Nitrogen transport and transformation in the saturated-unsaturated zone under recharge, runoff, and discharge conditions

Water level (WL) changes are an important factor in the fate and transport of nitrogen in the saturated-unsaturated zone. In this study, the fate of nitrogen was investigated under simulated conditions of replenishment, runoff, and discharge. Three patterns of water level changes of ascent, stability, and descent were simulated under laboratory conditions to study nitrogen transport and transformation. Three columns (I, II, and III) were used to simulate the conditions of replenishment, steady water level, and discharge, respectively. The nitrate-nitrogen (NO3 (-)-N), nitrite-nitrogen (NO2 (-)-N), and ammonia-nitrogen (NH4 (+)-N) concentrations observed at different depths were compared among the three columns at 46.5 and 251.5 h. The results indicated that the NO3 (-)-N concentration decreased with time in both the saturated and unsaturated zones of the three columns (columns I, II, and III). The maximum decreasing concentrations of NO3 (-)-N in the three columns were 14.3, 37.97, and 38.17 mg/L, respectively. However, NH4 (+)-N in the saturated zone increased with time, whereas the NH4 (+)-N concentration decreased in both the saturated and unsaturated zones of other columns. No significant change in NO2 (-)-N concentration was observed in the experiment. These results suggest that water level changes must be considered in the remediation of groundwater nitrate pollution in the field.