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 nearby surface water levels, precipitation, floods, and groundwater pumping. The associated change in saturation state and vertical movement of water through soil layers can profoundly influence dissolved organic matter (DOM) sources to shallow groundwater. To investigate the effects of water table fluctuations on DOM supply 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 a layered soil column at varying rates to simulate water table fluctuations within zones of more mineral or organic-rich soils over a period of 16 days. Flooding events were simulated by inundating the top of the column with local surface water. The effects of repeated wetting and drying events on carbon mobilization, DOM quality, and geochemical responses were measured. During initial water table fluctuations, redox potential near the soil-Mineral Interface was relatively stable but declined after subsequent wettings. DOC concentrations were consistently greater in shallow soil layers, and DOC-rich water moved downward into deeper layers after each wetting and draining cycle. As DOC concentrations in pore waters increased, SUVA at 254 nm also increased. Water table fluctuations had a measurable effect on the humification, fluorescence, and freshness indices of DOM. This study shows the influence of multiple saturation events on carbon mobilization and shallow groundwater biogeochemistry in unique floodplain soil sequences.National Science Foundation (NSF EAR-1752995)Friends of Orton HallMcKenzie & Brecher Scholarship FundNo embargoAcademic Major: Earth Science