氢氧同位素在地下水流系统的重分布:从高程效应到深度效应

Redistribution of hydrogen and oxygen isotopes in groundwater flow systems: From altitude effect to depth effect

大气降水的氢氧同位素含量具有高程效应,降水入渗后参与地下水循环,其高程效应如何受地下水流系统的影响转化为地下水氢氧同位素的深度效应?现有研究对于这个问题缺少定量认识。文章构建单向倾斜盆地和双峰波状盆地的稳态地下水循环理论模型,采用MODFLOW模拟剖面二维地下水流场、采用MT3DMS模拟重同位素分子的对流-弥散过程,得到地下水D和18O含量的空间分布,探讨了氢氧同位素高程效应在地下水流系统转化为深度效应的机理。结果表明:在单斜盆地,补给区大气降水D和18O含量的高程效应转化为排泄区地下水δD和δ^(18)O值随埋深增大而指数型衰减的深度效应;在双峰波状盆地,当含水层渗透性相对入渗强度较大时(K_(0)/w=1 000),仅发育一个区域地下水流系统,在区域地下水的排泄区δD和δ^(18)O随埋深增大呈现S形曲线分布;当含水层渗透性相对入渗强度较小时(K_(0)/w=250),双峰波状盆地发育多个局部地下水流系统,区域地下水的排泄区δD和δ^(18)O随埋深增大呈现S形曲线,而局部地下水排泄区的δD和δ^(18)O随深度增加呈单调衰减趋势。本研究从理论上推进了地下水流系统对溶质运移影响机理的认识,揭示了氢氧同位素对地下水流系统的指示作用。

The hydrogen and oxygen isotopes in precipitation have the altitude effect, and they participate in the groundwater circulation after the infiltration of precipitation. How does the altitude effect of hydrogen and oxygen isotopes in precipitation transform to the depth effect of hydrogen and oxygen isotopes in groundwater under the influence of groundwater flow systems? The existing research lacks quantification for this problem. In this study,the steady-state groundwater flow theoretical models represented by unidirectional inclined basin and bimodal wavy basin are constructed. The MODFLOW and MT3DMS programs are used to simulate the two-dimensional groundwater flow field in the profile and the convection-dispersion process of heavy isotope molecules to obtain the spatial distribution of D and18O values in groundwater and discuss the mechanism of altitude effect of the hydrogen and oxygen isotopes transforming to depth effect in the groundwater flow systems. The results indicate that in the monoclinal basin, the altitude effect of D and18O content in precipitation in the recharge area is transformed to the depth effect of δD and δ^(18)O values in groundwater through the regional groundwater flow system which exponentially decreases with the increasing water table depth in the drainage area. In the bimodal wavy basin, when the permeability of the aquifer is relatively larger than the infiltration intensity(K_(0)/w=1 000),only one regional groundwater flow system develops, and the distribution of δD and δ^(18)O presents a S-shaped curve with the increasing water table depth in the discharge area of regional groundwater. When the permeability of the aquifer is relatively smaller than the infiltration intensity(K_(0)/w=250), multiple local groundwater flow systems develop in the bimodal wavy basin. The δD and δ^(18)O indicate an S-shaped curve with the increasing water table depth in the discharge area of regional groundwater, while in the discharge area of local groundwater, they show a monotonic attenuation trend with the increasing water table depth. This study theoretically advances the understanding of the influence mechanism of groundwater flow systems on solute transport, and reveals the indicative role of hydrogen and oxygen isotopes in groundwater flow systems.