The Hydrochemical Characteristics And Source-sink Effects For Atmospheric CO₂ Of Small Karst River Under The Influence Of Anthropogenic Activities

Atmospheric CO₂ concentration plays an important role in controlling global climate.The greenhouse effect and the global carbon cycle were become important problems of atmospheric environment,and have arouse attention of governments around the world and the researchers.How to accurately evaluate the CO₂consumption by the rock weathering and carbon emission been a significant part of the research of global climate change.In traditional carbon cycle models,it has only considered the parts in which carbon dioxide is circulated between the land,seas and atmosphere.However,river was the connecting link between land and sea,and solutes produced by chemical weathering were transported to the oceans by rivers.Therefore,the Atmospheric CO₂ consumption by the rock weathering and the cycle processes of the riverine carbon emission in the river basin has recently become a focus of the research of global climate change.However,there was few researches about the influence of anthropogenic activities on the cycle processes of the riverine carbon,even though the anthropogenic activities may lead to the CO₂ consumption by the rock weathering of river be overestimated and release more carbon into the atmosphere.The chemical composition of river water can be used to discriminate the ion sources of solute,identifying the geological factors,human activities influence on river water chemistry,which can provides indications of the chemical weathering process of rock.The Rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake were heavily influenced by anthropogenic activities,for about coal mine,agricultural activities and urban domestic sewage.Water samples were collected in the 2017 and 2018,and analyzed for major ions in the Rivers flowing into the Aha Lake,to understand ion compositions and solute fluxes as related to rock weathering and associated CO₂ consumption rates.Based on the ion content and chemical characteristics analysis of the samples,the water chemical characteristics,the controlling mechanisms on the ion chemistry and its distribution rules of the three main rivers flowing into the Aha Lake were analyzed,as well as their origins and impact factors.Then,the impacts and inputs of anthropogenic activities,the key chemical weathering processes of the river basin and the influences of rock weathering from atmospheric CO₂ consumption were been discussed.And we also quantify the different sources contributed to the dissolved load and estimate rock weathering rates and atmospheric CO₂ consumption flux.Moreover,we also estimate the CO₂ emissions from the air-water interface and explore the role what the small catchment paly in the global carbon cycle.The results are showed as follows:?1?There were obvious differences in pH,EC and TDS in the rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake that were influenced by different types of anthropogenic activities.Aqueous pH values in the rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake were mildly alkaline,with an average of 7.78,8.10 and 8.02,respectivrly,revealed that the riverine total alkalinity was mainly composed of bicarbonate alkalinity.The order of EC and TDS?Total Dissolved Solids?was Youyu River?29?Baiyan River?29?Jinzhong River.The TDS of the Youyu river averaged 690.90 mg·L^–1,revealed that the river water was contaminated(TDS?29?500 mg·L^–1).?2?All of the samples of the Rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake that were influenced by different types of anthropogenic activities have the same cations characteristics but with the different anions characteristics.The cations decreased in the order:Ca²⁺>Mg²⁺>Na⁺>K⁺>NH₄⁺.However,major anions of Youyu river decreased in the following order:SO₄²>HCO₃^– > Cl^–> NO₃–> F^–.The value of major anions of Baiyan river and Jinzhong river decreased in the following order:HCO₃^–>SO₄^2–>Cl^–>NO₃^–>F^–.?3?The water chemical characteristics were obvious differences in the rivers?Youyu River,Baiyan River and Jinzhong River?.The Ca²⁺and SO₄^2–had higher value in the sampling spots which was flow through where rich in coal resources,and decreased significantly from upstream to downstream,revealed the