Supply Of Atmospheric Precipitation In Arid Regions Infiltration

Precipitaion falling on the surface of the earth accounts for almost all of thewater entering into the soil. Precipitation infiltration research is an important basicwork of evaluation and scientific management of groundwater resources. Accurateassessment of the rate of precipitation infiltration is the basic work for developingwater resources planning and sustainable groundwater exploitation scheme in the aridand semi-arid area where the water resources is very shortage. Previous studiesfocused on the areas of plains, piedmont basin or the inland river basins rather than ofthe extremely arid Gobi. With the rapid development of siting and sitecharacterization for deep geological disposal of high level radioactive waste (HLW),precipitation infiltration in the area of extremely arid Gobi is pain more and moreattention.In this thesis, the key topic of the precipitation infiltration mechanism in theextremely arid Gobi area is concentrated on the condition of geological settings atXinchang-xiangyangshan area in Gansu Beishan preselected region for geologicdisposal. Permeability of soil in the unsaturated zone is Carryed out, and theapplication of tritium tracing approach and Chloride ion tracing approach to theresearch of precipitation infiltration at Xinchang-xiangyangshan area in GansuBeishan preselected region as well. Some progresses have been made in the aspects ofapproaches and theories of the research of precipitation infiltration in the extremelyarid Gobi area by means of comprehensive experiments and analysis. For the firsttime to get the the values of precipitation infiltration recharge in the research area andobtain the new understanding of characterization of precipitation infiltration recharge.The following results and understandings have been got.1. The soil permeability parameters in the unsaturated zone in research area wereobtained, and the permeability features of soil and its affective factors have beenidentified.(1) The permeability coefficient of soil is quite different in the research area.Permeability of the gullies zone is better than that of the slope zone, and the slope zone better than lower zone. The average values are about4×10-5m/s,1.6×10-5m/s and7.69×10-6m/s, respectively.(2) The primary affective factor of soil permeability is the diameter of particlecomposition (>0.5mm and <0.1mm). The other affective factors, such as total ioncontent, pH value and the diameter of particle composition (0.1mm~0.5mm) are theminimum.2. The important progress has been made in the aspect of the application ofhistorical tritium tracign approach to study the precipitation infiltration recharge.(1) Tritium content profile in the unsaturated soil (including free tritium andbound tritium) with the depth has been obtained. The change of Tritium content inthe soil with depth has been observed and has close relationship with the soil particlecomposition, mineral composition, water content, organic matter content and the soilpermeability.(2) The bound tritium tracing approach has been presented and its application tothe research of precipitation infiltration in the extremely arid Gobi area has beenconducted. It has been shown from the research results that a set of isolation andextraction technique of bound tritium from unsaturated soil has been established andStudy of the precipitation infiltration recharge by using the bound tritium tracingapproach is theoretically feasible.3. For the first time to obtain the precipitation infiltration recharge rate ofunsaturated soil at Xinchang-xiangyangshan area by using different approaches.(1) The average precipitation infiltration recharge rate was about0.007mm/d~0.018mm/d by using tritium migration experiment in situ. But the value is bigger thanthat of other approaches because of the evaporation.(2) The annual recharge rate of each experiment point has been calculated from0.05mm/a to1.57mm/a by using based-on-unsaturated-zone chloride mass balance(CMB) and the average value is about0.9mm/a with the standard deviation of0.63mm/a.(3) The annual precipitation infiltration recharge rate at the Xinchangxiang-Yangshan area is only about0.287mm/a by using based-on-saturated-zone chloride mass balance (CMB) which accounts for less than1%of the average annualprecipitation.