Groundwater Contamination from Leaking UST

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Groundwater Contamination from Leaking USTs
Prevention versus Restoration
Mohammad Al-Suwaiyan Civil Engineering
Department King Fahd University of Petroleum
Minerals
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Presentation Layout

• Introduction
• Complexity of groundwater Contamination
  Assessment
• need for modeling
• associated difficulties, uncertainties

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1,400,000,000,000,000,000 m3
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1,400,000,000,000,000,000 m3 3.7
37,500,000,000,000,000 m3
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1,400,000,000,000,000,000 m3 3.7
37,500,000,000,000,000 m3 23
8,300,000,000,000,000 m3
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1,400,000,000,000,000,000 m3 3.7
37,500,000,000,000,000 m3 23
8,300,000,000,000,000 m3 1.5
126,250,000,000,000 m3
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Distance to sun 150,000,000 km
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5 million
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Background

• Ground water a main source of potable water
• Contamination of ground water
• Hydrocarbon leaks and associated problems

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Laws related to GW contamination

• Legislations related to air or SW pollution were
  enacted in 1940s and 50s
• 1980 CERCLA or superfund act was passed
  establishing 15 billion to pay for clean up of
  abandoned hazardous waste sites which can be
  recovered by EPA from responsible parties
• What is the reason for the time lag?

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Leaking
Residual hydrocarbon
Tank
Mobile hydrocarbon
Ground-water flow
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Leaking
Residual hydrocarbon
Tank
Mobile hydrocarbon
Ground-water flow
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Leaking
Residual hydrocarbon
Tank
zone
Mobile hydrocarbon
Vadose
Water table
Hydrocarbon components
dissolved in ground-water
Ground-water flow
Saturated zone
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How Can a Hydrocarbon Exist

• Residual phase held by capillary adsorptive
  forces
• Vapor phase
• Free phase
• Dissolved in groundwater

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Typical Phase Distribution After Spill
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Important Notes

• Due to the hidden nature of the subsurface
  environment many think it will degrade any
  contaminant given enough time.
• Contaminants that enter the subsurface will
  eventually make it to the aquifer.
• Remediation and cleanup of subsurface became an
  important issue.

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Factors Influencing Fluids Distribution

• Soil Hydraulic Properties
• Displacement head
• Pore size distribution index
• Residual saturation

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Factors Influencing Fluids Distribution Continued

• Fluid Properties
• density
• Surface tension
• Viscosity
• Solubility
• Volatility

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How is a spill discovered?

• Accidental detection
• Product could appear in a monitoring well
• Is using monitoring wells effective measure?

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Monitoring Well
Air
Oil
Water
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Problems associated with using MW to detect leaks

• Free product may not appear in MW even with
  significant LNAPL in formation
• LNAPL thickness in MW varies with the condition
  of the water table
• Sudden appearance and disappearance of LNAPL in
  MW is observed in the field

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Conclusion
Monitoring wells are easy to install and to use
but they are not reliable methods to detect and
quantify LNAPL leaks
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Remediation Project

• Control source
• Complete site characterization
• Design remediation process system
• Evaluate effectiveness

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Remediation Project

• Control source
• Complete site characterization
• Design remediation process system
• Evaluate effectiveness
• CAN ANY OF THE ABOVE BE DONE WITHOUT THE HELP OF
  MODELING?

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Modeling needs 1) Water flow model 2) Dissolved
pollutant model 3) Model to handle various phases

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Water flow model
Using water mass balance and Darcy equation a
Water flow model can be developed However it has
limitations and its results will influence
performance of next models
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Groundwater modelingflow equation
K exhibits large variation that is impossible to
represent which leads to non accurate velocity
field
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Dissolved pollutant model
Using mass balance for each species and
including Various influencing processes a model
capable of prediction distribution of dissolved
pollutant model can be developed However it has
limitations and its results will influence
design and operation of treatment system
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Contaminant Transport modelingtransport of
dissolved contaminants
Processes that can be included dispersion,
advection, reactions, Sorption kinetics.. How
close can we represent the actual
processes? Velocity field from flow model is
input to this model.
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Contaminant Transport Mechanisms

• Advection contaminant is carried with flowing
  water
• Flow velocity
• Solubility
• Hydrodynamic Dispersion spreading of
  contaminants
• Concentration gradient
• heterogeneity

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Attenuation Mechanisms

• Adsorption
• Retardation factor
• Reaction
• Decay
• Oxidation/reduction
• Biodegradation

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Modeling various phases
This is the most difficult part of modeling but
it can Be done as shown in the next
slide, However modeling real life cases is
impossible i.e. modelingit has limitations and
its results will influence design and operation
of treatment system
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Modeling Different PhasesContaminants as
distinct phases
For each phase similar equation will have to be
solved along With proper auxiliary and boundary,
initial conditions
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Conclusion 1

• Modeling is an essential tool for aquifer
  assessment remediation

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Conclusion 2

• Results obtained by modeling are rarely accurate
  which directly affects the effort for remediation

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Conclusion 3

• Verifying model prediction are practically
  impossible due to the nature of the subsurface
  environment

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Subsurface restoration
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Remediation Priority

• Stop bleeding Control source
• Know subsurface figure out extent
• Develop implement cleanup plan

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Sources

• leaking tanks
• Free product (Mobile phase)
• Residual phase

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Subsurface Extent

• challenging and difficult
• hidden nature
• inherited heterogeneity
• Naturally will involve heavy modeling

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Cleanup planning

• mobile phase
• Pump-and-treat
• Number of wells
• Locations
• Flow rates and time variations
• All are selected through modeling

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Cleanup planning

• residual phase (potential sources) SVE
• Through promoting mass transfer to vapor phase
  then extraction
• How close can actual processes be modeled
• Enhancing biodegradation IBD
• How close can the behavior of microorganisms be
  modeled
• Operation variables are selected through modeling

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Cleanup process complicating factors

• Initial moisture distribution
• Natural heterogeneity
• process may trigger worse situation through
  creation of new channels which facilitate
  contaminant transport

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Supporting reports

• NRC committee on groundwater cleanup alternatives
  investigated 77 sites in which treatment
  processes were conducted for a long time
• Only 8 sites reached cleanup goals
• In some cases the situation became worse

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Case of Brunswick,N.J.

• Subsurface contamination by a computer company
  was discovered
• After 6 years of Cleanup the process was stopped
  thinking it was complete
• After 3 years contamination came back worse than
  ever before

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Conclusion

• Characterization, analysis, design and operation
  of cleanup systems relies heavily on modeling
• Subsurface clean-up process is usually
  difficult, costly and has very low success rate

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Recommendation

• More effort should be given to prevention
• Meaningful regulation to the process of design,
  installation, monitoring and maintenance of USTs
  should be developed

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THANK YOU