CEC has collaborated with Burge Environmental of Tempe, Arizona, in the development and deployment of a sensor system called MiProbe that was supported by a series of grants from the U.S. Department of Energy.
The sensor is part of a full package of environmental sensors and data management tools that incorporates telemetry, the Cloud, and computer-generated graphics to bring the dynamics of contaminated site management to life. The system is unique in many ways, but one feature that makes it truly special is its use of biofilm as the sensor itself. The sensor also has a metabolic gas capture capability that gives it a second level of application in line with the current interest in Natural Source Zone Depletion (NSZD). Taken separately, the system’s features include:
Microbial Sensing Capabilities
The microbes and the associated electrode-support structure comprise a revolutionary redox sensor that is both instantaneous in reporting to the Cloud and robust to the point where, for all practical purposes, it has an operational life of several years.
A brief explanation of how it works: a biofilm/electrode combination with associated circuitry generates a steady-state voltage that is held by the electrode, noting that every redox state has an associated voltage. For example, the voltage is highest in an anaerobic environment, but it drops upon the encroachment of aerobic conditions because electrons are drawn away; this change is then recorded and transmitted. The system can work in reverse with voltages rising as anaerobic conditions develop. The sensor data may allow for better management and cost control associated with use of reagent applications (e.g. for providing oxidizing or reducing conditions in the subsurface).
It is also possible to understand the metabolic turnover rate in the environment. Based on the voltage output for the sensor (turning it on and off via remote control and allowing the voltage to drain and recover), a rate of substrate consumption can be calculated. This then factors into natural attenuation petitions or the progress of remedial intervention with oxidative or reductive processes as noted.
Metabolic Gas Capture Capabilities
The application targets a growing interest in documenting NSZD as a means of more enlightened management of complex sites with non-aqueous phase liquids (NAPLs). At present, the focus is on Light NAPLs (LNAPLs) like petroleum hydrocarbons. Depending on the subsurface conditions, LNAPLs will naturally attenuate through aerobic and anaerobic pathways while generating carbon dioxide and methane as metabolic gas end products. Capturing representative samples of these metabolic gases can be useful in calculating the Time of Remediation (TOR) of palpable contaminant masses in the subsurface. Of course, overlaying intervention activities such as oxidants or temperature inputs on this process can accelerate the TOR, and this will be recorded.
Other Applications and Features
Because MiProbe is sensitive to microbial activity, it can detect an uptick in electron flow as a function of a change in substrate availability. This would manifest when, for example, a dissolved-phase hydrocarbon plume impacts the sensor, which has major applications in managing UST sites or accidental releases. Conversely, a lack of electron flow could indicate a lack of bioavailability of contaminants.
The bioavailability application is important in sediments work in support of Monitored Natural Recovery (MNR) strategies. In effect, if a contaminant is unavailable, a case can be made for limited environmental impacts. Additional sediment and landfill management applications include the use of the redox sensor components to allow for characterization and modeling of the water exchanges between capped sediments or landfills and surrounding sources of water. Applications for ecological monitoring are also of interest and are ripe for further exploration by interested parties.
All of these analytical results have been obtained with extremely high reproducibility. The MiProbe system can be deployed several ways, including direct insertion into the subsurface or into monitoring wells, or as part of a floating deployment configuration. Also, the system is solar powered with real-time data transmitted using either cellular or satellite communications. In locations where communications are difficult, a data logging option is available.
This information on the MiProbe System is also available as a downloadable brochure on the Environmental Site Investigation and Remediation page of CEC’s website: http://www.cecinc.com/enviro_site_redevelopment.html.