<![CDATA[Nitrate is difficult to manage in shallow urban aquifers where polluted rivers and mixed land uses stress the same groundwater. This study examined the extent to which Kali Belik contributes to the long-term rise in nitrate at Umbul Pace, a spring-fed well supplying the SPAM TOYAGAMA system in the Yogyakarta-Sleman volcanic aquifer, Indonesia. The analysis combined water-quality sampling, spatial analysis, and a calibrated flow-and-transport model (RMSE = 1.39 m) over a 2.375 km² domain. The model showed Kali Belik losing water to the aquifer, yet the evidence does not support the river as the main nitrate source. Nitrate at Umbul Pace rose from 1.93 mg/L as NO?-N in 2001 to 16.5 mg/L in August 2025, while the river never exceeded 4.61 mg/L. Even with the river load doubled in the model, the concentration at the well reached only 5.01 mg/L. Backward particle tracking (77-123 days) delineates a local capture zone dominated by settlements, agriculture, and livestock facilities, pointing to persistent anthropogenic loading and land degradation along the recharge corridor as the more plausible explanation. Aquifer protection therefore cannot rely solely on the river: travel-time zoning, land-use controls, and nitrate treatment at the wellhead are needed.]]>
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