Water table dynamics in coastal aquifer sediments alter nitrogen fate: Observations from soil column experiments

Water tables in coastal aquifers respond to a variety of hydrologic forcings, including precipitation, coastal flooding, and tides. The water table response to these forcings has the potential to impact water quality by affecting the fate and transport of nitrogen, particularly in coastal environments where nitrogen can accumulate in soils and water. To investigate the urban and agricultural reactions involving N that occur near the water table, a meter-long column containing reconstructed coastal soil and aquifer layers from a Mediterranean site was made. We continuously monitored in-situ redox potential, soil moisture, and water pressure and collected frequent pore water samples for analysis of dissolved organic carbon (DOC) and dissolved inorganic nitrogen species over 16 days while imposing water table fluctuations by injecting local groundwater rich in nitrate-N (~15 mg/L). In-situ redox potential in shallow soils (40 cm depth) ranged from -600-600 mV, which is indicative of alternating conditions favorable for aerobic and anaerobic respiration. Redox potential increased upon saturation and declined again as soils drained, with more subtle changes occurring during the first wetting and drying cycle and greater changes occurring during repeated cycles. Pore water analysis shows mobilization of DOC and ammonium-N in shallow soils and removal of nitrate-N in sandy aquifer layers. More specifically, DOC, nitrate-N, ammonium-N, and nitrite-N were greatest in the organic soils and decreased down the column into the sandy aquifer layers. Toward the end of the experiment, the column was inundated with seawater collected from the Mediterranean to simulate a flooding event, causing an increase in all N-species concentrations below 10 cm as seawater transported the nitrogen and DOC contaminants to depth. In contrast, when the column was flooded from the bottom, nitrate-N concentrations decreased as the soils became saturated, oxygen was depleted, and denitrification occurred. Overall, we see how water table dynamics impact the fate and transport of nitrogen in groundwater as soils are repeatedly saturated from above and below.