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Microchemical sensors developed at Sandia National Laboratories were tested at the Nevada Test Site as part of the Advanced Monitoring Systems Initiative program. Two sensors, the chemiresistor sensor and the surface-acoustic-wave (SAW) sensor, were evaluated in the tests. Both sensors rely on sorption of chemicals onto polymer films to produce a change in an electrical signal that can be recorded and calibrated, but different transduction mechanisms are used. The primary purpose of the tests was to evaluate the feasibility of using these devices in potentially long-term, unattended applications such as long-term monitoring of subsurface contaminants. A complete monitoring system was developed that provided real-time monitoring of the sensors via the internet. Engineering issues such as sensor packaging, data acquisition, power requirements, and telemetry were addressed during the development and testing of the sensor systems. In addition, issues such as data processing, noise, and interferences from fluctuating environmental variables were also encountered and evaluated during the field tests. Results showed that both sensors could be operated remotely and continuously for long-term monitoring applications using commercial data-acquisition systems and custom-designed packaging. Both the chemiresistor and SAW sensors experienced drift in the signal and were impacted by fluctuations in temperature and humidity. However, results from the chemiresistor showed that exposure to large concentrations of contaminants (e.g., trichloroethylene) overwhelmed the fluctuations caused by temperature and humidity variations. Results also showed that the chemiresistor sensor exhibited better stability and sensitivity than the SAW sensor for the conditions and analytes that were tested, which was contrary to initial theoretical predictions. a a a a a

Field Demonstrations of Chemiresistor and Surface Acoustic Wave Microchemical Sensors at the Nevada Test Site