Tracing groundwater nitrate sources in an intensive agricultural region: Integration of self-organizing map (SOM) and end-member mixing model tool (EMMTE)

Abstract

The traceability of groundwater nitrate pollution is crucial for controlling and managing polluted groundwater. This study integrates hydrochemistry, nitrate isotope (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), self-organizing map (SOM), and EMMTE models to identify the sources and quantify the contributions of nitrate pollution to groundwater in an intensive agricultural region. The results indicate that the NO₃^--N concentration in 74% of the groundwater samples exceeded the WHO standard of 10 mg/L. According to the results of EMMTE modeling, soil nitrogen (68.4%) was the main source of nitrate in Cluster-1, followed by manure and sewage (16.5%), chemical fertilizer (11.9%) and atmospheric deposition (3.3%). In Cluster-2, soil nitrogen (60.1%) was the main source of nitrate, with a significant increase in the contribution of manure and sewage (35.5%). The considerable contributions of soil nitrogen may be attributed to the high nitrogen fertilizer usage that accumulated in the soil in this traditional agricultural area. Moreover, it is apparent that most Cluster-2 sampling sites with high contributions of manure and sewage are located around residential land. Therefore, the arbitrary discharge and leaching of domestic sewage may be responsible for these results. This results provide useful assistance for the continuous management and pollution control of groundwater in this area.