Oxygenation of aquifers with fluctuating water table: A laboratory and modeling study

Intertidal sediments host many biogeochemical processes which drastically affect coastal ecosystems. Dissolved Oxygen (DO) plays a major role in many of these biogeochemical reactions, and is of primary importance for intertidal groundwater quality. Groundwater level of intertidal zone fluctuates regularly corresponding to the tidal cycle. During the fluctuation of water table, the oxygen-rich air could be entrapped in the pore space of sediments, and hence serve as an important oxygen source to replenish oxygen-depleted groundwater. Although oxygenation enhanced by fluctuated water table has been studied in river banks, no study has been conducted in ocean intertidal sediments under tide-induced groundwater table fluctuations, and quantification of interphase oxygen mass transfer is lacking. In this study, combined laboratory experimental and numerical modeling approach is developed to study the mechanism of oxygenation of aquifers with regularly fluctuated water table. Two column experiments are conducted with regular multiple drainage-imbibition periods to mimic the Oman sea tidal cycles controlled by 3.5 and 5.75 mL/min of pumping rate respectively. The numerical model is developed and calibrated by the column experiment data, and used to simulate two additional columns by increasing the entrapped air saturation to 0.25 from 0.15 and using 30 ppt of saline water respectively. The results suggest that the larger water table fluctuation leads to 35% more dissolved oxygen, and higher entrapped air saturation enables 22% more oxygen transferred to the anoxic aqueous phase and significantly enhanced the oxygenation of the aquifer, especially the deep zone below the water table. Dissolved oxygen is 4.81% less in the column with 30 ppt of saline water than with fresh water.