1. Web-based Class Project on Geoenvironmental Remediation
Permeable Reactive Barriers
Prepared by:
This is a presentation of our web-based class project on geoenvironmental remediation. Our topic is permeable reactive barriers
With the Support of:
Report prepared as part of course
CEE 549: Geoenvironmental Engineering
Winter 2013 Semester
Instructor: Professor Dimitrios Zekkos
Department of Civil and Environmental Engineering
University of Michigan

2. More Information
More detailed technical information on this project can be found at:
For more information, or to read our paper please visit the website.

3. Overview Physics Chemistry ZVI case study Application GAC case study
Types of barriers
Construction methods
Advantages and disadvantages

4. Introduction First implemented in 1991
Reactive media dependent on contaminants
Common types:
Zero Valence Iron (ZVI)
Granular Activated Carbon (GAC)
Limestone
Oxygen Releasing Compounds (ORC)
Passive technique
(University of Newcastle Australia, 2012) for the 3D photo
The first PRB was implemented in 1991 so it is a relatively new remediation technique. The reactive media is dependent on the contaminant present on site. The most common reactive media is Zvi and it accounts for 55% of all PRBs.
Some other common reactive media include Granular Activated Carbon, Limestone and Oxygen Releasing Compounds.
This is a passive technique, so no extra energy is needed as an input. The hydraulic conductivity is designed to be greater than the surrounding soil, which drives the contaminated groundwater plume toward and through the PRB.

5. The Physics Funnel and Gate Continuous Wall
Funnel and Gate and Continuous wall are the two most common techniques for PRBs.
Within the barrier the contaminants are either adsorbed, chemically or biologically degraded. Once the chemical process is complete, the remediated water continues downstream via natural flow.

6. The Chemistry Common Materials: Reactive Processes:
Zero Valent Iron (ZVI)
Granular Activated Carbon (GAC)
Reactive Processes:
Abiotic Reduction
Biotic Reduction-Oxidation
Chemical Precipitation
Sorption or Ion Exchange
Two most common reactive materials are ZVI and GAC. Other examples can be found in the table next to the bullets. These however are not as commonly used as ZVI and GAC so their effectiveness is unknown.
There are four types of reactive processes....
In abiotic reduction, ZVI is the primary reactive material used. The contaminants react with the Zvi and degrade to less harmful compounds that either precipitate out or flow through the barrier.
Biotic reduction/oxidation promotes microbial growth such as sulfate reducing bacteria to degrade contaminants.
In Chemical Precipitation the reactive media reacts with contaminants to form a solid state and precipitate out of water. : Limestone is a common material used.
In Sorption/Ion Exchange reactive materials act as a binding surface for the contaminant, so they stay within the barrier while the water continues to flow through. ZVI and activated carbon are common sorption reactive media.
(Bronstein, 2005)

7. Application Types of Contaminants Applicable Soils Organic Inorganic
Broken down and removed
Inorganic
Precipitates, adsorbed or transformed to non-toxic
Hydraulic Conductivity most important soil parameter
Reactive material k > soil k
Geochemical properties pH
Minerals
The two classifications of the contaminants are organic and inorganic. Organic contaminants can be broken down into elements and compounds that are then remediated from the plume. Inorganic contaminants such as metals cannot be broken down but can only change speciation. Therefore remediation strategies include precipitation, adsorption and transformation into non-toxic forms.
As far as applicable soils, the most important factor is the hydraulic conductivity. The reactive media must be greater than the soil conductivity for the plume to flow through under natural hydraulic gradients. Geochemical properties of the soil such as pH and mineral content need to be analyzed because these factors can cause precipitation and clogging.

8. Types of Barriers ZVI GAC Limestone ORC
Treats: organic-halogenated hydrocarbons, inorganics and metals
Degrades or precipitates out
GAC
High adsorption for organic compounds
Potential re-use after cleaning
Limestone
Effective in reducing certain metals
Inexpensive
ORC
Typically used with microorganisms
Aerobic environment  Microbiological growth
ZVI is an oxidized compound that passes its electrons to the contaminants when they come in contact with one another.
GAC is a chemically stable material and has a high adsorption capacity because of its large internal surface area. The effectiveness of GAC on inorganic compounds has yet to be evaluated. There is a possibility of reuse of this material through acid washing, phosphate extraction and microbial regeneration.
Limestone is used in the remediation of anionic and cationic compounds. Its effective in reducing the solubility of certain metals. And is inexpensive.
Dissolved oxygen content is very low in groundwater. Oxygen releasing compounds can be used to create an aerobic environment and allow for microbiological growth. This growth enhances the PRB and assists in degrading the contaminant.

9. Types of Barriers Funnel and Gate Continuous wall Impermeable sides
Reactive material forming middle
Velocity in PRB greater than natural velocity
Continuous wall
Trench perpendicular to groundwater flow
Simple
Covers entire width of plume
Funnel and Gate and Continuous wall are two main types of PRBs.
The Funnel and Gate has impermeable sides along the length of the contamination plume that funnel the groundwater towards the reactive media. The groundwater is then forced to flow through the media causing the velocity to be higher than the natural groundwater velocity. The higher velocity needs to be properly analyzed when determining the residence time required for the reactive media. Since the funnel and gate method does not require reactive media to be placed along the entire length it uses less reactive material, so if expensive material such as ZVI is needed, the funnel and gate may be preferred.
The continuous wall covers the entire length of the plume and is a trench perpendicular to groundwater flow. It is usually anchored to an impermeable layer beneath to reduce underflow. This process may be preferred to the funnel and gate because the construction process is more simple and inexpensive than the funnel and gate system. It also ensures that the entire plume is treated and there is no flow around the PRB.
It is important to design either method to have a PRB width wide enough to allow for the appropriate chemical processes to take place.
(ITRC, 2005)

