Water table fluctuation effects on fate and transport of nitrate in the subsurface

Water table fluctuation (WTF) has been demonstrated to influence solute transport in subsurface, but yet the importance of WTF for solute movement has generally been ignored. In this study, laboratory tests were conducted to explore the effects of WTF on the transport and fate of nitrate. Two types of soil columns with fluctuating and static water table were used. The results showed that pore-water velocities, dissolved oxygen and redox potential at different locations were increased and decreased when the water table declined and increased. In this study, nitrate loss via denitrification was not the same above (unsaturated zone) and below (saturated zone) the water table, and the pattern of influence was different between the unsaturated and saturated zone. In the unsaturated zone, WTF enhanced the increasing trend of NO3--N concentrations and the decreasing trend of NH4+-N concentrations. The effect became more significant at the location being closer to the water table. In the saturated zone, WTF slowed down both the decreasing trend of NO3--N and the increasing trend of NH4+-N; the farther the distance from the water table, the more notable the effects. Furthermore, at both the unsaturated and saturated zones, the NO3--N concentrations would decrease with the increase of water table, and the NH4+-N concentrations would, however, increase. When the water table declined, the trends would become opposite. These results suggest that WIT should be considered when monitoring subsurface fate and transport of surface-applied NO3- in shallow ground water.