Tracing Nitrate Contamination Sources and Transformations in a Rural−Urban Karst Groundwater System in North China Using Multiple Isotopes and Simmr Modeling

In temperate karst aquifers under intensive anthropogenic impacts and high heterogeneity, groundwater contamination tracking has predominantly focused on nitrate, but inadequate evidence for co‐occurring ammonium sources undermines accurate nitrogen pollution assessments. This study pioneered the application of a isotope approach within the groundwater flow framework of the Jinan Spring Catchment, constructed a novel ammonium‐nitrate isotope tracing system (, , and ) for full‐form source tracking, and implemented the Bayesian mixing model (Simmr) to quantitatively apportion nitrate sources in karst groundwater. This integrated system enabled simultaneous NH4+‐N and NO3−‐N pollution source identification, enhancing the resolution of key nitrogen cycling pathways, particularly ammonium‐dominated nitrification. The NO3−‐N was the dominant form of inorganic nitrogen in karst groundwater (0.4−42.2 mg/L), and demonstrated an overall upward trend from 1958 to 2019. and nitrate isotope both confirmed that NH4+‐N and NO3−‐N primarily originated from ammonium fertilizer and soil nitrogen. The Simmr model quantified ammonium fertilizer (28.4%−58.3%) and soil nitrogen (23.1%−62.7%) as primary contributors to groundwater NO3−‐N. Hydrochemical and dual nitrogen isotope evidences revealed that mineralization and re‐nitrification of soil nitrogen, nitrification of ammonium fertilizers, and mineralization‐immobilization‐turnover of nitrate fertilizers all promoted nitrate accumulation in karst groundwater. Groundwater flow analysis identified mixing between shallow and deep karst groundwater as the primary mechanism for nitrate attenuation of spring waters in the urban discharge area, with denitrification playing a negligible role. These findings provide new insights into nitrogen behavior in temperate karst groundwater, offering valuable guidance for water resource management and protection in similar karst systems.