Correcting porewater concentration measurements from peepers: Application of a reverse tracer

This work describes the routine application of a reverse tracer to passive diffusion sampler deployments. The reverse tracer provides evidence of the extent of equilibrium for any peeper cell and an indication of the diffusivity change with depth associated with physical changes in the sediment. Quantitative measurement of the reverse tracer also allows for the correction of measured data within cells that may not have reached chemical equilibrium, thereby removing the requirement of a priori knowledge of the time to concentration equilibrium. Investigations that use ion chromatographic techniques that incidentally measure Br^- or other conservative tracers can apply this method with no additional analytical requirements. Our approach opens up peeper applications to shorter duration deployments as required for marine sediment sampled by submersibles and deployments in environments with uncertain or unknown equilibration times such as highly heterogeneous sediments. We present a novel peeper design as well as the theory and application of this technique along with an example from a field deployment. We conclude that corrections that assume a simple approximation of exponential tracer decay are a considerable improvement over the reliance on observed measured values from porewater dialysis samplers. Moreover, even without using the reverse tracer to correct measured cell concentrations, the reverse tracer can point to significant divergences from concentration equilibrium and is useful to determine which cell measurements should be discarded.