10. Construction Methods Slurry trenches, hydrofracturing, etc Depends on:
type of configuration
depth of contaminant
Slurry trench: 27m depth
Hydrofracturing: 90m depth
Typically key-in bottom
Width of PRB
Residence time
Hydraulic properties
As important as the permeability
Slurry trenching and hydrofracturing are two techniques used in placing the PRB.
The slurry trenching technique is an inexpensive method to design either a continuous or a funnel and gate PRB. However the depth that the trench can reach is reliant upon the strength of the surrounding soil. Also the slurry needs to be flushed out of the PRB so it does not interfere with the permeability. Therefore if the contamination is deep or in an urban environment a hydrofracturing method may be preferred because of its ability to reach deep locations and have minimal ground disturbance.
The construction process is just as important as properly analyzing the permeability in the design of the PRB. Any failure or incomplete fill of the reactive material can provide the contaminants with an area to bypass the PRB causing the effectiveness of the PRB to be greatly reduced.
(Day et al., 1999)

11. ADVANTAGES DISADVANTAGES
Less expensive than pump and treat
ZVI:
Highly reactive with inorganic and organic
Adaptable
Used in combination
No health hazards
Long term conditions not evaluated
ZVI:
Contamination coating
Silica or NOM reduces iron reactivity
Creation of ZVI not environmentally friendly
An advantage of the PRB is it is more cost effective than conventional pump and treat technologies. The PRB should not require operation or maintenance costs, and it doesn’t require any energy demand because it operates under natural hydraulic gradients. However long term costs and efficiencies have not been greatly evaluated for full scale PRB.
An advantage in using the Zvi as the reactive material in the PRB is it is highly reactive with both inorganic and organic contaminants. The Zvi is also adaptable because it has different sizes and chemical states that can be adapted to different loads of contaminants and the amount of water flux. The Zvi can be used in combination with other methods of remediation such as bioremediation to be more effective in removing contamnants. Also, Zvi is a safe material to handle because it is not hazardous.
A disadvantage to using ZVI is if the contaminants begin to coat the particles. This can cause the permeability to decrease or can reduce the iron’s reactivity. The reactivity can also be reduced if it comes into contact with any Silica or Natural organic matter. Also, the creation of the Zvi material itself is not environmentally friendly.
An advantage to using GAC as the reactive material is it is relatively inexpensive because it comes from low cost natural products. It is used best when remediating organic or heavy metal contaminants. And the GAC is chemically stable.
A disadvantage to using GAC is that there is limited data on field studies involving GAC as a remediation technique. Also GAC is highly dependent on temperature and other environmental parameters. Like Zvi surface coating can reduce the adsorption capacity of the GAC.
GAC
Limited data on field studies
Highly dependent on temperature
Changes in adsorption capacity
GAC:
Relatively inexpensive
Organic and heavy metals
Chemically stable

12. Case Study: ZVI Pilot Scale
Background
Site Layout
East Helena, Montana
Lead smelter
Arsenic, selenium, lead, cadmium and zinc
Groundwater flow through alluvial deposits
The first case study we would like to discuss is a ZVI pilot scale PRB that was used in East Helena, Montana. This site had a lead smelter that was in use for over 100 years leeching arsenic, selenium, lead, cadmium and zinc into the environment. The groundwater would carry these contaminants through an alluvium deposit that had a fine grained volcanic ash deposit beneath.
(EPA 2006)

13. Case Study: ZVI Pilot Scale
PRB Design
Continuous trench
9.1 x 13.7 x
L x D x W in meters
ZVI filled partial depth
Entire construction took 6 days
The PRB design was a continuous trench with dimensions of 9 meters long with a depth of 13.7m deep and a with of 2meters and was installed using a slurry method. This design was a “hanging wall” design because there was a remaining meter between the wall and the fine grained volcanic ash. The Zvi reactive material was placed in the trench until the height was above the max ground water table and sand was placed in the remaining height. The entire construction process (including the removal of the slurry mixture) took 6 days.
(EPA 2006)

14. Case Study: ZVI Pilot Scale
Results
Profile view
Two years of monitoring
Initial arsenic concentration: >25 mg/L
Final:
99% removal of As from groundwater
No change in k from build up
After two years of monitoring the initial levels of greater than 25 mg per liter of arsenic were approximately 99% reduced from the groundwater. However the researchers found that the hanging wall design allowed for some contaminants to bypass beneath the PRB. Therefore it is important to properly analyze the soil’s permeability or to key in the PRB to an impermeable boundary.
(Wilkin et al, 2009)

15. Case Study: GAC Modeling
Background
Profile
Laboratory and Modeling
Removal of Cd
Case study located near a riverbank
Model parameters:
Aquifer bed depth
Hydraulic conductivity
Average Cd depth
Rainfall periods
The next study I’ll cover
(Di Natale et al., 2008)

16. Case Study: GAC Modeling
Continuous trench
Width determined by groundwater velocity and mass transfer coefficient
Movement of Cd plume using advection and dispersion code

17. Case Study: GAC Modeling
Results
Contaminant Level over 7 months
Cd <0.005 mg/L for 7 months
Peak concentration at the center: desorption and resorption
self-cleaning
PRB dampen concentration peaks
not as a complete removal
Conclusions
(Di Natale et al , 2008)
Peak concentration at center when clean groundwater caused desorption and resorption of Cd
GAC PRB should be used to dampen conc peaks. It may not be effective for complete removal, but can be used to keep contaminant levels below the regulated limit

18. Any Questions?