| The Badan Jaran Desert?BJD?,located in the west of the Inner Mongolia Autonomous Region,has an extremely arid climate.The distributions of lakes and megadunes in the desert attract a large number of scholars,who have obtained many achievements about the formations of megadunes and the sources of water supply to lakes.Some issues such as the source of lake water supply,the groundwater recharge,the time of salt accumulation in lakes,and the law of salt migration between lakes and groundwater are still unclear and needed to be studied.It is of great significance to analyze the lake evolution,to correctly estimate the water resources in arid areas,to probe the law of paleoclimate evolution,and to protect the fragile ecological environment in the northwest by resolving these issues.In this study,the spatial variation trends of lake hydrochemical types and total dissolved solid?TDS?of lakes are analyzed by using 313 sets of water samples from 115 lakes from September 2009 to April 2016.The two lakes in the Sumujilin lake system were taken as the subject investigated.The three-dimensional unsteady flow numerical model and coupled numerical model for three-dimensional groundwater flow of variable density and solute transport were used to explore the groundwater recharge mechanism to lakes,the groundwater recharge,the salt accumulation time and the change characteristics of fresh-saline transition zone.The results can provide references for the study of lake water supply and lake evolution in the BJD.The main results drawn in this dissertation are listed as follows:?1?The mainly hydrochemical types of lake water in the BJD are Na-Cl-?SO4?-?CO3?,Na-Cl-?CO3?-?SO4?,Na-CO3-Cl-?SO4?,Na-Cl-CO3-?SO4?,Na-Cl-?CO3?,Na-Cl-CO3,Na-?Mg?-Cl-SO4,Na-?Mg?-Cl-SO4.The values of pH for the lake water fall in the range of 9-11.The TDS generally shows a trend of lower in the sourtheast and higher in the northwest.?2?On basis of the in-situ continuous measured meteorological data,the improved Penman formula was used to calculate the surface water evaporation for the Sumujilin lake system.The values of evaporation are 1,059.402 mm,1,027.767 mm,1,063.514 mm,and 997.306 mm for2011,2012,2013,and 2014,respectively.?3?The total recharge of groundwater to the two lakes in the Sumujilin lake systems reaches5.866×106 m3,accounting for 80.55%of the total supply during the fouty-five simulation periods from May 7,2011 to December 31,2014.The total amount of direct recharge from precipitation is9.714×105 m3,accounting for 8.76%of the total recharge,and the total amount of precipitation infiltration is 1.186×106 m3,accounting for 10.69%of the total supply.The salt accumulation time in Lake Sumujilin is estimated to be equal to 4,650 years more than 4 thousand years by the current supply intensity of lakes and Cl-concentration in groundwater.?4?According to the simulation of different scenarios,there is a very narrow fresh-saline transition zone about 2.5 m in width when the longitudinal dispersivity is set as 0.5 m.The salinity sharply increases from less than 1 g/L to more than 120 g/L.The concentration of the aquitard layer in the bottom of the lake is sharply reduced from 90 g/L to less than 10 g/L witnin the 1.5 m fresh-saline transition zone in the lake bottom.The migration range of“brine wedge”spreads mostly from 5 to 25 m from the beginning of brine downward and forward invasion to temporary stability.When the longitudinal dispersivity is 10 m,the width for the fresh-saline transition zone reaches or even is more than 5 m.The transition is gentler than that from the longitudinal dispersivity of 0.5 m.When the hydraulic conductividy becomes larger,the salinity of lake water mainly moves downward. |
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