Characteristics of groundwater replenishment in deep and thick unsaturated zone based on chloride tracer method
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Abstract:
The continuous decline of groundwater level in northern China,the thickness of vadose zone continued to increase,so the law of water movement in the vadose zone has changed significantly.The maximum depth of previous experiments and researches on the vadose zone was mostly 7-10 m.Due to the lack of research on the law of water movement and precipitation recharge in the deep vadose zone,lag effect of deep water movement and the process of groundwater recharge problems have not been well solved. A significant research has been carried out on geological drilling in the typical deep vadose zone of the piedmont plain of North China.During drilling,a built-in ring knife sampler was installed at the front of the sampling drill pipe,and a built-in ring knife was pressed into the soil by a non-circulating hydraulic method.The rotary drill pipe disconnected the bottom soil mass and the built-in ring knife sampler simultaneously consolidated the soil sample during sampling to obtain deep undisturbed soil samples with high integrity.The soil structure parameters,water content,and chloride concentration of undisturbed soil samples were tested,and the chloride ion tracing method was used to study the law of water movement in the 45m thick vadose zone.At the same time,the history of precipitation recharge in the deep vadose zone was reconstructed and the intensity of precipitation recharge was evaluated. The results show that :(1) At different depths of vadose zone profile,the chlorine content in soil water is significantly different,and negatively correlated with water content.(2)There are three chloride ion peaks in the depth range of 5.50-28.25 m at the profile of the vadose zone.The first chloride ion peak was affected by evapotranspiration in the semiarid area.The formation of the second and third chloride ion peaks was related to precipitation and the lithologic structure of the vadose zone,which reflected two dry precipitation periods in the recharge history,and the peak mainly occurred in the transition section from coarse-grained stratum to fine-grained stratum.(3) The total age of precipitation recharge recorded in the 45 m vadose zone section was 72.12 years.During this period,the recharge intensity experienced three high and low fluctuations,reflected three relatively dry and wet climate processes.Meanwhile,the average infiltration recharge intensity was 0.096 m/a,and the precipitation infiltration recharge accounted for 18.1% of the average annual precipitation. The vertical infiltration law of water in the deep vadose zone was multiple pulses under the superposition of precipitation and gradually move to the deep,so it need a long-term process to recharge groundwater.When the upper part of the vadose zone was coarse and the lower part was fine,the water migration was hindered and the ion components were accumulated.On the contrary,the water migration was faster and the accumulation of ionic components was less.
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