Contaminant Fate and Transport

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Contaminant Fate and Transport

• CIVE 7332
• Lect 4

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Contaminant Transport Equation
C Concentration of Solute M/L3 DIJ
Dispersion Coefficient L2/T B Thickness of
Aquifer L C Concentration in Sink Well
M/L3 W Flow in Source or Sink L3/T n
Porosity of Aquifer unitless VI Velocity in
I Direction L/T xI x or y direction
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Analytical Solutions of Equations

• Closed form solution, C C ( x, y, z, t)
• Easy to calculate, can often be done on a
  spreadsheet
• Limited to simple geometries in 1-D, 2-D, or 3-D
• Limited to simple sources such as continuous or
  instantaneous or simple combinations
• Requires aquifer to be homogeneous and isotropic
• Error functions (Erf) or exponentials (Exp) are
  usually involved

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Numerical Solution of Equations

• Numerically -- C is approximated at each point
  of a computational domain (may be a regular grid
  or irregular)
• Solution is very general
• May require intensive computational effort to get
  the desired resolution
• Subject to numerical difficulties such as
  convergence problems and numerical dispersion
• Generally, flow and transport are solved in
  separate independent steps (except in
  density-dependent or multi-phase flow situations)

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Domenico and Schwartz (1990)

• Solutions for several geometries
• Generally a vertical plane, constant
  concentration source. Source concentration can
  decay.
• Uses 1-D velocity (x) and 3-D dispersion (x,y,z)
• Spreadsheets exist for solutions.
• Dispersion axvx, where ax is the dispersivity
  (L)
• BIOSCREEN (1996) is handy tool that can be
  downloaded.

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BIOSCREEN Features

• Answers how far will a plume migrate?
• Answers How long will the plume persist?
• A decaying vertical planar source
• Biological reactions occur until the electron
  acceptors in GW are consumed
• First order decay, instantaneous reaction, or no
  decay
• Output is a plume centerline or 3-D graphs
• Mass balances are provided

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Domenico and Schwartz (1990)
y
Plume at time t
Vertical Source
x
z
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Domenico and Schwartz (1990)

• For planar source from -Y/2 to Y/2 and 0 to Z

Y
Flow x
Z
Geometry
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Instantaneous Spill in 2-D

• Spill source C0 released at x y 0, v vx
• First order decay l and release area A

2-D Gaussian Plume moving at velocity V