Tài liệu Numerical simulation for the assessment of groundwater safe yield in red river delta, viet nam

1 ABSTRACTS In Red River Delta, there are many important economic regions located on the coastal areas. The growth of both the economy and the population in these areas during the last decades, which is based on the availability of water resources, and the extensive exploitation have strongly increased the pressure on this finite and vulnerable resource. Since surface water is unevenly distributed and increasingly affected by human activities, groundwater has become the major source of potable water. This thesis therefore focused on contributing to the improvement of groundwater management in RRD by determining groundwater safe yield using modelling method. Nam Dinh Province was selected to be the study area. In the northern areas of Nam Dinh Province, due to the the high concentration of contamination in the groundwater aquifers, surface water is used as the main source of water for the region. Groundwater usage in these areas is mainly for other purposes than domestic use. Small scale groundwater extraction can be found near the main rivers with shallow wells in the uppermost aquifer due to the high level of contamination. Deeper aquifers are the main subject for extensive extraction in the province. Significant extractions are found in the south of Nam Dinh, where large freshwater lens in Pleistocene aquifer are located. Therefore in this thesis, the Pleistocene aquifer is the main area of interest. It was decided to use the Visual MODFLOW for the setting up of a numerical model of the Nam Dinh Area. The 3D hydrogeological structure for the model was created from strata data of 83 boreholes, including 27 boreholes from Nam Dinh, 15 boreholes from Ha Nam, 17 boreholes from Ninh Binh, 23 boreholes from Thai Binh and 1 from Thanh Hoa. Natural neighbors method was used to interpolate the elevation of surface and bottom of the layers. The 3D structural model was built with 5 layers, representing 4 aquifers and 1 aquitard, including Upper Holocene aquifer (qh2), Lower Holocene aquifer (qh1), Pleistocene aquitard (qp2), Pleistocene aquifer (qp1) and Neogen aquifer (n) with 2 the order from top to bottom respectively. The finite difference grid used for the model has the size of 181 x 160 (181 columns x 160 rows, the size of each cell is approximate 400m x 400m). Observed water heads from 19 wells were used to calibrate and verify the model. The normalize root mean squared of the calibrated model is 6.296%, which is considered to be good enough to simulate the future uses of groundwater for the study area. To show the impacts of groundwater utilization on the qp aquifer in Nam Dinh Province, scenarios of future situations were simulated using the calibrated model. Extraction rates were extrapolated until 2050 and three versions were considered: 1) Extraction rises constantly until 2050 in the same manner than from 1994 to 2012; 2) Extraction stays on a 2012 level, which means that no additional water is going to be extracted; 3) Extraction gradually declines to 0 by 2050. The simulation results of these scenarios were used to estimate the groundwater safe yield for Nam Dinh Province. It showed that the safe yield for the area is estimated to be 70300 m3/day. 3 DECLARATION I hereby certify that the work which is being presented in this thesis entitled, “Numerical simulation for the assessment of groundwater safe yield in Red River Delta, Viet Nam” in partial fulfillment of the requirement for the award of the Master of Science in Integrated Water Resource Management, is an authentic record of my own work carried out under supervision of Associate Professor Dr. Vu Minh Cat and Dr. Bui Du Duong. The matter embodied in this thesis has not been submitted by me for the award of any other degrees or diplomas. Date: 28th November 2014 4 ACKNOWLEDGEMENTS This thesis was completed at Faculty of Water Resources Engineering, Thuy Loi University. First and foremost, I would like to thank my advisor, Assoc. Prof. Dr. Vu Minh Cat for his invaluable guidance. I am deeply grateful that I had the opportunity to learn from his knowledge. I would like to thank Dr. Bui Du Duong for his helpful contribution to the hydrogeology field and his supervision of my study. I am very grateful to Assoc. Prof. Dr. Pham Quy Nhan, MSc. Dang Tran Trung and MSc. Tran Thanh Le for their valuable advices and help with the preparation of spreadsheet data for the boreholes and well logs. To Ms. Mariette van Tilburg, I am very thankful for the English corrections, suggestions for my thesis and for providing me useful writing resources. Lastly, a word of thanks is extended to NICHE-VNM-106 project team for providing a 18 months MSc scholarship. Again, I would like to express my sincere gratitude for all these valuable help! 5 TABLE OF