Exposure times rather than residence times control redox transformation efficiencies in riparian wetlands

- Interactions between hydrology and biogeochemistry in riparian wetlands.
- Identification of system specific exposure and residence times.
- Evaluation of system specific Damköhler numbers to asses transformation efficiency.
- Surface/groundwater couplings control transformation efficiency and the relation between exposure and residence times in riparian wetlands.

SummaryThe concept of Damköhler numbers have been extensively used in the discipline of chemical engineering and lately increasingly found its application into environmental science in order to describe the integrated behavior of hydrological systems with respect to their physical transport and biogeochemical transformation capabilities. Defining characteristic time scales of transport and reaction, as part of the Damköhler concept, however is not trivial especially for non-well mixed systems like catchments where physically controlled transport and biogeochemical moderated reactions can be highly variable among individual flow paths. Often, system specific residence times alone are not useful to describe the timescales of transport in the Damköhler concept, because it neglects that degradation of redox-sensitive compounds depend on dynamically changing and non-uniformly distributed hydro-biogeochemical boundary conditions that either facilitate or suppress biogeochemical reactions. In this study an approach is presented that highlights the importance to specifically distinguish between residence and exposure times if system specific transformation efficiencies are evaluated. We investigate the inter-relationship between residence and exposure time distributions for different biogeochemical processes in a virtual wetland environment that is exposed to different hydrological conditions. The relationship between exposure and residence times is mathematically described by a composition matrix that linearly relates the two identities to each other. Composition matrices for different hydrological conditions are analyzed by using the singular value decomposition technique. Results show that especially the type of couplings between the surface and subsurface flow domain control how exposure and residence times are related to each other in the wetland system and that timescales of residence and exposure typically differ by orders of magnitude. Finally, results also indicate that the assessment of system specific transformation efficiencies can be very error-prone if residence instead of exposure times are being used to derive system specific Damköhler numbers.