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Conceptual Site Models for ISCO

Compared to conventional remedial technologies, in-situ chemical oxidation (ISCO) requires a very well developed conceptual site model (CSM). The CSM is a living model that summarizes site conditions, and provides an overview of the distribution, mass, concentration, and fate and transport of contaminants of concern, potential receptors and exposure pathways, and land use data available for a given site. The CSM is developed using data from the initial investigation performed at the site and is continually updated throughout the lifecycle of the cleanup to reflect new information as it becomes available. This plan must be reviewed, updated, and incorporated into each stage of the remedial strategy as the design progresses. In some cases, remedies fail because of an incomplete or improper CSM and/or failure to integrate the information presented in the CSM into the design of the remedy. This section provides an overview of key CSM elements needed to adequately describe the site and common pitfalls in site characterization that can lead to suboptimal designs of ISCO treatment systems. For ISCO projects, Soil Oxidant Demand is an important parameter that should be integrated ito the CSM.

A comprehensive CSM should be developed and updated as new information is gathered so that it can be used as an engineering management tool from the initial site characterization through remedial action operation and long-term management. Regular analysis of the CSM to refocus remedy selection, design, and implementation will lead to a more cost-effective site cleanup.

Key CSM Elements and Potential Impacts to ISCO Designs

It is important to have a thorough understanding of the CSM when designing and applying ISCO treatment technologies. A detailed understanding of geochemical and lithologic characteristics of the site, flow and mass transport, and transformation and retardation of contaminants and the proposed oxidants is required to ensure adequate distribution and contact of the oxidant with the COCs. Failure to address these components in the design can have a negative impact on technology performance. Specifically, a CSM should take into consideration the site-specific factors listed in Table 1.

CSM Element Description
Nature and extent of contamination Several factors help to determine the horizontal and vertical locations to introduce oxidants as follows:
  • Age and origin of COCs, COC physical and chemical properties (e.g., organic carbon- water partition coefficient [Koc], solubility)
  • Mass of COCs, horizontal and vertical distribution of COCs, and heterogeneity of COC distribution
  • Presence and distribution of non-aqueous phase liquids (NAPLs) smear zone vs. clay lens
Human and ecological health risks
  • Risks presented by COCs, as well as risks associated with the introduction and persistence of the oxidants (which can influence treatment endpoints, number of applications required, etc.)
Fate and transport of the COCs
  • Determine how it impacts the location of injections, concentrations of oxidants, flowrates, and method of introduction into the aquifer
Site-specific infrastructure and characteristics Several factors influence injection locations and overall strategy as follows:
  • Consider urban vs. rural environment
  • Presence of buildings and utilities
  • Proximity to nearby receptors
  • Current and future land use

Hydrogeology Several factors determine the approach that will be used to introduce the oxidants into the aquifer as follows:
  • Lithology (lithologic units, heterogeneities, grain size, permeability, presence of bedrock, etc.)
  • Hydrogeology (gradients, confined or unconfined conditions, saturated thickness, conductivities, flux, Darcy velocity, groundwater flow velocity, anisotropy, etc.),
  • Mineralogy (e.g., could contribute to temporary metals mobilization)

Hydrogeochemistry
  • Document dissolved oxygen (DO), oxidation reduction potential (ORP), pH, and buffering capacity.
  • Determine soil organic matter to estimate the fraction of organic carbon (foc) and distribution coefficients (Kd).
  • Geochemistry in background (uncontaminated) and contaminated areas should be determined.


Several of these elements can have a significant impact on ISCO design and successful introduction and distribution of ISCO reagents into the subsurface (see Table 2).

CSM Element Design Impact
Hydraulic conductivity and aquifer anisotropy
  • Groundwater and oxidant flow follows the path of least resistance. Low conductivity regions may not be adequately treated. Additional or targeted injections may be required in those regions
Lithology
  • Fracturing or other enhancements may be required in low permeability aquifers to facilitate oxidant distribution
  • Heterogeneities will influence reagent flow pathways and contact with COCs
Presence of NAPL, smeared, or sorbed contaminants
  • Impacts oxidant demand
  • Contributes to substantial rebound if only dissolved phase is treated
  • Contributes to back diffusion (especially from low permeability areas)
  • Mobility will impact type and extent of treatment
Horizontal extent of contamination
  • Impacts degree of treatment, which could include only the source area, a portion or all of the dissolved phase plume, or a combination of both
Vertical extent of contamination
  • COCs distributed across regions having low hydraulic conductivities will be more difficult to treat requiring injection strategies that isolate these low permeability zones and/or increase fluid distribution (e.g., hydraulic or pneumatic fracturing)
  • Depth of contamination will influence cost and design (i.e., direct push, recirculation wells, aboveground recirculation, etc.)
Subsurface utilities and conduits
  • Potential pathway for groundwater and reagents, may cause reagents to flow into undesirable locations (e.g., streams, sewers) rather than contacting the COCs
    Potential direct impact to subsurface utilities. Important to check compatibility with utility corridors
    Potential pathway for volatile gases generated, either from degradation byproducts
Presence of aboveground structures
  • Vapor recovery may be required to mitigate risks associated with vapor intrusion when gas is generated (e.g., application of hydrogen peroxide) or heat evolution is a concern
  • Aboveground structures may pose access issues for ISCO injections



 

 
 
 
 

 

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