Validation of environmental military threshold values for explosives in soil

Firing practice and munitions testing inevitably result in the dispersion of Energetic Materials (EM) in the Canadian environment. As noted by the Auditor General in a report released in 2003, sites contaminated by energetic substances may present a risk for the ecological and/or human receptors that must be addressed by the Department of National Defence (DND). At the request of DND, the Applied ecotoxicology group of the Biotechnology Research Institute developed a set of Canadian military training soil quality guidelines (MTSQGs) that were actually ecological and human health military threshold values for EM. These values are now renamed Soil Concentrations for Military Training Sustainability (SCMTS). These values have been updated to consider the recent CCME methodology for ecological receptors. Also, the validation of the proposed values has been initiated. The validation process would serve to answer some questions regarding the threshold values for ecological receptors. Validation included sampling of various plants found at or near the training area and of accompanying soil in order to estimate contaminant transfer to biomass. The use of field bioassays allowed a preliminary estimation of the transfer from soil to soil invertebrates. In addition, reports of biological effects observed following laboratory exposure to field-collected soils were used to verify the safety of the derived guidelines. Even if laboratory experiments employing field soils do not really represent the real exposure conditions, they certainly are worth considering since they permit the evaluation of biological effects in tightly controlled conditions. From the literature search, it was found that there were no data concerning new ecological receptors, except in the case of RDX. The recalculated soil contact values for the 2,4- and 2,6-dinitrotoluene isomers are similar to the previous calculated values The update process led to a soil threshold value for NG that can be applied with higher confidence, relying on minimal information concerning the toxicity of NG for plants and soil invertebrates. The recalculated values for the other compounds are less conservative than the original MTSQG, mainly because the BCF values were averaged instead of being selected. Also, it was not feasible to derive SCMTS for secondary and tertiary consumers because of the lack of toxicity data for such consumers (insectivores, predatory mammals or birds) and the lack of bioaccumulation data for secondary consumers (small mammals). The check values for most compounds are driven by human health considerations and cannot therefore be modified without new information on human exposure. The resulting SCMTS are presented below. The “practical” validation could be carried out only for HMX and NG because the levels of other explosives in the sampled soils were generally under the limit of detection. On the average, the tissue accumulation noted in plant samples was not higher than predicted by the models. Generally, accumulation from the soil was lower than expected, but in some occasion high levels of HMX or NG were found in plant tissues. Laboratory and in situ bioassays with earthworms exposed to contaminated soils indicated that explosives may not always be toxic to the same extent in the field as in the laboratory. This point out to the necessity of considering data obtained following weathering in the derivation of guidelines. Results were also not always predictable, possibly because of the interference caused by other toxic compounds in the test soils (notably heavy metals) but perhaps also to other factors, such as non-homogeneous distribution of EM contamination. Accumulation of energetic materials from field soil was low and inconsistent, NG was found in earthworm tissues in only one instance, although many soil samples with high levels of NG were tested. The general conclusions drawn from the validation process were as follows: Firstly, the proposed SCMTS appear sufficiently protective, as the EM-related toxic effects found in the field soils were lower than expected. Secondly, the original values were found to be overprotective in some cases, because the normal weathering process leads to a reduced availability of some compounds (notably HMX). This has been somewhat corrected in the updated values (SCMTS) by incorporating data obtained through weathering/aging of soils. Also, it was discovered that the bioavailability of NG maybe reduced under the form present in explosives formulations (NG adsorbed to nitrocellulose). Nevertheless, it is not possible to readjust the SCMTS without conducting a larger field study. Few studies have been devoted to the effects of the interactions between each individual component of explosive mixtures. It is generally assumed that the toxicities would be additive, but this could not be verified in the case for the mixture of NG and nitrocellulose. Future work should now be focused on this aspect of environmental contamination.

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