The Genesis and Hydrochemical Formation Mechanism of Karst Springs in the Central Region of Shandong Province, China

With the intensification of human activities, the water resource environment in the karst mountainous area of central Shandong has undergone significant changes, directly manifested in the cessation of karst spring flows and the occurrence of karst collapses within the spring basin in the Laiwu Basin. To support the scientific development and management of karst water, this study utilizes comprehensive analysis and deuterium-oxygen isotope test data from surveys and sampling of 20 typical karst springs conducted between 2016 and 2018. By integrating mathematical statistics, correlation analysis, and ion component ratio methods, the study analyzes the genesis, hydrochemical ion component sources, and controlling factors of typical karst springs in the Laiwu Basin. The results indicate that the genesis of karst springs in the Laiwu Basin is controlled by three factors: faults, rock masses, and lithology, and can be classified into four types: water resistance controlled by lithology, by faults ,by basement and by rock mass. The karst springs are generally weakly alkaline freshwater, with the main ion components being HCO3- and Ca2+, accounting for approximately 55.02% and 71.52% of the anion and cation components, respectively; about 50% of the sampling points have a hydrochemical type of HCO3·SO4-Ca·Mg. Stable isotope (δ18O and δD) results show that atmospheric precipitation is the primary recharge source for karst springs in the Laiwu Basin, and there is varying degrees of evaporative fractionation and water-rock interaction during the groundwater flow process, resulting in significantly higher deuterium excess (d-excess) in sampling points on the southern side of the basin compared to the northern side, indicating clear differentiation. The water-rock interaction during the karst groundwater flow process and human activities are the main controlling factors for the formation of hydrochemical components: the dissolution of carbonate rocks, mainly calcite, is the primary source of HCO3-, SO42-, Ca2+, and Mg2+; the dissolution of salt rocks and reverse cation exchange adsorption contribute to the weak surplus of Ca2+ and Mg2+ and the deficit of Na+ and K+ ions in karst springs; calculated saturation index (SI) values indicate that gypsum, aragonite, calcite, and dolomite in the springs are saturated, while the SI value of salt rock is unsaturated. The mixing of urban domestic sewage, agricultural planting activities, and the use of manure also contribute to the formation of Cl- and NO3- ions in karst springs.