CONTENTS ABSTRACTS 1 DECLARATION 3 ACKNOWLEDGEMENTS 4 LIST OF ABBREVIATIONS 7 LIST OF FIGURES 8 LIST OF TABLES 11 CHAPTER I – INTRODUCTION 12 1.1. Background information 12 1.2. Objective & scope of study 14 1.3. Previous studies & state of knowledge 15 1.3.1. In the world 15 1.3.2. In Viet Nam 16 1.4. Methodology 18 1.5. Structure of thesis 20 CHAPTER II – CHARACTERISTICS OF NAM DINH PROVINCE 21 2.1. Physical settings of Nam Dinh Province 21 2.1.1. Geographical location 21 2.1.2. Topography 22 2.1.3. Climate conditions 22 2.1.4. Surface water bodies 24 2.1.5. Soils and land use 25 2.1.6. Population & Socio-Economy 26 2.1.7. Water supply and groundwater utilization 26 2.2. Geological characteristics of study area 28 2.2.1. Structural characteristics 28 2.2.2. Stratigraphy characteristics 32 2.3. Hydrogeological characteristics 2.3.1. Hydrogeological units 37 37 6 2.3.2. Groundwater dynamics 39 2.3.3. Groundwater salinity 42 CHAPTER III – CONSTRUCTION OF NUMERICAL MODEL TO ASSESS GROUNDWATER SAFE YIELD IN NAM DINH PROVINCE 44 3.1. Introduction of MODFLOW model 44 3.2. Model setup 47 3.2.1. Construction of 3D structural model 47 3.2.2. Model parameters 52 3.2.3. Boundaries conditions 54 3.2.4. Groundwater extraction 57 3.3. Model calibration 60 CHAPTER IV – SIMULATED RESULTS AND RECOMMENDATIONS 66 4.1. Scenario simulation 66 4.1.1. Extraction rises constantly 66 4.1.2. Extraction remains constant 67 4.1.3. Extraction gradually reduces 69 4.1.4. Estimation of groundwater safe yield 70 4.2. Discussion of model results 72 CONCLUSIONS AND RECOMMENDATIONS 74 REFERENCES 76 APPENDICES 78 7 LIST OF ABBREVIATIONS Generral abbreviations MONRE Ministry of Natural Resources and Environment NAWAPI National Center for Water Resources Planning and Investigation NDWRPI Northern Division of the National Center for Water Resources Planning and Investigation N, S, E, W North, South, East, West RRD Red River Delta UNICEF United Nations International Children’s Emergency Fund Technical abbraviations m bgl Meter below ground level m asl Meter above modern sea level TDS Total dissolved solids (mg/L) 8 LIST OF FIGURES Figure 1.1: Steps of study.............................................................................................19 Figure 2.1: The outline map of Nam Dinh Province ......................................................21 Figure 2.2: Monthly averaged data for temperature, precipitation and potential evaporation in period from 1959 to 2007, measured at Van Ly station, coastal area of Nam Dinh ......23 Figure 2.3: Land use distribution in Nam Dinh province, status 2007. ............................25 Figure 2.4: Bar chart showing official data for communal and private water supply (ws) in Nam Dinh from 2005 to 2009 ......................................................................................27 Figure 2.5: Quaternary geology and topography of the Red River delta and adjacent areas (Source: Tanabe et al. 2006) ............................................................................29 Figure 2.6: Geological Sketch map including major structural features and basis boundaries of the Holocene, Pleistocene and Neogene sediments (Source: NAWAPI) ....30 Figure 2.7: Sketch map showing location (orange line) of typical hydrogeological cross section of Nam Dinh Province .........................................................................31 Figure 2.8: Cross section from Vu Ban to Hai Hau (140x vertical exaggeration, modified after Hoc et al., 2003) ................................................................................31 Figure 2.9: Time-series of monthly averaged Groundwater level of Holocene (qh), Pleistocene (qp) and Neogene (n) aquifers. ...................................................................41 Figure 2.10: Contour map of the hydraulic groundwater heads (m asl) in Pleistocene (qp1) aquifer in the Nam Dinh province in May 2010 (left) and November 2010 (right) (Source: NDWRPI)...................................................................................................................41 Figure 2.11: Salinity distribution map (TDS) in qp pore water. ...............................43 Figure 3.1: Three-dimensional finite difference grid used in MODFLOW ......................45 Figure 3.2: Sketch map showing locations of boreholes used to construct the 3D structural model .........................................................................................................48 Figure 3.3: Finite difference grid and extent border of the model ............................48 Figure 3.4: 3D hydrogeological structure of Nam Dinh Province ............................49 9 Figure 3.5: Sketch map showing bottom elevation contours of 5 layers: a) Upper Holocene aquifer, b) Lower Holocene aquifer, c) Pleistocene aquitard, d) Pleistocene aquifer, e) Neogen aquifer .....................................................................52 Figure 3.6: Hydraulic conductivity (m/day) for each aquifer: a) Upper Holocene, b) Lower Holocene, c) Upper Pleistocene, d) Lower Pleistocene, e) Neogene ............53 Figure 3.7: Distribution of artificial well locations in study area .............................58 Figure 3.8: Estimated groundwater withdrawal rate from 1994 to 2009 in Nam Dinh Province .....................................................................................................................59 Figure 3.9: Sketch map showing the locations of groundwater monitoring wells in Nam Dinh Province ...................................................................................................61 Figure 3.10: Calculated versus measured water heads in Q108, 109 and 110..........62 Figure 3.11: Best fit simulation water level contour in qp aquifer (December 2012) ...................................................................................................................................63 Figure 3.12: Calibration residuals histogram ............................................................63 Figure 3.13: Calculated versus measured water heads in Q221-Q229 from 2010 to 2012 ...........................................................................................................................64 Figure 3.14: Scattered plot showing the relation between calculated and observed head ...........................................................................................................................65 Figure 4.1: Estimated groundwater level until 2050 at Q109 with a constant rise of extraction as postulated until 2012 ............................................................................66 Figure 4.2: Sketch map showing the groundwater level contour in qp aquifer in December 2050 with a constant rise of extraction as postulated until 2012 .............67 Figure 4.3: Estimated groundwater level until 2050 at Q109 with a constant level of extraction as in 2012 .................................................................................................68 Figure 4.4: Sketch map showing the groundwater level contour in qp aquifer in December 2050 with a constant level of extraction as 2012 ....................................68 Figure 4.5: Estimated groundwater level until 2050 at Q109 with a constant decline of extraction...............................................................................................................69 10 Figure 4.6: Sketch map showing the groundwater level contour in qp aquifer in December 2050 with a constant decline of extraction ..............................................70 Figure 4.7: Estimated groundwater level until 2050 at Q109 in different scenario ..72 11 LIST OF TABLES Table 2.1: Stratigraphy and Hydrostratigraphy of the strata in the Nam Dinh area. 32 Table 3.1: Amount of water wells in Nam Dinh in 1999 and 2009 ..........................57 Table 3.2: Table for statistic parameters of calibrated model result .........................65 Table 4.1: Estimated safe pumping rate in Nam Dinh Province ...............................71 12 CHAPTER I – INTRODUCTION 1.1. Background information In Red River Delta, there are many important economic regions located on the coastal areas. The growth of both the economy and the population in these areas during the last decades, which is based on the availability of water resources, and the extensive exploitation have strongly increased the pressure on this finite and vulnerable resource. Since surface water is unevenly distributed and increasingly affected by human activities, groundwater has become the major source of potable water [1]. Understanding and quantifying groundwater resources, especially in coastal areas, is a very complex and difficult task, considerably more problematic and uncertain than surface water hydrology. Various studies have been conducted using different types of models, including empirical, probabilistic and deterministic models. Since empirical models are limited in scope and probabilistic models require large data sets and cannot be used to solve many problems in practice (e.g., effects of a future pumping) [11] , numerical deterministic models are increasingly applied, especially within GIS environments. However, in contrast to the developed countries (e.g., Europe, Australia, Japan, and North America), where a vast majority of projects have been carried out due to the availability of a wide range of information and where technical and financial resources are available [12] , few projects have been in Viet Nam with very limited results. This has led to an inadequate understanding of the aquifer system characteristics as well as to unwise groundwater management in Viet Nam, especially in RRD. This thesis therefore focused on contributing to the