# Introduction to Groundwater Flow Modeling Prof. Dr. Halil Önder Fall 2008.

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Introduction to Groundwater Flow Modeling Prof. Dr. Halil Önder Fall 2008

Halil Önder2 Objectives and Scope Objectives: To understand the processes of groundwater flow in an aquifer To build models which enable the forecasting of the aquifer’s response to planned activities To provide the students with the necessary tools they need for predicting the behavior of local groundwater as well as a regional aquifer system, which are subject to natural and man-made excitations To present practical examples which will be of use in groundwater development and management.

Halil Önder3 Topics covered in the course The aquifer The essentially horizontal flow approximation Components of the groundwater balance The motion equation in confined and phreatic aquifers Storativity and mass balance equations for confined, phreatic and leaky aquifers Boundary conditions The complete mathematical statement of a forecasting problem in aquifers Review of solution methods. Fresh water-salt water interface in coastal aquifers Mathematical models of aquifer management

Halil Önder4 Hydrologic cycle Schematic representation of the hydrologic cycle

Halil Önder5 Hydrologic cycle An engineering view of the hydrologic cycle

Halil Önder6 Geologic Formations Classification on the basis of water storing and transmitting capability:  Aquifer [water-bearing formation (bed, deposit, stratum), groundwater reservoir, groundwater basin]  Aquitard [semi-pervious (leaky) formation]  Aquiclude [impermeable formation]  Aquifuge [non-porous, impervious formation)

Halil Önder7 Geologic Formations: Classification based on the origin and physical properties Consolidated formation (Rocks)  Igneous rocks (granite, basalt)  Sedimentary rocks (sandstone, lime stone)  Metamorphic rocks (marble, quartzite) Unconsolidated formation (soil)  Gravel  Sand  Clay

Halil Önder8 Water storing and transmitting capability

Halil Önder9 Subsurface distribution of water

Halil Önder10 Types of aquifers

Halil Önder11 Various simplified aquifer boundaries

Halil Önder12 Types of Interstices The portion of subsurface geological formation not occupied by solid matter is the void space. It is also called as pore space, pores, interstices, and fissures. The interstices can be grouped in two classes: Original Interstices  They are created by geological processes at the time the rock was formed, mainly in sedimentary and igneous rocks Secondary Interstices  They are developed after the rock was formed, mainly in the form of fissures joints and solution passages

Halil Önder13 Various types of interstices

Halil Önder14 Roles (Main Functions) of Aquifers Source of Water  Renewable resource  Non-Renewable resource Storage Reservoir  Aquifer storativity-  Porosity Conduit  Aquifer transmissivity  Permeability, Hydraulic Conductivity Filter Control of Base Flow Water Mine  Nonrenewable resource  One-time-reserve

Halil Önder15 Groundwater Budgets A groundwater system consists of a mass of water flowing through the pores or cracks below the earth’s surface. This mass of water is in motion. Water is constantly added to the system by recharge from precipitation, and water is constantly leaving the system as discharge to surface water and as evapotranspiration. The total amount of water entering, leaving, and being stored in the system must be conserved. An accounting of all the inflows, outflows, and changes in storage is called as water budget Human activities affect the amount and rate of movement of water in the system, entering the system, and leaving the system. The changes resulting from human interventions must be accounted for in the calculation of the water budget

Halil Önder16 Groundwater Balance 1. Groundwater Flow and Leakage  Inflow and Outflow through Aquifer Boundaries  Leakage through Semipervious Layers 2. Natural Replenishment from Precipitation 3. Return Flow from Irrigation and Precipitation 4. Artificial Recharge (to enhance infiltration)  Surface spreading methods  Artificial recharge through wells  Induced recharge 5. River-Aquifer Interrelationships 6. Springs 7. Evapotranspiration 8. Pumpage and Drainage 9. Change in Storage 10. Regional Groundwater Balance

Halil Önder17 Schematic representation of unconfined ground water balance

Halil Önder18 Groundwater System 3 SYSTEM Inputs Excitation Cause Outputs Response Effect Undesirable Desirable Uncontrollable Controllable DetectionIdentification Prediction 12

Halil Önder19 Inputs Uncontrollable Inputs:  Natural Recharge from Precipitation Irrigation return flow Controllable Inputs:  Policy Variables Pumping and Injection Schedules Artificial recharge

Halil Önder20 Outputs Desirable Outputs:  Reduction in Subsurface Outflow Undesirable Outputs:  Drying of a wetland  Drying of a spring

Halil Önder21 System Characteristics System Characteristics:  Aquifer Geometry Thickness Lateral extent  Hydraulic Parameters Hydraulic conductivity Transmissivity Porosity Storage coefficient Specific yield

Halil Önder22 Groundwater Systems and Three Unique Problems Detection (or instrumentation) problem  The set of inputs to the system is unknown. It involves determining the inputs given both the responses of the system and the system outputs. The determination of the recharge or leakage in semi-confined or unconfined aquifers, from the response properties of the aquifer system, is an example of instrument problem.  This problem is not considered an important problem in groundwater hydrology

Halil Önder23 The identification (or inverse) problem  It involves determining the parameters, which govern the response of the system in particular this, requires determining the coefficients of differential equation which describes the response of the system. The determination of the transmissivity and the storage coefficient of a confined aquifer are typical examples.  Identification is an extremely important problem in groundwater hydraulics. Groundwater Systems and Three Unique Problems

Halil Önder24 The prediction problem  This requires the prediction of the fluctuating head in the aquifers, given a range of groundwater recharge and pumpage patterns both in temporal and areal distribution, with known geometry and properties of the aquifer.  Solving the prediction (forecasting) problem means solving a model in order to obtain the future distribution of water levels, or of piezometric heads produced in a specified aquifer with known geometry and properties by an anticipated natural replenishment and by any planned schedule of future pumping and artificial recharge, as envisaged in a proposed management scheme. Groundwater Systems and Three Unique Problems

Halil Önder25 Management concept SYSTEM Intputs Outputs Uncontrollable Controllable Undesirable Desirable SYSTEM Intputs Outputs Uncontrollable Controllable Undesirable Desirable User Judgment Judgment Description

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