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of the device as mass is accumulated (micro-cantilever devices). These devices are
very sensitive to the presence of various volatile and semi-volatile chemicals. How-
ever, the sensitivity of the device depends on high-frequency excitation or extremely
small changes in shape; these aspects have not been tested in prolonged geologic
environments.
Finally, the fourth category of devices reviewed was optical sensors. These include
fiber optical sensors, colorimetry, and infrared sensors. These sensors rely on changes
in electromagnetic radiation (e.g., visible, infrared) to detect and identify the presence
of chemicals. The sensitivity of these sensors to VOCs can be good, and a TCE fiber
optical sensor integrated with a cone penetrometer already exists. Its use in long-term
applications still requires testing. Colorimetry is a simple and quick method to detect
changes in color in solutions mixed with the sample, but it requires manual interven-
tion. The infrared sensor appears to be useful for detecting combustible hydrocarbons
(e.g., methane, propane), but the devices reviewed were not amenable for real-time,
in-situ applications involving other lower volatility VOCs.
The most viable sensors for in-situ chemical sensing appear to be electrochemical
sensors, specificallyconductometricsensors, basedontheirsimplicityandrobustness.
Reports from the U.S. EPA (1992, 1995) have indicated that polymer-absorption
and metal-oxide-semiconductor sensors are viable candidates for use at underground
storage tanks. In addition, fiber-optic sensors and mass sensors (SAW devices in
particular) also appear to be viable candidates for in-situ applications. The general
issue among all of these sensors is that few, if any, have been tested and demonstrated
in long-term geologic environments.
In U.S. EPA(1992, 1995), they report that interference from water and methane are
concerns for sensor technology because they can trigger false positives in geologic
environments; therefore, tests on potential candidate sensors should be performed to
determine if methane and water vapor significantly impact the signal from the sensors.
In addition, the ability to discriminate among different chemical species needs further
investigation among the polymer-absorption and SAW sensors, especially in geologic
environments and applications. The ability to retrieve quantified information from
the in-situ sensors such as contaminant characteristics and location may prove useful
for the end user to make informed decisions regarding treatment and remediation.

ACKNOWLEDGMENTS

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed
Martin Company for the United States Department of Energy’s National Nuclear
Security Administration under contract DE-AC04–94AL85000.

REFERENCES
Berger, T., H. Ziegler, and M. Krausa, 2000, “Development of Electrochemical Sensors for the Trace
Detection of Explosives and for the Detection of Chemical Warfare Agents,” SPIE Proceedings,
Vol. 4038, p. 452, AeroSense 2000, April 24–28, 2000, Orlando, Florida.