Study On Hydrochemical Characteristics And Carbon Sink Of Degassing Process In Karst Area Of Donghe River,Hunan Province

Rivers are important channels for geological and geochemical cycles.As an important carrier of geochemical information,rivers can reflect the content and composition of various elements in river water,thus reflecting the various characteristics of the basin.The karst carbon sink is driven by the water cycle,which causes the water body to exchange with atmospheric CO₂ gas.Therefore,accurately evaluating the CO₂ exchange flux at the water-air interface of the karst water becomes the key to accurately evaluate the karst carbon sink effect.Both surface water system and groundwater system have the exchange of CO₂ between water and gas interface.Among them,karst groundwater is often exposed on topography with large drop,such as halfway up the mountain,the groundwater is exposed to waterfalls easily,and there is a significant degassing effect in the falling process.In the past,the calculation of carbon sinks does not deduct the loss of this part.The total carbon sink flux in the basin is estimated to be too large.Therefore,it is very important to study this process,combined with the characteristics of the typical subtropical carbonate basin,and further study of the water degasification's effect of falling water during the karst carbon cycle,to determine its changing law,and to analyze the process of CO₂ water-gas interface exchange,carbon credit can be calculated more accurately.The quantity has important scientific significance for the global carbon cycle research and has important practical and scientific basis for our response to global climate change.Taking the Dalongdong Underground River in the Donghe River Basin of Hunan Province as an example,the waterfalls at the only catchment outlet of the underground river are selected as six monitoring points in the study area,i.e.from the front of the dam to the entrance of the Dongnei Reservoir,and from the bottom of the waterfall to the surface flow section.Sampling and monitoring of the study area?including different seasons,day and night monitoring,rainfall impact?were conducted in April 2018 and August of the same year.Hydrochemical parameters were recorded and measured on site.Gas samples were collected by floating box method.Gas samples,main anions and cations and ¹³C isotopes were tested indoors.Then the analysis data are sorted out and the flux is calculated.The results show that:?1?The concentration of CO₂ in the air has a significant difference in spatial scale.Before the water-falling process,the concentration of CO₂ in the deep points X6 and X5 in the cave is relatively large,reaching about 1300 ppm,and at the hole point X4 at the dam,the concentration is reduced to 700.60ppm.After the falling water process,the CO₂ at point X1does not decrease compared with the hole point X4,but it may be due to the strong degassing during the falling water process and the increase of CO₂ in the air caused by water atomization in the air.The surface flow segment is stable at around 400 ppm.From the whole process,the concentration of CO₂ in the air is decreasing.?2?The carbon isotope?¹³C_CO2 in the air also has time and space scale differences.The composition of CO₂ is different at different sampling points.With the degassing process,the isotope values are gradually positive,from near-18.00‰in the hole is biased to the surface flow near-9.00‰.It shows that the source composition of CO₂ in the air varies from time to time.The air CO₂ concentration has a significant negative correlation with?¹³C_CO2.When the CO₂ concentration in the air decreases,the corresponding?¹³C_CO2 value increases.The?¹³C_CO2 in the cave is about-18.00‰,and the?¹³C in the basin carbonate rock is about 0‰,indicating that the air CO₂ in the deep part of the cave is mainly derived from the CO₂ of the soil in the karst area.The hole point X4 is more positive than the cave.Because the hole is wide,a large amount of air is mixed in the hole.At the same time,the utilization of the photosynthesis of the mountain vegetation makes the?¹³C_CO2 align in the hole.The?¹³C_CO2value at the point X1 after the waterfall falls is compared with the hole point X4 is negative,mainly due to the degassing of the falling process.The?¹³C_CO2 values of the point X2 and X3of the surface flow are close to and negative from the?¹³C value of CO₂ in the atmosphere,indicating that the main source of CO₂ in the air at this time is the atmosphere,and is also affected by the degassing of karst water in certain carbonate rocks.,resulting in a?¹³C mean that is biased towards atmospheric CO₂.During the day and night monitoring period,the variation of point X4 was negative at night and positive during the day.This may be the preferential use of light carbon for photosynthesis by plants around the hole during the day,and then affect the value of point X4 in the cave.Point X1 showed a single-peak curve during the whole day and night monitoring period.The first half was nighttime,and the value gradually became negative,and then the next day gradually became positive.During degassing,the?¹³C_DIC in water gradually became heavier,and X1 increased by 0.76‰compared with X4.The two observation points before and after the short-distance process show a large gap.The change of?¹³C_DIC indicates that the C source in the water has obvious changes.?3?The heavy rainfall process has an effect on the entire degassing process,and the effect at point X1 is greater than X4.Point X1 can reflect a certain day and night change law,but often the regular trend of the day is shorter?for example,obvious changes or extreme values appear at 13:00¹5:00?,and these parameters are often directly affected by light.Due to the particularity of the terrain,the point is located in the canyon,and the time period when the point is directly exposed to the sun is only around 11:00¹5:00.On the right,the shorter sunshine shows a strong change,which leads to the enhancement of plant photosynthesis and changes in DO,HCO₃^-,and?¹³C_DIC,indicating that point X1 is very vulnerable to external conditions.?4?There is a certain regularity in the degassing phenomenon caused by the falling process of the waterfall.The existing data can show the degassing phenomenon existing when the underground river is discharged to the hole?X4?,and there is obvious degassing in the short time of the falling process.The X1 section after the falling process is mainly composed of CO₂ absorption.That is to say,before the fall of the waterfall,it is the?source?of atmospheric CO₂,the falling process is the?source?of atmospheric CO₂,but the fall of the waterfall is the regular feature of atmospheric CO₂?sink?.?5?Based on the day and night monitoring data,the exchange flux of CO₂ is calculated,and then the annual flux change is estimated,which will improve the calculation accuracy.The results show that the X4 section before the fall of the waterfall shows CO₂ degassing,which is the"source"of atmospheric CO₂.The monthly flux is about 99.50 tCO₂,and its flux is 2701.11 tCO₂/yr.The X1 section after the fall of the waterfall shows CO₂ absorption.As a?sink?of atmospheric CO₂,the monthly flux is 350.52 tCO₂,and its flux is-2912.76 tCO₂/yr.The whole process is generally characterized by CO₂ degassing,its degassing flux is about910.64 tC/yr,and the flux of carbonated weathered carbon formed by carbonation in X4section is 2996.29 tC/y,deducting the flux of degassing.Finally,the entire underground river to the end of the water formation process formed a net flux of karst carbon sink of 2880.04 tC/yr.Among them,the total amount of CO₂ degassing accounts for 31.62%of the total net weathering carbon sink of carbonate rocks,which is much larger than the corresponding value of rivers in carbonate rocks area,indicating that the degassing effect of falling water produced by waterfalls is much more intense than the CO₂ exchange at the water-gas interface of general rivers.