This project aims to bolster the existing ESTCP monitored natural attenuation (MNA) protocol, specifically for sites with explosives-contaminated groundwater. To this end, a robust demonstration/validation of a tertiary line of evidence for MNA will be conducted, and a practical software decision tool for screening sites for the potential application of MNA remedies will be developed. The tertiary line of evidence will help distinguish groundwater plumes where true mass removal processes are occurring from plumes where nontransformative physical processes dominate.

Technology Description

A field demonstration will be conducted at four sites to support the development of a new tertiary line of evidence that is related specifically to the transformation of aqueous explosives contamination. This line of evidence will involve the application of a specific analytical method, gas chromatography-isotope ratio mass spectrometry (GC-IRMS), to spatially characterize stable isotope enrichment effects in groundwater plumes of explosives contamination. A limited demonstration of this approach was conducted as part of an earlier ESTCP-funded project (ER-9518), but that study was limited to a single explosive constituent—2,4,6-trinitrotoluene (TNT)—at a single site, Louisiana Army Ammunition Plant. In this project, investigators will (1) extend the evaluation of field-scale stable isotope enrichment effects to other aqueous munitions constituents besides TNT; (2) correlate enrichment effects with site environmental conditions by evaluating primary and secondary MNA lines of evidence, derived chiefly from historical site data; (3) assess the effectiveness of any existing in-place remedies, especially remedies that lead to enhanced bioremediation, by evaluating the stable isotope enrichment effects in the vicinity of such remedies relative to background enrichment effects; and (4) codify these findings and the key steps from the ESTCP MNA protocol into a managerial software decision tool to support the identification of sites where MNA is likely to be a successful remedy by itself, or as part of a combination of remedies.

Implementation Issues

Earlier MNA efforts at explosives-contaminated sites have been hampered by a general lack of tertiary lines of evidence, mainly because transformation products of explosives are short-lived and difficult to quantify with consistency. Therefore, establishing a mass balance for explosives and their transformation products in the field is extremely difficult. This project will help establish a strong tertiary line of evidence for explosives contaminant transformation by utilizing GC-IRMS, a method that focuses on the analysis of reactants rather than products. Furthermore, the project will help streamline the MNA evaluation process by supplying an easy-to-use MNA remedy screening software tool. Thirty-year life-cycle cost savings at individual sites, as a result of MNA implementation, could range from 31% when compared with passive remedies (e.g., mulch biowalls) to 71% when compared with active remedies (e.g., pump-and-treat). (Project Completed - 2008)