improvement of groundwater management in RRD by determining groundwater safe yield using modelling method. However, within the limited time, it is not feasible to construct the model for the whole Delta. In this study, Nam Dinh Province was selected to be the case study. There are 3 reasons for this selection: 13  Nam Dinh province is located in the south of the Red River flood plain. In general, the physical settings of the province are quite in common with that of the Red River Delta: 1) Adjacent to the ocean in the SE border; 2) The SW border is where clastic sediments of the Cenozoic meet the Mesozoic graben shoulders of the elongated Red River graben; and 3) Agriculture dominated land use with an extensive system of dykes and canals constructed to irrigate the paddy fields with river water.  Nam Dinh Province is facing the same groundwater problems with RRD, including: 1) Continuous declining of groundwater tables on a regional scale; 2) Potential salinization of coastal groundwater resources by seawater intrusion; 3) Potential pollution by unsuitable handling of domestic, agricultural and industrial waste, waste water and sewage; and 4) Unwise management and inadequate understanding of the aquifer system characteristics leaded to serious problems, such as the drying up of shallow wells, land subsidence, etc.  Nam Dinh is the province which has the highest groundwater drawdown rate compare to the other provinces/cities in Red River Delta [14]. (For more details, see Chapter 2) Nam Dinh Province is principally endowed with a subtropical climate and abundant surface water resources. However, economic and social developments, combined with the population growth over the last decades, have strongly increased pressure on available water resources. Surface water is the traditional water source and still used in Northern and Eastern areas of Nam Dinh. In South and West of Nam Dinh deeper groundwater resources has been increasingly exploited for domestic as well as economic utilization. Monitoring data since 1995 showed that the groundwater extraction exceeded the recharge resulting in drawdown of groundwater levels in Pleistocene and Neogene aquifers up to 0.6 m/year. A regional abstraction cone has developed in Pleistocene as well as the underlying Neogene aquifer. In this area, the natural 14 coastward directed groundwater flow has turned towards the centre of the abstraction cone with horizontal apparent velocities of 0.6 m/year. This results in migration of brackish and saline groundwater from East of Nam Dinh as well as from the ocean towards the fresh groundwater area. The consequences are that nowadays hand pumps must be replaced by electric pumps in order to pump the groundwater to the surface and, thus, increasing the costs for water supply. Groundwater will be increasingly important for Nam Dinh’s future water supply strategy, since surface water is vulnerable and increasingly affected by climate change, untreated sewage water and industrial waste water. Thus, sustainable groundwater exploitation and management is crucial for life, development and the environment in Nam Dinh. 1.2. Objective & scope of study The overall goal of this thesis is to provide a rough assessment on the safe yield of groundwater for Nam Dinh Province on the basis of numerical modeling method. To achieve the overall goal of the research, these following specific objectives have been addressed:  Clarify the spatial characteristics of groundwater in Nam Dinh.  Investigate the impacts of pumping activities on the flows and hydraulic heads of groundwater in Nam Dinh.  Roughly estimate the safe groundwater yield for Nam Dinh.  Proposing recommendations to improve groundwater management for the province by analyzing the simulation results. In the northern areas of Nam Dinh Province, due to the the high concentration of contamination in the groundwater aquifers, surface water is used as the main source of water for the region. Groundwater usage in these areas is mainly for other purposes than domestic use. Small scale groundwater extraction can be found near the main rivers with shallow wells in the uppermost aquifer due to the high level of contamination. Deeper aquifers are the main subject for extensive extraction in the province. Significant extractions are found in the south of Nam Dinh, where large 15 freshwater lens in Pleistocene aquifer are located. Therefore in this study, the Pleistocene aquifer was selected as the main area of interest. This study examines certain groundwater-related items in great details for the whole province, in which, groundwater pollution and salinization process were not taken into account due to the limitation of time and collected data. Furthermore, detailed evaluation of alternative groundwater management strategies was also included in this research. 