| The vadose zone is located below the earth's surface and above the phreatic surface.It serves as a connection and transformation link between atmospheric,plant,soil,and surface and groundwater.It is considered an important component of the geosphere and has the sensitive interactions with hydrosphere,biosphere,and atmosphere,greatly impacting on the environment of human survival.This study was conducted in the Achnatherum splendens grassland of the Qinghai Lake Basin.Precipitation,dustfall,and vadose zone samples were collected to investigate transport in vadose zone water using stable isotopes of hydrogen??2H?and oxygen(?18O),and chloride,as environmental tracers.We have concluded that:?1?The precipitation-weighted annual mean?18O and?2H of precipitation in the Qinghai Lake Basin were-10.59 and-70.27‰,respectively,and the local meteoric water line?LMWL?was?2H=7.95?18O+11.98.The total solid deposition in the basin was 279.35 g/m2·yr,including a total chloride deposition of 1.33 g/m2·yr.Precipitation was the main source of chloride in the research area,accounting for86.5%of total chloride deposition.The?2H and?18O values of soil water in the vadose zone plotted close to but below and to the right of the LMWL,the local evaporation line of soil water was?2H=4.74?18O-20.14,and the slope?4.74?was significantly lower than that of the LMWL?7.95?,thereby indicating that soil water in the vadose zone was mainly recharged by local precipitation.Strong evaporation occurred before the precipitation infiltrated and recharged the soil water.?2?The soil water and chloride content and isotope values in the soil profile varied across different depths.The joint effect of precipitation and water absorption by plant roots meant that soil water content at depths of 0?100 cm decreased gradually,with increasing depth;however,chloride content gradually increased.Chloride content at depths of 100?300 cm was lower than that at the surface layer,due to preferential flow recharge;however,soil water?18O values fluctuated.At depths greater than 300cm,soil water content gradually increased with increasing depth,whereas chloride content gradually decreased due to the impact of groundwater.Seasonal variation in soil water?18O values of the vadose zone indicated that,under the influence of evaporation,surface soil water had relatively higher?18O values than precipitation did in each season.The soil water?18O values were lower in winter than in other seasons,especially at depths of 0?300 cm,which may have been due to low winter temperatures;this led to isotope fractionation during water solidification.Furthermore,the local evaporation line of soil water?S-LEL?was?2H=4.74?18O-20.14,whose slope?4.74?was smaller than that of the LMWL?7.95?.This indicated that the soil water in this area was mainly contributed by local precipitation,and strong evaporation occurred prior to precipitation infiltration and recharge of the soils. ?3?In this study,the chloride content of groundwater was 50.25 mg/L,similar to that of deep vadose-zone soil water?51.68 mg/L?,but greater than that of precipitation?3.02 mg/L?and lower than the average chloride content of vadose-zone soil water?276.43 mg/L?.This indicated that the source of groundwater recharge in the research area consisted of preferential flow of precipitation and piston flow of upper soil water.The annual mean infiltration recharge from precipitation to the vadose zone soil water and groundwater was 9.93 and 26.46 mm/yr,accounting for 2.34 and 6.23%of the local precipitation,respectively.The infiltration recharge from preferential and piston flow accounted for 90.31 and 9.69%of the total infiltration recharge to groundwater,respectively,indicating that preferential flow was the main source of groundwater recharge in the Achnatherum splendens grassland studied.The chloride accumulation age of soil water in the vadose zone profile was 50.92 years,indicating that the main source of groundwater recharge in the Achnatherum splendens grassland in the Qinghai Lake Basin was precipitation since 1963.The results of this study were compared with those of other relevant studies.The recharge mechanisms of vadose zone soil water and groundwater varied between different regions and were controlled mainly by the spatiotemporal distribution of precipitation,topographic characteristics,and soil structure.These findings provide a reference and guidance for research on the circulation and evolution of vadose-zone water in the Qinghai Lake Basin. |
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