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

Abstract

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 δ¹⁵N-NH₄⁺ range (-3‰ to +10‰). 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 δ¹⁵N-NO₂^- compared to δ¹⁵N-NO₃^- indicated the significance of autotropic nitrite reduction, controlling the DIN composition, correlated with NO₂^- accumulation and net DIN attenuation. The δ¹⁵N: δ¹⁸O 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, δ¹⁵N-NH₄⁺ values and δ¹⁵N: δ¹⁸O ratio linked N contamination to the leachates source. The encounter with the oxidized groundwater promoted intensive nitrification. δ¹⁵N-NO₂^- values in the groundwater were lighter than both δ¹⁵N-NH₄⁺ and δ¹⁵N-NO₃^- by 22‰ to 62‰, 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.

Keywords: DIN; Landfill leachates; Nitrite reduction; Nitrogen attenuation; Nitrogen isotopes; Vadose zone.