Why Won’t This Stuff Just Go Away?
Written By Keith Gaskill, L.P.G., Project Manager & Geochemist, EnviroForensics, in collaboration with Stephen R. Henshaw, P.G., President & CEO, EnviroForensics.
As seen in the December 2011 issue of Cleaner & Launderer.
Environmental cleanups are most often a complex undertaking with both soil and groundwater contamination. A certain amount of creativity is required to complete a site cleanup within acceptable timeframes, regulatory requirements, and of course, budget.
Ideally, once a cleanup begins, it ends when all contaminants have been removed. Sounds simple enough. Many times, cleanup projects start very well and appear to be heading toward closure (even under budget) but the cleanup appears to stop working.
Why did it stop working?
Among the challenges requiring creativity in cleaning a site is getting the last bit of contamination out of the ground. Oh sure, the first 90 to 95 percent of the contamination is removed very quickly and effectively and this makes everyone happy. But…as time passes, the effectiveness of the cleanup seems to dwindle making everyone unhappy. The level of contamination may continue to slowly decrease, but never quite get to closure levels. So, with the time and money still running, and the levels of contamination very slowly decreasing; this leads to a curve expressed below:
This is termed an “asymptotic curve,” a line that gets closer to zero with time, but never quite touches zero.
It’s not so much that it stopped working, as it is that it reached a point where it is no longer cost effective to continue. We’re no longer getting our best return of our cleanup dollars. As an example, perhaps the soil impact is stuck in the pore spaces between sand grains and applied vacuum can no longer reach it. If soil impact is continually loading contaminants to groundwater, a balance between continued release to the groundwater and cleanup will not be met. Asymptotic cleanup concentrations are common, and with luck the asymptotic level will be reached below regulatory cleanup levels… but that result is uncommon.
What went wrong?
The difficulty with the selecting a cleanup is that we are asked to make detailed and expensive decisions based on a limited amount of information or data. Data is gathered from soil borings and monitoring wells that get inferred for some distance across the site. It is simply not cost effective to complete a soil boring or collect a groundwater sample every one square foot section of a site area. A consultant must learn to manage a certain level of uncertainty and select a cleanup technology that best fits the results of the site investigation. A good consultant knows when the level of uncertainty is too high and will need to gather more data to properly design a cleanup plan.
Do we need to change the cleanup technology?
With a multitude of remediation technologies available for use, it’s certainly important to follow a logical, yet creative path toward the right selection. With all remediation technologies, getting to the contaminants in the soil and groundwater is the number one goal. Without that, the technology chosen makes no difference and will be ineffective. In some cases, it’s required to plan a change in the type of technology being used over time to best address the level of contamination present.
For instance: Technology A is used because the level of contamination is high and it effectively drops the level of impact to a low to moderate (but not low enough) concentrations. Technology A is no longer suited, and is ineffective in treating low concentrations which leaves impact in the subsurface and closure cannot be reached (asymptotic level has been reached). So we change to Technology B at this point to treat the lower concentrations to a level and therefore closure can be attained.
How can we get to closure?
Asymptotic curves present a very real challenge to attaining closure. Cleanup projects are expensive, such that decisions made on the way to cleaning a site need to be made with closure in mind. Closure is met when state or federal agencies and property owners agree that the concentration of the contaminants meet their requirements.
Closure may be met when all the concentrations of the contaminants are below cleanup levels and therefore, additional risk to human health and the environment has been removed. This is an ideal situation and may not always be attainable. This condition is can be very costly and the responsible party may not have the funds to reach a “regulatory clean” site.
Many state agencies have moved to a “risk based closure” system. In this type of closure, the risk to human health and the environment is managed by eliminating exposure pathways to people and demonstrating that the contaminants are not posing any significant threat. This “leave in place” option may require a deed restriction stating that potable water wells cannot be drilled onsite or that the site can only be zoned commercial or industrial. Risk based closures may also require an environmental restrictive covenant requiring long term action such as keeping a competent asphalt cover over impacted soil or long term monitoring to assure regulators that a groundwater plume remains stable.
The ultimate goal of any cleanup project is to reach closure in the most cost effective and least time consuming fashion, while still being protective to people and the environment. By utilizing creative application of cleanup technologies and modern risk-based methods of closure, the asymptotic concentration curve is not as problematic as it once was.