Recharge source and hydrogeochemical evolution of shallow groundwater in a complex alluvial fan system, southwest of North China Plain

摘要

Many cities around the world are developed at alluvial fans. With economic and industrial development and increase in population, quality and quantity of groundwater are often damaged by over-exploitation in these areas. In order to realistically assess these ground-water resources and their sustainability, it is vital to understand the recharge sources and hydrogeochemical evolution of groundwater in alluvial fans. In March 2006, groundwater and surface water were sampled for major element analysis and stable isotope (oxygen-18 and deuterium) compositions in Xinxiang, which is located at a complex alluvial fan system composed of a mountainous area, Taihang Mt. alluvial fan and Yellow River alluvial fan. In the Taihang mountainous area, the groundwater wasrnrecharged by precipitation and was characterized by Ca-HCO_3 type water with depleted δ~(18)O and δD (mean value of -8.8per thousand (δ~(18)O). Along the flow path from the mountainous area to Taihang Mt. alluvial fan, the groundwater became geochemically complex (Ca-Na-Mg-HCO_3-Cl-SO_4 type), and heavier δ~(18)O and <5D were observed (around -8per thousand δ~(18)O). Before the surface water with mean δ~(18)O of-8.7per thousand recharged to groundwater, it underwent isotopic enrichment in Taihang Mt. alluvial fan. Chemical mixture and ion exchange are expected to be responsible for the chemical evolution of groundwater in Yellow River alluvial fan. Transferred water from the Yellow River is the main source of the groundwater in the Yellow River alluvial fan in the south of the study area, and stable isotopic compositions of the groundwater (mean value of -8.8per thousand δ~(18)O) were similar to those of transferred water (-8.9per thousand), increasing from the southern boundary of the study area to the distal end of the fan. The groundwater underwent chemical evolution from Ca-HCO_3, Na-HCO_3, to Na-SO_4. A conceptual model, integrating stiff diagrams, is used to describe the spatial variation of recharge sources, chemical evolution, and groundwater flow paths in the complex alluvial fan aquifer system.