This paper is the result of hydrogeological survey engineering.

Objective Gannan area in the upper reaches of Dongjiang River Basin is a typical water shortage area in China. The distribution of acid groundwater intensifies the lack of regional domestic water.

Methods In this paper, Xunwu area in the upper reaches of the Dongjiang River basin is selected as the study area. Shallow groundwater is systematically collected, and the hydrochemical characteristics including pH value, spatial distribution, hydrochemical types and action characteristics are analyzed by using hydrochemistry and mathematical statistics methods. From the perspective of water cycle, the causes of acidic groundwater are discussed, including atmospheric precipitation (acid rain and human activities), aeration zone characteristics, aquifer lithology and water-rock interaction.

Results The pH value of groundwater in the area is concentrated between 5.0 and 7.0, showing overall acidity; The spatial distribution has obvious correlation with geomorphic characteristics and circulation conditions: middle and low mountains<hills, recharge area<discharge area; The hydrochemical types are dominated by low salinity HCO₃ type with weak acid neutralizing capacity. Its pH value is HCO₃ type (6.0)>HCO₃−Cl type (5.05)>SO₄ type (4.8)>Cl type (4.5); Among different aquifer media, the pH value of pore fissure water of clastic rock is the highest, while that of fissure water of magmatic rock is the lowest.

Conclusions Acidic groundwater is an important manifestation of hydrochemical characteristics, and its origin is mainly controlled by water cycle process: acid rain infiltration is the main source of H⁺, weak buffer capacity of aeration zone and low acid neutralization capacity of aquifer are important circulation conditions, sulfide oxidation in water-rock interaction contributes significantly, while carbonic acid dissociation is relatively secondary. Acid rain and human activities are important external influencing factors. The research results have a guiding role in understanding the origin of acidic groundwater, rational utilization and protection of groundwater resources in this area.