Deciphering Nitrate Sources and Biogeochemical Pathways Along Intermediate Flow System in Volcanic Aquifer of the Denpasar Basin, Bali, Indonesia

<title>Abstract</title> <p> Nitrate source and transformation pathways within volcanic aquifers remain poorly understood, particularly where subsurface flow paths intersect mixed land use patterns. Moreover, relying on concentration of pollutants overlooks the extent of contamination. This research aims to evaluate spatial distribution of nitrate and associated hydrochemical indicator and to identify nitrate origins and transformation process along the groundwater flow path in the Denpasar Basin. A total of 70 samples were collected and analyzed for nitrate and chloride parameters by ion chromatography, nitrogen origins by dual nitrate isotopes (δ ¹⁵ N-NO ₃ ⁻ and δ ¹⁸ O-NO ₃ ⁻ ), and δ ¹⁸ O-H ₂ O using isotope ratio mass spectrometer. The results showed hydrochemical concentrations increased progressively along the groundwater flow path from high and middle elevation areas to the low elevation zone. Hydrochemical ratio and dual isotope fingerprinting estimate that agriculture fertilizers are the primary source of nitrogen in the high and middle altitude recharge zone, whereas domestic sewage acts as the primary contaminant source in the urbanized lowland discharge zone. The variability of nitrate in the urban region clusters within a specific δ ¹⁸ O-H ₂ O composition, demonstrating that an intermediate flow system regulates contaminant transport. Natural attenuation is evidenced by a robust microbial denitrification slope of 0.53. However, highly oxygenated recharge maintains dissolved oxygen &gt; 2 mg/L, restricting denitrification to micro-anoxic matrices while inducing localized oxygen isotope exchange during re-nitrification. These findings demonstrate that incorporating multi-tracer approaches creates a reliable approach to capture early-stage aquifer degradation and formulate sustainable groundwater management strategies. </p>