GROUNDWATER RECHARGE AND WATER-ROCK INTERACTION FROM UNSATURATED ZONE OF BADAIN JARAN DESERT

The investigation of unsaturated zone pore water plays an important role in soil science, environmental sciences, and hydrogeology. While rainwater moves toward the water table from the land surface, it can potentially record climatic and environmental changes in the unsaturated zone. In this study, two 6-m cores from the Badain Jaran desert (NW China) were collected to explore the history of recharge, natural geochemical processes and anthropogenic influence using directly extracted moisture. The extraction from these low moisture sediments (1-5% by wet weight) were conducted using immiscible liquid displacement techniques without affecting its water chemistry. Extracted pore water were then analyzed for major anions, cations and trace elements. Results show enrichment in pore water chemistry in the top 1-2 m where strong temperature and moisture fluxes occur. The enrichment in cations relative to chloride is primarily due to silicate mineral dissolution during infiltration. High nitrate and low iron concentrations indicate the overall oxidizing environment, which allows the mobility of oxyanions, such as uranium, arsenic and chromium. Ratios of sulfate over chloride in two profiles showed increasing trends since 1990s, which might suggest higher industrial activity and sulfur emissions. The calculated groundwater recharge rates using the chloride mass balance are negligible (between 1.5 and 3.0 mm/year) in this arid region. The modern rainfall infiltration signature contrasts with that of the underlying groundwater body, which has a distant, regional recharge signature. This reconnaissance study demonstrates the potential for a new geochemical approach to studying geochemical processes in the unsaturated sediments in semi-arid/arid environments due to both natural and human influences. The use of directly extracted water, rather than extraction by dilution, facilitates an improved understanding of hydrological and geochemical processes as water moves from the surface to groundwater.