The Effect Of Allogenic Water On Karst Inorganic Carbon Flux In Li River Basin

Allogenic water has low hardness and p H value and thus is beneficial to the dissolution of carbonate rocks.Therefore,river water in the Li River basin,a typical large karst basin,was investigate to explore the above mentioned process in this study.The geologic background of this basin was studied by collecting the hydrogeological data and investigating the karst hydrogeology characteristics in the field.Water chemistry and isotope geochemistry have been investigated to revea l the difference and mechanism of allogenic water-rock interaction and carbonate weathering in the basin and quantitative the effect of al ogenic water on karst carbon sink.In this study,the main course and several tributaries of Li River were selected as the sampling sites.Field sampling and monitoring of the river discharge rate was conducted every three months from April 2017 to January 2018,in order to study the temporal and spatial variation of ions in river water and their origin.The controls of different factors on dynamics of river water solutes also are discussed.The variation and origin of DIC in river water and the influencing factors were analysed.The rate of rock weathering in the Li River basin and the related CO₂ consumption and flux were estimated.The relationship between allogenic water and carbon sink and the accuratekarst carbon flux in this basin was revealed by comparatively analysing the difference of the karst carbon flux obtained by theoretical and actual monitoring calculation.The main conclusions are as follows:?1?The hydrochemistry of water samples from the Li River basin is of HCO ₃-Ca type.The cation concentrations in karst water decrease in the order of Ca²⁺,Mg²⁺,K⁺and Na⁺,successively.Ca²⁺is the main cationin the river water with an average proportion of 66.18%,followed by is Mg²⁺,which accounts for 26.39%of the total cations,and the proportion of Na⁺and K⁺is less than 8%.HCO₃^-is the main anion in the river water,which accounts for 60%⁹0%of the total anion,with an average of83.68%.The following is SO₄^2-with an average of 8.14%of the total anions.The propotion of NO₃^-and Cl^-were less than 10%.The cation composition of allogenic water is different from karst water and decrease in the order of Mg²⁺,Ca²⁺,Na⁺and K⁺,successively.Mg²⁺occupies about 51.06%of the total cations,Ca²⁺is 39.7%,Na⁺is 8.18%,and K⁺is less than 2%.HCO₃^-is the main anion.Ion source analysis indicates that Ca²⁺,Mg²⁺and HCO₃^-are mainly derived from rock weathering especially the carbonate rock weather.K⁺,C l^-and Na⁺are mainly derived from the weathering of silicate rock and the inputs of human activity.SO₄^2-and NO₃^-mainly originates from the imputs of human activities,such as the use of fertilizers and pesticides.TDS concentration is positively correlated with the distribution area of carbonate rocks in the basin.?2?The DIC concentration of water samples from the Li River basin ranges between 18.3²07.4 mg/L,with a average of 79.18 mg/L,the allogenic water has DIC concentration of 18.3⁷9.3 mg/L,with a average of 35.88 mg/L.The inhomogeneity of DIC spatial distribution is mainly controlled by the geological background of the basin and related to the distribution area of differentlithology.The?¹³C_DIC values of water samples from the Li River basin range between-14.55‰and0.6‰,with an average of-7.1‰.Influenced by the river discharge rate,?¹³C_DICIC value is positively correlated with DIC concentration in the rainyperiods,but negative correlation is shown in the dry periods.The DIC in water is mainly sourced from the dissolution of the rock weathering by CO₂ from atmosphere and soil.In addition,because the exogenous acid released by human activities involved in the process of rock dissolution and weathering,the rock weathering rate is accelerated,thus change the regional carbon cycle.?3?The weathering rate of silicate rocks in the Li River basin was smaller than that of carbonate rocks.The weathering rate of silicate and carbonate rocks were 1.57t/(km².a^-1)and 8.16 t/(km².a^-1),respectively.The rock weathering rate in the basin is small because of the strengthened vegetation protection.The allogenic water promotes karstifi cation,the concentration of DIC gradually increases and the karst carbon flux increases with the allogenic water flowing into the karst area.Due to the increasing carbon sink by allogenic water,the karst carbon flux obtained by actual monitoring calculation is 0.87 times higher than that obtained by the oretical calculation.Therefore,the role of exogenous water cannot be ignored when calculating karst carbon sink in karst area.The consumption of atmospheric CO₂ in the Li River basin is 240.909×10³ t CO₂/km².a,which is 48 times higher than the global average of 5.026×10³ t CO₂/km².a,a suggestive of the important role of the Li River basin in balancing the global carbon recycle.