Groundwater%20pumping%20to%20remediate%20groundwater%20pollution

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Groundwater pumping to remediate groundwater
pollution

• March 5, 2002

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TOC

• 1) Squares
• 2) FieldTrip McClellan
• 3) Finite Element Modeling

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First Squares

• Oxford Dictionary says
• a geometric figure with four equal sites and
  four right angles

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Squares

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

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Flownet
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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

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Flow nets (rules)

• Flowlines are perpendicular to equipotential
  lines
• One way to assume that Qs 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

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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
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Second McClellan Airbase
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Piping system
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Groundwater extraction wells
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Waste water treatment plant
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How to determine the spacing of wells?

• Determine feasible flow rates
• Determine range of influence
• Determine required decrease of water table
• Calculate well spacings

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Confined Aquifer

• Well discharge under steady state can be
  determined using

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Unconfined Aquifer

• Well discharge under steady state can be
  determined using

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Unconfined Aquifer

• Well discharge under steady state WITH surface
  recharge can be determined using

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What is optimal well design ?

• In homogeneous soil

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In heterogeneous situation

• Wells have flow rate between 1 and 100 gpm
• Some wells are in clay, others in sand

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Finite Difference method

• Change the derivative into a finite difference
  D

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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

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Approach to numerical solutions

• 2) Write the flow equation in algebraic form
  (using finite difference or finite elements) for
  each node or block

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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

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1-D example

• Boundaries h left 10, h right 3
• Initial conditions h 0
• K is homogeneous 3
• Delta x 2