Tracing the cycling and fate of the explosive 2,4,6-trinitrotoluene in coastal marine systems with a stable isotopic tracer, 15N-[TNT]

2,4,6-Trinitrotoluene (TNT) has been used as a military explosive for over a hundred years. Contamination concerns have arisen as a result of manufacturing and use on a large scale; however, despite decades of work addressing TNT contamination in the environment, its fate in marine ecosystems is not fully resolved. Here we examine the cycling and fate of TNT in the coastal marine systems by spiking a marine mesocosm containing seawater, sediments, and macrobiota with isotopically labeled TNT (¹⁵N-[TNT]), simultaneously monitoring removal, transformation, mineralization, sorption, and biological uptake over a period of 16 days. TNT degradation was rapid, and we observed accumulation of reduced transformation products dissolved in the water column and in pore waters, sorbed to sediments and suspended particulate matter (SPM), and in the tissues of macrobiota. Bulk δ¹⁵N analysis of sediments, SPM, and tissues revealed large quantities of ¹⁵N beyond that accounted for in identifiable derivatives. TNT-derived N was also found in the dissolved inorganic N (DIN) pool. Using multivariate statistical analysis and a ¹⁵N mass balance approach, we identify the major transformation pathways of TNT, including the deamination of reduced TNT derivatives, potentially promoted by sorption to SPM and oxic surface sediments.