Study On Material Source And Carbon Sink Of Karst Water In Typical Karst Hill (Valley) Depression Basin

Karst groundwater is an important groundwater resource in southwest China.In the process of groundwater migration,the hydrochemical composition are affected by the geological background and human activities.Therefore,clarifying the chemical characteristics and material sources of karst groundwater can reveal the interaction mechanism between groundwater and environment.In addition,vegetation restoration and human activities have an impact on the weathering of rock carbon sinks,in particular karst carbon sinks.Meanwhile,the carbon sink effect generated by rock weathering in the process of water-rock action is an important part of the"missing sink".To discuss the impact of vegetation restoration and human activities on karst carbon sink is of great significance for the accurate assessment of the carbon sink effect generated in the process of rock weathering and its in-depth study.Based on the typical karst in typical karst hill(valley)depression basin-Shiqi River basin as the research object,through the early data collection,field investigation and indoor experiment method,the characteristics and material sources of groundwater in the study area was qualitatively and qualitatively analyzed by Gibbs figure,Piper diagram,the end-member analysis and ion ratio such classic hydrogeochemical analysis method and theory of hydrogen,oxygen,carbon isotopes.Then,the solute load method and normalized difference vegetation index were used to study the karst carbon sink effect difference between 2004 and 2018 and the influencing factors were discussed.Finally,the dissolution rates of the test plates under different land use types in the basin in 2018 were calculated by the carbonate-rock-tablet test.The main research results are as follows:Through the statistical analysis of the conventional ion content in the groundwater,it was found that the pH value of the groundwater in Shiqi River basin was between 6.06-8.07,the total solid solubility was between 18.21 mg·L^-1 and 336.28mg·L^-1,and the charge of ions and ions in the water body was basically balanced.Ca²⁺,Mg²⁺,and HCO₃^-were the main ions in groundwater,with concentrations ranging from 2.61 to 108.7,0.54 to 27.61,and 8.1 to370.74mg·L^-1,respectively.The groundwater was consistent with the characteristics of high calcium and weak alkalinity of karst water.Then,by using Gibbs diagram,piper diagram,end-member analysis,and ion proportional coefficient,the hydrochemical characteristics and material sources of groundwater were further analyzed.The results showed that Ca²⁺and Mg²⁺were mainly controlled by the weathering of limestone and dolomite in which carbonic acid was involved.Na⁺mainly comes from the dissolution of silicate rocks.At the same time,Ca²⁺and Na⁺undergo cationic exchange adsorption in the groundwater flow process.K⁺,Cl^-,and NO₃^-were mainly affected by agricultural fertilizer application and domestic wastewater discharge.Through hydrogen and oxygen isotope analysis of groundwater in Shiqi River basin,it was known that atmospheric precipitation was the main recharge source of groundwater in this region,which had a certain influence on the chemical characteristics and material sources of local groundwater.And groundwater was isotopic exchanged with CO₂ produced by atmospheric,soil,or carbonate decomposition.Carbon isotope of the endmember analysis showed that soil CO₂ and carbonate rock weathering was the main source of dissolved inorganic carbon in basin groundwater,followed by the effects of photosynthesis and respiration of aquatic plants,and the contribution of atmospheric CO₂to dissolved inorganic carbon in groundwater can be neglected because its partial pressure was much smaller than that of soil and groundwater.Through solute load method to calculate basin karst carbon sink intensity of 2004 and 2018were 116.65 and 95.17 t CO₂·km^-2·a^-1 respectively.And the vegetation normalization index indicated that the vegetation coverage of the basin in 2018 had been greatly improved compared with that in 2004,indicating that compared with the natural restoration of forest land,the impacts of human activities such as land use change and village expansion have a dominant influence on the intensity of karst carbon sink.The carbonate-rock-tablet test was used to calculated that under different land use modes in the basin,the dissolution rate of each layer of grassland was higher than that of forest land,and the dissolution rate of grassland at 20cm underground was higher than that at 50cm underground,while that of forest land was the opposite,which was mainly affected by the root length of different plant species.The karst carbon sink strength in 2018 was calculated as 102.18 t CO₂·km^-2·a^-1 by the carbonate-rock-tablet test,which was similar to that calculated by the solute load method,indicating that the above two methods were reliable for the estimation results of the karst carbon sink intensity of the basin.