Other potential benefits of using this guidance include the following:

• identification of site complexities early in the remediation process, thereby maximizing the potential benefits of effective management
• reduction in potential technical, regulatory, and procedural barriers to using various remediation management approaches that exist but are infrequently practiced (including continued remediation, monitoring, institutional controls (ICs), alternate concentration limits (ACLs), or technical impracticability (TI) waivers)
• demonstration of precedent for implementing site management approaches in sixteen case studies
• return of all or part of the site to beneficial reuse earlier in the remediation life cycle as remediation milestones are reached, while recognizing that additional time will be needed to achieve overall site objectives
• reduction in environmental footprint and other benefits to society, while complying with environmental regulations that protect human health and the environment
• familiarity with existing guidance on a variety of tools, technologies, and approaches to use at different steps of remediation and postremedy management, which can rule out ineffective remedial technologies and minimize the gathering of unnecessary data
• references to existing federal and state guidance and stakeholder engagement practices that are relevant for complex sites
• assistance with long-term management at sites that may require decades to reach site objectives, including development of site-specific long-term management plans for remediation systems, ICs, maintenance, and monitoring
• streamlined process for stakeholder engagement with owners and regulators of complex sites, which studies show reduces the cost of remediation and long-term management

The USEPA began developing policy regarding complex sites (also termed “mega-sites”) in the early 2000s (Means 2001). Although USEPA initially used the terms “mega” and “complex” sites somewhat interchangeably; for example, see National Advisory Council for Environmental Policy and Technology Superfund Subcommittee 2003 (USEPA 2003), USEPA eventually preferred the term “mega-site” and did not use the term “complex site” in regulations or guidance. Only one state uses the term “complex sites” in its environmental programs (Washington Department of Ecology 2017). Europe recognized the concept around the same time (Darmendrail et al. 2004). Problems with these sites were soon recognized at the highest levels of USEPA management (Horinko 2002):

The remaining number of Superfund sites that have not reached the completion stage includes area-wide groundwater sites, mining sites, sediment sites, and federal facility sites. The size and complexity of these remaining sites generally indicate longer project durations and increased costs required to complete cleanup construction. There is now a greater number of federal facilities and very large and complex sites (sites exceeding $50 million in cleanup costs) as a percentage of National Priority List (NPL) sites not yet completed than ever before. Of the remaining 675 final NPL sites that have not reached construction complete, 138 are federal facilities and an additional 93 sites are very large and complex sites.

The National Research Council has also described complex sites as sites that require long-term remediation based on site-specific technical or regulatory complexities. Both USEPA and European definitions of “mega-site” include an economic criterion that NRC’s does not. NRC describes complex sites as follows (NRC 2013):

Although progress has been made in remediating many hazardous waste sites, there remains a sizeable population of complex sites, where restoration is likely not achievable in the next 50-100 years. Although there is no formal definition of complexity, most remediation professionals agree that attributes include areally extensive groundwater contamination, heterogeneous geology, large releases and/or source zones, multiple and/or recalcitrant contaminants, heterogeneous contaminant distribution in the subsurface, and long time frames since releases occurred. Additional factors that contribute to complexity include restrictions on the physical placement or operation of remedial technologies and challenging expectations (such as regulatory requirements, cleanup goals, and community expectations). The complexity of a site increases with the number of these characteristics present.

Complex sites constitute a small percentage of all sites, but consume a large proportion of cleanup costs. USEPA recognized in 2006 that a few complex sites consume much of the agency’s cleanup budget (Bodine 2006):

The largest and most complex Superfund sites must be managed as multi-year construction projects. This is particularly true of the “mega-sites” with estimated costs over $50 million. USEPA-funded mega-sites consume the majority of resources. In fiscal year 2005, approximately 50% of the Superfund obligations for long-term, ongoing cleanup work were committed to just eleven sites. The Agency expects to have a similar situation this year.

In 2011, the Strategic Environmental Research and Development Program and the Environmental Security Technology Certification Program (SERDP/ESTCP) compiled data from 588 Army, Navy, Air Force, and Defense Logistics Agency sites where groundwater remediation will not be completed until at least 2022. The greatest costs for cleanup of these sites were associated with source zones, groundwater, landfills, underground storage tanks, and surface disposal areas. Among these sites, 90% had cost-to-completion estimates of less than $14.3 million, and 10% of the sites had cost-to-completion estimates ranging from $14.3 to $122.2 million. Thus, 31% of the sites accounted for 80% of the total cost-to-completion (Vogel 2015).

Sites with a high cost of remediation also tend to have a high environmental footprint, including consumption of large amounts of energy and other resources, emission of greenhouse gases and criteria pollutants, and waste generation. Promoting more efficient approaches to managing complex sites improves the sustainability of remediation programs and is consistent with the objectives of green and sustainable remediation (GSR).

