The Influence Of Vadose Zone’s Texture And Thickness On Groundwater Recharge From Precipitation Infiltration And Isotope Fractionation Effects

As an important component of water resources,groundwater plays a pivotal role in water supply,especially in the arid and semi-arid northwest of China,where the fragility of the ecological environment and the proliferation of negative problems highlight the extraordinary importance of groundwater resources,and the quantitative evaluation of groundwater recharge is a prerequisite for the sustainable use of groundwater resources.The main source of groundwater recharge is atmospheric rainfall,and the pattern and intensity of rainfall and the characteristics of the unsaturated zone regulate the intensity and dynamic changes of groundwater recharge process.Therefore,it is of great theoretical significance and practical value to carry out research on groundwater recharge based on hydrogen-oxygen stable isotope tracing under the influence of different lithologies and different thickness of the envelope zone for accurate evaluation and management of groundwater resources.In this paper,four unsaturated zone media of fine sand,medium sand,coarse sand and gravelly sand were used in 2 m,3 m,4 m and 5 m inclusive gas zone thickness conditions,and the following main results were achieved under the observation of rainfall and soil column outflow water in both hourly and daily time-scale by using in situ scale observation tests,model calculations,and theoretical analysis:1.The main influencing factors for stable isotopes in atmospheric precipitation and the relationship between rainfall and recharge between different soil columns in the experimental area were elucidated.The stable isotopes of hydrogen and oxygen in atmospheric precipitation in the test area are mainly influenced by temperature,and rainfall also plays a slight role;in the two rainstorm infiltrations with an interval of about 2 days,the first rainfall infiltration mode is pulsed recharge,with a significant peak in recharge in time;the second is pressure-driven recharge,and the intensity of recharge decreases with the decrease of lithological particles and the increase of burial depth.2.The differences and reasons for the stable isotopes of outflowing water from different unsaturated zone media and soil columns with different thickness of the gas envelope are elucidated.The depletion of stable isotopes of effluent water from fine sand,medium sand,coarse sand and gravelly sand is caused by the combination of evaporation and infiltration fractionation,with evaporation increasing as the lithological grain decreases and infiltration fractionation decreasing;the depletion of stable isotopes of effluent water with the increase of the thickness of the inclusion zone is caused by the increase of adsorption.3.The fractionation mechanism of stable isotopes in the process of infiltration and the influencing factors were elucidated.The fractionation of stable isotopes during infiltration is caused by the matrix potential gradient,and the larger the generated matrix potential gradient is,the stronger the fractionation effect is.The fractionation is weakened with the increase of adsorption as the lithology particles become finer and the thickness of the gas envelope increases.