Shallow groundwater is vulnerable to major pollutants, such as nitrate-nitrogen (NO₃-N). The pollution characteristics of NO₃-N need to be understood to protect the quality of shallow groundwater. As already known, shallow groundwater contamination by NO₃-N is closely linked to agricultural fertilizers, domestic animals, and sewage. Correspondingly, nonpoint pollution sources such as fertilizers and sewage should be characterized by land use type. In this study, shallow groundwater contamination was examined according to three land use types (forest-dominant, agricultural-dominant, and mixed areas). Principal component analysis was adopted to extract two chemical components (Cl and SO₄) related to NO₃-N among a total of 11 components. Cluster analysis was performed through a self-organizing map by applying three water quality clusters to the three land use areas. Finally, the origin of NO₃-N contamination was evaluated based on the ratio of [NO₃-N]/[Cl]. As a result, the forest-dominant area was characterized by a natural origin of NO₃-N with a [NO₃-N]/[Cl] ratio of 0.45; the agricultural-dominant area was indicated by NO₃-N from nitrogen fertilizer with a [NO₃-N]/[Cl] ratio of 0.56; and the mixed area was designated by NO₃-N from sewage with a [NO₃-N]/[Cl] ratio of 0.34.