Hydrochemistry and variation of δ13C DIC of the Qingmuguan underground river in Chongqing

In order to determine the source of dissolved inorganic carbon and the matter in the Qingmuguan underground river system, the underground river system is monitored from April, 2010 to January, 2011, and the chemical components, the dissolved inorganic carbon content and δ13C DIC as well as their influencing factors are analyzed. Furthermore, detailed analysis is made to some monitor points in the ploughing season in 2010. The results show that(1)the type of the Qingmuguan underground river water is Ca-HCO3 that is affected mainly by water-rock action and rainfall; (2) the molar ratio of (Ca2++Mg2+)/HCO3- in the groundwater varies from 0.6 to 0.72 and is averaged 0.67,whichindicating the carbonate dissolution is controlled by CO2, HNO3 and H2SO4 in the soil under the C3 vegetation cover; (3)the HCO3- from carbonate dissolution by carbonic acid account for 56.16% to 81.25% of the total HCO3- concentration in groundwater with a mean percentage of 66.96%,whilethe HCO3- from carbonate dissolution by sulphuric and nitric acids make up 18.75% to 43.84%of the total HCO3- concentration in groundwater with a mean percentage of 33.04%; (4)the δ13C DIC in groundwater varies from -8.17‰ to -13.68‰ with an average value of -10.53‰,the average value of δ13C DIC in ploughing and dry season is -9.25‰ and -12.29‰ respectively, which indicates there is a significant perturbation to the cycling and export of inorganic carbon by agricultural activities in karst underground river system.