High resolution site characterization: the path to successful remedies and reduced cost

Full text: The passage of CERCLA in the United States in 1980 created, nearly instantly, what is today a $9 billion annual industry in that country which is still growing. Knowledge of the fate and transport of industrial chemicals in the subsurface was extremely limited in the 1980s and most of the early practitioners in the new industry came from, or were influenced by, the water supply and wastewater disposal fields. The conceptual models and scales of measurement from these fields were inappropriate for assessment and remediation of chemicals in the subsurface as recognized by a few early pioneers like C.V. Theis and John Cherry. These models and approaches became embedded in the field and are still prevalent today. In the 1980s, pumping and treatment (P&T) of groundwater comprised between 80% and 90% of Superfund groundwater remedies. In large measure these remedies failed to meet the goal of aquifer restoration and as of 2011 P&T comprised only 20% of remedies. Understanding of the nature of contaminant behavior in the subsurface has increased tremendously, largely due to academic research, and new, more effective investigatory and remedial technologies have been developed. The 14^th Edition of the Superfund Remedy Report (November 2013) stated that "…the continued increase in the selection of in situ groundwater technologies suggests a role for recently-developed characterization techniques, such as high-resolution site characterization (HRSC)". HRSC has great promise to shorten project life cycles, improve Conceptual Site Models, increase remedy effectiveness and reduce life cycle costs. Data which are Insufficient in terms of quantity, proper placement and sample scale (usually too large) lead directly to erroneous Conceptual Site Models which in turn lead to bad decisions and remedies that either don't work, underperform or are overdesigned. These situations result in the waste of money and other resources, a lack of protectiveness, as well as failing to alleviate the owner's liability. HRSC requires making measurements at scales that encompass the spatial structure of the key variables (e.g., hydraulic conductivity, hydraulic head, contaminant concentration, geochemistry etc.), and while direct sensing tools are an important part of HRSC, these tools alone are insufficient. HRSC requires small sampled volumes and close sample spacing of both high and low permeability media, in the source zone and in the dissolved plume. It may be necessary to identify all of the contaminant phases: Non Aqueous Phase Liquid (NAPL), solute, gas and sorbed. It is critical that data sets provide clear insight into the mechanisms sustaining the plume. For example, is NAPL still present in the source zone? If so, where is it located and how much is present in a relative sense? Is a significant quantity of contaminant mass present in the immobile pore water in low permeability layers within the flow system? If so, is this mass located throughout the footprint of the dissolved plume? What fate mechanisms are significant: biodegradation, abiotic degradation, hydrodynamic dispersion, retardation etc.? HRSC is best applied using transects normal to the direction of transport. Application of Triad Approach principles reduces the time and costs associated with HRSC. This means using real time measurement tools, managing and interpreting data in near real time, using dynamic work strategies and a life cycle CSM. Another important concept borrowed from the Triad Approach is the use of collaborative data platforms. This essentially means using screening level toos such as MIP and LIF to reduce the overall spatial uncertainty associated with the data and then using definitive tools such as the WaterlooAPS, soil coring and onsite laboratory analyses to manage the analytical uncertainty and the relational uncertainty. This approach can both speed up site characterization and lower the cost. Appropriate use of technologies such as Waterloo^APS, Membrane Interface Probe, LASER Induced Fluorescence, on-site laboratories, geophysical techniques, detailed chemical profiling of aquitard materials and others facilitate the collection of the types and density of data which allow for the level of understanding of site conditions which is required for successful and cost effective selection, design and implementation of the combinations of remedial technologies required to meet remedial goals. In addition, the ability to adequately determine the flux of contaminant mass across key planes is taking on greater importance in terms of assessing the effectiveness of remedies as well as assessing the level of risk posed to potential receptors. Research has shown that the vast majority (e.g., 75%) of contaminant mass flux occurs through a very small portion (e.g., 5 to 10%) of the plume cross sectional area. Identifying these zones allows for targeted remedial efforts to control mass transport, both reducing cost and increasing protectiveness. HRSC results allow for more "surgical" application of remedial technologies thereby making remedies more cost effective. However, application of a single "high resolution" technology does not result in a high resolution investigation. Multiple tools and techniques are required to address the various questions posed by a given site. (author)