TNT Incorporation and Mineralization by Natural Microbial Assemblages at Frontal Boundaries between Water Masses and in Underlying Sediments in Coastal Ecosystems
The magnitude of 2,4,6,-trinitrotoluene (TNT) source terms for active firing ranges and reported slow degradation rates in terrestrial ecosystems suggest that these ranges have the potential to negatively impact adjacent coastal waterways. However, significant concentrations of TNT and other energetics are rarely detected in these coastal environments, indirectly suggesting that natural attenuation may be a viable remedial alternative. In previous work, we found that rates of TNT incorporation into microbial biomass and mineralization to carbon dioxide (CO2) might be rapid enough to account for such loss of range source material (or breached unexploded ordnance [UXO]) across the salinity gradient in estuaries.
This project was conducted in two phases. The Phase I objective was to determine whether environmental conditions at frontal boundaries enhanced TNT metabolism rates by natural microbial assemblages relative to those conditions found at other points along the salinity gradient. This effort was completed in 2011. The Phase II objective is to determine the primary biogeochemical factors that control TNT metabolism by natural microbial assemblages in coastal systems. By correlating standard water quality measurements with degradation rates, we may predict turnover times for energetics released into hydrodynamically similar, UXO-impacted ecosystems where access to site samples may be limited. Three data gaps will be addressed as part on an integrated sampling scheme of DoD-relevant field sites: 1) determining energetic metabolism rates for ecosystem types where we currently lack data; 2) correlating biogeochemical water quality data with degradation rates; and, 3) measuring TNT incorporation rates into bacterial DNA using sediment samples.
Phase I efforts focused on whether TNT metabolism was enhanced by the same biogeochemical features that enhanced aromatic carbon metabolism at frontal boundaries. Water column and nepheloid samples were collected along estuarine salinity gradients especially across salinity fronts between water masses and salt wedges. Rates of 14C-TNT incorporation into microbial biomass and mineralization to 14CO2 were normalized to heterotrophic carbon metabolism and to transformation rates of other terrestrial aromatic organic matter. These values helped determine TNT lability in natural ecosystems relative to other common and naturally present forms of organic matter and identified areas of enhanced degradation between water masses in Kahana Bay, Lower Mississippi/Gulf of Mexico, and Charleston Harbor.
Phase II efforts will determine biogeochemical controls of TNT metabolism that are straightforward for a site manager to measure (e.g., DO, salinity) and those involving dissolved organic carbon quality and character that are initially more complicated to evaluate but can be simplified once the relationships are established. These parameters will be compared with our standard measures of bacterial energetic metabolism, as well as, new methods for determining TNT incorporation rate into bacterial DNA. These evaluations will be made on ecosystem types where there is limited data including a wet tropical reef and lagoon (Florida Keys), southern temperate lagoon (Albemarle-Pamlico Estuarine System), and northern temperate tidal bay and river (Great Bay Estuary). Ultimately this project will produce degradation rate data that be used to predict attenuation in terms of energetics (per type of ordnance) that can annually degraded by natural assemblages in a particular ecosystem.
By identifying the specific areas and microbial mechanisms associated with the attenuation of aromatic energetics in coastal waters, the likelihood of public and regulatory acceptance of natural attenuation as a viable remedial alternative will increase. Documenting (and validating) natural pollution abatement mechanisms can support continued use of active Department of Defense ranges. (Anticipated Project Completion - 2015)
In Phase I, we found evidence that these water mass interfaces are sites of enhanced bacterial metabolism and TNT mineralization when examining salt wedges (Gulf of Mexico, Charleston Harbor), a frontal boundary (Charleston Harbor), and mixing experiments between freshwater and marine end members (Gulf of Mexico, Charleston Harbor, Kahana Bay). Two to 10-fold increase in bacterial growth rates across relatively short distances (meters) of a salt wedge (vertical fronts in Gulf of Mexico and Charleston Harbor) and a river convergence (horizontal front, Charleston Harbor) drove a large variation in degradation potential across stratified water masses. Likewise, TNT and phenanthrene mineralization rates were highest on both sides of a river convergence. Experimentally mixing freshwater and marine end members (Kahana Bay) resulted in higher rates of bacterial production (+62%), phenanthrene mineralization (+68%) and TNT mineralization (+33%) though the effect on production was time dependent (possibly a response to osmotic shock). In summary, we found that TNT and aromatic degradation was enhanced at the interfaces between water masses of different salinity. The results of this study can be found in the Phase I report and in a peer-reviewed publication.
Points of Contact
Dr. Michael Montgomery
Naval Research Laboratory
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