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15 Management of Groundwater Resources

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15.1 Basic Concepts in Managing Groundwater Resources ... Clogging. Suspended silt. Reaction, precipitation. Growth of algae, bacteria. Dissolved gases ... – PowerPoint PPT presentation

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Title: 15 Management of Groundwater Resources


1
15 Management of Groundwater Resources
  • Strategies and Modeling

2
Introduction
  • Basic Concepts in GW Management
  • Safe yield
  • Artificial recharge
  • Conjunctive use
  • Basic elements of digital groundwater models
  • Governing equations
  • Numerical procedures
  • Finite difference method
  • Software

3
15.1 Basic Concepts in Managing Groundwater
Resources
  • Groundwater represents 80 90 of water supply
    in Saudi Arabia (GCC)
  • Increase in mining groundwater ? a need to manage
    effectively
  • Approach water balance in a basin
  • mass balance equation
  • RN Qi T Qo Qp ?S
  • RN Recharge Qi Surface water inflow
  • T Transpiration Qo Outflow to surface
    water
  • Qp Total pumping rate ?S Change in Storage

4
Basic Concepts in Managing Groundwater Resources
  • Safe Yield
  • The rate of groundwater extraction from a basin
    for consumptive use over an indefinite period of
    time that can be maintained without producing
    negative effects
  • Goal of SY
  • Achieve a long-term balance between
  • use replacement
  • i.e., Stop declines in water table

5
Basic Concepts in Managing Groundwater Resources
  • Safe Yield in other words
  • Limit pumping to the amount of groundwater that
    may be safely harvested each year
  • Rules of thumb of SY
  • Annual extraction should not exceed average
    annual recharge
  • Pumping should not lead to deterioration in water
    quality

6
Case study, Central Arizona, USA
  • Prescott Active Management Area (AMA), Central
    Arizona (1997)
  • To answer the question
  • Is the quantity of pumped water from PAMA below
    SY?
  • Used three methods
  • Tracking water levels
  • Evaluating annual water budget
  • Modeling groundwater flows

7
Case study, Central Arizona, USA
  • Data
  • Water levels 1940 -1994, 82-98, 94-98
  • Results
  • Gradual decline in wl in 75 of the wells
  • Water budget for 1995-1997
  • Withdrawal 17,850 acre-feet/year
  • Natural discharge 4,850 acre-feet/year
  • Avg. annual recharge 13,900 acre-feet/year
  • ? storage decreases at rate 8800 af/y
  • DEPLETION
  • GW flow model
  • Confirmed current abstraction exceeded safe
    yield
  • Simulate future conditions
  • Test scenarios

8
15.2 Management Strategies
  • Overexploitation of groundwater can lead to
  • land subsidence
  • Sinkholes
  • Saltwater intrusion
  • Costs of pumping
  • Solutions
  • Reduce pumping?! Demand has to be met!
  • Artificial recharge increase the quantity of
    available gw
  • Conjunctive use replace groundwater supply by
    other sources

9
Artificial Recharge
  • Todd (1980)
  • Augments the natural infiltration of
    precipitation or surface water into ground by
    some method of construction, the spreading of
    water, or a change in natural condition
  • Used to
  • Replenish depleted supplies
  • Prevent or retard saltwater intrusion
  • Store water underground

10
Artificial Recharge
  • Induced recharge
  • Wells next to rivers
  • (figure 15.3)
  • Direct recharge
  • Recharge pits (expensive, small recharge
    capacity)
  • Dams
  • Shafts expensive, small recharge capacity
  • Spreading basins
  • Wells (deep aquifers)
  • (figure 15.4)

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13
Artificial Recharge
  • Recharge rates
  • 0.5 15 m/d
  • Problems
  • Clogging
  • Suspended silt
  • Reaction, precipitation
  • Growth of algae, bacteria
  • Dissolved gases

14
Conjunctive Use
  • Coordinated use of surface and groundwater to
    meet some specified water demand in a given area
  • Objective
  • Maximize net benefits
  • Net benefit Revenues - Costs
  • Minimize costs
  • Minimize degradation of the environment

15
15.3 Introduction to Groundwater Modeling
16
Flow Equations and Numerical Methods
  • Basic flow Equations
  • Groundwater Flow Simulation
  • What is a MODEL?
  • Why Model?
  • Modeling Protocol
  • FD equation of flow
  • MODFLOW

17
Basic Flow Equations
  • Confined aquifers
  • Unconfined aquifers

18
Groundwater Flow Simulation
  • Whats a MODEL?
  • Physical Models (sand tanks)
  • Mathematical Models
  • Analytical
  • Numerical
  • FDM
  • FEM

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20
FD grid (mesh-centered)
21
FD grid (block-centered)
22
FE elements
23
Why MODEL?
  • Predictive requires calibration
  • Interpretive to organize field data no
    calibration
  • Generic theoretical studies
  • Revise geology?!

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25
Modeling Protocol-1
  • Purpose of the model
  • Develop a conceptual model
  • Select a computer code
  • Model design
  • Calibration

26
Modeling Protocol-2
  • Sensitivity analysis
  • Model verification
  • Prediction
  • Predictive sensitivity analysis
  • Postaudit
  • flowchart

27
Formulating A FD Equation
  • governing differential equation replaced by
    difference equation
  • domain discretized by a grid (rows, columns)
  • dimension of cells can vary (coarse, dense)
  • model layers geologic units

28
Discretization of a 3-D system
29
Deformed grid
30
Cell i,j,k
31
Derivation Of The FD Equation
  • SUM OF FLOWS INTO AND OUT OF ANY CELL
  • TIME RATE OF STORAGE
  • ?
  • ADDITIONS OF WATER FROM SOURCES OR SINKS

Mathematically
32
Final form of the FD equation
33
Final Eq. (matrix form)
vector of unknown heads
  • coefficient matrix

vector of constant heads
  • mathematical solution of this system provides
    hydraulic head for given time step

34
MODFLOW family of codes
  • Modflow Packages (overhead)

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37
VModflow Illustrative Example
  • site is located near an airport
  • Geology
  • upper sand and gravel aquifer
  • clay and silt aquitard
  • lower sand and gravel aquifer
  • Relevant Site features
  • A plane refueling area
  • Municipal water supply well field
  • Discontinuous aquitard zone

38
Example, contd.
  • Problem
  • fuel spilled in refueling area.
  • plume of cont. developed in the upper aquifer
  • Required
  • Build a groundwater flow model
  • Assess potential impact of fuel contamination on
    wells

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41
Example, contd.
  • MODULE I Model Input
  • MODULE II Running Visual MODFLOW
  • MODULE III Output Visualization
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