Tackling Tough Contaminants
Successful remediation of recalcitrant compounds starts with comprehensive site characterization
Webster's defines recalcitrant as "to be stubbornly disobedient, obstinately defiant of authority or restraint, difficult to manage or operate, not responsive to treatment."
There are numerous case studies of failed remedial systems, which in turn can be linked to the remedial design team not fully understanding the site conditions. Often the most effective way to clean up these recalcitrant sites is source/migration control rather than intrusive remediation. The best solution can only be determined if the site is properly understood.
When it comes to environmental releases, recalcitrant compounds include chlorinated solvents, methyl tertiary butyl ether (MTBE), and metals such as hexavalent chromium (CrVI), arsenic, etc. Chlorinated solvents such as tetrachloroethylene (PCE); trichloroethylene (TCE); and 1,1,1 trichlroethane (1,1,1 TCA) are recalcitrant because they are relatively insoluble and they are denser than water, and therefore in the pure phase these solvents will migrate in the subsurface by density rather than groundwater flow direction. Although these compounds are relatively insoluble in water, they are highly soluble when compared to drinking water standards. A small amount of chlorinated solvents in the subsurface can contaminate a large amount of groundwater, often making successful remediation difficult to achieve.
As for the metals CrVI and arsenic, they are found in various forms that exhibit variable levels of toxicity and mobility. They are also subject to stringent drinking water standards, which create remedial challenges as well.
The need to attain low compliance criteria for recalcitrant compounds in turn drives the need to better understand the site so that informed decisions can be made and a successful remedial approach can be selected. The few complications listed above, concerning the solubility of chlorinated solvents and the toxicity and mobility of CrVI and arsenic, make it clear that spending significant sums of money on a remediation system at a site that is not well understood will ultimately be a waste of money. Pump-and-treat systems, once the preferred remedial approach, are no longer regarded as viable solutions for chlorinated sites because they often do not remove all of the contaminant. Even if a remedial system removes the majority of the contaminant mass, the minor fraction remaining can continue to create non-compliant conditions or pose risk to human health and the environment.
Not only are recalcitrant compounds difficult to understand and remediate, environmental liabilities are often viewed by industry as a regulatory requirement that only negatively effects the bottom line. Cleaning up contamination from a process used generations ago at a site that might not even be actively used, or was simply acquired in a merger, does not garner the corporate budget and resources that a product that generates revenue can. Consequently, consultants are regularly pushed to restrict costs in order to solve the problem, which can lead to inadequate remediation plans. But subsurface contamination can have serious impacts on business, both financially and in regards to public perception. Many of the recalcitrant compounds are considered carcinogens, creating health risks for people. Companies can be sued for millions of dollars and corporate images can be tainted. Recent Hollywood films such as A Civil Action and Erin Brokovich have exemplified such a turn of events.
The Solution: Comprehensive Site Characterization
Comprehensives site characterization, rapid adaptive site characterization, and "the triad approach" are all variations of the same thing. The U.S. Environmental Protection Agency's (EPA) triad approach incorporates systematic planning, dynamic work plans, and real-time measurement technologies. Employing these three aspects to site characterization provides sufficient data to develop a comprehensive conceptual site model that provides a thorough understanding of site risks, along with the need for and appropriateness of remedial measures. In sum, this approach to site characterization eliminates uncertainty, which translates into cost savings. This becomes increasingly important when dealing with recalcitrant compounds such as chlorinated solvents.
The first aspect of the comprehensives site characterization approach, systematic planning, involves the development of a preliminary conceptual site model (CSM). This model is based on any existing data for the site and research (e.g., review of geologic maps, regulatory file reviews, etc.). The CSM is used to determine data gaps and make assumptions about the site, which allows the investigators to develop a work plan to fill data gaps and test assumptions.
A dynamic work plan, the second aspect of this approach, is not focused on addressing particular locations. Instead, explorations are guided by the data collected and therefore exploration locations, although originally planned, are adjusted in the field in order to acquire the necessary data to achieve the site goals. Traditionally, subsurface data is collected through several field investigation programs carried out over an extended period of time. Using a comprehensive approach and a dynamic work plan, all the necessary data are collected in a single field mobilization, which reduces the project timeline and ultimately reduces the project cost.
