Hydrogeochemical evolution characteristics of Baotu Spring in Jinan City, based on long-term monitoring

Baotu Spring in Jinan City is of great significance in the fields of ecological regulation, history and culture, tourism economy, etc.; therefore, it is urgent to study its hydrogeochemical evolution characteristics and genesis mechanism, so as to provide a scientific basis for the high-quality protection of Baotu Spring. Based on the long-term hydrogeochemical monitoring data of Baotu Spring, its water chemical dynamic characteristics as well as its hydrogeochemical evolution over past 60 years have been analyzed in this study.

The results show that the chemical composition of Baotu Spring presented a significant regularity from 1958 to 2022. The average contents of main ions in Baotu spring were ${\rm{HCO}}_3^{-}$, Ca²⁺, ${\rm{SO}}_4^{2-}$, Cl⁻, ${\rm{NO}}_3^{-}$, Na⁺, Mg²⁺ and K⁺ in a descending order, among which ${\rm{HCO}}_3^{-}$ and Ca²⁺ were the dominant ions of anion and cation in spring water. The variation coefficients of ${\rm{SO}}_4^{2-}$ and Na⁺ contents were relatively large, indicating the varieties of groundwater environment in different periods. The variation coefficients of ${\rm{HCO}}_3^{-}$, Ca²⁺ and Mg²⁺ contents were relatively small, showing that the source of related ions was stable. The specific gravity of ${\rm{SO}}_4^{2-}$, Cl⁻, and Na⁺+K⁺ gradually increased, and the hydrochemical type evolved from HCO₃-Ca to HCO₃·SO₄-Ca. In recent years, the hydrochemical type has shown the HCO₃·SO₄-Ca·Na type in several periods, indicating that the water chemical components of Baotu Spring are increasingly complex and diversified. The water chemical components of Baotu Spring in the Gibbs chart showed a trend of upward shift to the right, indicating that this spring was mainly controlled by water-rock interaction, and was increasingly influenced by other factors such as human activities. The ion ratio method indicates that the mineral weathering dissolution of Baotu Spring shifts from carbonate rock to silicate rock, and the ratio of (K⁺+Na⁺)/Cl⁻ is generally below and gradually away from the line of 1∶1, indicating that there are other sources of Cl⁻ different from those of Na⁺, and the sources have increased year by year. The contents of Ca²⁺ and ${\rm{HCO}}_3^{-}$ ions in Baotu Spring show a gradual increasing trend, which indicates that the dissolution of carbonate rock has increased gradually. In addition, the Chlor-alkali index (CAI) shows that the water-rock interaction in Baotu Spring is dominated by the dissolution of carbonate rock, while the dissolution of gypsum and silicate rock plays a secondary role, but has gradually increased, indicating that the spring recharge area in the southern part of the gypsum and silicate rock stratum has a stronger recharge effect on groundwater runoff in Baotu Spring. The cation exchange in Baotu Spring is weak on the whole, but it has gradually increased over the years, and the cation exchange is stronger in the dry season than the wet season. The mineral saturation index of Baotu Spring is in a supersaturation state as a whole, and its dispersion becomes larger with the passage of time. Over the years, the mineral saturation index of Baotu Spring during the wet season has been generally higher than that during the dry season and normal season, but the performance is various in different periods. From 1950s to 1980s, the mineral saturation index of Baotu Spring during the wet season was lower than that during the dry season, and the mineral saturation index of Baotu Spring during the wet season was higher than that during the dry season after 1990s, indicating that with the increase of human activities in different historical periods, the source of groundwater and the water-rock interaction changed in dry and wet seasons. The contents of ${\rm{NO}}_3^{-}$/Cl⁻ and Cl⁻ increased first and then decreased under the influence of agricultural activities and domestic sewage discharge. In 1950s–1960s, the contents were greatly influenced by agricultural activities. Since 2000s, with the continuous improvement of spring protection and ecological environment, agricultural activities have gradually weakened the effect, while the influence caused by emission of domestic sewage and other human activities have been strengthened by degrees. Due to the influence of other human activities other than gypsum dissolution, the contents of ${\rm{SO}}_4^{2-}$ have increased gradually since 2010s.

In summary, in the quasi-natural state from 1950s to 1960s, the contents of chemical components of Baotu Spring were relatively low. From 1970s to 1980s, the contents were increasingly influenced by agricultural activities, industrial and mining activities and domestic sewage discharge. From 1990s to 2000s, the contents of ${\rm{NO}}_3^{-}$, Cl⁻ and ${\rm{SO}}_4^{2-}$ gradually increased under the comprehensive influence of human activities such as agricultural pollution in the recharge area, industrial and mining activities, domestic waste landfill, etc. Since 2010, the pollution of industrial and mining enterprises and domestic life has alleviated, and the protection measures of Baotu Spring such as ecological groundwater recharge have become normal. The contents of Ca²⁺ and Mg²⁺ in non-carbonate karst hydrolysis increased gradually, and the contents of Cl⁻ and ${\rm{SO}}_4^{2-}$ affected by human activities increased significantly. The research shows that the water chemical components of Baotu Spring have mainly come from the water-rock interaction since 1958, and the influence of human activities has been increasing continuously. In different historical periods, different human activities (agricultural activities, industrial and mining activities, groundwater recharge, etc.) have different effects on the hydrogeochemical evolution of Baotu Spring.