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Deterministic and stochastic modeling of groundwater flow and solute transport in the heavily-stressed Bangkok coastal aquifer, Thailand, and investigation of optimal management strategies for possible aquifer restoration Manfred KOCH1 and Phatsaratsak ARLAI1, 2 1Department of Geotechnology and Engineering Hydrology, University of Kassel, Germany 2now at: Department of Water Resources, Nakhon Pathom Rajabhat University, Nakhon Pathom, Thailand Contents of Presentation 1. Introduction 2. Literature Review 3. Hydrogeological Overview of Study Area 4. Statistical and stochastic Approaches to assess reasonable calibrated Parameters in a complex Multi-Aquifer System 5. Modeling Flow and Transport for sustainable Yield Estimation of Groundwater Resources in the Bangkok Aquifer System 6. Numerical Modeling as a Tool to investigate the Feasibility of artificial Recharge to prevent possible Saltwater Intrusion into the Bangkok coastal Aquifer System 7. Numerical Investigation of Density-driven Flow Effect in transient State to the Seawater and vertical Saltwater Intrusion of the Bangkok-Multilayered Aquifer System 8. Integrating an Groundwater Management Optimization Module and a variable Density Flow and Transport Model to investigate sustainable Restoration Schemes for the Bangkok Aquifer 9. Summary 1. Introduction problem: Increase of groundwater pumping in the last decades History of groundwater withdrawal Hydraulic heads in 1999 Land subsidence in 1999 (~1m) Saline plume migration in 1999 1. Introduction 1.1 Objectives a. Discriminate unequivocal sources of saltwater contamination in the Bangkok aquifer system b. Future possible sustainable management strategies for the present and future safeguard of the groundwater resource will be simulated and then determined. 1.2 Scope of Study a. Setting up 3D groundwater flow model for Bangkok aquifer using MODFLOW-96 (Harbaugh and McDonald, 1996) b. Examine the density effect in Bangkok aquifer system • Constant density: MODFLOW-96&MT3DMS embedded in PMWIN (Chiang and Kinzelbach, 2001;2005) • Variable density: SEAWAT-2000 (Langevin et. al., 2003) c. Apply models to data in layer 3, 4 and 5 to simulate • Present day situation of seawater intrusion or/and saltwater from vertical leakage sources in these coastal aquifers and • Future saline plumes under several possible management strategies to alleviate a future deterioration of the groundwater resources 2. Literature Review 2.1 Saltwater intrusion investigations in the multiple-aquifers system underneath Bangkok and adjacent provinces a. AIT: the saltwater contamination in PD and NL originate from vertical leakage of adjoining aquifers, connate water and the seawater intrusion. b. Kogkusai Kogyo (1995): the contaminations intrude from the Bangkok clay and the seawater intrusion. 2.2 Density effects on the solute transport a. Hydrodynamic dispersion α 1/density contrast b. Significant density-affected vertical plume movement can occur with relatively low concentrations c. With density contrasts of ~ 0.3%, the variable density may have an effect on the plume migration 3. Hydrogeological Overview of Study Area 3D – Geological Map Profile of Bangkok Aquifers System Recharge (Sanford and Buapeng, 1996): a. Insignificant amount of recharge occurs at the center of this plain b. Main recharge exists at the basin flank of aquifer 4. Statistical and stochastic Approaches to assess reasonable calibrated Parameters in a complex Multi-Aquifer System 4.1 Model Implementation Model domain and Finite Different grid of layer 5 Model Area: = 208*256 km2 = 53248 km2 Flow model Model Grid: One layer =52*55 = 2860 •Dirichlet´s BC. at the basin flanks, 1st 9 layers = 25740 layer, and gulf of Thailand 3D of FD of BK aquifers Conceptual Model Modeled layer 5 4. Assess reasonable calibrated Parameters 4.2 Effective approaches to assess the reliable parameters 4.2.1 Conventional (forward) trial&error approach: The head in 1999 is selected to be the target of the steady state calibration a. Quantitative assessment Kogkosai Kogyo, 1995 AIT, 1997 The current model 4. Assess reasonable calibrated Parameters 4.2 Effective approaches to assess the reliable parameters 4.2.1 Conventional (forward) trial&error approach: b. Qualitative assessment RMS/head losses is 4%, 3%, 4% for layer 3, 4 and 5, respectively (5%, Anderson and Woessner, 1992) (a) (b) (c) Observed-(a solid line) and computed (dashed line) heads in 1999 4. Assess reasonable calibrated Parameters 4.2 Effective approaches to assess the reliable parameters 4.2.2 Statistical regression (inverse) approach Yobbi (2000) 0.02 MODFLOW&UCODE (Poeter and Hill, 1998) to compute rcs and cc, scrutinizes uncertainties/ non-uniqueness problem in the calibration. Relative scale sensitivity of T-16 subzones 0.02 Relative scale sensitivity of Vk-14 subzones Subzones of transmissivity in layer 3, Phra Padeang 4. Assess reasonable calibrated Parameters 4.2 Effective approaches to assess the reliable parameters 4.2.2 Statistical regression (inverse) approach The matrix of cc unveils the non-uniqueness problem. (cc < 0.95, Hill,1998) Correlation coefficient matrix of transmissivity (left panel) and vertical leakance (right panel) in layer 3, 4 and 5. 4. Assess reasonable calibrated Parameters 4.2 Effective approaches to assess the reliable parameters 4.2.3 Stochastic modeling using MC-simulations and validation of stochastic theory: (a) validate the pure stochastic formula 1*σY2 and 2*σY2 Simulated variograms of head in layer 3 (left panel), 4 (middle panel) and 5 (right panel) with variances and correlation lengths as specified in above table. Correlation length at 95% fits better than at 66% of sill to Gelhar´s formular. 4. Assess reasonable calibrated Parameters 4.2 Effective approaches to assess the reliable parameters in the Bangkok aquifers model 4.2.3 Stochastic modeling using MC-simulations and validation of stochastic theory “ ”: (b) investigate the calibration parameters which are responsible for the systematic misfit error a. 180 realizations of lnT are generated and run for the Monte Carlo simulations. b. 180 Monte Carlo simulations with randomly disturbed pumping rates of varying magnitudes (30 - 80 % of the reference value) are performed. c of stochastic pumping < of the stochastic transmissivity field. (a) (b) (c) A realization of MC- simulation of T in layer 3 (a), 4 (b) and 5 (c) 5. Modeling Flow and Transport for sustainable Yield Estimation of Groundwater Resources in the Bangkok Aquifer System 5.1 Transient calibration: Dirichlet solute BC. at Gulf of Thailand and the Bangkok clay Monthly observed (red) and computed (blue) heads from 2000 to 2002 at wells PD 93 (left panel), NL2 (middle panel), NB61 (right panel) in layer 3, 4 and 5 Observed (red) and computed (blue) saline concentration (mg/l) in 1995 in layer 3 (left panel), 4 (middle panel) and 5 (right panel). 5. Flow and Transport modeling for Sustainable Yield Estimation 5.2 Sustainable yield estimation Sustainable yield concept 1stscheme: Projected pump for the next 30 yrs based on Sustainable yield the average pumping trend from 1983 to 2002 2 pump schemes for layer 6 to 9 2ndscheme: Projected pump Trial&error pumps in layer 3 to 5 for the next 30 year relied on the double average pumping trend from 1983 to H3,4,5>0.75*HDec,2002 2002 NO and/or Conc3,4,5 < 250 mg/l YES S.Y. for 1stscheme: Decrease pump 1.2%, 1.2% and 1.9% /yr for layer 3, 4 and 5 5. Flow and Transport modeling for Sustainable Yield Estimation 5.2 Sustainable yield estimation Unmet water demand (umd) is the residual water demand for which the sustainable yield cannot serve the expected future water demand and must be supplemented by surface water. (a) (b) (c) Unmet water demand (umd) in 2012 (a), 2022 (b) and 2032 (c) in various provinces 5. Flow and Transport modeling for Sustainable Yield Estimation 5.2 Sustainable yield estimation Saline plume (mg/l) distribution in layer 3 (left), 4 (middle) and 5 (right panel) in 2032. 