This document synthesizes the knowledge of DNAPL site characterization and remediation acquired over the past several decades, and provides guidance on concurrent characterization of contaminant distributions, hydrogeology, and attenuation processes to allow for improvements in the following areas:
An integrated site characterization (ISC) approach, under which data of adequate resolution are collected to fully characterize a site, significantly reduces uncertainty and enables the development of cost-effective solutions to manage contaminated sites. By applying proven scientific principles, investigation approaches, and characterization tools, a rigorous three-dimensional conceptual site model (CSM) can be constructed to more effectively support environmental management decision making.
The recent innovations and advancements in site characterization allow for more effective site management, remedy selection, and remedy implementation (Stroo et al. 2012). Adequate subsurface characterization of sites where DNAPLs have been released is essential to the development of effective groundwater protection strategies. Site characterization and research conducted over the past decades, together with basic scientific concepts, demonstrate the following understanding of subsurface science:
In understanding contaminant fate and transport in the subsurface, the complexity of subsurface geologic conditions, and especially the system heterogeneity, can cause DNAPL to persist. This remnant DNAPL may persist at the interface between strata with contrasting permeability or in interbedded zones that are difficult to flush. Diffusive interchange between higher- and lower-permeability zones slows the propagation of dissolved-phase contaminants during early stages of plume development and then sustains the dissolved-phase concentrations over extended periods. Moreover, contamination that reaches low-permeability zones can be highly resistant to treatment. Collectively, these processes fall under the term matrix diffusion. In many older sites that initially contained DNAPL, matrix diffusion processes have driven the bulk of the contaminant mass into lower-permeability zones (ITRC 2011b; Payne, Quinnan, and Potter 2008; Stroo et al. 2012; NRC 2013).
Whether a remediation effort succeeds or fails depends largely on a thorough understanding of, and a remedy implementation tailored to, site-specific geology and the specific properties of the chemical contaminants. Subsurface cleanup challenges and limitations are less related to differences in remedial technologies and more related to the difficulty of identifying and targeting treatment in the most affected geologic zones (Stroo et al. 2012).
This emerging understanding holds that site conditions are dynamic, such that the evolution of a site and the trajectory of that evolution over time are as important as the site’s current state. Key to understanding source and plume behavior are the nature of the release, the composition of the contaminant, the history of the contaminant (in particular DNAPL) migration, the current contaminant distribution between chemical phases and geologic units, and the manner in which the contaminants are being redistributed. Furthermore, as this understanding has developed into a consensus, maintaining a useful
Historically, subsurface characterization has typically involved the use of monitoring wells screened over large vertical portions of the aquifer; however, this approach is limited in that data collected from typical long-screen (generally 5–20 feet long) monitoring wells cannot inform the detailed hydrostratigraphy that controls DNAPL behavior at the centimeter-to-millimeter scale. Reliance on monitoring well data has resulted in poorly performing remedies and unacceptably low predictions of plume behavior and exposure risks (Stroo et al. 2012; NRC 2013).
The complex processes of multiphase flow in porous media, which control DNAPL migration and distribution, are especially sensitive to subsurface lithologic heterogeneity and secondary porosity. Recent advances have demonstrated that geology and variations in permeability are important controlling factors in contaminant distribution, when DNAPL is present and when contaminant transport is sustained by back-diffusion (Sale et al. 2008). The now broad recognition of complex geologic control over DNAPL migration and groundwater transport has rendered the decades-old concept of highly dispersed plumes in homogeneous media to the category of major oversimplification.
Based on the above concepts and their strong interrelationships, environmental management of DNAPL sites requires a well-developed, comprehensive, holistic (and heuristic) CSM. This requires the use of updated concepts in site characterization, better investigation strategies, and improved characterization tools. The recent advances in subsurface characterization tools and techniques included in this guidance are key to improved site management decision making and ultimately to better remedy performance. The overriding regulatory challenge will be to familiarize regulatory project managers with the new characterization tools and CSM approaches and demonstrate their benefits and reliability.
This guidance addresses the problems expressed in the introductory paragraphs of this section by describing an effective ISC approach—one that aligns data on contaminant distribution (and other biochemistry and geochemistry issues) with site geologic heterogeneity and groundwater flow conditions, at a spatial resolution appropriate to the site-specific remedial objectives. A key element is the identification of contaminant distribution with respect to low-permeability vs. transmissive zones, as suggested by Stroo et al. (2012) and NRC (2013). In its simplest form, an ISC approach might involve overlaying well logs with existing site chemical data to correlate contaminant concentrations with geologic units at as fine a scale as data are available. In more complex applications, an ISC approach can involve real-time field screening techniques and temporary exploratory borings to provide vertical profiling of subsurface conditions.
