Evolution of geochemical processes driven by hydrogeological processes in the hierarchically nested groundwater flow...

In large basins in arid and semi-arid regions, hierarchically nested groundwater flow systems are generally formed. Due to the great thickness of aquifers, the depth and number of boreholes are limited, which makes understanding the evolution of hydrogeochemical processes in hierarchically nested groundwater flow systems challenging. This study reveals the evolutionary mechanisms of geochemical processes driven by hydrogeological processes in the thick aquifer in the endorheic region of the Ordos Basin, China, based on samples collected stratified from deep boreholes in the recharge and discharge zones, combined with samples from domestic and artesian wells. The results show that hydrodynamic conditions in different hierarchical groundwater flow systems vary widely, and the intersection zone of adjacent groundwater flow systems flowing in opposite directions forms a hydraulic trapping zone with extremely slow flow rates. The dominant geochemical processes in different hierarchical flow systems are controlled by hydrodynamic conditions and evolve driven by hydrogeological processes. In the local groundwater flow system, carbonate dissolution and cation exchange mainly occur; in the intermediate and regional groundwater flow systems, the slowdown of flow rates and the increase in flow distances along the flow paths increase the reaction time of the geochemical processes, resulting in carbonate minerals reaching saturation and the evolution of the dominant geochemical process to the dissolution of evaporites with higher solubility products. In addition, redox reactions such as sulfide oxidation and bacterial sulfate reduction are limited, and sulfate enrichment and carbonate precipitation occur in the hydraulic trapping zone. This study proposes a research approach to identify the hydrogeochemical processes in hierarchically nested groundwater flow systems based on limited boreholes, and provides new insights into groundwater environment management in water-scarce areas.