The International Information Center for Geotechnical Engineers

Permeable Reactive Barriers - The Applicable Soils and Types of Contaminants

THE APPICABLE SOILS AND TYPES OF CONTAMINANTS

Applicable Soils

When considering if a Permeable Reactive Barrier will be adequate to reduce the contaminants in the ground, the surrounding soil properties need to be analyzed. The PRB must be designed to be more permeable than the surrounding aquifer soils, so identifying the permeability of the soil is important. (Thiruvenkatachari et al., 2008) For example, the commercially available granular activated carbon (GAC) (Aquacarb 207EATM) has a hydraulic conductivity of approximately 0.001m/s. (Di Nardo et al., 2010) When the reactive material has a higher hydraulic conductivity than the surrounding soils the contaminated groundwater moves under a natural hydraulic gradient through the reactive material. (Di Nardo et al., 2010) Once the permeability of the soils is considered, a PRB can ideally be placed in any soil because the mechanical properties of the soil primarily affect the flow of the water. However, the soil chemical and physical properties will affect the construction of the barrier discussed in the Construction section of this report. The geochemical properties of the soil will also determine the type of reactive media to be used in the PRB. For example, groundwater contaminated with PCE may not be best with a ZVI PRB. The ZVI causes a series of reduction reactions which can lead to precipitation and result in reduced hydraulic conductivity of the PRB (Di Nardo et al., 2010). Adsorption by an activated carbon PRB, alternatively, can immobilize the PCE in the barrier to circumvent contaminant precipitation and the corresponding issues such as clogging. (Di Nardo et al., 2010) Certain interactions between contaminants and reactive media can induce unintentional precipitation due to increased pH. (Thiruvenkatachari et al., 2008)

 

Types of Contaminants

Analyst estimate that there are between 300,000 and 400,000 sites in the USA contaminated with a wide variety of toxic chemicals representing cleanup cost in the range of 500 billion to 100 trillion dollars. (Thiruvenkatachari et al., 2008) “Many of these sites experience groundwater contamination of complex mixtures of chlorinated solvents, fuels, metals, or radioactive materials.” (Thiruvenkatachari et al., 2008)

Groundwater contaminants originates from two categories of sources, point sources and distributed non-point sources.

 Figure 6 Common Contaminants

Figure 6: Examples of common contaminants found in contaminated groundwater (Thiruvenkatachari et al., 2008)

 

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. (Thiruvenkatachari et al., 2008) Conversely, most inorganic contaminants themselves are elements and cannot be broken down but can only change speciation. Therefore remediation strategies in removing inorganics include precipitation, adsorption and transformation into non-toxic forms. (Thiruvenkatachari et al., 2008)

 

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