Subtropical Small Watershed River Water Chemistry And Strontium Isotopic Characteristics Of Weathering Instructions Significance

In this paper, we take the upstream basin of Ganjiang River in the southern Jiangxi Province and Hongfeng Lake in the middle Guizhou Province as the research objects. The dissolved elements of watersheds were traced and a clear contrast for rock weathering contribution of different lithologies to river was observed through calculating water chemical constitutes and dissolved Sr of two small basins. The relationships between water chemical characteristic of river and human activity, the lithologies and atmosphere precipitation were also discussed, and the contribution of every end is calculated by using the end member equation of ⁸⁷Sr/⁸⁶Sr.The main conclusions are showed as follows:1.The rainwater of the upstream basin of Ganjiang River represent the characteristics ofthe silicate weathering , while the rainwater and groundwater in Hongfeng Lake watershed represent the characteristics of the carbonate rock weathering.2.On the basis of the principal composition analysis, several conclusions could be drawn: The upstream basin of Ganjiang River has enrichment in Na⁺+K⁺, poor in Ca²⁺+Mg²⁺ and the concentration of Si is high.While that of Hongfeng lake watershed is poor in Na⁺+K⁺, rich in Ca²⁺+Mg²⁺and the concentration of Si is low. The source of the major ions of river in the upstream basin of Ganjiang River is close to the end members of granite weathering by HCO₃^-, basalt weathering by HCO₃^-granite weathering by H₂SO₄, and that of Hongfeng Lake watershed is close to the end members of carbonate weathering by HCO₃^- and carbonate weathering by H₂SO₄ after making the standardization of major elements .3.Basing on the model of major element balance equation, conclusions are showed as follows: the proportion of source from the weathering of silicate rock is 69% in the upstream basin of Ganjiang River, which reflects the dominant inputs of its water chemical component coming from the weathering of silicate rock. The proportion of source from the weathering of carbonate rock is 83.4% in the Hongfeng Lake watershed, which reflects the dominant inputs of its water chemical component coming from the weathering of carbonate rock. In addition, according to the relationship among SO₄^2-/Na⁺ , NO₃^-/Na⁺ , Cl^- /Na⁺ ,we find that the influence of human activity is low in the upstream basin of Ganjiang River ,and that is relatively high in the Hongfeng Lake watershed .4.Deduced from the balance equation of dissolved Sr, the source of Sr in both river basins most are from the rock weathering excluding the atmosphere precipitation, the proportion of source from the weathering of silicate rock, the carbonate rock ,evaporated rock for dissolved Sr in the upstream basin of Ganjiang River is 48.3%,27.4%,4.3%. and relative value of that in the Hongfeng Lake watershed is 6.4%,75.5%,0.5%.5.The relationship between ⁸⁷Sr/⁸⁶Sr and 1/Sr in two small watersheds shows prominent positive correlation, and the plots between Mg/Na^*,Ca/Na^*, Mg/Ca, HCO₃^2-/HCO₃^-+SO₄^2-and⁸⁷Sr/⁸⁶Sr indicate that the weathering of silicate rock mainly controls the water chemistry components in the upstream basin of Ganjiang River, the weathering of carbonate rock mainly controls the water chemical components in the Hongfeng Lake watershed.6.According to end member simulated equation of Sr isotope, the contribution of the atmospheric deposition in the upstream basin of Ganjiang River to the ⁸⁷Sr/ ⁸⁶Sr is about 9.7 % and the rock weathering's contribution of ⁸⁷Sr/⁸⁶Sr is 90.3 % ,and those of which are 8.3% and 91.7% in Hongfeng lake watershed. The data above indicates that the elements in the upstream basin are from atmospheric precipitation and rock weathering, which is called the mixed type, while the elements in the Hongfeng Lake are from rock weathering. Moreover, studying on soil section in Wang Jiazhai in the Hongfeng Lake in the middle Guizhou Province shows that: the grain size frequency distribution curves and mineral components indicate that the weathering crust and the lower bedrock have well inheritance relations.7.Through calculating of the parameters of Sr isotope and the basin, the results are showed as follows: the weathering rate of the upper reaches of Ganjiang River is 0.28 (keq/hm²·a),and that is 0.35 (keq/hm²·a) in the Hongfeng Lake watershed. The weathering rate of the Hongfeng Lake watershed is slightly higher than that of the upstream basin of Ganjiang River. But compared with the result of Duanlei(Duan lei,2002), the numerical value is still small. The possible reason is that the time of sampling is in the monsoon and the contributions of elements in the river are in a low status.