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Permeable Reactive Barriers - Types of Barriers and Their Construction

TYPES OF BARRIERS AND THEIR CONSTRUCTION

Site Characteristics

Certain site characteristics such as hydrogeology, geochemical, hydrochemical, contaminant distribution, and microbiology must be evaluated prior to installation. (Bronstein, 2005) Hydrogeology, geochemical and contaminant distribution affect the location, configuration and reactive media of the PRB. Microbes present in the site can enhance the effectiveness of PRB. (Yeh et al., 2010) The depth, hydraulic conductivity, porosity, dispersivity, etc. characteristics of an aquifer must be well established to determine the PRB placement because the barrier efficiency differs with varying working conditions. (Di Natale et al., 2008) Factors surrounding the site, including: weather, surrounding surface water bodies, and surface conditions, must also be assessed because these variables will alter the water flux and can potentially increase the contaminant concentration. (Di Natale et al., 2008)

The contaminant type undergoing remediation and its distribution pattern must be considered to determine the appropriate medium and PRB characteristics, such as width and configuration. The geochemistry of the contamination site can affect the reactivity of the PRB material and change the lifespan of the barrier. (Bronstein, 2005) Therefore a geochemical study needs to take into account the pH levels, redox potential, dissolved oxygen, and minerals present for the specific site. (Bronstein, 2005)

 

DESIGN

Configuration

The two most conventional setups for a PRB are continuous wall and funnel-and-gate.  (Thiruvenkatachari et al., 2008) (Day, O’Hannesin, & Marsden, 1999) The continuous wall configuration is the most common configuration as of 2005. (ITRC, 2005) Both configurations are proven to be effective, but one might be more preferable than the other depending on the contaminated site.

 

Figure 7 Funnel Gate vs Continuous wall

Figure 7: Typical configurations of a Permeable Reactive Barrier (ITRC Mining Waste Team, 2005)

 

A continuous wall configuration places a PRB in a trench perpendicular to groundwater flow. It is the simplest to install and typically covers the entire width and depth of the contamination plume. (Day et al., 1999) The characteristics of the site (water flow and the type of contaminant) will determine the width of the PRB to ensure proper reaction time and velocity. The continuous configuration is recommended to be anchored to an impermeable layer to reduce the potential of groundwater underflow. (Bronstein, 2005)

Funnel and gate systems consist of impermeable sides, such as sheet piling or slurry walls that divert contaminated groundwater into a reactive gate. (Bronstein, 2005) This ensures that the entire groundwater in the site goes through the reactive material to remove the contamination. (Day et al., 1999) Typically, the ratio of the length of the funnel to the length of the gate is less than six. (Day et al., 1999)  The groundwater velocity through this gate is several times higher than the natural velocity. (ITRC, 2005)

In comparing the continuous wall to the funnel and gate system, a continuous wall configuration is sometimes chosen because it minimizes the potential for bypass around. (Vogan, 1999) A funnel and gate configuration is preferred when the reactive material is expensive because the funnel and gate utilizes less reactive media than the continuous wall. (Thiruvenkatachari et al., 2008) The construction cost of a continuous type barrier is significantly cheaper than the funnel and gate system. (Thiruvenkatachari et al., 2008)

The width of the PRB should be wide enough for the residence time of the contaminated flow to be long enough for the chemical process to take place. In reduction processes, the design width is a function of groundwater velocity and the residence time needed to reduce the contaminant concentration. (Richards, 2008) When designing the width (W) for an adsorption process, it should satisfy the inequality:

EQUATION 1

The flow of the contaminants is not always continuous, so a period of desorption may take place within the barrier causing contaminants to be released downstream. (Di Nardo et al., 2010) Designing a wider barrier can allow this process to be slowed preventing any contaminant concentration peaks from occurring and improving the overall long-term performance. (Di Nardo et al., 2010)

Types of Reactive Material (Physical Properties)

ZVI:

ZVI is an oxidized compound that passes electrons to contaminants when they come in contact with one another. ZVI treats contaminants such as organic-halogenated hydrocarbons, inorganics, and metals. The reaction tends to degrade or precipitate out contaminants when reduced by the ZVI. (Thiruvenkatachari et al., 2008)

GAC:

Activated carbon is a chemically stable material with a high adsorption capacity for organic compounds because of its large surface area. The effectiveness of GAC on inorganic compounds has yet to be evaluated. There is a possibility of repeated use of this reactive material through phosphate extraction, acid washing, and recently, microbial regeneration. (Thiruvenkatachari et al., 2008)

Limestone:

Limestone is used in the remediation of anionic and cationic compounds. It is also effective in reducing the solubility of certain metals. This material is inexpensive. (Thiruvenkatachari et al., 2008)

ORC:

Dissolved oxygen content is very low in groundwater. Oxygen releasing material can create an aerobic environment and allow for microbiological growth. This growth enhances the PRB and assists in degrading the contaminant.  (Yeh et al., 2010)

CONSTRUCTION

The Permeable Reactive Barriers can be constructed using conventional excavation techniques, a slurry mixture to prevent collapse, or by hydrofracturing to place the reactive material. (Day et al., 1999) The type of construction method depends on the type of configuration being used in the design of the PRB as well as the depth of the contamination. It is also “good practice to key the PRB into an underlying low-permeability layer to ensure complete capture and as a safeguard in the event the permeability of the PRB is reduced.” (ITRC, 2005) However it is not necessary to key into a low-permeability layer because it is not the goal of the PRB to prevent flow. (ITRC, 2005)

One of the more common methods of constructing a PRB is to use a biodegradable slurry that allows the trench or excavation to remain open during construction (ITRC, 2005). This method is preferred over using sheet piling due to cost comparisons. (ITRC, 2005) However when creating shallow trenches, hydraulic shoring or trench boxes can still be used for temporary support (ITRC, 2005).

The most common polymer used in a biodegradable slurry is guar gum because this solution is more easily degraded than a bentonite solution. (Day et al., 1999) The reactive material is placed in the trench while the biodegradable slurry remains in place. (Day et al., 1999) Recirculation wells are then placed surrounding the trench to break down the slurry mixture with enzymatic breaker fluid. (ITRC, 2005) It is important that the slurry material is degraded because, while in place, it affects the permeability through the reactive medium. A trench can reach a depth of 27m (90 feet) and a thickness of 0.6m (2 feet) using this method of excavation. (ITRC, 2005) However the stability of these trench/slurry walls relies on shear strengths of the soil and other soil characteristics which can vary significantly within a site. (ITRC, 2005)

Figure 8 Bio-slurry excavation

Figure 8: Construction of a slurry reinforced PRB trench (Day et al., 1999)

 

Another method of construction is using vertical hydrofracturing which allows the PRBs to be placed deeper than excavation or slurry supported trenches. (ITRC, 2005) For example, a continuous wall’s reactive material can be placed deeper than 90m (300 feet) and up to 0.2m (9 inches) thick with hydraulic fracturing. (ITRC, 2005) This method would be ideal in urban areas for deep contamination because this method does not disrupt the surface as much as trenching. (ITRC, 2005)

Figure 9 Hydrofracturing

Figure 9: Construction of a PRB using the hydrofracturing technique (Tinker Air Force Base, 2006)

 

The trench construction is just as important as the site characteristics and determining the type of reactive material. This report covers a case study where the construction process led to a failure in the PRB causing some contamination to be transported downstream in higher concentrations than preferred. See the ZVI Case Study, (Richards, 2008), for more details. 

 

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