A Climatic Threat: Sustainable Remediation is not just about “Sustainability”

Lake Wakatipu near Glenorchy, South Island, New Zealand

Within the contaminant remediation technical practice, the concept of “sustainable remediation” has developed over the past 20 or so years as an engineering approach by which the use of “green” or natural processes are key components of a contaminant mitigation strategy. Beyond the technical nuance, numerous government agencies, working groups, and the public have adopted “sustainable remediation” as a culture, if not the “fight song,” of the modern remedial philosophy. The trend toward a low energy, low carbon, low water-use remedies to remove or at least reduce contaminant occurrence in groundwater, soil, bedrock, and vapor beneath a chemically affected site should be applauded by all. However, the working definition of sustainability often differs depending on one’s practice and technical perspective. In many ways, the term “sustainable remediation” is somewhat of a paradox in that we don’t want chemical impacts on soil and groundwater to be “sustained,” and our goal is to quickly reduce the threat of a contaminant occurrence on potential environmental and human receptors. But with many, if not most so-named sustainable remedies and approaches, time matters, and often, substantial time is necessary for site mitigation to be completed.

For treating contaminated groundwater as an example, the devised “sustainable” remedy performs under ambient or hydraulically passive conditions that require no input of energy (e.g., groundwater pumping) to move and treat the affected groundwater in situ. That is, remedies are developed whereby physical, biological, or chemical processes are applied to, or enhanced within, a groundwater plume so that contaminants migrate into and through the applied or engineered subsurface treatment zone (which is often composed of natural materials that provide necessary chemical reduction) under normal, and not forced, groundwater flow conditions. Providing that the contaminated water is in contact within the “enhanced” treatment zone in an aquifer for a sufficient amount of time (i.e., a sufficient “residence time”), the contaminant mitigation process – e.g., chemical destruction, reduction, or change of toxicity – would be considered a success if the plume is mitigated to the intended risk management level before it migrates off-site or reaches an environmental or human receptor.

Sustainable remedies, including bioremediation (enhancing specific microbial processes in an aquifer), phytoremediation (using plants to capture or reduce the mass of dissolved chemicals), or one of many passive chemical manipulation processes (e.g., via geochemical oxidation or reduction) often take a very long time – years to decades – to adequately mitigate a groundwater plume. This is because of the great amount of time (that is, many decades) for a chemical source to desorb from earth materials and migrate with slow moving (often at velocity rates of tenths of a foot per day) groundwater. Because of the extensive time component that may be necessary to treat a chemical plume under this construct, two critical areas of awareness should be raised.

First, we should be cautious that risk management priorities are not being diminished for the sake of remedying “greenness.” That is, the first priority should always be to consider the risk management needs of a site and, if environmental and human receptors are in harm’s way, to work within the appropriate laws, regulations, and societal needs to reduce the risk quickly, effectively, and economically. Substituting “green” or natural and sustainable means in place of appropriate risk mitigation needs should not be our pursuit.

Secondly, and the primary purpose of this blog, is the concern that “sustainable remedies” may not necessarily be durable or resilient if hydrological, biological, or chemical conditions change, either chronically or acutely, as climate-induced shifts become dominant. Over the 40 years since the US Superfund Law was enacted, passive in situ remedies have developed, have improved, and have shown themselves to generally be successful in mitigating at least some of the chemical mass that has been released to soil and groundwater. For example, the first commercial permeable reactive barrier, or PRB, for treating chlorinated volatile organic compounds, such as trichloroethylene (TCE) was installed at a northern California industrial site in late 1994 and continues to successfully remove contaminants from groundwater now nearly 30 years later. The concept of “sustainable remediation” had not yet become part of the industry lexicon. Still, the site was designed[1]to perform even if hydraulic gradient and groundwater flow conditions changed for any reason. However, it is not clear, as there is no known regulatory mandate, that all such remedies consider changes to hydrology, biology, or chemistry that may accompany changes to climatic conditions – changes that may alter groundwater flow conditions, allow seawater to rise and affect coastal groundwater quality, or change subsurface temperatures that would affect biological and phytoremedial schemes.