impacts of coal mines distributed in the basin.The Na⁺,K⁺,Cl^–had higher value in the Jinzhong river than the other two rivers,revealed the impacts of urban domestic sewage.The Baiyan river had a relatively higher Cl^–?NO₃^–compositions,which were affects both coal mines and domestic sewage.?4?The Rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake were affected both carbonatite rock weathering and anthropogenic activities,mainly affected by carbonatite rock weathering.The contributions of anthropogenic inputs were calculated approximately 15.48%,14.51%,13.10%in the Youyu River,Jinzhong River and Baiyan River,respectively and the contributions of carbonatite rock weathering were 15.48%,14.51%,13.10%,respectively.The ion sources analysis have shown that the Ca²⁺?Mg²⁺?HCO₃^–of the Rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake that were influenced by different types of anthropogenic activities have the same sources,mainly derived from rock weathering,but the Na⁺,K⁺,Cl^–,SO₄^2–,NO₃^–of the rivers come from a variety of anthropogenic activities input.?5?The ions ratios analysis indicates that the hydrochemical indices can be used to differentiate the influence of different types of human activity.The rivers subject to an obvious influence by the mining industry have relatively high Ca²⁺and SO₄^2-as well as relatively high Ca²⁺/Mg²⁺,SO₄^2-/Mg²⁺,and(Ca²⁺+SO₄^2-)/Mg²⁺ratios;the rivers subject to an obvious influence by urban sewage discharge have a relatively high?Na⁺+K⁺+Cl^-?/Mg²⁺ratio.When the NO₃^–concentration is relatively high,the rivers are subject to an obvious influence by agricultural activities but a relatively small influence by urban sewage discharge and have relatively high NO₃^-/Na⁺, NO₃^-/K⁺,and NO₃^-/Cl^-ratios?6?The correlation analysis indicated that the primary weathering process of the rivers flowing in Aha Lake was weathering by the H₂CO₃ and H₂SO₄.The deduction method and ion balance analysis showed that carbonate rock weathering by the H₂CO₃ and H₂SO₄ in the Youyu river were made up 30.31%and 69.69%,respectively,and were 65.03%,34.97%and 60.08%,39.92%in the Jinzhong river and Baiyan river,respectively.?7?Calculations show that the small river has the higher weathering rates but lower CO₂ consumption for the reason of the affected by the anthropogenic activities.And sulfuric acid has a certain effect on the river basin chemical weathering rate.For our calculations,the rock weathering rates within the Youyu river was177.33t·?km²·a?^-1,faster than the Jinzhong river(135.68 t·?km²·a?^-1),Baiyan river(100.07 t·?km²·a?^-1)and other rivers in the word.Then the atmospheric CO₂consumption rates by rock weathering within the Youyu river was169.07×10³mol·?km²·a?^-1,whilelowerthanthoseoftheJinzhong river(666.40×10³mol·?km²·a?^-1)and Baiyan river(417.16×10³mol·?km²·a?^-1).The atmospheric CO₂ consumption rock weathering was highly overrated because of the anthropogenic activities.The atmospheric CO₂ consumption rates by rock weathering within the Youyu river,Jinzhong river and Baiyan river were been overestimated about 593.20×10³mol·?km²·a?^-1,358.36×10³mol·?km²·a?^-1 and277.19×10³mol·?km²·a?^-1,respectively.?8?Calculations show that the small rivers that affected by the anthropogenic activities have the higher CO₂ emission rate and flux.The calculation results shown that the CO₂ emissions from the air-water interface were 813.28 mg·?m²·h?^-1,577.63mg·?m²·h?^-1 in the Youyu river and Jinzhong river where was significantly affected by anthropogenic activities,respectively.And the CO₂ emissions were significantly higher than that was in the Baiyan river(197.17 mg·?m²·h?^-1)where was less affected by anthropogenic activities.The CO₂ emissions flux from the air-water interface in the rivers?Youyu River,Baiyan River and Jinzhong River?flowing into the Aha Lake were 41.65×10³mol·a^-1,50.56×10³mol·a^-1 and 9.42×10³mol·a^-1,respectively.?9?The CO₂ emissions rates from the air-water interface in the rivers were significantly lower than the atmospheric CO₂ consumption rates by the rock weathering(Youyu River:11141×10³mol·a;Baiyan River:21483×10³mol·a^-1;Jinzhong River:31654.00×10³mol·a^-1).These karst rivers that were significantly affected by anthropogenic activities were plays a role of atmospheric CO₂“Sink”in the global carbon cycle research.