Hydrochemical characteristics of groundwater and spatio-temporal evolution of carbonate weathered carbon sink in Tianjin Plain
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Abstract:
Currently, owing to the significant impact of carbonate weathering processes, the chemical composition of groundwater in the Tianjin Plain exhibits a distinct horizontal zonation pattern, both from north to south and from north to east. This zonation is a result of complex geological and hydrogeological interactions that shape the unique characteristics of the groundwater in this region. The pH value of the groundwater typically ranges between 7.0 and 8.5, classifying it as neutral to slightly alkaline. This pH range is influenced by various factors, including the mineralogy of the aquifer materials and the presence of dissolved ions. One notable feature of the groundwater in the Tianjin Plain is its high total dissolved solids (TDS) content. This elevated TDS level is primarily attributed to the abundance of specific ions, notably sodium (Na+), chloride (Cl?), bicarbonate (HCO?3)，and calcium (Ca2+) ions. In order to study the hydrochemical characteristics of groundwater in Tianjin Plain and the spatial and temporal evolution characteristics of carbonate weathered carbon sinks, ArcGIS antidistance weight spatial evolution analysis, cluster analysis and SPSS data statistical analysis were conducted by the sampling data from 2020 to 2022 in the research area. It is found that in terms of spatial and temporal evolution characteristics, the mass concentration of Na+ and Cl? increased gradually from 2020 to 2022, and the content of salt water in the groundwater between 116°63′E-116°84′E、117°12′E-117°47′E, Ca2++ Mg2+to HCO?3+SO24? was less than 1. In the latitudinal spatial distribution of ions in Tianjin Plain, the mass concentration interval of cations Na+ and Ca2+ in groundwater is [104 mg/L,4 812.21 mg/L], Ca2+[38.3 mg/L,713.67 mg/L], and the mass concentration interval of anions Cl?and HCO?3 is [73.6 mg/L, 11 282.49 mg/L], [78 mg/L, 539.46 mg/L], respectively. In terms of the influence of salinity water intrusion, by comparing and analyzing the concentration ratio of Cl? and other ions (Na+, Ca2+) in groundwater, it is found that the salinity degree of groundwater is high near the coastal area, and further empirical analysis found that seawater intrusion has a significant impact on the chemical composition of groundwater (Δ [Na+]=4 708.21mg/L,Δ[Cl?]=11 208.89mg/L). In terms of characteristic analysis of carbonate weathered carbon sink, the weathered carbon sink of groundwater in the western area of Tianjin Plain decreased, while the eastern area increased, and the mass concentration of Ca2+ and Mg2+had a significant positive effect on carbonate weathered carbon sink (B [Ca2+]=0.910,p=0.03; B[Mg2+]=0.312,p=0.09). The results further verify the weathering of groundwater carbonate in Tianjin Plain and the relationship between the chemical characteristics of groundwater. In terms of spatial and temporal evolution characteristics, the mass concentration of Na+ and Cl? showed a trend of increasing year by year from 2020 to 2022, indicating that the salt water content in groundwater was gradually increasing. In terms of the impact of brine intrusion, the coastal area, namely the area with higher longitude, showed a higher salinity of groundwater, indicating that seawater intrusion had a significant influence on the chemical composition of groundwater. In terms of the characteristic analysis of carbonate weathered carbon sink, the carbonate weathered carbon sink of groundwater in the western area of Tianjin Plain decreased, while the amount of groundwater carbonate in the eastern region increased, and the mass concentration of Ca2+ and Mg 2+ had a significant positive effect on the carbonate weathered carbon sink. Compared with southwest karst region and the loess plateau region, it is found that although these areas of rock dissolution and carbon sink effect in common, but the Tianjin Plain rock type, dissolution rate and climate conditions, which leads to the carbonate weathering carbon sink characteristics and influencing factors, also has certain uniqueness.
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