Research On Evaluation Of Groundwater Evaporation Based On Bromide Tracer Method

Groundwater evaporation is a crucial constituent part of hydrological cycle.Accurately estimating the process and quantity of groundwater evaporation is of great significance to the evaluation of groundwater resources,to solve the soil salinization problem and to the calculation of ecological water consumption of natural vegetation.Most of the previous studies focus on the evaluation of groundwater evaporation under different conditions,but the upward transport process of water caused by atmospheric evaporation in the vadose zone is not fully considered,which is not conducive to deeply understand the process and mechanism of groundwater evaporation.Chemical tracer is one of the important tools to study groundwater.In this paper,bromide is selected as tracer to evaluate groundwater evaporation.The variably saturated numerical simulation,indoor soil column experiment and field tracer experiment were conducted to analyze the controlling factors of evaluating groundwater evaporation by bromide tracer method,and the feasibility and applicability of the method were discussed finally.Combined with the actual conditions,the conditions of tracer injection are optimized to provide theoretical basis for the application of the method.This paper draws the following conclusions:(1)The lithology of the vadose zone and the water table depth are the main factors affecting the bromide transport in vadose zone profile.The shallower the water table depth,the finer the soil lithology,the greater the groundwater evaporation intensity,and the faster the transport rate of bromide.The migration and distribution of bromide show significant differences with changes in the characteristics of the vadose zone,indicating that it has a higher sensitivity as a tracer in response to the changes and can effectively indicate changes in groundwater evaporation intensity.(2)By comparing the actual quantity of groundwater evaporation and the calculated results,the effects of natural factors(water table depth,vadose zone lithology)and human factors(injection depth and injection concentration of tracer)on the evaluation of groundwater evaporation by the bromide tracer method are summarized.The bromide transport fast in the vadose zone and the peak concentration is obvious when the water table is shallow and the vadose zone is consisted of fine sediments,and the calculation error is relatively small.The injection depth of tracer has little effect on the transport of bromide concentration peak;however,since the water content increases with depth,the average water content of the tracer transport interval increases with the injection depth.Hence,the calculated results of groundwater evaporation increases and deviates from the actual value;the injection concentration of tracer only affects the concentration of bromide peak on the vadose zone profile,and will not change the movement of it,so it has little effect on the evaluation result of groundwater evaporation.(3)The bromide tracer method can effectively indicate groundwater evaporation under different conditions,but infiltration is the main interference factor leading to the errors in evaluation results;hence the method is more suitable for quantitative research on groundwater evaporation in arid and semi-arid areas.The water table depth,lithology of the vadose zone and the injection conditions,etc.have uncertain effects on the evaluation results.When the water table depth is shallow and the vadose zone is consisted of fine sediments,the accuracy of the evaluation results is relatively high.In the application of bromide tracer method,a moderate injection depth should be chose according to the actual conditions of vadose zone,and a large injection concentration is suggested to be selected to extend the evaluation period and improve the accuracy of evaluation.