Mother nature knows best

Sites that distribute, handle and use petroleum products are everywhere. Not surprisingly, sites contaminated with petroleum LNAPLs are among the most common class of brownfields worldwide, ranging in size from the corner gas station to large industrial properties like Willow Run. Determining how to best manage the legacy contamination on these properties is a challenge with widespread global reach. However, we often focus on managing these sites solely in terms of getting released product back out of the ground to the maximum extent practicable (MEP) without much concern for whether it is an activity that will provide any beneficial reduction in risk. Even worse, we rarely consider whether narrowly focused management strategies might end up creating more contamination and risk than they can realistically mitigate.

Concerns about climate change and sustainability are all around us. Although their actions don’t usually (ever?) bear this out, even my young(ish) children understand the concept of reducing our footprint and trying not to make things worse! Despite the increasing pervasiveness of these ideas in society, it remains rare to see them find their way into our remediation work in a meaningful way.

Can we incorporate these ideas into our management of petroleum NAPL sites to implement strategies that are protective of human health and the environment and more sustainable? That’s what we did at Willow Run. All it takes is an openness to the significance of natural petroleum NAPL biodegradation, reframing MEP in that light, and broadening our consideration of costs to include things like environmental footprint and remedial risk.

Traditionally, aggressive intervention has been the favored approach at LNAPL sites. For years the go-to remedies were focused on aggressive source removal (actual or perceived), such as “dig-and-dump" or pump-and-treat. Other newer methods include energy/resource intensive options such as high vacuum multi-phase extraction or in-situ thermal treatment. While all these remedies can be effective at achieving specific site goals, they also can be costly, carry a significant carbon footprint, and can generate a fair bit of risk to workers and communities in their own right.

Beyond dollars and cents, selection of an appropriate remedy should also consider the environmental and social costs. From an environmental perspective, the remedy “costs” include carbon footprint, energy intensity, water intensity, waste generated and/or disposed, and associated direct and indirect emissions. From a social perspective, the remedy may have an impact related to the footprint in a given community, may delay or prevent transition of the site to beneficial community use, not to mention environmental justice issues that may arise based on the relative impact to under-served communities. For example, soil may be moved from more affluent to less affluent communities depending on the location of the disposal facility, effectively transferring the environmental impact from one jurisdiction to another. Engineered remedies can obviously carry a significant financial cost, but consideration of these other factors makes it clear they’re also far from being “free” in terms of their social and environmental impacts.

Another consideration: The Willow Run site perfectly illustrates how the implementation of appropriate controls can address all concerns related to exposures or contaminant migration with even a very large LNAPL site. And a large proportion of petroleum NAPL sites will be in the same boat. Does it not follow then that this large proportion of LNAPL sites can be made safe for reuse without incurring the potentially significant economic, social and environmental costs of engineered remediation systems?

Mother Nature isn’t exactly helpless in dealing with contaminants introduced through human activity – particularly organic materials such as petroleum LNAPLs.

"In a process called Natural Source Zone Depletion (NSZD), naturally occurring microbial communities directly consume LNAPL, sometimes at rates that rival or exceed engineered systems, without any human or mechanical intervention."

In addition, microbes are able to access contamination in small or isolated pore space that would be difficult or impossible to access via conventional remedial approaches. And microbes don’t require engineering, heavy machinery, or huge amounts of power to get the job done. This was a big part of the strategy at Willow Run: confirming that there was robust NSZD activity and using that as a basis for assessing the net benefit and costs of other alternatives over NSZD alone.

We know there is little benefit in our efforts to recover LNAPL at many sites in terms of saturation reduction or risk mitigation. We find most LNAPL areas to be stable in extent, having long ago lost their ability to expand into areas that aren’t already contaminated. Where we can remove LNAPL from the ground, it is often a small fraction of what is left behind with its composition unchanged. This was the case at Willow Run; although there was an extensive LNAPL-contaminated footprint, it had already stabilized and the recoverability of the LNAPL was low enough that recovery efforts were likely to effect a negligible change in conditions site-wide.

Bottom line: There will be many sites where engineered systems do not provide a significant incremental benefit over NSZD alone that might balance the various costs involved in their implementation. Where LNAPL has stabilized, recoverability is low (i.e., de minimis LNAPL transmissivity and/or past the point of diminishing returns), and controls can effectively mitigate risks, NSZD will almost always be the most sustainable option that carries the least remedial risk.

Willow Run is an example of an LNAPL site where the implementation of a site management strategy based on NSZD and controls was technically appropriate, allowed significant costs to be avoided, and allowed the return of the site to beneficial use much sooner than would have been realistically possible otherwise contaminated. Where we can remove LNAPL from the ground, it is often a small fraction of what is left behind with its composition unchanged. This was the case at Willow Run; although there was an extensive LNAPL-contaminated footprint, it had already stabilized and the recoverability of the LNAPL was low enough that recovery efforts were likely to effect a negligible change in conditions site-wide.

My experience tells me there are a lot of Willow Runs out there.

Meet the Author

Matt Rousseau is a Senior Environmental Engineer and NAPL Technical Lead with GHD based in Canada. Matt has been consulting for over 20 years, with a focus on NAPL sites specifically for most of that time. His work focuses on the preparation of Conceptual Site Models (CSMs), the design of NAPL site investigation programs, the evaluation of NAPL mobility/recoverability/stability, natural source zone depletion (NSZD) assessments, and the development of NAPL remediation and management strategies with a focus on sustainable risk-based solutions. Matt regularly provides training related to NAPL behaviour, site characterization and remediation strategy having helped develop related technical guidance documents in the U.S. (ITRC LNAPLs and LNAPL Update Teams), Canada (FCSAP, Department of National Defence), and Australia (CRC CARE Technical Report 46 and associated online masterclass). Matt is currently active in ITRC teams related to NAPL site management as an author and trainer and with ASTM as a contributor to a new draft standard pertaining to the detection and quantification of NAPL biodegradation (NSZD). Matt has B.A.Sc. (1997) and M.A.Sc. (2000) degrees in Environmental Engineering from the University of Windsor in Windsor, Ontario, Canada.