Validation of Colorimetric 2D DET Techniques for Concurrent Measurement of Ammonium and Nitrate/Nitrite in Sediment Porewater

This study presents new colorimetric two-dimensional diffusive equilibrium in thin films (2D DET) techniques for the concurrent measurement of ammonium (NH 4 –N), nitrate (NO 3 –N) and nitrite (NO 2 –N) in sediment porewaters. The methods are salinity-independent and suitable for freshwater and marine environments, with working ranges of 10–1800 μmol/L for NH 4 –N, 100–∼1000 μmol/L for NO 3 –N, and 25–1000 μmol/L for NO 2 –N. However, performance is influenced by extreme pH (≈3 and ≈9), and interferences from glycine (500 μg/L) and NO 2 –N affect NH 4 –N and NO 3 –N determinations, respectively. The techniques were applied to sediments from a municipal wastewater stabilization lagoon to characterize two-dimensional distributions of dissolved inorganic nitrogen (DIN). Results revealed strong spatial heterogeneity in porewater DIN, identifying depth-specific sources and sinks associated with microbial activity under light and dark conditions. Coupled with ex situ core incubations, the profiles demonstrated active nitrogen cycling, including NH 4 –N-oxidation and NO 3 –N release to the overlying water. These results demonstrate that colorimetric 2D DET provides a robust tool for high-resolution assessment of porewater DIN dynamics and biogeochemical processes in aquatic sediments.