西江水氢氧同位素组成的空间变化及环境意义

Spatial Variation and Environmental Significance of δ^(18)O and δD Isotope Composition in Xijiang River

2014年6月和2015年1月,在西江各干流和支流分别采集了54个水样,对水的氢氧同位素组成进行了分析.结果表明,西江水体δ18O和δD存在明显的线性关系,这种相关关系旱季显著于雨季,西江水体旱、雨季18O、D值均沿大气降水线分布.氘盈余(d)值雨季大于旱季,主要是由于雨季降水多通过落水洞、竖井、漏斗等岩溶形态直接补给地下河,与围岩接触的时间较短,而旱季降雨较少,地表河系统受储存在岩溶裂隙、孔隙中的地下水体补给.由于受海拔高度及蒸发效应的影响,西江干流红水河—浔江段和支流右江—郁江段δ18O和δD值整体上表现为沿流程逐渐偏正的趋势,而由于桂江支流上游到下游海拔差异不大,流程简短、加上桂江支流水量少于西江干流和右江—郁江支流,桂江支流未出现从上游到下游逐渐偏正现象.通过δ18O与高程的关系建立了二者之间的线性回归模型,揭示了雨季西江水体随高度的变化率为:-0.44‰·(100 m)-1,旱季为-0.45‰·(100 m)-1,反映西江流域降水的高度效应,这对流域水循环过程的研究具有重要的水文地质意义.

The H and O isotope composition of the Xijiang water was investigated on 54 samples collected from the mainstreams and main tributaries in June 2014 and January 2015.It was found that in the Xijiang river, there was a remarkable linear relationship between δ18 O and δD. This relationship was more significant in the dry season. In both seasons, the 18 O and D values distributed along the meteoric water line, which indicated that precipitation was the source of the Xijiang River. Due to the direct water recharge through karst forms （i.e. sinkholes, vertical shaft and funnel） to the underground river, the d-excess value was higher in the rainy season. While during the dry season, water recharge came from the storage in fissures and pores. Affected by altitude and evaporation effect, along the mainstream Hongshui River-Xunjiang and tributary Youjiang-Yujiang reach, the δ18 O and δD values were gradually approaching positive value in general. However, along the short tributary Guijiang reach, the altitude difference was not dramatic, and the discharge was less, the same phenomenon was not observed. By establishing a linear regression model between δ18 O and altitude, it revealed that the change rate of Xijiang water with altitude was -0.44‰·（100 m）-1 in the rainy season, and -0.45‰·（100 m）-1 in the dry season. The result reflected the height effect of the precipitation, which has a considerable meaning for the study of hydrological cycle in a river basin.