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Indonesian coastal alluvial plain has a large groundwater storage capacity. However, human perturbation to the groundwater environment has resulted in groundwater contamination. Nitrogen species are parameter those usually detected in Indonesian coastal groundwater. The main objective of this study was to evaluate the possible sources of ammonium (NH4 +-N), as one form of nitrogen species, using both chemical properties and geographic information. The study site was Indramayu, a developing rural area located in the northern part of West Java. In August 2017, 20 groundwater and river water samples were collected from sites representative of various types of land-use. The chloride (Cl-) concentration in the samples ranged from 15 mg/L to 12,000 mg/L. Most of the locations represented fresh-brackish to brackish-salt water conditions, indicating the influence of seawater on the groundwater samples. Furthermore, analysis of several nitrogen species identified high NH4 +-N concentrations in the samples. Based on the relationship between Cl- and NH4 +-N, samples were clustered into three groups. The first and second groups exhibited linear correlations, while in the third group, no relationship between these two parameters was identified. The land-use associated with each group was then evaluated. The main NH4 +-N source in the first group was identified as organic sediment, where NH4 +-N may be released through ion exchange with sodium, facilitated by the high salinity conditions. The source of NH4 +-N in the second group was suggested to be a combination of sediment, fertilizer, and human and animal waste. Finally, it was hypothesized that the source of NH4 + in the third group is mainly human waste. [ABSTRACT FROM AUTHOR]