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Subsurface Fate and Transport of Contaminants. Contaminant Transport. Describes mechanisms to move contaminants from source to receptor Important to calculate dose in risk analysis process. Solutes in saturated media can be transported by three mechanisms. Diffusion Advection Dispersion.

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Presentation Transcript
contaminant transport
Contaminant Transport
  • Describes mechanisms to move contaminants from source to receptor
  • Important to calculate dose in risk analysis process
solutes in saturated media can be transported by three mechanisms
Solutes in saturated media can be transported by three mechanisms
  • Diffusion
  • Advection
  • Dispersion
diffusion
Diffusion
  • Diffusion - spreading of solute due to concentration gradient, minor at most GW velocity
  • Diffusion controlled mass transport occurs if the concentration of a species is greater in one location than an adjacent location (concentration gradient)
  • Fick’s second law used to calculate flux
advection
Advection
  • Advection - transport with bulk flow of groundwater at average velocity of GW.
  • Mass added to stream tubes remains in stream tubes, other processes move mass between stream tubes (diffusion, dispersion,
  • Dominant transport mechanism
dispersion
Dispersion
  • Mechanical Dispersion - mechanical mixing due to the velocity variations as groundwater moves through tortuous pathways
  • Solute transport by advection and dispersion requires flow of groundwater to carry solutes along with liquid flow
  • At most fluid flows advection and dispersion dominates over diffusion
transformation processes
Transformation Processes
  • Sorption
  • Radioactive decade
  • Chemical transformation
  • Volatilization
  • Colloid transport
  • Biotic
contaminant transport8
Contaminant Transport

Advection

Conc.

Sorption, Dispersion

Sorption, Dispersion

And Degradation

Time

mathematics
Mathematics

Change in mass storage with time =

Mass inflow rate - mass outflow rate + mass production rate

mass balance
Mass Balance

qx

dz

dy

dx

mass balance 1 dimensional
Mass Balance - (1-Dimensional)

Change in mass storage with time =

Mass in/out due to advection, dispersion, diffusion

and sources and sinks

mass balance 1 dimensional12
Mass Balance (1-Dimensional)

Divide by xyz and let x -------> 0

F = mass flux rate of contaminant due to

advection, dispersion, and diffusion, mass area-1 time-1

r = source or sink term

source or sink terms
Source or sink terms

R (mass/vol-time):

Biotic

Radioactive Decay

Sorption

solution requires
Solution Requires
  • Definition of parameters
  • Suitable numerical or analytical solution
  • Boundary and initial conditions
  • Analytical solution possible if 1-D, source or sink terms linear, boundary and initial conditions known
solution to mass balance including advection and dispersion only
Solution to Mass Balance Including Advection and Dispersion Only

C = the concentration at time t and distance x

Co = original concentration

L = distance

t = time

erfc = complimentary error function

DL = dispersion coefficient

vx = linear velocity

solution to mass balance including diffusion only
Solution to Mass Balance Including Diffusion Only

Solution:

C = the concentration at time t and distance x

Co = original concentration

x = distance

t = time

erfc = complimentary error function

D* = effective diffusion coefficient

example diffusion process
Example – Diffusion Process

Assume Landfill A contains Na+1 = 10,000 mg/l and Ca+2 = 5,000 mg/l and assume Landfill B contains Fe+2 = 750 mg/l and Cr+3 = 600 mg/l. Landfill A has a 6 m clay liner under the waste and Landfill B has 3 m of clay under the waste. Assuming diffusion is the only process affecting solute transport, which of the four species will break through the clay layer in either of the landfills first? How long will that take?

given
Given

Given: Effective diffusion coefficients D*:

solution
Solution

The erfc(z)* function has non-zero values only at z values less than 3. To solve this problem assume times and calculate at edge of clay layer for each case and keep changing the time until z has a value of 3. WHY?

*Z=x/2((D*t)^0.5)

increase time to 12 years
Increase Time to 12 Years

Fe+2 will break through first

decrease time to 11 years
Decrease Time to 11 Years

Fe+2 will break through ~ 11 years

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Last updated April 1, 2014 by Dr. Reinhart

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