Due to this concern, we have considered the potential changes to “hydrogeobiochemistry” of the groundwater system that may be forced due to systematic environmental shifts. Science has already assessed how hydrologic shifts in certain environments can affect reduction-oxidation chemistry, for example. We know how excessive or reduced rainfall recharge can influence the dissolved oxygen content of the subsurface – a key condition that affects the performance ability of many in situ remedies, including bioremediation and metal-enhanced dechlorination (i.e., using granular zero-valent iron to degrade dissolved chlorinated solvent contaminants). And we have a long history of monitoring groundwater quality and salinity shifts in coastal settings where phytoremediation and other in situ remedies may be considered. But to our knowledge, we have seen little change to the historical remediation design approach. The new paper in the journal Remediation titled: Climate-influenced hydro biogeochemistry and groundwater remedy design: A review is authored by researchers from the United States., The United Kingdom and Australia have attempted to put into perspective these considerations. An excerpt from this new paper follows below; the full citation and DOI index are provided for Open Access retrieval.

If a so-called “sustainable remedy” cannot stand up to changes in its environmental setting, the intended purpose of “sustainability” cannot be achieved. In this respect, perhaps we should reconsider this term and ask ourselves if “Durable and Adaptable REmediation” is more representative of our pursuit or whether that is too much of a “dare” to the remedial practice status quo.

 “The process of designing a remedy for contaminated groundwater historically has not commonly included climate-future, hydrologic, and biogeochemical aquifer characteristics. From experience, the remedy design process also has not consistently nor directly integrated or projected future hydrologic and biogeochemical effects of the human-induced or developed environment—aka the anthropogenic influence—on potential remedy performance. The apparent practice of (1) not regularly assessing anthro-influenced hydrological (termed here as anthrohydrology) or biogeochemical characteristics (collectively hydro biogeochemistry) of a site and (2) rarely accounting for future climatic shifts as design factors in remedy design may be due, in part, to the general practice-level view that groundwater remediation systems (whether in situ or ex-situ) have seldom been anticipated to last more than a few years (or one or two decades at the most). Second, methods to reliably and quantitatively estimate site-specific, climate-future shifts in groundwater conditions using global and/or regional climate models and the resultant impacts on contaminant plume characteristics have not been readily available.

The authors here suggest that while the concept of remedy design resilience and durability, within an envelope of climate change and anthropogenic influence, has been discussed in some technical circles as a component of “sustainable remediation,” we have found that direct application of these technical concepts in quantifiable terms remains rare. By incorporating the potential influence of future hydro biogeochemical scenarios into remedy design, however, the design process could account for reasonable climate-induced influence on the groundwater system for a given site. These scenarios could then be applied within the remedy selection process to assess performance durability under potentially changing hydrologic, biological, and chemical conditions.”

“We recommend that the gap between contaminant behavior and remediation performance under climate and anthropogenic influence should be addressed to deliver greater confidence that remedies will provide durable improvement in groundwater quality under various environmental and climatic conditions. Reassessing the CSM by more completely acknowledging the influence of anthropogenic behavior and processes on the hydrologic and biogeochemical environment will undoubtedly lead to more questions. Still, it will also promote a more complete understanding of the total environment for which contaminant remedies are intended to successfully perform until contaminant risk has been mitigated.”

The full article can be found in the Open Access version of the journal Remediation published by Wiley:

Warner, S. D., Bekele, S., Nathanail, C. P., Chadalavada, S., and Naidu, R. (2023). Climate-influenced hydrobiogeochemistry and groundwater remedy design: A review. https://doi.org/10.1002/rem.21753

Reference

[1] Warner, Scott D. "Two decades of application of permeable reactive barriers to groundwater remediation." Permeable Reactive Barrier Sustainable Groundwater Remediation; Naidu, R., Birke, V., Eds (2015): 25-39.