1.3. Previous studies & state of knowledge 1.3.1. In the world Worldwide applying of modelling methods to study the subjects of groundwater began in the 19th century, especially in industrialized countries like the former Soviet Union (Russia and others Republic countries now), the US, France, Canada, Denmark ... History of groundwater modelling can be divided into two stages: The first stage lasted from the 19th century to the late 50s of the 20th century. At this stage, modelling method was applied to study the basic seepage problem. Several laboratories, which studied soil permeability, were formed in the Soviet Union as VNIIG, VODGEO, MGRI. This stage is characterized by a strong development of EGDA method (Analogue EGDA), which was developed and used predict groundwater dynamics, balance of groundwater in irrigated areas in Davogia, Dovongie, North Keprad, Central Asia, Ukraine. The second stage extended from the 60s of the 20th century to the present. In this stage, modelling method was strongly developed and applied to interpret many complex problems as: Assessment of exploitable reserves of underground water, Prediction of groundwater dynamics in irrigated areas, etc. Developed countries like the United States, Russia (former USSR), Denmark, Canada, Australia... have used numerical models to serve the integrated management of water resources. Countries in Asia, Southeast Asia have also applied the model to assess and manage water resources in their territory. For the last decades, several methodologies for groundwater management and vulnerability assessment have been developed to deal with groundwater problems. 16 These issues include: groundwater depletion, groundwater contamination, land subsidence, and seawater intrusion. Thomsen et al. (2004) presented a highly advanced approach which included spatially dense geophysical/hydrogeological mapping, dense perforation grids as well as numerical modelling, GIS and advanced database technology for groundwater management and protection in Denmark [17] . Other advanced approaches include quantitative aquifer modelling for entire catchment areas using numerical groundwater flow modelling as a basis for sustainable aquifer exploitation and vulnerability assessment [1], [9] . However, in contrast to the availability of data in developed countries, the reduced availability of the necessary input data in developing countries require that a significant part of the basic input variables has to be estimated. As a result, these approaches lean towards a growing uncertainty [12]. 1.3.2. In Viet Nam In Vietnam, several studies have dealt with groundwater problems. Minh, (2004) used weak Galerkin finite element method to simulate the movement of saltwater wedges into the coastal aquifer in Hoa Khanh, Tien Giang [13]. Postma’s research in 2007 under the Danida Enreca Project provided a set of sources as well as mitigations for arsenic contamination in the groundwater of the Red River Delta [15]. Nguyen (2011) used finite element modelling for the assessment of seawater intrusion into the ground aquifer in Thai Binh province. Erban (2013) provided an extensive study on arsenic releasing into deep groundwater in the Mekong Delta, which is linked to pumping-induced land subsidence [8] . However, there has been less improvement in groundwater management in Vietnam, since groundwater problems were still reported to happen in many areas of Vietnam according to MONRE, 2013 [14]. Previous studies in Nam Dinh Province: Since the 1990s, groundwater resources in the Red River Delta (RRD) including Nam Dinh area were subject of mapping and exploration projects carried out by 17 governmental authorities. Moreover, Vietnamese universities have published several scientific studies about groundwater related issues in Nam Dinh and upstream areas, partly in cooperation with international partners. The most relevant studies and information sources are namely: “Characteristics of geology, natural resources and geochemical environment of sediment on tidal estuary of Xuan Thuy and Nghia Hung, Ha Nam Province” – report by staffs at Hanoi University of Mining and Geology, 1995. "Geological mapping for Nam Dinh area ratio 1:50.000" – project by Division 47 of Hydrogeology – Geology Engineering, 1996. In this project, 27 boreholes were drilled, including 8 boreholes in Holocene aquifer (qh), 17 boreholes in Pleistocene aquifer (qp) and 2 boreholes in Neogene aquifer (n). Strata data collected from the boreholes was used for the mapping process. Slug tests were also conducted in 26 boreholes to determine the hydrogeological parameters of the aquifers and the chemical composition of water [6]. "Planning of water supply and sanitation in Nam Dinh province in 1999 – 2010 period" – report by the Center for Water and Sanitation, Nam Dinh Department of Agriculture and Rural Development, 1999. "Assessment of groundwater