Complex sites may have a greater potential for land reuse and other societal benefits. Economic benefits can also accrue from remediating complex sites. One study found that brownfield site assessment or remediation can increase nearby residential property values by 5 to 15% and (based on anecdotal surveys) reduce crime (USEPA 2016b). Site remediation can mitigate damage to public resources and improve resource availability, restoring clean drinking water, hunting, fishing, and recreational land uses. Site remediation can also decrease public health risks, reduce stigma, improve commerce, and avoid the loss of cultural practices. Site remediation can mitigate these effects even when sites are not restored to a condition that allows for unlimited use and unrestricted exposure (for instance, at Brownfields sites). Restoring the beneficial use of land, surface water, groundwater, and other natural resources can benefit municipalities, rural residents, tribes, and others.

It is sometimes difficult for site owners, regulators, and stakeholders to agree on a reasonable time frame for remediation for a complex site, as well as the degree of site complexity. For example, the ITRC authoring team for this guidance includes federal agency and private party site owners, state and federal regulators, stakeholders, remediation contractors, and academics. At the beginning of the team’s process, these participants were surveyed about complex sites and time frames for remediation. Results, shown in Table 1, indicate a wide disparity in opinions:

Table 1. ITRC team survey results: time frames and complex sites (Appendix A)

*Note: survey responses total more than 100% because 5 survey respondents selected multiple answers.

Timely remediation is frequently valued by RPs, regulators, and stakeholders. Because reasonable restoration time frame is not defined in regulations, early stakeholder engagement on this issue can help to build consensus. Lack of consensus-building on restoration time frame can result in site objectives that do not reflect site complexities and are therefore unrealistic or unachievable.

NRC recommends adaptive site management at complex Superfund sites, noting that “adaptive management is not synonymous with ‘trial and error’” (NRC 2005). The adaptive site management process is instead a means to “…learn from, test, assess, and modify or improve remedies with the goal of meeting long-term objectives” (NRC 2007). Adaptive site management practices have been implemented at many sites, including the Watervliet Arsenal, New York (ITRC 2012).

USEPA guidance describes a process similar to adaptive site management. For example, the Groundwater Remedy Completion Strategy describes an iterative remediation process as follows (USEPA 2014b): “The schedule should reflect the dynamic and iterative nature of a groundwater strategy’s implementation. With ongoing data collection, evaluation of the performance metrics and remedy assessment, the completion strategy process and findings should be updated periodically to facilitate open and transparent communications within the project team and external stakeholders.” Elements in the stepwise completion strategy are stated as follows (USEPA 2014b):

• Understand current site conditions.
• Design site‐specific remedy evaluations.
• Develop performance metrics and collect monitoring data.
• Conduct remedy evaluations using site‐specific metrics.
• Make management decisions.

ITRC’s process for adaptive site management, as outlined in this guidance, is similar to the USEPA process and to NRC’s recommendations.

Although adaptive site management can be used at any site, it is particularly useful at complex sites. Adaptive site management does not need to be applied sitewide—simple issues can be addressed in parallel using existing processes. Because of the technical uncertainties of applying remediation technologies in complex settings, several iterations may be needed at complex sites. Compared to simpler sites, complex sites may use the same remediation technologies, but require multiple remediation technologies across the site and throughout the remediation process. Complex sites may also require more diagnostic tools and data collection to guide technology implementation. It may be appropriate to reevaluate site objectives to align them with achievable goals.

Adaptive site management can be used at any stage of site remediation. The first steps of the adaptive site management process could be considered prior to the remedial alternative evaluation (such as the CERCLA feasibility study or RCRA corrective measures study). The adaptive remedial strategy could then be documented in the final remedy decision document. At other sites, a remedy may already be in place and adaptive site management practices could be integrated where appropriate. Adaptive site management goes beyond what is required under CERCLA or RCRA in assessing restoration potential, developing interim objectives, developing a long-term management plan, and following clear decision logic to refine the CSM and to adjust or even reevaluate the remedy. Note that the CSM may be refined at other times as a result of new data gathering. For simplicity, this detail is not shown in Figure 1. Site managers can also implement relevant phases of adaptive site management during remedy planning and implementation (remedy selection, remedial design, remedial action) or during long‑term management, which includes postconstruction phases of remediation management, monitoring, and evaluation of remedy protectiveness. Early stakeholder involvement and continuing engagement after remedy selection and throughout the remedy has proven successful at complex sites).

Sites that have already selected a remedy may choose to integrate existing site operations into the adaptive management process by establishing interim objectives. Interim objectives can be developed for each component of the remedy, along with specific, relevant, and time-bound performance metrics. The next steps are remedy implementation, monitoring, and evaluation. Remedy performance is evaluated routinely against performance metrics to keep the remediation on track to achieve interim objectives. If performance is not acceptable, the remedy can be optimized, if feasible, replaced with a contingency plan, or reevaluated. Remedy reevaluation is a process that consists of refining the CSM to better describe site complexities, reevaluating site objectives if needed, and modifying the remedy.

Adaptive site management ends when all interim objectives have been met, site objectives have been achieved, and the closure process has been initiated. If a “no further action” site objective is unachievable or otherwise not applicable, then the remediation process may include long‑term management of the site or portion of the site.