Soil and sediment layers having sharply contrasting transport, redox conditions, and nutrient and contaminant loads are directly juxtaposed in floodplains, creating interfaces and conditions ideal for exchange of oxygen, carbon, and reactive solutes. Transport and mixing across these interfaces, driven by fluctuating hydrologic conditions (common in floodplains), as well as by chemical gradients (diffusion), give rise to zones of intense reactiveity that can profoundly influence the biogeochemical function of floodplains and river corridors. For example, these processes mediate carbon and iron retention or release and the compositions of ground- and surface water. Yet, our knowledge about molecular- to floodplain scale hydro-biogeochemical process coupling and the overall function of these interfaces remains limited.<br/><br/>The SLAC Floodplain Hydro-Biogeochemistry Science Focus Area project is studying the coupling between hydrological perturbations and biogeochemical responses across laterally extensive subsurface interfaces in the riparian floodplain of a snowmelt-dominated headwater catchment, Slate River, Gunnison County, CO, and in laboratory analog systems. In each case, we are using hydrological and reactive transport modelling to integrate signals from hydrological, microbiological, and geochemical measurements. We will present results showing that evapotranspiration, seasonal river flows, and the presence of ponded water drive bidirectional vertical flows, with major implications for export of micronutrients and contaminants from metal- and nutrient-rich floodplains to the surrounding watershed. Upward flows of oxic water and downward flows of sulfidic iron-rich water maintain diverse microbial communities and enable robust biogeochemical cycles. Moreover, discharge of colloidal and dissolved organic carbon, HS<sup>-</sup>, Fe(II), and other reactive products across anoxic-oxic interfaces promotes the establishment of reducing conditions in adjacent (or surrounding) aquifers. The intense biogeochemical reactivity of gravel-bed floodplains and their connectivity to rivers makes them important ecosystems within watersheds.

Hydro-biogeochemical dynamics of subsurface interfaces in riparian floodplains

ACS Fall 2021

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