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: R N + Q i – T – Q o – Q p = ∆S R N = Recharge Q i = Surface water inflow T = Transpiration Q o =Outflow to surface water Q p = 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: 1. Tracking water levels 2. Evaluating annual water budget 3. 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-foot/year  Natural discharge:4,850 acre-foot/year  Avg. annual recharge:13,900 acre-foot/year   storage decreases at rate: 8800 acre-foot/y (1 acre-foot/year = 1234 m 3 /day) 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

12. 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: 1. Replenish depleted supplies 2. Prevent or retard saltwater intrusion 3. Store water underground

13. Artificial Recharge Induced recharge Wells next to rivers

14. Artificial Recharge Direct recharge Recharge pits (expensive, small recharge capacity) Dams Shafts expensive, small recharge capacity Spreading basins Wells (deep aquifers)

15. Large volumes of reclaimed water, which has undergone advanced secondary treatment, are reused through land-based applications in a 40-square-mile area near Orlando, Florida. These applications include citrus crop irrigation and artificial recharge to the surficial aquifer through rapid infiltration basins.

16. Artificial Recharge Recharge rates 0.5 – 15 m/d Problems: Clogging Suspended silt Reaction, precipitation Growth of algae, bacteria Dissolved gases

17. 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

18. Case Study Al-Madinah Water supply

19. Paleo- valleys Wadi Al-Aqiq Al- Madinah

20. W Figure 5.9. Conceptualization of hydraulic conductivity (K) distribution in lavas.

21. Result of pumping tests RemarksST m 2 /dDrowdown m Q m 3 /d3 rd Wellfield Group Well no 4.5x10 -3 79.22.62 1037 1354 No drawdown, in 24 hours.02 1440 1336 No drawdown in 48 hours.02 1339 2305 1.2x10 -4 32.761.6 1440 2328 No drawdown after 24 hours~ 0 1771 3372 No drawdown in 72 hours~ 0 965 3386

22. Table 6.2. Summary of groundwater chemistry in Abar Al-Mashi Basaltic aquifer [values are in milligrams per liter] MeanMinimumMaximum Number of samples 67950098430TDS 1079840150017mmhos/cmConductivity 15110821830NaSodium 53830KPotassium 421512930CaCalcium 2184330MgMagnesium 14811024530SO4Sulfate 1947135530ClChloride 41730NO3Nitrate 1138521030HCO3Bicarbonate 927017230CaCO3Alkalinity 7.97.58.230Ph 36.13538.539 CC T

23. General Head Boundary Drain Boundary Al-Madinah No-Flow Boundary y No-Flow Boundary y

24. Table 7.5. Emergency abstraction plans during a temporary halt in desalinized water supply. Can aquifer support? DurationTotal Abstract ion (m 3 ) Amount lost (m 3 ) IncidentCase  Yes  3 days  15 days  30 days 130,00080,000First phase discontinued I  Yes  No  3 days  15 days  30 days 250,000200,000Second Phase discontinued II  No  3 days  15 days  30 days 330,000280,000First and second phase discontinued III

25. 15.3 Introduction to Groundwater Modeling

26. Flow Equations and Numerical Methods Basic flow Equations Groundwater Flow Simulation What is a MODEL? Why Model? Modeling Protocol FD equation of flow MODFLOW

27. Basic Flow Equations Confined aquifers: Unconfined aquifers

28. Groundwater Flow Simulation What’s a MODEL? –Physical Models (sand tanks) –Mathematical Models  Analytical  Numerical –FDM –FEM

30. FD grid (mesh-centered)

31. FD grid (block-centered)

32. FE elements

33. Why MODEL? Predictive:requires calibration Interpretive: to organize field data; no calibration Generic: theoretical studies Revise geology?!

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

36. Modeling Protocol-2 Sensitivity analysis Model verification Prediction Predictive sensitivity analysis Postaudit flowchart

37. Code verified? Define Purpose Field data Conceptual Model Mathematical Model Analytical Solutions Numerical formulation Computer program no CODE SELECTION yes Model design Field data calibration Comparison with field data verification prediction Presentation of results Postaudit Field data

38. 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

39. Discretization of a 3-D system

40. Deformed grid

41. Cell i,j,k

42. 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

43. Final form of the FD equation

44. Final Eq. (matrix form) mathematical solution of this system provides hydraulic head for given time step coefficient matrix vector of unknown heads vector of constant heads

45. MODFLOW family of codes Modflow Packages (overhead)

47. 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

48. 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

50. Recharge = 10 cm/year 18 m 12 m 6 m 0 m 2000 m Aquifer Aquitard Aquifer

51. Example, contd. MODULE I: Model Input MODULE II:Running Visual MODFLOW MODULE III:Output Visualization