Simulation of Flow and Transport Processes of a Confined Aquifer using Groundwater Modeling System applying Grid Approach

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1 Smulaton of Flow and Transport Processes of a Confned Aqufer usng Groundwater Modelng System applyng Grd Approach Master of Engneerng Cvl Engneerng Department, Assam Engneerng College, Guwahat Abstract Throughout the world groundwater s an mportant source of water supply. Groundwater table s decreasng n many parts of the world due to overexplotaton and unplanned pumpng n the aqufer. Another problem n groundwater s extensve contamnaton due to varous pont and non-pont sources. Ths current study presents a new smulaton method by usng the groundwater smulaton software (GMS). Ths method proposes by lnkng user frendly commercal groundwater smulaton software GMS wth MATLAB for smulaton of nonunform aqufer. GMS s a complete program for buldng and smulatng groundwater models. It s one of the most sophstcated groundwater modellng software through whch we can smulate the flow and transport processes. The smulaton packages MODFLOW and MT3DMS avalable n Groundwater Modellng System (GMS) are used to smulate the flow and transport processes n an aqufer. Two partal dfferental equatons, the flow equaton and the transport equaton have been used to smulate both the flow and transport processes of the contamnants n the aqufer. Keywords: Groundwater contamnaton; Non-unform aqufer; Groundwater modellng system (GMS); Flow and transport processes; MATLAB; Smulaton. 1. Introducton Groundwater contamnaton has emerged as one of the Naton s prmary envronmental concerns. It s the occurrence, dstrbuton and movement of contamnated water below the surface of earth that bascally relates to water accumulaton through generatons by water nfltratng nto the earth s crust from all natural and artfcal water sources. The sources may be seen n any three phases of water forms whch proves to be the basc necessty of the entre manknd makng t utterly vulnerable to every possble causes of explotaton. Causes of explotatons are nnumerable startng very frst from excessve use of fertlzers and accumulaton of toxc materals n all water sources. Ths never endng process leaves traces of some undentfed pollutant sources n an aqufer and polluton dstrbuton takes place due to steady or transent pumpng. Therefore, the pumpng should be done n such a way that the entre aqufer does not dry up due to the contnuous pumpng. Some famous yet effcent methodologes that have been put forwarded by some great people n tme are of great mportance wth ths dscusson. The Galeran method of approxmaton n conuncton wth the fnte element method of analyss was used to smulate the movement of groundwater contamnants [7]. A nonlnear smulatonoptmzaton model was developed to determne optmal strateges for contamnated groundwater remedaton. The nonlnear programmng algorthms avalable n MINOS (Modular In-core Nonlnear Optmzaton System) were used to solve the desgned problem. The management model was solved both for steady and transent condtons [1]. The dentfcaton of the unknown groundwater polluton sources was studed by usng lnked smulaton optmzaton approach. The groundwater flow and transport smulators were lnked to the nonlnear optmzaton model as an external module. The groundwater smulaton model SUTRA (Saturated Unsaturated Transport ) was lnked wth the optmzaton program MINOS. The computatonal effcency of these models has been evaluated usng hypothetcal sources dentfcaton problems [3]. Artfcal neural network (ANN) based methodologes, and a genetc 10

