Groundwater born Fe(II) affects concentrations of dissolved O2 in stream water

The interface between groundwater and surface water is a critical zone that influences ecohydrological and biogeochemical cycles within the surface water ecosystems. It is characterized by complex redox gradients. with groundwater-mediated inflow of reduced substances into the stream water. 
In this study, we have experimentally simulated the inflow of Fe(II)-rich groundwater with concentratrion of up to 1000 mmol L-1  into the open stream water of a flume system in order to quantify its effect on dissolved oxygen concentration in both the stream water and the hyporheic zone. The experimental setup consisted of 24 flumes, 12 of which were used for input of groundwater augumented with Fe(II), while the another 12 were used as controls, i. e., with inflow of Fe(II)-free groundwater. In addition, the experimental set-up provided the possibility to study the effects of fine sediment (coarse reference substratum (5% fine sediment content) vs. added fine sediment (35% fine sediment content) and low discharge (reference flow conditions vs. low discharge (drought) conditions within a threefold replicated, crossed design. All flumes had permanent groundwater input during the experiment. Fortnightly sampling campaigns were performed to analyze Fe(II), Fe(III),  and DO, concentrations in the porewater (hyporheic zone) and the open water over five consecutive weeks. 
Our results clearly indicate that Fe(II) inflow resulted in a decrease of DO concentrations both in the porewater and subsequently in the open water, with distinct effects of sediment porosity and discharge. Over the five weeks, the sustained decrease between 40 and 50% of DO concentrations was more pronounced in flumes with fine sediment than in flumes with coarse sediments. Our findings suggest that increasing the Fe(II) concentration in the hyporheic zone can affect the availability of oxygen, important in controlling biogeochemical and ecological processes, microbial activities, and aquatic life. The formation of oxygen-depleted subsurface and surface waters in freshwater ecosystems has been associated with nutrient-rich waters stimulating eutrophication and the subsequent reduction of river health. In conclusion, this study highlights the importance of considering the effects of hyporheic redox processes and Fe(II) in assessing the health of stream ecosystems.