Around the world, industries face significant technical and institutional challenges related to the restoration of contaminated groundwater. Even with billions of dollars in investment over 30 years, these challenges still exist, according to a 2013 report from the National Research Council of the National Academies. This report focused on the future management of complex contaminated sites, which are characterized by their large size, the presence of recalcitrant contaminants and complex hydrogeologic conditions like fractured rock and heterogeneous aquifers.

A 2004 estimate from the U.S. Environmental Protection Agency projected that expenditures for soil and groundwater cleanup at over 300,000 sites through 2033 may exceed $200 billion (not adjusted for inflation). This was likely an underestimation given the impact that emerging contaminants — including per- and poly-fluoroalkyl substances (PFAS) and those associated with coal combustion residuals — have had on industry since that time.

So, what makes the remediation of complex sites so costly and difficult to achieve? For most complex sites, the existing conceptual site model — the framework serving as the basis for remediation strategy and design development — often fails to account for the range of possible mechanisms and processes that sustain and control the persistence of groundwater contamination plumes. Many of these mechanisms and processes (e.g., matrix diffusion) are impediments to remediation success; however, others, such as long-term biotic and abiotic contaminant transformation processes, can be leveraged to benefit remediation efforts and reduce cost. In either case, identifying and quantifying the specific processes that affect contaminant behavior and remedy effectiveness for a given site are crucial to setting realistic objectives and designing cost-effective remedial actions.

Focused remediation goes beyond targeted remediation, which implies a spatial emphasis (e.g., focusing wells screens on a specific interval informed by high-resolution site characterization data). Focused remediation occurs when the sensitivities and dynamic processes at work in the impacted groundwater system — not just site conditions at a given point in time — are established as the primary drivers for the technical remediation strategy.

PROGRESS — Progressive Remediation Strategies — is the Burns & McDonnell approach to focused remediation. PROGRESS involves the development and utilization of a process-based conceptual site model (CSM) throughout the remediation project life cycle. This CSM properly quantifies, incorporates and accounts for relevant subsurface processes during remedy selection, design, implementation and optimization. For instance, the challenges associated with low permeability contaminant mass storage zones, and the long-term back diffusion of contaminants from these storage zones, have become significantly more appreciated and understood over the past 10 years. However, quantification of back diffusion rates and incorporation of this parameter into the CSM has not occurred on a wide scale. In some cases, this has resulted in the establishment of unrealistic remedial objectives and poorly informed technology selection, design and implementation.

An appropriate understanding of contaminant distribution between mass storage, mass transfer and mass transport zones can reveal significant opportunities for optimization of a site remediation strategy. If mass storage zones must be addressed, specific remedial technologies, delivery techniques or optimization measures may be required, and efforts can be properly focused on this aspect of the CSM. For many sites, however, it may be more practical and appropriate to focus remediation efforts on mass transport zones, in which case a targeted, flux-based remedy can be implemented, often at a far lower cost than what would be required to treat the entire saturated thickness of the plume.

Focused remediation of complex sites requires a well-planned and executed performance monitoring program that provides the data needed to gauge performance and optimize the remedy and monitoring program. Proactive data evaluation and monitoring program optimization prevents dollars from being wasted on inconsequential data while providing the information needed to minimize remedy duration and cost. These evaluations can also be used to support transitions from active to passive remedies.

 

This post is one of a series explaining Progressive Remediation Strategies (PROGRESS). Through PROGRESS, targeted, cost effective remedies are implemented. But this is just one component of PROGRESS and its comprehensive, next-generation approach to remediating complex sites.

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John Hesemann is a principal remediation engineer at Burns & McDonnell, where he leads the firm’s remediation technical practice. He leads multidisciplinary project teams and supports nationwide remediation strategy development, technical execution and thought leadership.