Comparison of effects of inset floodplains and hyporheic exchange induced by in-stream structures on solute retention

The pollution of streams and rivers is a growing concern, and environmental guidance increasingly suggests stream restoration to improve water quality. Solute retention in off-channel storage zones, such as hyporheic zones and floodplains, is typically necessary for significant reaction to occur. Yet, the effects of two common restoration techniques, in-stream structures and inset floodplains, on solute retention have not been rigorously compared. We used MIKE SHE to model hydraulics and solute transport in the channel, on inset floodplains, and in structure-induced hyporheic zones of a third-order stream. We varied hydraulic conditions (winter base flow, summer base flow, and stormflow), geology (hydraulic conductivity), and stream restoration design parameters (inset floodplain length and presence of in-stream structures). The in-stream structures induced hyporheic exchange for approximately 20% of the year (during summer base flow) while inset floodplains were active for approximately 1% of the year (during stormflow). Flow onto inset floodplains and residence times in both the channel and on the floodplains increased nonlinearly with the fraction of bank with floodplains installed. The fraction of streamflow that flowed onto the inset floodplains was 1-3 orders of magnitude higher than that which flowed through the structure-induced hyporheic zone. Yet, residence times and mass storage in the hyporheic zone were 1-5 orders of magnitude larger than that on individual inset floodplains. In our modeling, neither in-stream structures nor inset floodplains had sufficient percent flow and residence times simultaneously to have a substantial impact on dissolved contaminants flowing downstream.