Article pubs.acs.org/est
Stable Isotopes of Nitrate Reflect Natural Attenuation of Propellant Residues on Military Training Ranges Geneviève Bordeleau,*,† Martine M. Savard,‡ Richard Martel,† Anna Smirnoff,‡ Guy Ampleman,§ and Sonia Thiboutot§ †
Institut National de la Recherche Scientifique, Centre Eau, Terre et Environnement (INRS-ETE), 490 de la Couronne, Quebec City, QC, G1K 9A9, Canada ‡ Geological Survey of Canada, 490 de la Couronne, Quebec City, QC, G1K 9A9, Canada § Defence Research and Development Canada - Valcartier, 2459 Pie-XI Boulevard North, Quebec City, QC, G3J 1X5, Canada ABSTRACT: Nitroglycerin (NG) and nitrocellulose (NC) are constituents of double-base propellants used notably for firing antitank ammunitions. Nitroglycerin was detected in soil and water samples from the unsaturated zone (pore water) at an active antitank firing position, where the presence of high nitrate (NO3−) concentrations suggests that natural attenuation of NG is occurring. However, concentrations alone cannot assess if NG is the source of NO3−, nor can they determine which degradation processes are involved. To address this issue, isotopic ratios (δ15N, δ18O) were measured for NO3− produced from NG and NC through various controlled degradation processes and compared with ratios measured in field pore water samples. Results indicate that propellant combustion and degradation mediated by soil organic carbon produced the observed NO3− in pore water at this site. Moreover, isotopic results are presented for NO3− produced through photolysis of propellant constituents, which could be a dominant process at other sites. The isotopic data presented here constitute novel information regarding a source of NO3− that was practically not documented before and a basis to study the contamination by energetic materials in different contexts.
1. INTRODUCTION In the past decade, contamination of soils, groundwater and surface water by energetic materials (EMs) has been recognized as an important environmental issue. The presence of various EMs in soils and water has been documented at ammunition production facilities,1 as well as on military training ranges throughout Canada and the United States.2,3 At military training facilities, firing positions of antitank ranges are among the sites most heavily impacted by the presence of EMs.3,4 In this context, double-base propellants are used, which are composed of nitroglycerin (NG) embedded within a nitrocellulose (NC) matrix. During ammunition firing, incomplete combustion causes significant amounts of propellant residues to be deposited at the soil surface, mainly behind the firing position. The residues contain small quantities of readily soluble NG and nitrate (NO3−), as well as fragmented propellant particles.5 Following rainfall or snowmelt, NG can leach out of the propellant particles by dissolving in infiltration water; however, the rate and extent of dissolution is limited by the stable, non-water-soluble NC matrix.6 Depending on the prevailing environmental conditions, various degradation processes can affect the fate of propellant constituents (NG, NC) at the soil surface, in the unsaturated zone, and in groundwater. Up to now, very few studies have aimed at identifying degradation processes that could be significant under environmental conditions. However, based on © XXXX American Chemical Society
laboratory studies focused on remediation strategies, a series of potential processes that could participate in natural attenuation of propellants can be inferred; these mainly include photolysis,7 biodegradation,8,9 hydrolysis,10 and reduction mediated by soil organic matter.11,12 In all, two pathways have been identified for NG degradation, namely a reductive and a hydrolytic pathway;10,13 both result in the release of the functional groups as either nitrate (NO3−) or nitrite (NO2−); the latter is then oxidized to NO3− in presence of oxygen. While NO3− is less toxic than NG, it is very stable in aerobic conditions and is one of the most common contaminants in shallow aquifers.14 On training ranges where NG is detected in soils, the presence of NO3− concentrations above background concentrations in water from the unsaturated zone (pore water) can therefore be an indication that attenuation of NG is occurring.5,15 However, other sources such as fertilizers, septic waste or atmospheric NOx could also contribute to the detected NO3− loads. To distinguish the NO3− produced from propellant degradation and other sources, stable isotopes of nitrogen (N) and oxygen (O) contained in the molecule can be used. The fundamentals for this approach imply that chemical, Received: January 29, 2013 Revised: June 20, 2013 Accepted: July 1, 2013
A
dx.doi.org/10.1021/es4004526 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
and readily dissolves in water.5 Therefore, it must be considered as a potential contributor to the NO3− load in groundwater.
