Sources and Genesis of Subsurface Brine in Sua Pan, Botswana
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摘要: 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.Abstract: The salt lake of Sua Pan, one of the largest salt lakes in Botswana, is important resources of natural alkali. To identify the sources and genesis of subsurface brine in this salt lake, groundwater and surface water near this lake, in addition to subsurface 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, 18O and 37Cl, 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, 18O and 37Cl 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 37Cl is 0.04‰-0.06‰). Relationships of TDS-γNa/γCl indicate that leaching of halite affects the formation of this subsurface brine (γNa/γCl≈1), and the age of 14C (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 evaporation and concentration of surface water and halite leaching by groundwater.
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Key words:
- subsurface brine /
- hydrogeochemical characteristics /
- isotopes /
- inverse simulation /
- geochemistry
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表 1 Sua盐湖水样采集信息
Table 1. Water samples collected from Sua pan
样品编号 SW01 SW02 GW03 GW04 BW06 SW07 经纬度 26°11.224′E 26°22.055′E 26°27.126′E 26°28.373′E 25°57.871′E 26°14.439′E 20°12.798′S 20°05.645′S 20°30.683′S 20°30.517′S 20°27.852′S 20°17.889′S 类型 地表水 地表水 地下水 地下水 地下卤水 地表水 地点 Nata河 Nata河 Dukwi地区 Dukwi地区 Sua盐湖 Nata国家公园 采样深度 0~30 cm 0~30 cm >36.93 m >34.50 m 约20 m 0~30 cm 表 2 水样的水化学分析结果
Table 2. Results of hydrochemical analysis of water samples
指标 SW01 SW02 GW03 GW04 BW06 SW07 EC(μs/cm) 1 024.0 138.3 1 556.0 1 562.0 183 300.0 4 333.0 pH 8.80 8.89 7.16 7.08 9.50 8.83 温度(℃) 32.9 26.4 28.8 27.8 29.0 30.3 HCO3-(mg/L) 130.43 85.81 583.50 518.97 4 118.85 364.52 CO32-(mg/L) 49.52 0.00 0.00 0.00 15 445.06 210.80 Cl-(mg/L) 207.44 3.20 216.75 252.79 41 640.00 1 076.08 SO42-(mg/L) 20.58 0.90 93.22 95.40 12 480.00 97.90 K+(mg/L) 14.91 6.33 4.81 4.14 2 108.90 49.23 Ca2+(mg/L) 16.81 16.26 23.08 16.34 7.30 10.64 Mg2+(mg/L) 3.05 2.78 35.22 30.94 3.50 0.78 Na+(mg/L) 185.30 5.43 269.76 296.44 46 278.01 1 028.80 TDS(g/L) 0.630 0.121 1.229 1.218 112.655 2.841 阴阳离子平衡(%) 0.981 4.486 1.548 0.017 4.371 4.808 库尔洛夫式 Cl·HCO3-Na HCO3-Ca·Na HCO3·Cl-Na HCO3·Cl-Na Na-Cl Na-Cl 表 3 样品的37Cl测试结果
Table 3. δ37Cl values of water and salt crystals samples from Sua Pan
样品 δ 37Cl(‰) d值(‰) GW03 0.22 0.06 GW04 0.24 0.04 BW06 0.18 0.02 盐结晶 0.29 0.07 表 4 Sua盐湖卤水的14C年龄
Table 4. 14C age of brine water from Sua Pan
编号 现代碳百分数(%) 14C年龄(ka) 校正后年龄(ka) BW06 13.464 16.580±0.27 21.459 表 5 Sua盐湖卤水演化的地球化学模型(单位:mol/L)
Table 5. Geochemistry models for the evolution of brines in Sua pan
起点 终点 H2O 岩盐 方解石 钾长石 石膏 白云石 钠长石 CO2 SW01 BW06 -1.261e+4 1.538e-1 -1.891e-1 -3.514e-2 1.220e-1 -2.844e-2 3.514e-2 3.779e-1 SW02 BW06 -7.794e+4 1.418 -6.526e-1 -1.756e-1 1.578e-1 -1.605e-1 1.756e-1 -6.335e-1 SW07 BW06 -2.567e+3 - -1.320e-1 -7.751e-3 1.210e-1 -1.356e-3 7.751e-3 2.735e-1 BW06 -2.578e+3 - -1.302e-1 - 1.191e-1 -1.362e-3 - - GW03 BW06 -2.869e+3 1.271 -7.363e-2 4.542e-2 1.197e-1 -7.626e-2 -4.542e-2 -8.165e-2 GW04 BW06 -2.720e+3 1.242 -7.786e-2 4.661e-2 1.212e-1 -6.354e-2 -4.661e-2 -7.031e-2 注:负值表示从水溶液中迁移出来的摩尔量;正值表示进入水溶液的摩尔量. -
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