The use of real-time measurement technologies is the third aspect of the comprehensives site characterization approach. On-site analytical tools are a requirement of this approach because the dynamic work plan is directed by the data, which is developed in near real time. On-site analytical tools include field gas chromatographs (GC) for volatile organic compounds and petroleum constituents, and portable X-ray fluorescence (XRF) analyzers and spectrophotometers for metals. Samples are collected, delivered to the on-site field lab, prepared and analyzed, and the results are communicated to the field scientist monitoring explorations. This type of data analysis and reporting guides investigations. During the field program, the vertical and lateral limits of contamination are defined, site geologic and hydrogeologic properties are defined, and the conceptual site model evolves and becomes far more comprehensive. Monitoring well locations and depths are determined, data gaps are filled, and assumptions are adjusted to reflect actual site conditions. In the end, the uncertainty involved in remedial design and cost estimates is minimized, the project timeframe is reduced, and money is saved.
Case Study: Cleaning up a Former Paint Pigment Factory
The comprehensive site characterization approach's ability to remove uncertainty and reduce cost can be seen in the following case study from 2001, which involves a former paint pigment manufacturing facility.
Hexavalent chromium and lead contamination impacted about 30 acres on the site, which was in operation from the mid 1800s until the early 1900s. From the time of its discovery in the late 1970s until 2000, traditional investigation costs were estimated at approximately $1 million, which included installation of 138 monitoring wells and piezometers, multiple sampling events, and reports. Even after more than 20 years of evaluation, there were still many unanswered questions regarding groundwater flow paths, source areas, plume limits, etc. The most logical remedial solution involved a passive reactive barrier at the downgradient limits of the site in order to address the contamination. The preliminary estimates on the barrier were on the order of $5 million net present value.
Based on the numerous data gaps that existed, a conceptual site model and a work plan to address the data gaps and/or data uncertainties (e.g., vertical limits of CrVI plume) were developed. The Waterloo Profiler®, a state-of-the-art drive point tool that provides the capability to continuously sample groundwater for on-site analytical characterization with negligible sample cross contamination in even extremely high-concentration settings, was used in conjunction with an on-site mobile laboratory with a spectrophotometer to implement the dynamic work plan. Data were provided in "real-time," which guided the investigation. During the investigation, two unknown source areas were identified and removed and a portion of the CrVI observed was confirmed to be originating from an off-site source.
Ultimately, this program allowed the removal of key source areas that were previously unknown, determined that many well clusters at the site had been installed in such a manner that they "missed" the plume, and allowed for proper well placement to monitor the plume. Source remediation was completed in 2002. Since then the CrVI concentration has diminished dramatically across the majority of the site and the anticipated remedial approach is monitored natural attenuation.
The fieldwork, data analysis, and reporting were completed in less than a year at a cost of approximately $400,000. This represents a 60 percent cost savings compared to the traditional investigations conducted to date. However, the real value was that the client did not need to implement a multimillion dollar remedial system. If this system was installed without the knowledge gained during this investigation, the two source areas would have remained and the passive system would be necessary into the foreseeable future. As a result of this comprehensive site characterization approach, the only capital expenditure being proposed is regular groundwater monitoring to ensure natural attenuation is occurring. Furthermore, it is anticipated that the monitoring frequency will be reduced and may ultimately be discontinued over time.
Additional Case Studies
Six case studies of the successful implementation of the triad approach are documented in Technical and Regulatory Guidance for the Triad Approach: A New Paradigm for Environmental Project Management. These case studies document successful cleanup projects ranging from $300,000 cost savings at a dry cleaner site in Florida to $45 million cost savings at the Pine Street Barge Canal.
When dealing with recalcitrant compounds, investing in comprehensive site characterization will result in cost savings during remediation. As stated earlier, the best solution can only be determined if the site is properly understood.
This article originally appeared in the 11/01/2004 issue of Environmental Protection.
Russell A. Schuck, PG, is a senior hydrogeologist at Haley & Aldrich Inc. in Boston. He has more than 15 years of experience in groundwater investigations and remediation and has worked with the Waterloo Profiler and dynamic field programs on many projects over the last ten years.