In order to ensure a sustainable water resource for residents of the lower reaches of the Changhua River on Hainan Island, it is crucial to understand the chemical characteristics and formation mechanisms of their shallow groundwater. To achieve this, we collected and analyzed 100 groundwater samples using various methods. Our findings indicate that the groundwater is generally near-neutral to weakly alkaline, suggesting an oxidized environment. We identified 56 chemical types, with Ca•Na-HCO₃ and Ca-HCO₃ being the most common. Water–rock interactions—particularly with silicate rocks—are the primary factor influencing the water’s chemical characteristics, with Na⁺ and K⁺ coming from the leaching of silicate rocks, and Ca²⁺ and Mg²⁺ originating from the dissolution of carbonate rocks. Unfortunately, human activities such as agricultural practices and domestic sewage have had a significant impact on the groundwater, with NO₃⁻ levels surpassing SO₄²⁻ in most samples. These findings provide valuable insights into the development and protection of the shallow groundwater in this area.

The lower reaches of the Changhua River are located in the northwest of Hainan Island, and the shallow groundwater in this area is the main water source for residents living at both sides of the river. In order to sustain the exploitation and use of water resources in this area, it is essential to clarify the chemical characteristics and the formation mechanism of the groundwater. We collected 100 sets of shallow groundwater samples from the lower reaches of the Changhua River, and analyzed the chemical characteristics of the groundwater in shallow aquifers as well as its controlling factors by means of the methods including descriptive statistical analysis, Piper diagram, Gibbs diagram, and ion ratio. The results show that the overall characteristics of groundwater in the study area are near-neutral to weakly alkaline nature, indicating a generally oxidized environment. The groundwater involves in 56 chemical types, and the most predominant types are Ca•Na-HCO3 and Ca-HCO3, respectively. NO- 3 has become a macro component exceeding SO2- 4in most of the sampling groundwater, suggesting a significant impact of human activities in this area. At the same time, the chemical characteristics of groundwater in the study area are mainly affected by the water-rock interaction, while the water-rock interaction is mostly affected by the dissolution of silicate rocks and less by the dissolution of carbonate rocks and evaporite rocks. According to the ion ratio results, Na+ and K+ in the study area are mainly from the leaching of silicate rocks, Ca2+ and Mg2+ are mainly from the dissolution of carbonate rocks, in addition to a certain degree of cationic exchange. NO- 3 is mainly related to human activities, and the human activities are more affected by agricultural activities and domestic sewage than by industrial and mining activities. This study can provide a reference for the development and protection of shallow groundwater in the lower reaches of the Changhua River.