physical, and biological reactions can cause isotopic fractionation, i.e., favor either the light (14N, 16O) or the heavy (15N, 18 O) isotopes in the reaction products, with the inverse effect being observed in the reactants.16,17 Products with an isotopically lighter composition than the reactants are said to be depleted, and products with a heavier composition are said to be enriched, compared to the reactant. The fractionation effect and its magnitude are typical of a specific reaction occurring under specific conditions. Hence, NO3− produced from different sources may have different N and/or O isotopic values.17 Similarly, NO3− produced from a unique source (e.g., NG), but through different degradation processes, can also have different isotopic values.18 The use of both isotopes in a 2-D plot, in the so-called “dual isotopic approach”, increases the likeliness of source identification.17,19 The ranges of dual isotopic ratios representing the most common NO3− sources have been documented.17,20 However, very little is known about the ratios related to EM degradation. The few studies that have used stable isotopes to follow the degradation of trinitrotoluene (TNT) or hexahydro-1,3,5trinitro-1,3,5-triazine (RDX) confirmed the occurrence of isotopic fractionation in the reactants,21,22 as well as in the released NO3−.18,23,24 More specifically, RDX combustion results in NO3− with a slightly enriched N signature compared to the parent compound,24 which is analogous NOx produced from coal-fired power plants being enriched compared to coal.25 Inversely, RDX photolysis results in NO3− with a depleted N signature.18,23 This is a consequence of light (14N) isotopes in the functional groups forming weaker bonds with the central part of the molecule, compared to heavy (15N) isotopes.19 These weak bonds are then preferentially broken during photochemical degradation of EMs. Isotopic analyses may therefore represent a promising way of inferring and understanding natural attenuation of EMs. In the present study, the isotopic approach focuses on the product of NG degradation (NO3−), rather than on the reactant. This approach was adopted because NG concentrations in pore water are often very low,5,26,27 which would render isotopic analyses difficult, while NO3− concentrations are usually sufficient to allow analysis.5,18 The objectives of this study are therefore to (1) analyze the isotopic ratios of NO3− coming from the degradation of NG and NC through controlled processes, (2) determine whether these ratios can help identify the processes involved in natural attenuation of propellant constituents at an antitank firing position, and (3) evaluate whether isotopic ratios can be used to distinguish the sources of NO3− in groundwater of training ranges. For the controlled degradation experiments, specific processes were selected on the basis of whether or not they could contribute to the natural attenuation of propellants, and the concomitant production of NO3−, at the antitank firing position. The selected processes include photolysis at the soil surface7 and degradation mediated by soil organic matter in the unsaturated zone12 but exclude hydrolysis and biodegradation. Indeed, the pH of pore water at this site could not support hydrolysis, and previous biodegradation experiments with microorganisms isolated a few hundred meters from this site indicate that they consume the NO2 groups for their growth, so NO3− is not released in pore water.12 Finally, an additional process has been considered, namely the combustion of propellant. While combustion is not a natural attenuation process in itself, it generates NO3− which falls at the soil surface
2. EXPERIMENTAL SECTION 2.1. Experimental Approach. This paper presents the NO3− isotopic results obtained on laboratory and field water samples. The production and/or collection of these samples is described in detail elsewhere,5,7,12 but a succinct description is presented in sections 2.3 and 2.4. In this paper, the laboratory samples are analyzed for their isotopic ratios, and dual isotopic domains are delineated for each process. The isotopic ratios are also measured in pore water samples from the antitank firing range and are compared with the domains representing each degradation process. Finally, the laboratory and field samples are compared with the isotopic domains documented for other common sources of NO3− in shallow aquifers. 2.2. Chemicals and Reagents. Degradation experiments were carried out on double-base propellants, NC powder, and dissolved NG. Particles of two different types propellants were used, namely AKB 204 (61% NC, 37.5% NG, and 1.5% ethylcentralite), collected during the live firing of 84-mm CarlGustav ammunition, as well as Powder C (65% NC, 34% NG, 1% ethyl centralite), which was made for research purposes. Pure military-grade NC powder was obtained from GD-OTS (Valleyfield, QC). The NG solutions (15 mg/L) were prepared by stirring Powder C particles in ultrapure water for 7 days and then filtering the solution on a 0.45 μm membrane. 2.3. Laboratory Degradation Experiments. To obtain NO3− samples from propellant combustion, fresh AKB 204 propellant residues from the live firing of 84-mm antitank ammunition were collected. The training event took place on the same training range where pore water samples were collected. To collect the propellant residues, a series of rectangular aluminum traps containing a thin layer of distilled water were placed on the ground behind the firing position.5 As training proceeded, propellant residues were expelled rearwards from the shoulder launcher and landed in the traps. The content of the traps was then poured into a 4-L amber glass bottle and brought back to the laboratory where it was filtered on a 0.45 μm membrane. NO3− samples related to organic carbon-mediated NG degradation were obtained from experimental soil columns (height 117 cm, diameter 4.2 cm) maintained under unsaturated conditions.12 Experiments were performed on columns filled with either sand (
