Carbon delivery to floodplain aquifers in response to water table fluctuations: Observations from soil column experiments

Water tables in floodplain aquifers rise and fall over a variety of timescales in response to changes in recharge, discharge, floods, and water use. To investigate the effects of water table fluctuations on DOC delivery to groundwater, an experiment was conducted at two Mediterranean sites: a pristine forested stream and an urban coastal floodplain. Groundwater was pumped into and out of the bottom of the soil column at varying rates to simulate water table fluctuations over a period of 16 days. Flooding events were imitated by inundating the top of the column with water sourced from nearby surface water features. The effects of repeated wetting and drying events on carbon mobilization, DOM quality, and geochemical responses were measured. Preliminary analysis reveals strong downward movement of DOC from soil layers after wetting events. SUVA at 254 nm increased with DOC concentrations compounds within pore waters during wetting events. During initial water table fluctuations, redox potential near the soil-aquifer interface was relatively stable but declined after subsequent wettings. Forthcoming analyses will also examine changes in the humification, fluorescence, and freshness indices of DOM from excitation-emission matrices. This study shows the influence of multiple saturation events on carbon mobilization and shallow groundwater biogeochemistry in unique floodplains.