Since World War I, munitions have been manufactured in the United States using a variety of modern highly energetic materials, including propellants, explosives, and pyrotechnic materials. As a result, many sites now contain soil and groundwater contaminated with explosives. A recent study by the U.S. Army estimated that 1,300 installations worldwide containing approximately 5,500 individual sites contaminated with various compounds, including explosives, will require some form of remedial action to comply with environmental laws and regulations and ensure protection of human health and the environment.

The objective of this project was to develop in situ sensors capable of replacing conventional methods for measuring concentrations of TNT, RDX, HMX, tetryl, and nitrocellulose in groundwater at 20-50 parts per billion (ppb) levels with long-term stability (i.e., years).

Schematic Diagram of the In Situ Sensor

This technology is based on an electrochemical detection method using a carbon-fiber working electrode and square-wave voltammetry. The reduction potential provides the method selectivity for a specific explosive, and the current required per unit time determines the concentration of the targeted explosive in an aqueous media.

During this project, a submersible electrochemical probe at the end of a 50-foot-long shielded cable was developed using a carbon-fiber electrode assembly for the real-time monitoring of nitro-organic explosives in natural water buffered with sodium chloride. The reduction of the nitro moiety group allowed convenient and rapid square-wave voltammetric measurements of trace levels of nitroaromatic explosives (TNT, TNB and tetryl) down to levels of 50-100 ppb. A newly developed subtraction algorithm was used to minimize background contributions. A highly stable response was observed for prolonged (i.e., 10 hour) operations in natural water samples, indicating no apparent surface fouling. Detection limits for nitroamine explosives (HMX and RDX) and nitrocellulose were in the range of 1-2 parts per million. The peak potential for nitroamine explosives was directly superimposed onto the oxygen peak in the voltammogram resulting in a significant increase in detection limits. To successfully measure nitroamine compounds, oxygen removal will be required. Several potential interferences were investigated including monoand di-nitrotoluene, 4-amino-2,6-dinitrotoluene, trichloroethylene, nitrates, chromate, and perchlorate. Laboratory studies determined that only degradation products of nitroaromatic explosives containing a nitro group interfere with the measurement of the nitroaromatic parent explosive. This SEED project was completed in FY 2001.

A significant portion of the Department of Defense’s restoration budget goes for long-term monitoring costs. Most of the cost is associated with the collection of samples, disposal of purge water, and subsequent sample analysis. Use of in situ sensors can result in significant long-term cost savings. By coupling subtractive-square-wave voltammetry with a newly designed compact remote probe, nitroaromatic and nitroamine explosives can be monitored continuously and rapidly at large sample-instrument distances. The new monitoring capability has high sensitivity, speed, simplicity, reproducibility, and stability for nitroaromatic explosives.