Identification of nitrification and denitrification along groundwater flow paths using dissolved N2, Ar, and N2O in typical groundwater flow systems in the Qingyi River basin

Seasonally different precipitation infiltration under monsoon humid areas may drive changes of groundwater flow systems and possible nitrate transformation processes in groundwater. In this study, dissolved greenhouse gases, noble gases concentrations (N2 and Ar) and isotopes of N2O were used to quantitively identify nitrification and denitrification to reveal spatial and temporal characterization of nitrate transformation in typical groundwater flow profiles in the Qingyi River basin, east China. In dry and wet seasons, the recharge altitudes of groundwater were distinctive and dominant nitrate transformation processes differed spatially and temporally. According to the N2-Ar estimation, the recharge altitudes of groundwater in dry season were higher than those in wet season, indicating obviously less proportion of precipitation from lower altitudes and relatively increased proportion of recharge from regional recharge areas in dry season, whereas local groundwater flow systems were preferentially developed in wet season. Denitrification is commonly observed in groundwater during the dry season, with positive Excess-N2 concentrations and phenomena that N2O concentrations initially accumulate with progress of denitrification but later decrease due to enhanced N2O reduction. In the wet season, nitrification is the dominant process in groundwater, with only a small portion of groundwater exhibiting denitrification, resulting in positive Excess-N2 concentrations. In this case, N2O concentrations initially increase during nitrification but later decline due to incomplete denitrification. Quantitative results based on δSP-N2O isotopes indicated that the maximum contribution of nitrification in groundwater during the wet season ranged from 52.8% to 100%, with an average of 77.3%. The contributions from denitrification and N2O reduction in wet season are limited, which is consistent with results identified by nitrate and ammonium isotopes. Spatially, due to more reducing redox environment in regional groundwater flow systems, the denitrification progress (DP) in most groundwater in discharge zones exceeds 99%, with denitrified NO3− concentrations reaching up to 25.72 mg/L, significantly higher than the average DP values in recharge zones (27.7%) and transition zones (31.6%).