offers several advantages relevant to long-term management, including less sensitivity to spatial/temporal variability and correspondence with screening models that attempt to correlate source zone mass removal with downgradient plume behavior.

Several recent reviews have explored the relative performance of various techniques for measuring mass flux, which appear to be highly site-specific (EPA, 2009; ITRC, 2010; Kavanaugh and Deeb, 2011). Additional field research is needed to support the more widespread adoption of flux-based performance metrics, including (1) further clarification of the range of uncertainty associated with mass flux and mass discharge measurements, (2) continued refinement of specific aspects of the various techniques, including a better definition of the necessary preliminary site characterization, and (3) new measurement techniques.

Sensor Technology

Because existing monitoring and performance assessment tools are expensive, slow, and consist of point measurements, real-time measurements could provide data for management decisions including optimization of active remedies and assurance that either active or passive containment is effective. Recent advances in microelectronics, wireless communication technologies, and information technologies have produced potentially low-cost techniques to gather and process large amounts of data at very high spatial and temporal resolution. Such wireless sensor networking could be applied to a variety of subsurface systems.

Advances in the development of wireless sensor networks have been successfully applied to problems in infrastructure monitoring, weather and storms, volcanoes, air quality, agriculture, forestry, and ecology (e.g., Culler et al., 2004; Haenggi, 2005; Werner-Allen et al., 2006). Much of this work has highlighted the advantages of deploying a large number of inexpensive sensors to replace of a few highly accurate, but expensive sensors. While environmental monitoring has been considered an ideal application since the field’s inception, only a few projects have combined wireless sensing with subsurface monitoring, largely because of the technical difficulty and cost associated with monitoring VOCs in groundwater environments. For example, a study by EPA (2003) concluded that although a sensor might cost as little as $100 to manufacture, a fully developed multiparameter sensor suitable for long-term management applications would cost around $7,500. More recently, an ESTCP-sponsored project (Lieberman, 2007) evaluated sensors for monitoring VOCs, including Halogen-Specific Detector/Membrane Interface Probe systems and laser-induced fluorescence, based on ROST (rapid optical screening tool). However, despite some advances in detection capabilities, the relatively large costs (thousands of dol-



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