Wetland stream sediment redox biogeochemistry is subject to hydrological forcing and inter- and intra-seasonal variations. These variations are especially pronounced in intermittent streams where surface flow depends on shallow subsurface hydrological connectivity and precipitation. Intermittent stream reaches may experience a net loss of oxic stream water (influent conditions) that are influenced by precipitation events or a net gain of reduced subsurface fluids (effluent conditions) during prolonged dry periods. Effluent conditions lead to hyporheic water mixing with surface water and the potential for hot spots and hot moments of enhanced biogeochemical redox activity with consequences for broader-scale processes. For example, the oxidation of effluent Fe(II) near the sediment-water interface may result in iron flocculant which decreases longitudinal transport of Fe(II), while simultaneous depletion of dissolved oxygen may reduce the capacity for aerobic methane oxidation and increase the release of the greenhouse gas to surface waters. Such dynamics, however, remain poorly quantified. This combined pore- and meso-scale study aims to quantify the impact of redox variations associated with hydrological forcing on carbon remineralization pathways and the potential release of greenhouse gases from stream and wetland sediments. Vertical hydraulic stream gradients demonstrate that horizontal hyporheic flow is a major transport pathway linking pore- and meso-scales. A combination of voltammetric in situ semi-continuous measurements and depth profiling at discrete timepoints highlights the spatial distribution and temporal variability of biogeochemical redox processes. At a typically gaining reach, dissolved sulfides persist through wet and dry periods, at times occurring closer to the sediment-water interface than expected, while aqueous clusters of FeS display a transient signal. In contrast, extended drought-like periods induce the formation and persistence of iron floc mats at the oxic-anoxic interface that coincide with consistent production of organic-Fe(III) complexes. Preliminary monthly methane data collected during the same drought period reveal sustained micromolar-level concentrations throughout all sediment profiles and suggest a relatively constant flux to overlying stream water. Together, the observed redox variations correspond to a variety of potential pathways for carbon remineralization with implications for greenhouse gas emissions.

Effect of hydrological forcing on biogeochemical redox processes and greenhouse gas emissions

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.