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An On-Line Version of a Column First Published in:
Environmental Technology March/April 1998 Vol. 8 No. 2 Pages 14-15

By: David B. Vance

This column is about the impact of high technology on the prevention, assessment, and remediation of groundwater pollution. It falls into two realms; the first, is inexpensive real time communication systems and computer tools to manage and manipulate data, the second, advances in sensor technology that allow for the real-time collection of data in the field.

First, the application of communication and management systems. Tank monitoring technology is nothing new, systems are available that include leak detection probes as well as real time tightness testing on demand. Here the value of high technology lies in the communication, management and response to that information. With availability of inexpensive hardware and unlimited access to the World Wide Web these monitoring systems, distributed on a national or even international scale, can be administered from a central location by a single individual. This capacity reduces labor costs and increases the assurance of rapid response to product releases from storage tanks for all types. It reduces training costs, increases operator productivity, and provides third-party due diligence that is acceptable to regulators as well as financial partners and corporate managers.

The use of high tech communication and data management systems can also be applied to any other data that can be generated on-site in a digital format. Which leads us to the second portion of this discussion, advances in sensor technology. The National DOE Laboratories, NASA and university researchers are engaged in sensor technology commercialization. Much of that technology is first manifested in the medical arena, with environmental applications soon following.

Sensor technology for such basic physical parameters such as pressure, temperature, pH, flow rate, and water level have long been available. However, a new commercially available technology allows for the in-situ real-time determination of groundwater flow rate and direction. This development is more dramatic that it appears at first thought, it allows (for the first time) the direct measurement of groundwater flow. Prior to this groundwater flow has been an interpolated value, based on water levels in monitor wells, estimated hydraulic functions of the formation, and pumping rates. From those values groundwater flow is calculated. But real world heterogeneity’s and anisotropic conditions inherently make those calculations only approximations that can become increasingly inaccurate as smaller aquifer volumes are considered. This technology works by placing a heated element into the saturated subsurface via a bore hole. One vendor uses a plate as the heated element that allows for the determination of only the horizontal component of flow. Another uses a cylindrical heated element that resolves groundwater flow into three dimensions. This technology is available due to sophisticated miniaturized temperature detection arrays that came out of the National Laboratories. In conjunction with computer capabilities that allow for the rapid calculations required to convert spatial temperature data from the surface of the probe into actual groundwater flow information.

The next important area for sensor advancement is the capacity to generate real time analytical chemical data. The detection of hydrocarbon compounds is particularly difficult given that what is most useful is a sensor for the qualitative and quantitative measurement of contaminants that is reversible. Reversibility means that it can respond to changes up or down in contaminant concentration without having to utilize a one-time only analytical procedure. Approaches that are at least in the field testing stage include:

  • Laser induced fluorescence using ultraviolet light that in turn stimulates fluorescence when it strikes hydrocarbons. The color (or actually frequency) of the fluorescence is indicative of the type of hydrocarbon and the intensity the concentration. The UV laser can be tuned to frequencies that are ideal for particular organic compounds. It can provide semi-qualititiative and quantitative information on BTEX hydrocarbons.
  • The use of optical fiber systems appears to have much promise. One approach utilizes optical fibers that are coated with multiple analyte sensitive polymers that fluoresce under excitation light, the fluorescence signals are in turn interpreted into concentration data.
  • Micro-electro-mechanical technology is another approach that uses existing proven analytical technologies at ever decreasing physical scales. At its simplest the process involves sampling, injection, and mixing with reagents, followed by optical or electrochemical detection. As with full scale laboratory equipment this approach allows for the optimization of the detector to yield the highest degree of sensitivity, with concurrent selection of reagents to offer high selectivity towards the contaminants of concern. Detection limits in the parts per billion range are possible. Such units have already been developed with dimensions of 1.5 by 1 by 0.1 inch, and that size will continue to decrease. Using micro-mechanical technology a gas chromatograph the size of a telephone that includes capillary column, sample injector, and detector, with detection sensitivity of one part per billion, is currently available.

 The detection of soluble metals is an easier process. Electrochemical sensors are already developed for the detection of soluble metals in the parts per billion range. Micro-machining and photolithography technology is being used to fabricate ultra-miniture (and ultimately low cost) multichannel/multispecies microelectrode arrays.

Lastly, is a technology that is commercially available (for medical applications). However, it has promise as a significant advance in chemical detection systems, you will here more about this technology. It is called Surface Plasmon Resonance. It exploits a quantum optical- electronic phenomena in which light interacts with electrons that have been transferred to the surface of a metal. A chemical change in the near surface environment results in a change in electron distribution and the way in which the light interacts with a metal plate, the magnitude of that interaction is quantitatively related to the magnitude of the chemical change. The figure illustrates the basic components and configuration of this device. At this juncture the technology is being applied using immunoassay methodologies for medical applications.

With regulatory acceptance, the increased range of application, greater sensitivity and reliability of advanced sensors used in tandem with advanced communication and data management systems, will lower analytical, operational, and labor costs associated with groundwater environmental concerns. A prospect pleasing to industry and critical to the operational efficiency of the environmental service provider.

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Copyright 2008 David B. Vance
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If you have comments or suggestions, e-mail me at 2the4@2the4.net