suggested that annual savings of a few hundred to tens of thousands of dollars might be achievable, particular for sites where more than 50 samples are collected and analyzed annually (EPA, 2004). EPA subsequently issued a “road map” to assist managers with developing a site-specific LTMO program (EPA and USACE, 2005), including user-friendly software tools. Although the underlying concepts are fairly well established, additional documentation of successful case studies would clarify the range of potentially achievable cost savings.

Monitoring of Source Zone Contamination

The successful design of a source zone remediation program depends on sufficiently detailed knowledge of the spatial pattern of immobile source materials. A number of recent reviews have evaluated the variety of tools available to quantify the magnitude and spatial distribution of DNAPL (e.g., NRC, 2005; Mercer et al., 2010). These tools range from low-cost methods to infer the presence of DNAPL (as reviewed by Kram et al., 2001) to more extensive methods designed to delineate the spatial distribution of NAPL saturation to guide source zone remediation (e.g., Saenton and Illangasekare, 2004; Moreno-Barbero and Illangasekare, 2005, 2006). For the latter purpose, the partitioning interwell tracer test (PITT) has proven to be relatively effective (e.g., Annable et al., 1998; Brooks et al., 2002), although its deployment is hindered by high cost and need for relatively sophisticated interpretive tools.

As it is unlikely that complete removal of contaminant source material will be feasible for many complex sites, the transition to long-term management will depend not only on the amount of source mass removed, but on the rate at which mass is transferred between the source and plume compartments during the post-remediation period. One of the most promising recent developments in source zone management is the development of tools for measuring contaminant mass flux, either at localized monitoring points or as an integrated mass discharge across a control plane. Such knowledge of contaminant discharge is particularly useful in evaluating the potential for downgradient natural attenuation processes.

Conceptually, contaminant discharge is a calculated parameter that reflects both temporal and spatial averaging of the product of groundwater discharge (length per area per time) and contaminant concentration (mass per volume). Field methods include synoptic sampling (e.g., Einarson, 2006), passive flux meters (Annable et al., 2005; Basu et al., 2006), steady-state pumping (e.g., Buschek, 2002), recirculation flux measurements (Goltz et al., 2007), integral pumping tests (Bockelmann et al., 2001; Bauer et al., 2004), and modified integral pumping tests (Brooks et al., 2008). The use of flux measurements as an alternative to concentration-based metrics

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