5. Flow and Transport modeling for Sustainable Yield Estimation 5.2 Sustainable yield estimation Reasons why the S.Y. can not recover the water quality: The velocity vectors and H in layer 3 (a), 4(b) and 5 (c) 34% of inflow Bangkok clay 6% of inflow Gulf of Thailand Saltwater intrusion profile in 1990 and 1995 6. Investigate Feasibility Schemes to prevent Saltwater Intrusion 6.1 Investigation of the origins of the saline sources Observed chloride (mg/l) fingerprints for several profiles in 1990 Blue: c> 4000 mg/l Blue Green: mixing seawater- Green and vertical saltwater intrusion Pale green: c>1000 mg/l, green from upper layers to the layer 3 Red: c>1000 mg/l, from Red upper layers to the layer 5 Map of delineating four types of contamination zones inside Bangkok aquifers system 6. Investigate Feasibility Schemes to prevent Saltwater Intrusion 6.2 Numerical study of the possibility of aquifer restoration The best non-constructive scheme 1. 10th scheme: shut off pump in layer 5 to 9 (2012 to 2032) 2. 19th scheme: decrease pumping to 60% in shallow saltwater intrusion zone and shut off completely the pumps in deep saltwater intrusion zone (2012 to 2032) The best integration of non- and constructive scheme 1. 31th scheme: combination of recharge- and clean-up wells (2012 to 2032) 2. 32th scheme: combination of recharge and clean-up wells (2012 to 2032) 6. Investigate Feasibility Schemes to prevent Saltwater Intrusion 6.2 Numerical study of the possibility of aquifer restoration Optimal design of possible aquifer restoration schemes WOS (a) 19th scheme (b) 31th scheme (c ) Qualitative effects of the different schemes; WOS (a), the best „non-constructive“ scheme (b) and best integration of „non- and constructive“ scheme (c) 7. Numerical Investigation of Density-driven Flow Effects in transient State to the Seawater and vertical Saltwater Intrusion of the Bangkok Multilayered Aquifers System 7.1 Comparison of constant density-and variable density model of the calibrated parameters Constant density model-MODFLOW-96&MT3DMS and variable density model- SEAWAT-2000 are simulated for the long term (2003 to 2032) transient behavior of saline transport. Massive pumping and high transmissivities V is high: Dα V D α 1/Density effect (Koch and Zhang, 1992) Saline distribution of constant- and variable density in 2032 at modeled column 21 or UTM-X = 662000 m. 7. Investigation of Density-driven Flow Effects in transient State 7.2 Sensitivity analysis of hydrodynamic dispersion Sensitivity analysis of A (dispersivity) Five A cases; With pump • Outside major pumping zone ---> Plumes of variable density model sink deeper than one of constant density model • Greater A ---> Plumes sink deeper in both constant- and variable density model Simulated saline conc. of Without pump constant- (solid line) and • Massive uplift >>> density variable density (dashed effect line) in Dec, 2032) with different values of dispersivities. 7. Investigation of Density-driven Flow Effect in transient State 7.3 Sensitivity analysis of hydraulic anisotropy Sensitivity analysis of hydraulic anisotropy Kx/Kz Four Kz cases; With pump • Outside major pumping zone ---> Plumes of variable density model sink deeper than one of constant density model Without pump • Massive uplift >>> density effect The simulated saline conc. of constant- (solid line) and variable density (dashed line) in Dec, 2032) with different values of Kz. 7. Investigation of Density-driven Flow Effect in transient State 7.4 Effect of density-dependent flow and transport on the trial&error aquifer restoration management schemes Effect of variable density flow and transport on the effectiveness of the trial&error aquifer restoration management schemes Snapshots of simulated saline plumes (kg/m3) for Dec, 2032, using the