In situ measurement of redox sensitive solutes at high spatial resolution in a riverbed using Diffusive Equilibrium in Thin Films (DET)

Here we evaluate a new field methodology for the deployment of Diffusive Equilibrium in Thin Films (DET, protected in stainless steel holders) in coarse riverbed sediments based on that originally developed for fine-grained sediments and soils. Concentration gradients of NO3−, NH4+ and dissolved Mn were measured at cm resolution. We observed fine scale changes in NO3−, NH4+, and Mn concentrations in the river bed (0–30 cm) that were only evident at high resolution and compared them to profiles of NO3− and NH4+ obtained with low resolution multilevel piezometers. The range in concentrations of NO3− and NH4+ measured through DET was larger than those measured at coarse resolution through pore water sampling from multilevel piezometer in the riverbed over the 30 cm depth. According to the results, high resolution profiles of redox sensitive chemical species in riverbeds could help in identifying and resolving hotspots of biogeochemical activity. Measurements of NH4+ using DET were higher than measurements in pore water collected from the multilevel samplers. Further studies are needed to establish whether there is a systematic bias associated with either procedure for the measurement of NH4+.