Experimental study on the effect of subsurface freshwater-saltwater mixing on the permeability of coral sand
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摘要:
珊瑚砂渗透性是决定岛礁地下淡水储量的关键因素, 长期以来缺乏岛礁地下淡水形成过程中不同盐度溶滤液对珊瑚砂渗透性影响的分析研究。在实验室条件下利用3种盐度不同的溶液模拟淡水、过渡带水和海水, 分别对珊瑚砂进行了溶蚀和渗透试验, 分析了滤出液在溶蚀过程中的离子浓度变化以及珊瑚砂渗透系数的变化特征, 并基于X射线衍射分析和PHREEQC反向模拟, 探讨了珊瑚砂在不同盐度溶液中发生的主要水岩相互作用以及渗透性变化的可能原因。结果表明, 珊瑚砂渗透特性与砂体发生的溶蚀作用强弱有关, 一般来说, 溶滤液盐度越高, 溶蚀作用越强烈, 溶滤后砂体渗透系数增大。在纯水中, 珊瑚砂受溶蚀程度较小, 溶蚀前后渗透系数几乎不变; 在咸水中, 珊瑚砂发生了碳酸盐岩溶解和以Na+-Ca2+为主的离子交换作用等水岩作用, 且随盐度升高作用越强烈, 渗透系数越大。经过NaCl浓度为0.2, 0.4 mol/L的溶液溶蚀360 h后, 砂体渗透系数由0.58 m/d增加为0.64, 0.74 m/d, 推断溶蚀作用通过改变砂体整体孔隙度, 从而增加渗透系数。研究结果可为准确评估岛礁淡水资源储量及保障水资源可持续发展提供科学参考。
Abstract:Objective The permeability of coral sand is the key factor determining the groundwater reserves of coral islands. For a long time, the research on the influence of different salinity filtrates on the permeability of coral sand during the formation of freshwater lenses is quiet few.
Methods In this study, three kinds of solutions with different salinities were designed to represent freshwater, transition zone water and seawater to conduct dissolution and permeation tests on coral sand. Based on X-ray diffraction analysis and PHREEQC reverse simulation, the possible causes of water-rock interactions and permeability changes in coral sand in different salinity solutions were discussed.
Results Results showed that the permeability of coral sand was highly related to its dissolution in water bodies. The higher the salinity of the solution, the stronger the dissolution and the greater the permeability coefficient of the coral sand. The permeability coefficient of coral sand was almost unchanged in pure water. In saltwater, coral sand has undergone water-rock interactions, including carbonate dissolution and Na+-Ca2+ ion exchange, and the water-rock interaction is stronger with increasing salinity. After 360 hours of reaction under NaCl solutions with concentrations of 0.2 mol/L and 0.4 mol/L, the coral sand permeability coefficient changed from 0.58 m/d to 0.64 m/d and 0.74 m/d, respectively. It was inferred that dissolution could increase the overall porosity and thus the permeability coefficient by changing the sand particle size.
Conclusion The results provide a scientific reference for the accurate assessment of freshwater reserves and the sustainable development of water resources on coral islands.
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Key words:
- coral sand /
- different salinity /
- dissolution test /
- water-rock interaction /
- permeability
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图 2 试验装置示意图
1.砂柱; 2.广口瓶; 3.蠕动泵; 4.导管; 5.止水夹; 6.定水头供水瓶; 7.烧杯
Figure 2. Schematic diagram of the test device
图 3 滤出液离子质量浓度动态变化特征
R1.去离子水; R2.ρ(NaCl)为0.2 mol/L的溶液；R3.ρ(NaCl)为0.4 mol/L的溶液。其中，Na+, Cl-质量浓度为稀释后质量浓度，在R1、R2、R3三种溶液体系下分别稀释了10，100，100倍
Figure 3. Dynamic change characteristics of ions in different solutions during the test
表 1 珊瑚砂溶蚀过程中所发生的主要反应
Table 1. Main reactions that occur during the dissolution of coral sand
水化学反应 化学反应式 石膏溶解 CaSO4·2H2O→Ca2++SO42-+2H2O 方解石溶解 CaCO3+CO2+H2O→Ca2++2HCO3- 白云石溶解 CaMg(CO3)2+2CO2+2H2O→Ca2++Mg2++4HCO3- 离子交换 2Na++CaX(s)→2NaX(s)+Ca2+ Ca2++2KX(s)→CaX(s)+2K+ Ca2++2NaX(s)→CaX(s)+2Na+ 盐岩溶解 NaCl→Na++Cl- 
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表 2 模拟路径上可能反应相的交换量
Table 2. Exchange volume of possible reactive phases along the simulated path
方解石/10-3 mol 石膏/10-4 mol 白云石/10-3 mol 盐岩/10-2 mol CO2/10-3 g NaX/10-3 mol CaX2/104 mol KX/10-4 mol W1 -1.08 11.6 1.58 1.16 1.97 1.0 -6.55 3.07 W2 18.30 6.78 1.99 -4.95 19.7 36.9 -188 6.76 W3 22.50 12.8 2.13 -9.78 22.5 47.0 -237 3.81 注：负值代表沉淀，正值代表溶解
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表 3 全新世珊瑚砂渗透系数的测定方法及其范围
Table 3. Previous research on the permeability coefficient of Holocene coral sand
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