2 algorthm (GA) based lnked smulaton optmzaton methodology was developed that would facltate optmal dentfcaton for unknown groundwater polluton sources usng concentraton measurement data [10]. Three steps soluton methodology was descrbed to dentfy optmal dynamc montorng network desgn and dentfcaton of unknown groundwater polluton sources. SUTRA (Saturated Unsaturated Transport) model was used for smulatng the groundwater flow and transport processes [4].The methodology for optmal dentfcaton of unknown groundwater polluton sources was developed by lnkng flow and transport numercal smulaton wth classcal nonlnear optmzaton [5]. GMS was lnked wth MATLAB-based optmzaton procedure for solvng groundwater source dentfcaton problem [3]. In ths study, a new methodology has been proposed by lnkng the most advanced groundwater smulaton software GMS (Groundwater Modellng System) wth MATLAB for smulaton of nonunform aqufer by usng Grd approach. GMS s a complete program for buldng and smulatng groundwater models. GMS contans numerous numercal models and support features for modellng the groundwater envronment. The smulaton packages MODFLOW and MT3DMS avalable n Groundwater Modellng System are used to smulate the flow and transport processes.two partal dfferental equatons, the flow equaton and the transport equaton have been used to smulate both the flow and transport processes of the contamnants n the aqufer. When GMS executes MODFLOW and MT3DMS, t saves nput and output data n a number of fles n a modular pattern. Output data generated by MODFLOW s used by MT3DMS whle solvng transport model. So, for settng a MT3DMS smulaton there should be a MODFLOW smulaton. In ths study, a MATLAB code s developed to open and change the nput fles of MT3DMS. Ths code can smulate the flow and transport processes n an aqufer for any arbtrary concentraton source parameters. 2. Methodology Two partal dfferental equatons, the flow equaton and the transport equaton have been used to smulate both the flow and transport processes of the contamnants n the aqufer Groundwater Flow and Transport equaton A transent, two-dmensonal, areal groundwater flow equaton for a heterogenous, ansotropc and fully saturated aqufer gven by [2] be wrtten as- h h ( T ) S Q W (1) Where, S s the storage coeffcent; T = K b s the transmssvty tensor (L 2 T -1 ); K s the hydraulc conductvty (LT -1 ); b s the saturated thckness of the aqufer (L); h s the hydraulc head (L); t s the tme(t); Q s the pumpng rate per unt area ( LT -1 ); W s the recharge flux per unt area (LT 1 ); x and x are the Cartesan coordnates. The two-dmensonal solute transport gven by [6] be wrtten as - ( cb) t, 1,2 c ( bcv ) c' W c' Q bd. ) (2) Where, b s the saturated thckness of the aqufer (L); c s the concentraton of the dssolved chemcal speces( ML -3 ); D s thecoeffcent of hydrodynamcdsperson (second order tensor) (L 2 T - 1 ); c / s the concentraton of the dssolved chemcal n a source or snk flud ( ML -3 ); v s the seepage velocty n the drecton x ( LT -1 ); ŋ s the effectve porosty of the aqufer ( dmensonless); Q s the pumpng rate per unt area (LT -1 ); W s the recharge volume flux per unt area ( LT -1 ). 2.2 Study area The performance of the proposed methodology s evaluated for rregular boundary of twodmensonal llustratve study area. The confned aqufer of the study area has a sze of 1.62km 2 (1.8 km 0.9 km).the boundary of the aqufer s rregular and grd sze s consdered as 100m by 100m. There are 9 rows and 18 columns. The boundary condton of the aqufer s consdered as tme varant. Both north and south boundares are no flow boundares and east and west boundares are fxed head boundary. Flow boundary condton s assumed to be constant at the east and west sdes of the aqufer. At the west sde of the aqufer, the head s varyng from 100m to 97.5m. Smlarly the hydraulc head at the east sde s varyng from 88m to 90.16m. Insde the aqufer, ntal head s consdered as 100m.Ths study s carred out for the llustratve study area (see fgure.1)

3 The dfferent pumpng rates of water n the pumpng locaton are gven n the Table 2. Table 2. Pumpng rate of water n pumpng locaton. step Dschargng rate (m 3 /d) step Dschargng rate(m 3 /d) Fgure 1. Illustratve Study Area. There are three contamnant sources n the aqufer, whch are desgnated as S1,S2 and S3 as shown n the Fgure 1. The aqufer has horzontal hydraulc conductvty (Kxx) and vertcal hydraulc conductvty (Kyy) of 20m/day. The effectve porosty (ŋ) value of the aqufer s 0.3. The another parameter values of the aqufer are : longtudnal dspersvty( α L )=20.0 m ;transverse dspersvty ( α T ) = 9.6 m ; specfc storage = A pumpng well s located at the centre of the study area and frstly the rate of pumpng s consdered steady and then t s consdered as transent. There are eght observaton wells n the aqufer whch are desgnated as 1,2,3,4,5,6,7.The flow and transport smulaton are made for 2years usng 8 stress perods of 3 months each. There are three (3) contamnant sources n the aqufer. At the tme of smulatng the transport sources, only two pollutants are taken as actve and one s consdered as nactve The concentratons of the sources at dfferent locatons for the llustratve study area are gven n the Table 1. Table 1. Source flux for the llustratve study area. Step Source Flux ( gm/sec) S1 S2 S Model Development by GMS : Groundwater Modellng System (GMS) s the most effcent groundwater modellng software avalable. It s a complete program for smulatng groundwater models. In ths study, the flow and transport processes of a non-unform aqufer can be observed by usng the GMS. GMS contans two computer softwares, MODFLOW and MT3DMS for the flow and transport smulaton of the non-unform aqufer.gms frst runs MODFLOW and then t executes MT3DMS whle solvng the transport model. So for settng a MT3DMS smulaton, there should be a MODFLOW smulaton.after the executon of MODFLOW and MT3DMS, GMS saves nput and output data n a number of fles n a modular pattern. The nput and output fles of GMS are lnked wth MATLAB, to read the bnary fles produced by GMS. A code s developed n MATLAB to open and change the nput fles ofmt3dms. Snce bnary head fles cannot be opened n GMS, therefore, bnary head fle generated by MODFLOW can be opened n MATLAB and used to plot head contour. In ths way, bnary concentratons can also be opened n MATLAB and s used to plot contour for concentraton of contamnants. 3.Results and dscussons 3.1 Head dstrbuton when there s Steady pumpng by usng MODFLOW. Assumng the rate of steady pumpng as Q 1 =11500m 3 /d and the recharge rate as m/d, the head dstrbuton n the aqufer can be observed when there s a steady pumpng (See Fgure 2). 12

