Continuous monitoring of dissolved inorganic nitrogen (DIN) transformations along the waste-vadose zone-groundwater path of an uncontrolled landfill, using multiple N-species isotopic analysis

Landfill leachates contain a heavy load of dissolved inorganic nitrogen (DIN), posing a threat to water resources. Therefore, it is highly important to understand the processes that control its evolution (speciation, accumulation, or attenuation) during the percolation of leachates through the unsaturated zone, finally affecting the groundwater. However, tracking DIN transformations in this complex and inaccessible environment is challenging, and knowledge concerning this important topic under field conditions is scarce. The presented study used a unique monitoring system that allows sampling of repetitive samples from within the waste and the unsaturated zone. An array of 8 wells penetrating the underlying aquifer completed the spatial observation. Multiple N-species isotopic approach was applied to discern the dominating N-involving processes over the continuum - from the waste mound through the unsaturated zone and the underlying aquifer. Despite the considerable heterogeneity observed throughout the profile, the results provided a cohesive and valuable reflection of the evolution of the inorganic nitrogen pool in this highly contaminated environment. Leachates inside the waste had reducing characteristics with high accumulation of ammonium (up to 360 mg/l NH4+-N), and a distinct delta N-15-NH4+ range (-3 parts per thousand to +10 parts per thousand). The upper layers of the unsaturated zone underneath the landfill margins found to be aerated, promoting N oxidation which resulted in the accumulation of nitrate in the leachates (up to 490 mg/l NO3-N). Exceptionally high concentrations of nitrite (up to 126 mg/l NO2-N) were found as oxygen levels decreased in deeper sections of the vadose zone. Enrichment of delta N-15-NO2- compared to delta N-15-NO3- indicated the significance of autotropic nitrite reduction, controlling the DIN composition, correlated with NO2- accumulation and net DIN attenuation. The delta N-15: delta O-18 ratio implied co-occurrence of denitrification in the leachates, even in the more oxidized sections, further contributing to N-attenuation in the unsaturated zone. In the aquifer, delta N-15-NH4+ values and delta N-15: delta(18) ratio linked N contamination to the leachates source. The encounter with the oxidized groundwater promoted intensive nitrification. delta N-15-NO2- values in the groundwater were lighter than both delta N-15-NH4+ and delta N-15-NO3- by 22 parts per thousand to 62 parts per thousand, implying the co-occurrence of nitrification-denitrification processes. The effect of denitrification grew with decreasing dissolved oxygen (DO) levels below 0.5 mg/l towards the center of the plume, contributing to net DIN attenuation in the plume. The findings are significant for any consideration of the risk posed by DIN, as well as remediation measures, in a landfill environment and other sites with a heavy load of degrading organic matter.

Automated summary

This study investigates the evolution of inorganic nitrogen in landfill leachates and found that there is a high accumulation of ammonium in the waste, aerobic conditions in the upper layers of the unsaturated zone promote nitrification, and nitrite accumulates as oxygen levels decrease in deeper sections. The findings are significant for understanding the risk of DIN and implementing remediation measures in landfill environments.