An ISC approach can result in significantly greater effectiveness of site management and remedy decisions through improvements in characterization methods and tools. The benefits of applying an ISC approach may include the following:
ISC should lead to remedial decisions that are (1) protective of human health and the environment; (2) provide better and more protective remedial decisions; (3) consider the necessity of source control and proactive remediation; and (4) predict a foreseeable outcome. Thus, the ISC approach is more attractive to stakeholders who can anticipate sustained resources, cost-effective contaminant source remediation, and the preservation of regional aquifer systems.
Remediation practitioners must evaluate the benefits of investigation costs against the value of the outcome. Compared to other, more traditional characterization methods, ISC often incurs higher upfront costs; however, it may also result in a more accurate CSM and thus a more effective remedial strategy.
Understanding the heterogeneity of the subsurface, its influence on the transport of contaminants, and the fate of the dissolved and nonaqueous phase contaminants depends largely on delineating the distribution and frequency of fractures, faults, lithologic changes, mineralogy, grain size morphology and distribution, and other physical parameters of the subsurface. As well, the hydrogeology and chemical characteristics of the contaminants inform the CSM on the fluid dynamics and reactions. Historically, CSM development involved collecting and analyzing data for each individual parameter, so to keep characterization costs low and make remediation decisions early in the process, assumptions and generalizations were made about the physical characteristics of the subsurface and the chemical characteristics of the contaminants. As stated previously, this traditional approach led to poor quality remediation results and repeat treatment or to additional testing and refinement of the remediation systems. With ISC, however, a more accurate and realistic CSM can be developed and, consequently, the most effective remedial strategy can be chosen.
For example, in a heterogeneous aquifer where significant contaminant mass resides in relatively thin, low-K layers, a simplified site characterization may indicate a relatively homogeneous aquifer and fail to identify the low-K layers that act as a source of dissolved phase contamination for the higher-K (transmissive) portions of the aquifer. Thus, a chemical oxidation remedy, which does not address contaminant mass in the low-K zones, will fail regardless of how many injections of chemical oxidants are applied in the higher-K formation materials. If, on the other hand, an ISC method is used, the contaminant mass in the low-K layers may be identified and a more appropriate remedy chosen.
A well-informed remediation decision requires thorough characterization using data sets that are integrated among the geologic, hydrologic, and chemical characteristics of the subsurface. While higher-resolution data collection and interpretation leads to higher cost in the short term, the benefits of implementing ISC ultimately lead to lower costs to the overall project. As shown in Figure 1-1, use of a higher-resolution site characterization method (such as ISC) results in better decision making and more reliable performance when it comes to the remedial technology, thereby reducing the remediation cost through the end of the project. The figure illustrates that, following the Stage 1 preliminary investigation (similar costs regardless of characterization methods to be used), using ISC (dashed black line) for the Stage 2 characterization is initially more costly than using traditional characterization methods (solid blue line); however, the return-on-investigation (ROI) costs (shown as the difference between the dashed black and solid blue curves in Stage 3) are lower. Additionally, the lifetime of the project is likely shorter, thereby reducing ongoing liability for the site.
Improved CSMs and focused cleanups improve reliability and certainty of outcome while also reducing costs. For example, the costs of ISC and higher-resolution site characterization can be offset by optimizing a DNAPL source zone in situ remedy. Thus, a site characterization effort that employs the new methods and tools (such as ISC)—resulting in a more robust CSM, fewer permanent monitoring wells, and a focused remediation program—leads to reduced total life cycle costs and an improved ROI.
ROI: Traditional vs. ISC costs over the project life cycle.
Table 1-1 summarizes the EAB (enhanced anaerobic bioremediation) remedy cost for Well 12A, both with and without ISC. The cost without ISC was projected based on the actual cost of implementation, and adjusted over the treatment volume assuming a 50 ft treatment zone. The cost with ISC is the approximate total final costs based on a more robust CSM informed by high-resolution characterization. In this particular case, the characterization did not result in a significant change in the target treatment area, but did result in a significant change in the vertical interval for treatment. Reducing the target vertical interval for treatment from 50 ft to 12 ft (average), with a smaller portion of the site having a deeper treatment zone of 5 ft thickness, reduced the overall treatment volume by approximately 70%. This reduced the overall cost of the remediation by reducing the costs of the amendment, well installation, and labor for amendment injection for one full-scale injection event—from an estimated $4.66 million to $1.66 million. The cost of the higher-resolution characterization for the site was approximately $350,000. Even with this additional characterization cost, however, the project saved an estimated $2.65 million due to the substantial reduction in treatment volume.