4 Fgure days pont source contour Fgure 2. Head dstrbuton durng steady pumpng 3.2 Pollutant dstrbuton durng transent pumpng by usng MT3DMS A number of 2D transent transport smulaton s performed usng MT3DMS. The dstrbuton of the pollutant at each and every tme step s dfferent from the prevous step.it can be observed that the pollutant concentratons are movng towards the pumpng well after 90 days of the concentraton dstrbuton ( See Fgure 2) due to contnuous pmpng of water from aqufer. Lkewse, t can be observed that more and more pollutant concentratons are movng towards the well due to the contnuous pumpng after 180, 270, 360 days. Same stuaton has been observed as shown after 450 days ( See Fgure 3). It may be noted that the upto ths perod, the sources n the aqufer are actve. The concentraton dstrbuton after 540 days ( See Fgure 4) shows that the pollutant concentraton s decreasng near the pumpng well. Ths s because the sources are consdered as nactve now. Lkewse after 720 days, t can be observed that the pollutant concentraton slowly tends to dsappear from the aqufer as the sources were not actve for a longer perod (See Fgure 5). Fgure days pont source contour 13 Fgure days pont source contour Fgure days pont source contour 4. Concluson In ths paper, GMS has been proposed for the smulaton of flow and transport processes of a nonunform aqufer by usng the user-frendly commercal software Groundwater Modellng Sytem (GMS). GMS s theeffcent software platform for creatng groundwater smulatons. The two computer packages MODFLOW and MT3DMS avalable n GMS have been used n ths methodology to smulate the flow and transport processes n the non-unform aqufer. After lnkng GMS wth MATLAB, the concentratons of the nactve pollutant can be read. The performance of the proposed methodology s evaluated for rregular boundary of two-dmensonal llustratve study area.

5 The performance of ths methodology s encouragng even when appled to complcated problem also. Further studes wll be necessary to udge the full potental of the proposed methodology. 5. Acknowledgement I would lke to acknowledge the nestmable help gven to me by the entre staff of Cvl Engneerng Department, Assam Engneerng College, Guwahat, for ther knd help and co-operaton to carry out ths study. 6. References [1] Ahfeld, D.P., and MULVEY, J.M.(1986), Desgnng optmal strateges for contamnated groundwater remedaton, Advances n Water Resources, 9(2), [2] Bear, J.(1972), Dynamcs of Fluds n Porous Meda, Dover Publcatons,Inc.,New York. [3] Borah and Bhattacharya (2015), Soluton of source dentfcaton problem by usng GMS and MATLAB, ISH J.of Hydraulc Engneerng, 19(3), [4] Datta, B.,Chakrabart D., and Dhar,A. (2003), Identfcaton of unknown polluton sources usng lnked optmzaton smulaton methodology. Proc. Of Symposum on Advances n Geotechncal Engneerng (SAGE,2003). IIT, Kanpur, Inda, pp [5] Datta, D.C and Anrban Dhar(2009), Smultaneous dentfcaton of unknown groundwater polluton sources and estmaton of aqufer parameters, J. of Hydrology,376(1-2), [6] Datta, B.,Chakrabart,D.,and Dhar, A.(2011), Identfcaton of unknown groundwater polluton sources usng classcal optmzaton wth lnked smulaton, J.of Hydro-envronment Research5, [7] Freeze, R.A., and Cherry, J.A. (1979), Groundwater. Prentce-Hall Inc., New Jersy. [8] George F. Pnder(1973), A galerkn-fnte element smulaton of groundwater contamnaton on Long Island, New York, J.Of Water Resources Research, 9(16)1-13. [9] Lohman, S.W. (1972). Groundwater hydraulcs, U.S.G.S. Prof. paper, 708, 70. [10] Mahar, P.S., and Datta, B. (2001), Optmal dentfcaton of ground-water polluton sources and parameter estmaton. J. Water Res. Plannng Manage., ASCE, 127(1), [11] Sngh,R.M and Datta B, Jan A (2004), Identfcaton of unknown groundwater polluton sources usng artfcal neural network, J of Water Resources Plng and Mgmt ASCE 130(6):