constant- (a, c, e) and variable density model (b, d, f): WOS (upper-left panel), 19th scheme (upper-right panel) and 31th scheme (lower- left panel) 8. Integrating an Groundwater Management Optimization Module and a variable Density Flow and Transport Model to investigate sustainable Restoration Schemes for the Bangkok Aquifers 8.1 Optimal sustainable management Optimal sustainable management 1st scheme: Sustainable yield 4th scheme: Optimize the 5th,6th,7th scheme: Optimize water integration of non- and constructive trade off concept from the scheme 2nd scheme: Trial&error scheme 19 scheme--obtained from the 4, 2, 3 3rd scheme: Trial&error scheme 10 trial&error scheme 31 Re-simulate 7 schemes with SEAWAT-2000 Best:H-, Pollution recovery, invested costs Best scheme 8. Groundwater Management Optimization 8.2 Results Results for scheme 4 Applying GWM module embedded within MODFLOW-2000 to optimize the trial&error integrated non- and constructive scheme (31th scheme from chapter 6) Objective function and constraints Comparison of head recovery and monetary costs 51% savings Trial&error well scheme in layer 3 Optimal well scheme in layer 3 (a), (a), 4(b) and 5(c). 4(b) and 5(c). 8. Groundwater Management Optimization 8.2 Results Results for scheme 5, 6 and 7 Embedding GWM within MODFLOW-2000 optimizes the water trade off concept for three new schemes Objective function and constraints of 5,6,7 “Water Trade off Concept” = to examine the least-cost effective means through raise the water levels along the front of the seawater intrusion up to zero meter (MSL) either by shutting off the discharge wells or increasing freshwater injection close to the shoreline through in-lieu water supply, in order better restrain seawater intrusion or to reduce the polluted area of the “without scheme”. An example of in lieu delivered water supply cell- (white cell), recharge well(blue cell) candidates and withdrawal cells (orange cell) in layer 3 (a), 4(b) and 5 (c) of the scheme 5 to 7. 8. Groundwater Management Optimization 8.2 Results Quantitative analysis of variable-density effects of all schemes The 6rd scheme is the best scheme Summary of % averaged reduction of vertical saline plume pollution area, -seawater intrusion area and head recovery in layer 3 to 8 relative to the wos scheme 8. Groundwater Management Optimization 8.3 Discussion Choosing the best scheme The 3rd scheme is the best scheme Comparison of unmet water demand, averaged reduction of saline pollution area and total costs of each scheme 9. Summary • Conventional-, new statistical- and stochastic method are used to evaluate the set of calibrated parameters and show that the set of calibrated parameters are realiable • The sustainable yield concept can satisfy only the hydraulic constraints, but does not significantly improve the groundwater quality. • The saline leakage from the Bangkok clay appears to be the major source of contamination, while the horizontal seawater intrusion is the minor pollution source. • With the co-existing pollution sources, the groundwater management scheme needs complex „policy“- or „non-constructive“ measure as well as integrated „non-and constructive“ measures to remediate the saline pollution within the Bangkok aquifers system. Two best „non-constructive“, and integrations of „non- and constructive“ measurement are found through trial&error and constant density modeling. • Through the sensitivity analyses of the hydrodynamic dispersion and the aquifer anisotropy, it is found that the variable density of the contamination saline plumes does not appear to have a significant feedback effect on the hydraulic flow itself in the Bangkok multilayered aquifers. • From the 7 schemes in Chapter 8 it is discovered that the scheme 3 is the best among these 7 schemes with respect to the groundwater hydraulic, quality, invested cost and impact to existing groundwater users.
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