potential and prediction of depletion and salinity instrusion in Hai Hau – Giao Thuy, coastal areas of Nam Ding Province" – report by the Department of Water Management and Irrigation Works, Ministry of Agriculture and Rural Development, 2000. "Evaluation on characteristics of natural conditions and non-biological resources in coastal areas of Nam Dinh province" – report by Marine Natural Resources & Environment Survey Center, 2001. "Assessing the sustainability of the exploitation of groundwater resources in Vietnam territory. Strategic orientation for rational utilization and protection of groundwater resources by 2020" – state project by Bui Hoc et al, 2003 [10]. "General survey of groundwater resources in Nam Dinh province: planning for rational and sustainable uses" – report by Doan Van Canh and Le Thi Lai, 2003 [2]. 18 "Research and application of combinatorial methods of geology, geophysics, numerical modelling to investigate, assess salinity intrusion and search for fresh groundwater lenses or aquifers in coastal areas of Nam Dinh” – report by Nguyen Van Dan - NDWRPI, 2009 [4]. In 2010, within the framework of the project “Improvement of Groundwater Protection in Viet Nam” in Nam Dinh Province, 10 additional groundwater monitoring points were constructed. In total of 23 new monitoring wells, there are 4 wells in Holocene aquifer, 14 wells in Pleistocene aquifer, 4 wells in Neogene aquifer, and 1 well in Triassic aquifer. Slug tests were conducted in 2011 by National Center for Water Resources Planning and Investigation (NAWAPI) to determine the hydrogeological parameters of the Pleistocene aquifer in Nam Dinh Province. Many of these documents have proven to be very useful secondary data sources, which contributed vast important information for the construction, calibration and validation of the model (see Chapter 3). 1.4. Methodology Based on principle of water balance this research used the water balance equation to determine the groundwater safe yield. Numerical simulation of a hypothetical case was used to demonstrate the natural groundwater balance, effects of pumping and the dynamic development of the capture. In this thesis, as a systematic evaluation tool for groundwater flow, GIS program and the groundwater modeling program MODFLOW were integrated and used (Figure 1.1). At first, related information available in print or published on the internet were gathered and analyzed. To simulate the groundwater flow, regional hydrogeology, topography, hydrology, soil, land use and well data during the period of 1995-2012 were collected from NAWAPI under the Ministry of Natural Resources and Environment (MONRE), Vietnam. The aquifer system was identified and characterized to construct the 3D structural model. 19 Boreholes strata data Boundary conditions Geological maps 3D structural model construction Extraction Aquifer parameters 2D crosssections MODFLOW inputs Model set up Model Calibration & Verification MODFLOW outputs Scenarios simulation Groundwater safe yield determination & assessment Figure 1.1: Steps of study In order to use MODFLOW, initial conditions, hydraulic properties, and stresses were specified for every model cell in the finite-difference grid. Measured heads from the field were assigned directly to inland constant-head boundaries. After setting up the model as well as calibrating and verifying, different pumping schemes were used, based on which, corresponded simulations were executed. The results were analyzed to determine groundwater safe yield for different areas of Nam Dinh Province. Finally, recommendations were given to improve the groundwater management in the area. 20 1.5. Structure of thesis This thesis consists of 4 chapters: Chapter I - Introduction: This chapter is the introduction which contains the background, importance, objectives of the study and short description of the study area. A comprehensive review of literature and a description of study scope were also presented in this chapter. Chapter II - Characteristics of Nam Dinh Province: This chapter provides a comprehensive description on the natural conditions as well as socio-economic characteristics of the study area. This chapter also describes the current status of groundwater and the utilization of groundwater in Nam Dinh Province. Chapter III - Construction of numerical model to assess groundwater safe yield in nam dinh province: In this chapter, the numerical model set up will be presented. The simulation results will then be analyzed to estimate the impacts of pumping activities on groundwater flow and hydraulic head. Chapter IV - Simulated results and recommendations: This chapter presents the results of several pumping variations. Finally, overall conclusions, recommendations were given for improving groundwater management in the study area as well as possible works for the future studies.