Characteristics and causes of acidic shallow groundwater in Gannan area in the upper reaches of Dongjiang River Basin ——Take Xunwu area as an example

[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 pH value, spatial distribution, hydrochemical types and action characteristics of acidic groundwater are analyzed by using hydrochemistry and mathematical statistics methods. From the point of view of material basis, circulation conditions and external factors, the causes of control of water rock action, aeration zone medium, acid rain, human action and other factors on acidic groundwater are discussed. [Results]The pH value of groundwater in the area is concentrated between 5.0 and 7.0; The spatial distribution has obvious correlation with geomorphic characteristics and circulation conditions:middle and low mountains3 type (6.0)>HCO₃-Cl type (5.05)>SO₄ type (4.8)>Cl type (4.5); Among the pH values of groundwater types in different media, pore fissure water of clastic rock > pore water of loose rock > fissure water of metamorphic rock >fissure water of magmatic rock.[Conclusions]The interaction between water and rock is the main role in the formation and evolution of pH value, and the dissociation of carbonic acid is the main material basis for the source of H+; Aerated zone medium and acid neutralization ability are the main water circulation conditions affecting the formation of acidic groundwater; Acid rain and human factors are external factors that cannot be ignored. The research results have a guiding role in understanding the origin of acidic groundwater, rational utilization and protection of groundwater resources in this area.