Water tables in coastal aquifers respond to a variety of hydrologic forcings, including precipitation, coastal flooding, and tides. Water table fluctuations induce the flow of water and air across shallow organic-rich soils, which affects the supply of nitrogen (N), dissolved organic carbon (DOC), oxygen, and other reactive solutes, and leads to changes in water quality. My goal was to investigate reactive N transport near a fluctuating water table using a meter-long column containing reconstructed coastal soil and aquifer material from a Mediterranean site (Mataró, Spain). I continuously monitored in-situ redox potential, soil moisture, and matric potential and collected frequent pore water samples for analysis of dissolved inorganic N species and DOC over 16 days. Local groundwater containing high concentrations of nitrate-N (16.5 mg/L) was supplied to the column base. As the water table rose and fell, redox potential fluctuated widely from -600 to 600 mV within the zone of variable saturation. Redox potential typically 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 that nitrate was depleted near the zone of fluctuation, while ammonium, nitrite, and DOC were elevated, relative to groundwater entering the base of the column. When the water table rose, nitrate was transported up into soils from the groundwater, and concentrations fell as denitrification occurred in the presence of DOC. At the end of the experiment, the column was flooded with seawater at the top of the column. Seawater mobilized ammonium, nitrate, and DOC in the vadose zone, but nitrate did not accumulate beneath the water table, presumably due to enhanced denitrification. Seawater flooding therefore has the potential to mobilize accumulations of N in soils if an ample supply of DOC is not present. In the absence of seawater inundation, water table fluctuations may help remove large nitrate concentrations in groundwater by relieving the DOC limitation to denitrification. These processes are especially relevant for groundwater quality in agricultural and densely populated Mediterranean coastal settings, which are expected to experience new dynamics in water table elevation and soil saturation under a changing climate and rising sea levels. 

Our experimental setup consisted of a meter-long soil column that was placed into a large tank that was floods periodically over the course of 16 days. Within our column we had Rhizon porewater samplers placed every 15 cm starting at a 10 cm depth. We had three redox sensors placed at 10, 40, and 100 cm depths. Two soil moisture probes located at 20 and 55 cm and a tensiometer at 55 cm. We pumped fresh groundwater into our tank a we raised and lowered the syphon over here on the left to control the water table level. We began our experiment by doing a quick rise up to 30 cm depth and drop to 60 cm to saturate the aquifer material before letting things settle for a few days. This was then followed by several water table fluctuations on two day scales where we raised the water table up to 10 cm and back down to 60 cm with a final rapid raising and lowering on day 11. On day 14 we poured seawater collected from the Mediterranean into our column to simulate a flooding event from the sea.