Genetic mechanism of karst water revealed by hydrochemistry and isotopes in Longnan, Gansu province

 ObjectiveCarbonate strata are widely distributed and rich in karst water in Longnan area of Gansu province, which is one of the important local water resources. Finding out the formation mechanism of karst water in Longnan region is of great significance for sustainable utilization of karst water resources and protection of water source. MethodsWater chemistry and isotopes have tracer characteristics, which can reveal the process of recharge, runoff and discharge of karst water, and indicate the scope and change of karst water system. ResultsThe hydrochemical types of karst water in the study area are predominantly HCO₃-Ca and HCO₃-Ca·Mg types. The total dissolved solids (TDS) range from 156.61 mg/L to 362.07 mg/L, with an average of 254.68 mg/L, indicating low mineralization. The hydrochemical composition of the karst water is mainly influenced by water-rock interactions. For samples GW1, GW5, GW10, and GW11, the water chemistry is primarily derived from the dissolution of three minerals: aragonite, calcite, and dolomite. For the remaining samples, the composition is mainly influenced by the dissolution of aragonite and calcite. Additionally, gypsum and anhydrite minerals in the overlying layers provide small amounts of SO₄²⁻ and Ca²⁺ to the karst water. Stable hydrogen and oxygen isotopes indicate that all karst water recharge is from atmospheric precipitation, with weak water-rock interaction. This is likely due to intense groundwater flow and short residence times, which limit the extent of interaction with the surrounding rocks. ConclusionsThe mechanism of karst water in Longnan, Gansu Province can be summarized as follows: the karst water is recharged by atmospheric precipitation; the karst aquifer is dominated by fissure water, which forms convergence and rapid migration in large karst pipes; and the spring emerges in the water-conducting fissure or water-blocking fracture zone. Due to the rapid movement of groundwater, the TDS is low and the water-rock interaction is weak.