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

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 and ¹⁸O 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 and ¹⁸O content in precipitation in the recharge area is transformed to the depth effect of δD and δ¹⁸O 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 (K₀/w=1000), only one regional groundwater flow system develops, and the distribution of δD and δ¹⁸O 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 (K₀/w=250), multiple local groundwater flow systems develop in the bimodal wavy basin. The δD and δ¹⁸O 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.