海南岛东北海岸地下水游离CO_2对pH的影响

The Variation of Groundwater Dissolved Free CO_2 during Tidal Cycle and Its Impact on pH along East-northern Coast of Hainan Island

利用海南岛东北海岸测井地下水潮周期逐时取样实测数据，分析了游离CO2的变化特征及其与pH的关系。结果表明，游离CO2并不稳定，含量随测井条件而变化，主要来自大气输入，变化在0~17．6mg/L之间。测井地下水pH与其有密切联系，在QZ1测井对其的响应率δpH/δCO2依次为-0．17、-0．18，QZ4-1为-0．07；无游离CO2的pH比较稳定，与海水pH接近，且代表值8．33作为背景值；游离CO2所产生的H＋含量为1．35×10-8~8．24×10-8mol/L，贡献率在58．31%~94．63%之间，产生的pH在7．08~8．54之间，参与解离的H2CO3质量含量比例在0．033%以下；H2CO3饱和指数SI在QZ1测井为0．02~0．19，QZ4测井为0．032以下，远未达到溶解平衡；利用平衡方程计算得到可溶性SiO2的pH贡献，结果显示产生的H＋含量很低，产生的pH偏高，在8．55~8．76之间。从产生的H＋绝对含量对比来看，游离CO2明显高于可溶性SiO2。

The dissolved free CO2 is analyz ed in underwat er monitoring wells during tidal cycles along Hainan Island by use of the test data on samples. The results show dissolved free CO2 comes from air and is unstable subject to well environment variation and falls into the range 0~17.6mg/L, and have an significant relationship with pH and the response δpH/δCO2 is -0.17 and -0.18 in well QZ1 and -0.07 in well QZ4-1; the pH is very stable while no CO2 in wells and near to seawater pH and typical 8.33 is used as the background pH in all wells; H＋from dissolved free CO2 is 1.35×10-8~8.24×10-8 mol/L according to the equation H＋CO2=ΔH＋=H＋sample-H＋background with contribution percentage 58.31%~94.63%; Percentage of H2CO3 producing H＋ to total is under 0.033% and its SI index is 0.02~0.19 in well QZ1 and well QZ4 is below 0.032; dissolved SiO2’s contribution to pH is apparently lower than using balancing equations.