Advanced Geotechnical Engineering
ES4D8
Contaminated Land (Lecture 8)
Remediation Approaches
Mohaddeseh Mousavi-Nezhad
Room: D211
Email:m.mousavi-nezhad@warwick.ac.uk
31/05/2016
The University of Warwick
1
Purpose of Remediation
Purpose:
A- Hydraulic control of contaminated ground
water
Prevent contamination from spreading to
uncontaminated area
B- Treatment of contaminated groundwater
Reduce concentrations in ground water to
below cleanup standards
Remediation approaches
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Covering systems
In-situ air-sparging
In-situ chemical treatment
In-situ bio-remediation
Slurry walls
Permeable reactive barriers
Pump & Treat Technology
Cover systems (caps)
• Cover systems prevent physical contact and
exposure to waste.
• Sufficient cap may be enough thickness of soil
to prevent humans or animals from digging
into waste materials
• Reduce or remove precipitation infiltration
• Reduces or prevents transport of
contaminants to ground water by infiltrating
water.
Covering layers
Vegetation
Top soil
Protection layer
Granular or
geotextile filter
Drainage layer
Geomembrane/s
oil barrier layer
Geotextile gas
collection layer
60 cm
30 cm
Geomembrane/
overlaying
protective
geotextile
Covering layers
Vegetation
This layer is used to
• control erosion
• reduce infiltration by
evapotranspiration
Characteristics of layer:
• Shallow rooted plants
• Low nutrient needs
• Drought and heat
resistant
Covering layers
Top soil
layer
This layer is used to
• support vegetation
• protect underlying
layers
Covering layers
Also called “biotic
barrier”
90-cm layer of cobbles to
stop burrowing animals
and deep roots
Not always included
Covering layers
Filter layer
Prevents clogging of
drainage layer by fines
from soil layer
May be geosynthetic filter
fabric or 30-cm sand
Covering layers
Drainage layer
Prevents ponding of water
on geomembrane liner
Covering layers
Low permeable layer
Covering layers
Needed if waste will
generate methane
(explosive) or toxic gas
Similar to drainage layer:
30 cm of sand
Gas collection layer
In-situ air-sparging
•
In-situ air-sparging approach converts the groundwater contaminants from a
dissolved phase to a vapour phase that then rises to the surface.
• Effective at targeting volatile organic compounds (VOCs), e.g., petroleum
products.
• Leads to the production of gas and vapour which if left just stays on-site or
contributes to the atmosphere.
• The remediation technologies are generally combined with a system of
vapour extraction.
In-situ chemical treatment
• In-situ chemical treatment can
target contaminants in the
unsaturated soil, in the
groundwater, or both.
• Chemical treatment involves the
addition of chemicals to soil
and/or groundwater to oxidise or
reduce the contaminants.
• The chemical reactions can;
degrade contaminants, reduce
toxicity, change solubility, or
increase their susceptibility to
other forms of remediation.
• Introducing the chemical reagents is often through injection wells, as a
liquid solution.
Ex-situ chemical treatment
• Ex-situ chemical treatment allows for extremely effective remediation
because the chemicals can be mechanically mixed in but this involves
the excavation of soil and neglects groundwater contamination.
In-situ bio-remediation
• In-situ bio-remediation is the process of enhancing biodegradation.
• Biodegradation is the natural breakdown of pollutants in the soil by
microorganisms through aerobic (requires oxygen) or anaerobic (doesn’t
require oxygen, instead hydrogen) respiration.
• In-situ bioremediation has many variants, each with a different method of
introducing the oxygen or other reagent. E.g.,
 Bioventing is the injection of air flow into the unsaturated zone (the
soil above water table) through an injection well
 Biosparging is into the ground water.
Picture from http://gunsch.pratt.duke.edu/hgt
Vertical cut-off walls
• Slurry walls
• ….
Slurry walls
Slurry walls
Typical vertical section for slurry wall
“hanging” slurry wall for LNAPLS
Soil mechanics of slurry walls
During construction, wall stability maintained by higher head in trench than in
ground water:
Ground water
Slurry
Slurry density should be 0.25 g/cm3
lighter than emplaced backfill
Materials for slurry walls
SB (soil-bentonite) have lower K, are less expensive
Typical K = 10-7 cm/sec
Reported K’s as low as 5 x 10-9 cm/sec
CB (cement-bentonite) have greater shear strength,
lower compressibility
Use on slopes where strength is important
Use in areas where appropriate soils (for SB) are
not available
Additives to enhance CB and SB:
Fly ash to increase carbon for adsorption
Liners or sheet pile installed within wall to
decrease K
EPA review of slurry wall success
Reviewed 130 sites – 36 had adequate data:
8 of 36 met remedial objective
4 met objective except not yet for long term
13 appear to have met objective
4 appear not to have met objective
7 are uncertain
4 of 36 leaked and required repairs (leaks most often
at “key” with floor)
Potential sources of failure (leaks)
Construction:
• Improperly mixed backfill (CB, SB) Sloughing
or spalling of soils into trench
• Inadequate bottom excavation for wall key
Post-construction:
• Wall properties changed by freeze-thaw
• cycles Wet-dry cycles due to water table
fluctuation
• Degradation due to contact with chemicals
Permeable reactive barriers
• Permeable reactive barriers (PRBs) are a solution mostly used by civil
engineers to groundwater contamination.
• Physical barriers are set into the saturated and unsaturated ground.
• The permeable materials allow groundwater to continue to flow through but
introduces a filtration or reaction system.
• PRBs can be adapted to target different contaminants, e.g., organic
contaminants, heavy metals.
• The barriers are prone to become less effective over time but can often easily
be replaced.
Source of contaminant
Treated
groundwater
Permeable
treatment wall
Contaminated
groundwater
To be continued….