Representing multicompartment stream transport utilising exposure time

The fate of compounds in natural streams is heavily dependent on their exposure to different biogeochemical conditions during transport. Classifying biogeochemical conditions and the fate of contaminants in stream sediment studies allows for controls to be determined at point scale; however, the total contribution of exchange with sediments containing specific conditions at the reach scale remains challenging as the magnitude of exchange can be highly heterogeneous, and the overall contribution of the time spent in specific biogeochemical conditions   relative to the total transport time is unknown. To overcome the issues of upscaling point studies we present an exposure time-based modelling approach following Ginn (1999).  The approach allows for exposure velocities to control how the solute is tracked in the system; a velocity of one suggests that the exposure time to a certain biogeochemical condition increases at a rate of 1/time. The transport characteristics of the system are then identified by applying exposure velocities of zero or 1 to obtain the distribution of times spent in individual components of the stream-sediment system. These can be aggregated through convolution to give the total residence time, or individual components may be first modified to represent sorption and removed prior to aggregation after Höhne et al. (2021). We present a model that contains three zones – stream, benthic (shallow sediment) and hyporheic (deep sediment) zones under steady flow conditions and interpret a multi-tracer study from the Erpe River to demonstrate the utility of the model. The presented approach offers insights into the contribution of key individual components to stream transport, greatly improving our understanding of the controls on stream transport and contaminant removal.

References

Ginn, Timothy R. 1999. ‘On the Distribution of Multicomponent Mixtures over Generalized Exposure Time in Subsurface Flow and Reactive Transport: Foundations, and Formulations for Groundwater Age, Chemical Heterogeneity, and Biodegradation’. Water Resources Research 35 (5): 1395–1407. https://doi.org/10.1029/1999WR900013.

Höhne, Anja, Jörg Lewandowski, Jonas L. Schaper, and James L. McCallum. 2021. ‘Determining Hyporheic Removal Rates of Trace Organic Compounds Using Non-Parametric Conservative Transport with Multiple Sorption Models’. Water Research 206 (November). https://doi.org/10.1016/j.watres.2021.117750.