FACULTEIT WETENSCHAPPEN Opleiding Master of Science in de geologie Modelling the influence of the Abu Nakhla pond on the phreatic aquifer in Qatar: present and future scenarios Joren De Tollenaere Academiejaar 2014–2015 Scriptie voorgelegd tot het behalen van de graad Van Master of Science in de geologie Promotor: Prof. Dr. L. Lebbe Leescommissie: J. Claus, Prof. Dr. V. Cnudde VOORWOORD Eerst en vooral wil ik m’n promotor prof. dr. Lebbe bedanken voor de mogelijkheid om dit onderwerp als masterproef te doen. Ook wil ik hem bedanken voor zijn verhelderende uitleg over de resultaten van de modellen en over de hydrogeologie. Ik wil ook Jasper Claus bedanken voor het gebruik van zijn python scripts om de input van de modellen te maken alsook voor zijn inzicht en aanpassingen aan het model. Gert-Jan Devriese wil ik bedanken voor de morele steun en voor het gebruik van zijn bureau waar we vele leuke uren hebben doorgebracht. En als laatste wil ik Devlin Depret bedanken voor zijn rationele benadering in het steunen bij het schrijven van deze masterproef en zijn kritisch inzicht inzake de resultaten. i ii Table of contents 1. INTRODUCTION ......................................................................................................... 1 2. STUDY AREA ............................................................................................................. 3 3. GEOLOGY .................................................................................................................. 5 3.1 Structural geology .................................................................................................. 5 3.2 Surface geology ..................................................................................................... 5 3.3 Stratigraphy ........................................................................................................... 6 3.3.1 Umm er Radhuma Formation .......................................................................... 7 3.3.2 Rus Formation ................................................................................................. 7 3.3.3 Dammam Formation ...................................................................................... 10 3.3.4 Dam Formation ............................................................................................. 11 3.3.5 Hofuf Formation ............................................................................................ 11 3.3.6 Sabkhas ........................................................................................................ 12 4. HYDROGEOLOGY ................................................................................................... 13 4.1 General hydrogeology .......................................................................................... 13 4.2 Head distribution .................................................................................................. 14 4.3 Hydrogeological parameters ................................................................................ 19 4.4 Fresh and salt water distribution .......................................................................... 20 5. CLIMATE AND HYDROLOGY .................................................................................. 23 6. MODELLING SOFTWARE ....................................................................................... 29 6.1 Evolution of the modelling software ...................................................................... 29 6.2 MODFLOW .......................................................................................................... 29 6.2.1 Code concepts .............................................................................................. 30 6.2.2 Discretisation of the groundwater reservoir .................................................... 30 6.2.3 Finite difference approximation of the groundwater flow ................................ 32 6.2.4 Iteration with the Strongly Implicit Procedure ................................................. 34 6.2.5 Cell and boundary types ................................................................................ 35 6.3 MOC3D................................................................................................................ 36 6.4 MOCDENS3D ...................................................................................................... 37 iii 6.5 Packages............................................................................................................. 38 6.5.1 MODFLOW packages ................................................................................... 38 6.5.1.1 Basic package (.bas-file) ........................................................................................ 38 6.5.1.2 Block-Centred-Flow package (.bcf-file) .................................................................. 39 6.5.1.3 Well package (.wel-file) .......................................................................................... 40 6.5.1.4 Evapotranspiration package (.evt-file) ................................................................... 41 6.5.1.5 Strongly Implicit Procedure package (.sip-file)....................................................... 41 6.5.1.6 Name file (Infile.nam) ............................................................................................. 41 6.5.2 MOCDENS3D packages ............................................................................... 42 6.5.2.1 MOC package (.moc-file) ....................................................................................... 42 6.5.2.2 Densin.dat-file ........................................................................................................ 42 6.5.2.3 Name file (moc.nam) .............................................................................................. 42 7. MODEL CONSTRUCTION ....................................................................................... 43 7.1 Base model ......................................................................................................... 43 7.1.1 Location and discretisation of the study area ................................................. 43 7.1.2 Hydrogeological parameters ......................................................................... 46 7.1.3 Boundary conditions and initial head values .................................................. 48 7.1.4 Fresh and salt water distribution .................................................................... 51 7.1.5 Recharge ...................................................................................................... 53 7.1.6 Evapotranspiration ........................................................................................ 53 7.1.7 Time discretisation and closure criterion ....................................................... 54 7.2 Adding the pond into the base model ................................................................... 54 7.2.1 Sub-model one: Steady state flow with pond ................................................. 55 7.2.1.1 Discretisation of the Abu Nakhla pond ................................................................... 55 7.2.2 Sub-model two: Mimicking steady state with an unsteady state model .......... 57 7.2.2.1 Hydrogeological parameters .................................................................................. 58 7.2.2.2 Time discretisation and closure criterion ................................................................ 60 7.2.3 Sub-model three: From mimicking steady state to full unsteady state ........... 61 7.2.3.1 Boundary conditions and storage factors ............................................................... 61 7.2.3.2 Time discretisation and closure criterion ................................................................ 61 7.2.4 Sub-model four: Desiccation and dissipation of the Abu Nakhla pond ........... 62 7.2.4.1 Boundary conditions and hydrogeological parameters .......................................... 62 iv 7.2.4.2 Time discretisation and closure criterion ................................................................ 62 7.3 Sensitivity analysis ............................................................................................... 63 7.3.1 Horizontal conductivity of the upper aquifer ................................................... 63 7.3.2 Vertical conductivity of the gypsiferous Rus Formation .................................. 63 7.3.3 Storage factors .............................................................................................. 64 8. RESULTS AND DISCUSSION .................................................................................. 65 8.1 Base model .......................................................................................................... 65 8.1.1 Steady state flow ........................................................................................... 65 8.1.2 Unsteady state flow ....................................................................................... 66 8.2 Desiccation of the Abu Nakhla pond .................................................................... 68 8.2.1 Sub-model one: Steady state flow with pond ................................................. 68 8.2.2 Sub-model two: Mimicking steady state with an unsteady state model .......... 69 8.2.3 Sub-model three: From mimicking steady state to full unsteady state ............ 71 8.2.4 Sub-model four: Desiccation and dissipation of the Abu Nakhla pond ........... 73 8.3 Sensitivity