Subsurface Fe (II) affects concentrations of dissolved O2 in streamwater

The interface between groundwater and surface water is a critical zone influencing ecohydrological and biogeochemical cycles within surface water ecosystems. It is characterized by complex redox gradients, with groundwater-mediated inflow of reduced substances affecting the oxygen budget of stream water. In this study, we have experimentally simulated the inflow of Fe(II)-rich groundwater into the open stream water of a flume system to quantify its effect on dissolved oxygen concentration in the stream water and the hyporheic zone. Our results indicate that Fe(II) inflow substantially decreased DO concentrations in both the pore and open waters. Oxygen uptake rates increased from 7.4 up to 8.6 g O2 m-2 d-1 at a mean flow rate and from 1.7 to 1.9 g O2 m-2 d-1 at a low flow rate. This accounts for a contribution of Fe(II) input to the overall uptake rate of between 21 and 17%, respectively. Treatment with FeCl2 led to a substantial increase in DOC (> 60 mg L-1) suggesting a linkage between Fe(II) mobilization and the occurrence of DOC. This study highlights the need to consider the effects of hyporheic and riparian redox processes and subsequent inflow of Fe(II) into streams on the oxygen budget of stream ecosystems.