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 both the stream water and the hyporheic zone. The experimental setup consisted of 12 flumes, half used for input of groundwater augmented with Fe(II), while the other half served as a control. We studied the effects of coarse (6% fine sediment content) sediment vs. fine (28 % fine sediment content) sediment as well as and moderate (3 L s^-1) vs. low (0.5 L s-1) flow rate in a fully-crossed, 3-way-replicated design. Weekly sampling campaigns were performed to analyze Fe(II), Fe(III), DOC, and dissolved oxygen (DO) concentrations in the pore water (hyporheic zone) and in the open water over five consecutive weeks. 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 moderate flow rate and from 1.7 to 1.9 g O₂ m^-2 d^-1 at a low flow rate. This corresponds to a contribution of the Fe(II) input to the overall oxygen uptake rate in the flumes of 21 and 17%, respectively. Treatment with FeCl₂ also led to a substantial increase in DOC from ∼ 55 mg L^-1 in the control flumes to > 60 mg L^-1 suggesting a linkage between Fe(II) mobilization and the occurrence of DOC. In conclusion, 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 and the health of stream ecosystems.