The abundance of aging unexploded ordnances (UXOs) in marine continental margins and nearshore areas is a global scale concern. Though it is impractical to retrieve these UXOs in their entirety, it is critical to establish cost effective methods with which areas at risk may be monitored or evaluated for leakage and to measure concentrations of energetic compounds derived from UXOs to compare with water quality criteria.
Specific technical objectives of this project were to: (1) evaluate ethylene vinyl acetate (EVA) sampler uptake and offloading rates for the target munitions compounds as a function of ambient concentration and film thickness; (2) assess the stability of target compounds within the sampler film for deployment timescales; (3) optimize sampler geometries and film thicknesses for the detection of target field concentrations (below nanograms per liter); and (4) deploy samplers onsite to demonstrate mapping field distributions of target compounds and identifying accumulation zones.
In this effort, a novel passive sampling approach based on the introduction of samplers coated with EVA was evaluated to assess the efficacy of these samplers to fulfill the gap in marine water and porewater monitoring. These initial efforts were dedicated to parameterizations identifying the uptake and depuration rates to confirm sufficient deployment times. The ultimate test of the sampler involved sampling at sites of known UXO disposal where energetic compounds have been detected. Deployment in Halifax Harbor detected three of the target compounds in our laboratory analysis followed by confirmation and identification of several more energetic compounds confirmed by ALS labs.
The samplers exposed to contaminated waters underwent a rapid uptake within two hours of deployment in the kinetic uptake phase and reached steady state/equilibrium within 6 hours. TNT kinetics were slightly faster than those of RDX. It was discovered that by altering the percent acetate of the EVA film, the uptake of RDX could be enhanced significantly above expected predictions based on octanol water partitioning. This was attributed to the influence of the polar acetate group and likely due to dipole-dipole interactions between acetate and nitrate groups. This enhancement also was true for TNT though to a lesser degree. Compound integrity within the film was stable once refrigerated for up to three months and this can be extended if samples are frozen. Both effects of salinity and temperature were evaluated in order to improve targeted designs and back calculations of aquatic concentrations. LogKEVA-W varied inversely with temperature at -0.01units per oC increase. Salinity did not significantly affect equilibrium values. The extraction process was improved to two hours per sample from the time of arrival to the laboratory to the time of analysis. This could likely be further reduced with faster solvent reduction techniques.
ALS detected significant concentrations of our target compounds in addition to pentaerythrotol tetra nitrate (PETN), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (OTT), nitroglycerine (NG), 2,4,6 trinitrophenylmethylnitramine (tetryl). These compounds are in the ppb ranges and were not detected in earlier studies.
The second deployment in the Baltic Sea at a known munitions dumping site, Gdansk Deep, also yielded significant concentrations of TNT, RDX and two di-nitro TNT derivatives from our laboratory. Confirmation from ALS labs also identified the presence of tetryl, NG, OTT, and PETN in these samples. These results provide excellent confirmation of the utility of these non-specific samples wherein 2 to 5 g of film are sufficient to capture trace environmental concentrations of energetic compounds and their derivatives. The field tests of the samplers have shown promising results worthy of further investigation.
The EVA sampling approach provides in situ spatial and temporal sampling at biologically relevant concentrations of contaminants. The sampler shows promise for a broad range of hydrophobic contaminants, including TNT and RDX. The potential as a remediation technique may lead to minimizing exposure risk and avoiding costly dredging approaches.