西安灞河流域现代岩溶作用与CO_2吸收量

RESEARCH ON THE MODERN KARST PROCESSES AND ABSORBED AMOUNT OF CO_2 IN BAHE RIVER CATCHMENT OF XI' AN

本文探讨了灞河流域岩溶特征、ＣＯ２估算方法和ＣＯ２吸收量。观测资料表明，灞河流 域河水中ｐＨ值在冬春季高，夏秋季低，而ＨＣＯ３含量通常与ｐＨ相反。河水和黄土地下水中 ｐＨ值、ＨＣＯ３等化学成分含量与北方南部石灰岩区岩溶水基本相同，表明该区现代岩溶作用 明显，黄土地层也在不断吸收ＣＯ２，并对全球碳循环有重要影响。雨水富含ＣＯ２，其中约有 ８２％的ＣＯ２被岩溶过程吸收，１８％左右的随河水流失。根据一个流域ＣＯ２输入量和输出量，可 以计算岩溶过程中 ＣＯ２吸收量。计算表明，灞河流域现代岩溶过程中每年吸收的 ＣＯ２约为５６３２．８ｔ。

On the basis of one yea's observation of pH, the content of CO2, HCO3, Ca^2+, Mg^2+ in the water of the river, the ground water in loess, and the rain of Bahe River catchment area in Xi'an, this paper discusses the characteristics of modem karst processes and puts forward a method to calculate the absorbed amount of CO2 in the karst processes. The observation results are as follows. The indexes of the rain show that pH is from 6.30 to 6.46, the content of HCO3 is from 10.66 to 25.38mg/L, the content of Ca^2+ and Mg^2+ is very low, the content of free CO2 is from 3.46 to 34.12mg/L, and sometimes there is aggressive CO2. And the characteristics of chemical composition shows that the rain has obvious acidity and strong corrosion ability. The indexes of the river show that pH is higher than 8.30, the content of HCO3 is from 87.8 to 237.9mg/L, the content of Mg^2+ and Ca^2+ is high, and there is a little free CO2 only within three days after raining. As compared with the rainwarer, the pH and the content of HCO3 of the river water are higher than that of the rain water, which shows that the rainwater undergoes obvious karst processes and most of CO2 is consumed on the course of converging into the river. However, within three days after raining, the content of CO2 in the river reaches 1.8mg/L, which shows that CO2 has not been used up and about 18 percent has run off with the river. The chemical composition of the river water expresses seasonal changes. In the rainfall summer and autumn, pH is low, and the content of HCO3 is high. But this situation in winter and spring is opposite. The results show the karst processes are strong in summer and autumn, and weak in winter and spring. The reason is that there is free CO2 in the karst wader in summer and autumn. The indexes of the ground warer in the loess containing rich CaCO3 show that pH is from 7.46 to 7.76, the content of HCO3 is from 120.1 to 137.6mg / L, the contents of Ca^2+ and Mg^2+ are 32.6-38.5mg / L and from 4.8 to 6.2mg/L, there is no free CO2. Its chemical composition is similar to that of the river water, and as same as that of the karst water in the limestone area in the southern part of north China. The results show that the loess has obvious karst processes and absorbs CO2 constantly. The rate of the karst processes is high because the calcite in the loess is small and scattered, and the loess is loose and porous. The strong alkalinity and high degree of mineralization of the ground water in the other areas in the Loess Plateau show that the widely distributed loess has strong effect on the global CO2 cycle and it should be regarded as one of the most impohant areas for researching the subject. This paper gives a method to calculate the absorbed amount of CO2 in the karst processes in a catehment area. The free CO2 in the rainwater minus the free CO2 left after the karst processes is the absorbed CO2. It is hard to get the result by the denudation test of limestone tablets because there is not only dissolution of CaCO2 but also the deposit of CaCO3. So the method given here is worth vaining and improving. To calculate, there are two kinds of observation data needed. The first one is the total amount of CO2 got from the rain which can be calculated by using the free CO2 in the rain and the total discharge. The second one is the free CO2 left which can be got by observations. The annual average runoff discharge of the Babe River catehment is 6.092 hundred million m^3. The average content of the free CO2 in the rain is about 10mg/L. These are the basis to calculate the total CO2 got from the rain. The runoff discharge condoning CO2 left after the karst processes is 2.6 hundred million m^3, and the CO2 left is 1.8mg/ L. These are the bases to calculate left CO2 amount. Accordingly, the absorbed amount of CO2 in the karst processes in the Babe River catchment basin is 5 623.8t/a.