Carbon Delivery to Floodplain Aquifers in Response to Water Table Fluctuations: Observations from Soil Column Experiments
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Date
2023-08
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Publisher
The Ohio State University
Abstract
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.
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Keywords
Dissolved Organic Carbon, Nutrients in Aquifers, Dissolved Organic Matter, DOM, Floodplain Aquifers