|
Characterization Costs
|
Costs |
Notes
|
|
|---|---|---|---|
|
Without ISC |
With ISC |
||
|
Pre-Design Investigation |
$250,000 |
$250,000 |
Phase I/II |
|
High Resolution Source Area Investigation with Mass Discharge Estimate (Transect Method) |
|
$350,000 |
|
|
Mass Discharge Evaluation (GETS Pumping Test) |
$150,000 |
$150,000 |
|
|
Total Characterization Costs |
$400,000 |
$750,000 |
|
|
Remediation Unit Costs |
|
|
|
|
EAB-Treatment Volume |
Without ISC |
With ISC |
|
|
Target Area |
52,000 sf |
52,000 sf |
No change |
|
Target Thickness |
50 ft |
17 ft |
Two intervals – shallow (12 feet thick) and deep (5 feet thick) |
|
Target Volume |
300,000 cy |
90,000 cy |
~70% reduction in treatment volume |
|
EAB-Amendment Injection Costs |
Without ISC |
With ISC |
|
|
Total Amendment Cost |
$1,600,000 |
$450,000 |
|
|
Total Drilling Cost |
$1,320,000 |
$740,000 |
|
|
Total Injection Labor Cost |
$1,740,000 |
$470,000 |
|
|
Total EAB Full-Scale Remediation |
$4,660,000 |
$1,660,000 |
|
|
Overall Costs (Characterization + Remediation) |
Without ISC |
With ISC |
|
|
|
$5,060,000 |
$2,410,000 |
|
|
Cost Savings from ISC |
$2,650,000 |
|
|
|
Note: GETS = groundwater extraction and treatment system |
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Cost comparisons at Well 12A resulting from a reduced vertical interval for treatment, thereby reducing the amendment, well installation, and labor costs during the life cycle of the project.
This guidance describes how, with the current understanding of subsurface contaminant behavior, both existing and new tools and techniques can be used to measure physical, chemical, and hydrologic parameters to better characterize the subsurface. This guidance also provides a Tool Selection Worksheet that helps to screen out tools that are not applicable to specific data needs at a site. Links within the Tool Selection Worksheet navigate to descriptive information on tools and offers additional resources that, when applied properly, can improve the identification, collection, and evaluation of appropriate site characterization data. The expected result of using this guidance is a more accurate site-specific CSM, which can then be applied in the ITRC Integrated DNAPL Site Strategy (ITRC 2011b,
Chapter 2 of this document reviews DNAPL types and the characteristics that control their distribution, fate, and transport in the subsurface. Although these issues are addressed in peer-reviewed literature, they are also summarized in this document because they are crucial to designing an adequate characterization program.
Chapter 3 describes the characteristics of the subsurface that control the fate and transport of DNAPL and aqueous- and vapor-phase contaminants, information that should be considered when developing or revising a site-specific CSM. Application of these site characterization techniques will increase the level of understanding related to complex site contaminant behavior, leading to more practical and effective remedy implementation and reduction in long-term costs.
Chapter 4 describes the specific steps in an ISC process, which are as follows:
Chapter 4 also provides the Tool Selection Worksheet, as mentioned above, for the interactive selection of appropriate tools based on geologic, hydrologic, and chemical data needs at a site. The Tool Selection Worksheet links to more detailed descriptions of each tool—including its applicability, data quality capability, and limitations or challenges. Case study examples illustrate the application of ISC principles (for example, linking data collection objectives to data needs, resolution, and tools selection) at sites with various scales of heterogeneity and investigation complexity. Methods and models for managing, evaluating, and visualizing data are also included in Chapter 4. Evaluating the data against data collection objectives and updating the CSM will reduce uncertainty and support decision making. Data analysis is linked back to data collection objectives to identify any further data gaps and assess the value of additional site characterization.
Chapter 5 describes the regulatory challenges and benefits of applying an ISC approach to DNAPL sites.
Chapter 6 describes the perspectives of community and tribal stakeholders toward an ISC approach.
This guidance is a resource to inform regulators, responsible parties, consultants, community stakeholders, and other interested parties of the critical concepts related to characterization approaches and tools for collecting subsurface data at DNAPL sites.
This document revises and replaces the