Remediation techniques designed to clean-up contaminated soil and groundwater have evolved since promulgation of regulations associated with the Resource Conservation and Recovery Act (RCRA) and the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) in the 1970s and 1980s. There are now so many choices to consider when determining the “right” way to address contamination, and the “best” way depends on several important factors.

Factors that go into the decision of how to best address contamination include:

• Legal liability
• Type of contamination
• Magnitude of contamination
• Contaminant migration
• Risk tolerance
• Future site use
• Feasibility (chance for success)
• Regulatory involvement and program
• Time frame
• Cost

At the beginning of the remedial evolution, the common approach was “you contaminated it – you clean it up to pre-incident conditions”. These earliest techniques were essentially “removal actions” that involved excavation of grossly impacted soils and placement of those soils in an off-site landfill or incinerator and pumping groundwater and treating it in an above-grade treatment system.  The removal cleanups progressed to a “how clean is clean enough” approach and were targeting a specific cleanup level such as the US Environmental Protection Agency Maximum Contaminant Levels.

As remediation experts soon discovered, removal of 100% (or close to depending on site factors) of the contamination was generally not possible, particularly in groundwater. Groundwater pump and treat systems would operate five (5) to even 20 years or more and there would still be appreciable contamination left behind.  Although pump and treat systems hydraulically controlled the groundwater contaminant plume, residual soil and groundwater impacts were left behind at levels that were substantially higher than the concentrations deemed “safe” at the time.  As a result, additional remediation technologies were evaluated and refined.

As the state of the science evolved in the 1990s and early 2000s, there was increased focus on utilizing a combination of risk assessment techniques to establish site-specific clean-up criteria, and newer in situ and ex situ techniques for soil and groundwater cleanup.  Newer ex situ treatment for soils included technologies like thermal desorption and multi-phase or dual-phase extraction, while a number of in situ techniques became increasingly common and relied upon, such as soil vapor extraction (SVE) and air sparging (AS), chemical oxidation, groundwater circulation wells, permeable reactive barriers, and bioremediation.

From the mid-2000s to present, there has been a fundamental focus in remediation techniques toward in situ approaches such as various chemical oxidants and reductants, zero valent iron blends, and thermal remediation. These approaches are now commonly coupled with evaluating exposure potential and the use of exposure prevention measures to protect human health and the environment. Exposure prevention measures include the use of risk assessment, multiple lines of evidence, and use of deed restrictions, institutional controls, and engineering controls such as vapor intrusion mitigation systems.

Due to the shift in risk-based thinking to achieve remediation objectives, you must consider whether or not the source should be treated and the effects the source area will have on the contaminant plume over time. Risk based decisions alone are not typically viable over long periods of time if the environmental conditions continue to change because of factors such as contaminant migration, leaching and/or additional contaminant contributions, among other things.  Source areas can continue to dissipate contamination as it spreads from highly contaminated areas to less contaminated areas. Eventually the process will stabilize but depending on the contaminant and geologic setting it can take many years or decades. Source area cleanup is almost always an important part of the remediation and closure process.

A wide variety of cleanup techniques are available for reducing contaminant concentrations in the source area. The appropriate technique depends on a number of factors similar to those presented above as well as the source area location, geologic setting, type of contaminant, and remedial objectives. In determining how much source area cleanup is warranted, consideration must be given to long-term costs and obligations related to contamination that is left in place. These include:

• Operation and maintenance of remedial systems and engineered controls
• Monitoring (indoor air, groundwater)
• Financial assurance requirements
• Encumbrance of future land use options
• Development restrictions

In addition, there are potential ongoing legal liabilities, such as third party claims, associated with contamination which is left in place.

For more information on remediation techniques used to address soil and groundwater contamination, sign up to attend the webinar on May, 13 2020.