Groundwater pumping to remediate groundwater pollution

1. Groundwater pumping to remediate groundwater pollution March 5, 2002

2. TOC • 1) Squares • 2) FieldTrip: McClellan • 3) Finite Element Modeling

3. First: Squares • Oxford Dictionary says • “a geometric figure with four equal sites and four right angles”

4. Squares • Units within a flow net are curvilinear figures… • In certain cases, squares will be formed • Constant head boundary…

5. Flownet

6. Flownet • No flow crosses the boundary of a flowline ! • If interval between equipotential lines and interval between flowlines is constant, then volume of water within each curvilinear unit is the same…

7. Flow nets (rules) • Flowlines are perpendicular to equipotential lines • One way to assume that Q’s are equal is to construct the flownet with curvilinear squares • Streamlines are perpendicular to constant head boundaries • Equipotential lines are perpendicular to no-flow boundaries

8. Flow nets (rules 2) • In heterogeneous soil, the tangent law is satisfied at the boundary • If flow net is drawn such that squares exist in one part of the formation, squares also exist in areas with the same K a1 K1 K2 a2

9. Second: McClellan Airbase

11. Piping system

12. Groundwater extraction wells

13. Waste water treatment plant

14. How to determine the spacing of wells? • Determine feasible flow rates • Determine range of influence • Determine required decrease of water table • Calculate well spacings

15. Confined Aquifer • Well discharge under steady state can be determined using

16. Unconfined Aquifer • Well discharge under steady state can be determined using

17. Unconfined Aquifer • Well discharge under steady state WITH surface recharge can be determined using

18. What is optimal well design ? • In homogeneous soil:

19. In heterogeneous situation: • Wells have flow rate between 1 and 100 gpm • Some wells are in clay, others in sand

21. Finite Difference method • Change the derivative into a finite difference D

22. Approach to numerical solutions • 1) Subdivide the flow region into finite blocks or subregions (discretization) such that different K values can be assigned to each block and the differentials can be converted to finite differences

23. Approach to numerical solutions • 2) Write the flow equation in algebraic form (using finite difference or finite elements) for each node or block

24. Approach to numerical solutions • 3) Use “numerical methods” to solve the resulting ‘n’ equations in ‘n’ unknowns for h subject to boundary and initial conditions

25. 1-D example • Boundaries: h left = 10, h right = 3 • Initial conditions h = 0 • K is homogeneous = 3 • Delta x = 2