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Remedial Investigations and Feasibility Studies

Benefits of using a Triad approach to remedial investigations (RI) and associated feasibility studies (FS).

Remedial investigations (RI) and associated feasibility studies (FS) offer several opportunities for realizing benefits from a Triad approach. The primary purpose of an RI is to determine contaminants of concern and establish whether they pose unacceptable human or ecological risks. The primary purpose of feasibility studies is to provide the information necessary to correctly select a remedial alternative that will address contamination concerns.

A Triad approach to an RI can contribute in the following ways:

• Human/Ecological Risk Assessment Support. Because of the critical nature of risk conclusions for sites developed as part of the RI, the data contributing to a risk assessment must be able to withstand scientific scrutiny and potential legal challenges. Risk assessments are based on maximally exposed individual scenarios. This presupposes that information is available that quantifies "worst case" contamination scenarios, which in turn assumes that sufficient characterization information exists to identify areas of a site that present worst case scenarios. In this setting, limited analytical data, even if it is of the highest analytical quality, can lead to incorrect conclusions if areas of concern or contaminants of concern are missed because the CSM is incomplete. The Triad provides a cost-effective way of increasing data collection density and hence decision quality, without compromising overall data set quality.
  

  The Triad does this in three ways. First, by emphasizing CSM development as part of systematic planning, the Triad can produce judgmental sampling programs that are more likely to target and identify areas/contaminants within a given exposure unit that pose the greatest concern and/or represent the greatest uncertainty regarding the presence of contamination.

  Second, a Triad approach can assist in refining the CSM while data collection work is underway in ways that greatly improve the risk assessment process. For example, a Triad approach involving real-time measurement techniques can help determine if contamination is isolated to specific horizons in soils or sediments, and consequently focus follow-on sampling and analysis on those horizons. In a similar fashion, a Triad approach with in situ direct push probes can be used to identify likely preferential horizontal flow zones for contaminated groundwater movement, and focus groundwater sampling on those zones. Once judgmental sampling bounds contaminated areas, random sampling within those areas can supply samples that are sent off-site for more rigorous analyses. Statistical parameters (e.g., the mean and upper confidence level) for each distinct population can then be calculated from this data set, and then mathematically combined (if desired) by area or volume weight averaging to produce a statistically defensible (and highly confident) mean concentration for the area of concern or entire exposure unit.

  Third, through the judicious application of cheaper real-time measurement technologies, Triad-based sampling programs can increase overall spatial coverage for a site, using real-time measurement results to flag sampling locations where more rigorous data are required (i.e., locations that pose the greatest contamination concern). This in turn reduces the likelihood that relatively small but important areas of concern might have been missed.

• Contamination Extent Estimation. Preliminary estimates of contamination extent are crucial to feasibility study data needs, to an accurate evaluation of remedial alternatives, and to the development of a proposed plan. Traditional sampling and analysis programs are not very effective for estimating contamination extent. The number of samples that can be collected and analyzed is limited by the costs of standard fixed-laboratory analyses for most contaminants of concern for sampling programs that rely solely on these techniques. Sample number constraints prevent estimating contamination extent with any reasonable level of certainty. Contamination may be encountered that extends horizontally or vertically beyond preplanned sampling grid boundaries. Unbounded contamination footprints also complicate contamination extent estimation.

  A Triad approach can provide sampling data sets that consistently bound contamination footprints and that are of sufficient density to allow accurate contamination extent or volume estimation. Real-time measurement techniques combined with dynamic work strategies allow the progress of sampling to adapt to contamination footprints encountered during the course of work. Mixing cheaper, less rigorous analytical techniques with more expensive traditional methods increases the density of sampling possible, and reduces contaminated volume estimation errors. What is important to remember is that for the purposes of establishing contamination extent, accurately quantifying contamination levels at particular locations is not as important as being able to support yes/no decisions, i.e., at a particular location data indicate it is unlikely that contamination exists at levels of concern, or vice versa.

• Feasibility Study Support. Feasibility studies are often part of the remedial investigation process. Feasibility studies provide data that become part of the basis for evaluating the potential efficacy of remedial alternatives. Feasibility studies span the range of simply collecting media samples for bench scale testing, to implementing pilot technology verification programs. The Triad approach can be used to enhance the performance of feasibility studies. For example, in the case of media collection, real-time measurement technologies can provide information quickly enough to assist in selecting samples that have the characteristics necessary for a successful bench scale test (e.g., contamination present at levels that are of particular interest).

  In the case of pilot studies, real-time measurement programs can be used to fine tune and/or modify the pilot while it is in progress, increasing the likelihood that the pilot will produce useful results. The ability to monitor performance parameters for a pilot project is particularly important, since by their nature pilot projects involve remediation technologies whose site specific performance characteristics are initially poorly understood. For example, suppose a pilot involved the injection of a chemical to enhance in situ degradation of subsurface chlorinated solvents. The ability to cheaply monitor in "real-time" subsurface response to the intervention (e.g., movement of the chemical, reduction of contamination concentrations, presence of reaction products that might be of concern, etc.) at key monitoring points would provide significant insight into the likely performance of a fully deployed system.

• Method Applicability Studies. As will be described in the next section, the greatest cost savings that can be accrued from implementing a Triad approach occur during remedial design and implementation. This presumes, however, that real-time measurement technologies have been identified and their performance established for the contaminants of concern in the context of environmental conditions present at a particular site. The RI/FS provides the perfect opportunity for generating the types of measurement performance information (e.g., detection capabilities, interference concerns, precision, optimal sample preparation and cleanup methods, etc.) that will be critical to the successful implementation of a Triad approach during remediation. This information is typically produced through a carefully considered method applicability study that is integrated with overall RI data collection activities.

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