Transformation relationships of three waters in the Hailar River, Inner Mongolia based on δD - δ¹⁸O isotopes

Objective The mutual transformation between atmospheric precipitation, surface water, and groundwater constitutes a core component of the global water cycle. Driven by the dual effects of climate change and human activities, clarifying the "three-water" transformation mechanism is of great significance for the sustainable management of regional water resources. This study aims to take the Hailar River Basin in the Hulunbuir High Plain of Inner Mongolia as a typical area, to reveal the recharge sources, transformation paths, and renewal characteristics of surface water and groundwater in this region.

Methods Stable isotope technology (δD-δ¹⁸O) was comprehensively used to analyze the recharge sources of groundwater and surface water; groundwater dynamics methods were combined to quantify their recharge relationships; and hydrochemical analysis was employed to evaluate the residence time of different water bodies, constructing a multi-method coupled water cycle research framework.

Results Isotope analysis shows that groundwater and surface water share a common atmospheric precipitation source. Hydrochemical and dynamic verification indicates that the residence time of surface water is generally shorter than that of groundwater, and the renewal rate of shallow groundwater is higher than that of deep groundwater. Spatially, the upper reaches of the Hailar River are characterized by groundwater recharging river water, while the lower reaches transform to river water recharging groundwater. With the increase of groundwater depth, its renewal capacity shows a significant downward trend.

Conclusions This study reveals the temporal and spatial variation laws of "three-water" transformation in inland high plain areas, clarifies the dominant role of precipitation in the basin water cycle and the vertical attenuation characteristics of groundwater renewal capacity. The research results provide a scientific basis for the optimal allocation and sustainable utilization of regional water resources under the background of climate change.