances I numericaland computer technology, an interesthas arisen in developing a numericalsolution to describe the flow and dis-persion processes in selective with-drawal problems taking into accountdensity variation and the non-homogeneity of an aquifer. The objec-tives of the study are: 1) to develop anumerical model describing the flowand mass transport of salt pollutanttowards a pumped well in a density-stratified non-homogeneous uncon-fined aquifer, and 2) to provide theneeded guidelines for selective with-drawal from such groundwater reser-voirs to meet a desired water qualityconstraints.Materials and MethodsThe study involved both analytical andexperimental works. Numerical equa-tions governing the groundwater flowand solute transport in an aquifer sys-tem were analysed and solved, andexperiments to validate the numericalsolutions were carried out. The equa-tions were solved by the finite elementmethod using the Galerkin technique.A computer software, called SUTRA,was employed to generate results forvarious well depths and dischargerates. The experimental works wereperformed on a laboratory scalegroundwater system model with facili-ties to monitor the hydraulics head atseveral radial distances from the dis-charging well and the saltwater con-centration at several depths in the aqui-188fer. The head and concentration weremonitored throughout the period of anexperimental run.Results and DiscussionThe results show that the numericalsolutions are comparable with the ex-perimental observations with maximumdepartures of about 10% for the pres-sure head and 11% in the case of saltconcentration distributions. The salt-water concentration at various loca-tions in the aquifer when the shallowerwell is used is lower than the concen-tration due to a deeper well. In the caseof piezometric head profiles, abstrac-tion using the shallower well resultedin smaller drawdown, compared to thedeeper well. A smaller discharge rateinduced less saltwater into the aquifer.However, with long period of abstrac-tion the difference in the salt concen-tration at various points tend to dimin-ish. Although comparison of both so-lutions had shown the consistency thatis needed for the purpose of modelling,it is observed that all these discrepan-cies need to be investigated further. Inthis study, for the case of concentrationratio distributions, it was apparent thatthe values of the numerical solutionwere greater than experimental solu-tions, while for the case of pressurehead distributions; experimental solu-tions always had higher values. In real-ity, the situation may be different be-cause solute reaction processes areneither linear nor equilibrium con-trolled, and the numerical model maynot necessarily represent the true com-plexities of the reaction. Difficultproblems also arise when the concen-tration of the solute of interest isstrongly dependent on the presence ofnumerous other constituents that existin the porous media. Here, mineralogi-cal variability may be significant andmay affect the rate and of reactions,and yet be ignored in the mathematicalmodelling instead. At the same time, inmany groundwater flow systems, sorp-tion may also cause the retardation ofthe movement of the contaminants.Sorption refers to the uptake of thedissolved constituents from solution bythe porous medium. These phenomenamay have caused the numerical valuesof concentration distribution becominggreater than experimental solutions inthis study. Larger movement of soluteexperienced in the numerical modellingalso meant that higher velocity ofgroundwater flow in the porous media,and this could generate bigger draw-down in water table.ConclusionsThe numerical solution was developedfor the hydraulic head and concentra-tion distributions in two-dimensionalaxi-symmetric flow towards a partiallypenetrating well through an uncon-fined, non-homogeneous and isotropicaquifer. Comparisons between the nu-merical results and those from experi-mental tests indicate that the model canaccurately simulate the movement ofpollutant (saltwater) in the saturatedzone of a non-homogeneous uncon-fined aquifer.Benefits from the studyA numerical method to describe thephenomenon of selective withdrawal ofgroundwater. This will assist the usersin determining the rate and the durationof groundwater withdrawal so as toavoid the problem of saltwater intru-sionLiterature cited in the textNone.Project Publications inRefereed Journalslong Tu Yong.AbdulHalimGhazali,Suieyman AremuMuyibi.Salim Said,Aziz F. Eloubaidy. 2000. ModelingofApplied SCiences and Technologiessaltwater intrusion into a discharging wellin a non-homogeneous unconfinedaquifer. Under review Pertanika Journalof Science and Technology.Project Publications inConference ProceedingsNone.Graduate ResearchJong Tze Yong. 2000. Groundwatermodeling [M.sj. Universiti PutraMalaysia.189

Groundwater utilisation from density-stratified non-homogeneous unconfined aquifers

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Groundwater utilisation from density-stratified non-homogeneous unconfined aquifers

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