博茨瓦纳Sua盐湖地下卤水来源及成因

Sources and Genesis of Subsurface Brine in Sua Pan,Botswana

Sua盐湖是博茨瓦纳面积最大的盐湖之一,是南半球重要的天然碱资源.为进一步揭示该盐湖的成因,对盐湖区地下水、地表水以及晒盐场盐结晶进行了常规阴阳离子含量、D、18O和37Cl同位素组成及地下水14C年龄分析.水化学结果显示Sua盐湖的地下卤水具有富Na和K,贫Ca和Mg的特征.氢氧同位素关系和37Cl结果显示,附近河流地表水与Sua盐湖地下卤水具有密切的水力联系,而地下水对其补给作用较弱(37Cl差异为0.04‰～0.06‰).TDS-γNa/γCl关系揭示Sua盐湖地下卤水受到盐岩地层溶滤作用的影响(γNa/γCl≈1),而14C年龄(距今约2万年)表明古气候的波动变化是其重要影响因素.基于以上认识,运用PHREEQC软件对Sua盐湖地下卤水进行反向地球化学模拟,结果表明地表水的强烈蒸发浓缩以及附近地下水对盐岩地层的溶滤是控制Sua盐湖卤水形成的主要因素.

The salt lake of Sua Pan, one of the largest salt lakes in Botswana, is important resources of natural alkali. To identi- fy the sources and genesis of subsurface brine in this salt lake, groundwater and surface water near this lake, in addition to sub- surface brine water in the lake and salt crystals from solar salt field were collected and determined. The contents of major cation and anion, the isotope compositions of D, 180 and 37C1, and the 14C age of groundwater were calculated. It is found that the subsurface brine is characterized by enriched Na and K, and depleted Ca and Mg. Results of D, 18 O and 37 C1 show that surface water is closely connected with subsurface brine in Sua Pan, while the role of groundwater recharge on it is weak （the difference of 17C1 is 0. 04%o--0. 06N0）. Relationships of TDS-yNa/yCl indicate that leaching of halite affects the formation of this subsurface brine （yNa/Ycl ≈ 1）, and the age of 14 C （about 20 000 years ago）indicates that the fluctuation of ancient climate is considered to be an important factor of the formation of Sua Pan. Based on the above knowledge, inverse simulation of subsurface brine in Sua Pan is modeled by PHREEQC software, which further verifies that salt lake of Sua Pan is mainly the result of strong evap- oration and concentration of surface water and halite leaching by groundwater.