Geochemistry of sediment moisture in the Badain Jaran desert in Northwest China: Insights into groundwater recharge and past environmental changes

English Abstract:

Unsaturated zone pore water has the potential to record history of recharge and retain information related to environmental changes. In this study, two 6-meter cores from the Badain Jaran desert (Northwest China) were collected to explore this potential using directly extracted moisture. Pore waters in low moisture sediments (1-5%) were directly extracted using immiscible liquid displacement and then analyzed for major anions, cations and trace elements. The pore water chemistry was used to illustrate the recharge history, natural geochemical processes and anthropogenic influence. The calculated recharge rates at two sites from the conservative tracer Cl^- were low, 1.5-3.0 mm/year. Major cations in two profiles showed relative low concentrations in the unsaturated zone and high concentrations in the capillary fringe and into the water table. High nitrate and low iron concentrations indicate the overall oxidizing environment, which allows the mobility of oxy-anions, such as uranium, arsenic and chromium. The modern rainfall infiltration signature contrasts with that of the underlying groundwater body, which has a distant, regional recharge signature. In addition, ratios of SO₄^2- over Cl^- in two profiles showed increasing trends since 1990s, which might suggest higher industrial activity and sulfur emissions. This reconnaissance study demonstrates the potential for a new geochemical approach to studying geochemical processes in the unsaturated sediments in semi-arid environments due to both natural and human influences. The direct use of pore water improves the understanding of hydrological and geochemical processes as water moves from the surface to the capillary fringe zone and to groundwater.