The costs for initial characterization, remediation monitoring, and long-term, post-remediation monitoring of groundwater contaminated with explosives are increasing as live firing ranges, ammunition depots, ordnance test facilities, and other Department of Defense (DoD) sites come under ever closer scrutiny. Major elements of the expense for characterization and monitoring are collection, packaging, shipping, and laboratory analysis of samples.
The reference laboratory procedure for explosives is Environmental Protection Agency (EPA) Solid Waste (SW)-846 Method 8330. However, most samples test blank, wasting time and monetary resources on the laboratory procedure. Those samples that do test positive can be characterized by analyzing for just a few explosives, most notably 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), obviating the need for a complete Method 8330 analysis. Furthermore, the conventional approach is not well suited for monitoring active remediation processes such as pump-and-treat systems because turnaround times for laboratory results are too slow for process control.
Colorimetric and immunoassay field methods have been developed as faster, cheaper, more portable alternatives to the laboratory method. However, these techniques have limitations—most notably their inability to detect more than a single analyte or class of analytes per test. In this project, a new method based on surface-enhanced Raman spectroscopy (SERS) has been developed. Raman is a vibrational spectroscopic method that produces a unique “fingerprint” spectrum for each analyte, allowing the components of a mixture to be discriminated in a single test. Normally a weak phenomenon, the Raman signal is enhanced up to 106 by adsorbing analytes onto a noble metal surface (colloidal gold, in this case), rendering it suitable for trace analysis. This project was designed to field and demonstrate the potential of SERS to reduce the time, cost, and waste generated per analysis (versus laboratory and field methods) while providing discriminate quantification of multiple analytes (even within a chemical class) in a single measurement.
This project was designed to field and demonstrate the potential of SERS to reduce the time, cost, and waste generated per analysis (versus laboratory and field methods) while providing discriminate quantification of multiple analytes (even within a chemical class) in a single measurement.
This project entailed three demonstrations at two Army facilities, Alabama Army Ammunition Plant (ALAAP) and Umatilla Chemical Depot (UMCD). The demonstration objectives were to demonstrate (1) the general fieldability and ease of use of the SERS instrument; (2) capability for quantifying multiple explosives (TNT, 2,4-DNT, 2,4,6-trinitrobenzene (TNB), RDX, and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine [HMX]) in a single measurement; (3) capability for at-line remediation process monitoring; (4) capability for in-situ and ex-situ groundwater monitoring from a cone penetrometer (CPT) platform; (5) improved capability for discriminating explosives versus colorimetry; and (6) the cost benefit of the SERS technology. All six objectives have been met.
Collection of groundwater from wells at ALAAP and process water samples at the UMCD granular activated carbon (GAC) plant was coordinated with ongoing remedial investigation activities at the sites. A total of 56 water samples was collected—32 groundwater well samples at ALAAP, 18 GAC plant samples at UMCD, and 6 CPT groundwater samples at UMCD. Split samples of groundwater were analyzed by SERS, colorimetry, and EPA SW-846 Method 8330. Fundamental performance parameters established for the SERS method were the method detection limit, linear dynamic range, accuracy, and precision. Basic analytical SERS performance met the objectives of this project and was not far from the performance of the reference laboratory method. SERS performance exceeded the colorimetric methods for nitramine and nitroaromatic explosives.
The SERS instrument and methods are readily fieldable, durable (low maintenance), simple to set up and use, and very safe. Analytical performance was shown to be better than the other field methods but could be improved to meet or exceed that of the reference method. Advantages of using SERS include: reliable quantification of individual explosives in water samples at concentrations of regulatory relevance; faster results and lower cost than EPA SW-846 Method 8330; comparable speed, lower cost, simpler procedures, less matrix interference, and better discrimination of individual explosives as compared to colorimetry; and applicability to virtually any environmental water monitoring application such as groundwater well monitoring, expedited site characterization, and remediation process monitoring. The SERS method is ready for implementation at DoD installations and other sites. EPA’s recent acceptance of performance-based standards should allow many sites to deploy the technology.