Study on source-sink effect in the process of carbonate rock dissolved by acid rain:An example of typical karst regions in Guangxi

It is well acknowledged that the reaction between strong acids and carbonate rocks can release CO2,and then reduce the karst carbon sink.Acid rain is one of the sources of these strong acids.Carbonate rocks are widely distributed in Guangxi Zhuang Autonomous Region.Meanwhile,acid rain pollution is also common there,so that the reaction between acid rain and carbonate rocks is pervasive in this area.Seven acid rain monitoring sites in Guilin,Liuzhou,Hechi,Baise,Yulin,Nanning and Qinzhou represent the whole situation of Guangxi,where temperature,rainfall,and the degree of acid rain were studied in this work.The method of the carbonate-rock-tablet test is used to assess the intensity of the karst process at all sites.Precipitation parameters at these sites are collected at the same time.Based on the dissolution quantity of test blocks and precipitation parameters such as rainfall,rainfall duration,pH value and conductivity in a whole year(2014.5-2015.5),the factors of carbonate dissolution and source-sink effect of acid rain were analyzed.The results show that the dissolution quantity of test blocks is in accordance with the accumulative rainfall time which represents the dissolution time duration of rainfall to the test blocks.For a single acid rain,dissolution of the carbonate rock can be divided into two stages,(1) dissolution by strong acid with CO2 releasing;(2) dissolution by carbonic acid due to CO2 absorbing.The reaction between carbonate rocks and strong acid which is related to acid rain is a fast reaction.The result shows that the dissolution quantity of test blocks dissolved by strong acid is quite low,that is also the reason for the low correlation between dissolution quantity and the degree of acid rain such as pH and the rate of acid rain.Calculation results show that carbonate rocks dissolved by carbonic acid is the main process at all the sites.About 0.27% to 3.52% with the average of 1.96% of dissolution quantity should be deducted because of the acid rain in a whole year.It is lower than 10% to 30% which is calculated by river water chemistry.This is because of additional sources of the acid,such as weathering of sulfide deposits,and agricultural and industrial activities.The proportion carbonate rock dissolution is small in the study area,but it will increase exponentially while the pH value decreases as shown in computational formula.Therefore,the acid rain has a significant effect on karst carbon sink,which cannot be ignored.