The Impacts Of Groundwater On Lakes In The Badain Jaran Desert Relevant To Water Balance And Salts Variation

Badain Jaran Desert(BJD)is the second largest desert in China and has the famous landscape of the co-existing maga sand hills and many lakes.All the lakes within the desert are groundwater-fed lakes.Most of them are salt or saline lakes,a few of them are subsaline lakes.Groundwater influences the water balance and salt characteristics in the BJD lakes in multiple ways.This study mainly investigated the discharge forms of groundwater into the lakes,the impacts of groundwater on the water-salt balance of the closed lakes and semi-closed lakes.The distributed groundwater-collection zones(GCZ)of the grouped lakes were also investigated with the numerical modeling method.Surrounding the lakes and on lake-bed,groundwater can discharge out in a concentrated way as springs.There are 4 types of springs: channeled flow descending spring,seepage flow descending spring,islet ascending spring and sublacustrine spring.Among them,both shallow and deep sublacustrine springs have been observed in the basin of Sumujilin lakes.In this study,the general water balance and salt variation models of groundwater-fed lakes were developed.They were applied for the closed salt lakes,such as the South Sumujilin lake,and for the semi-closed lakes,such as the East Badain lake.A simplified water balance model of closed lakes has repeated the seasonal variations of the lake water level,showing a close-to-steady discharge of groundwater,from which the seasonal variation range of water level was maintained to be less than 0.3 m though influenced by significant evaporation.The salt accumulation time of the Sumujilin lakes and the Huhejilin lake,etc.,were estimated as values between 800 a to 7000 a,indicating a long formation and evolution history of the lakes that was over thousands of years.The development of the semi-closed subsaline lakes involved the existing of ephemerally overflow path,which could happen when the terrain divide between the lake and the lower stream basin has a relative height smaller than the half of the seasonal variation range of the lake water level.The low salinity is due to the equilibrium between the salts exported in the overflow-period and imported in the closed-period.A simplified water-salt balance model proved that the dynamic balance point of the semi-closed lakes depends on the portion of net evaporation loss of water in the total contribution of groundwater to the lake,in a linear manner.The model estimated total dissolved solids(TDS)in the East Badain lake for annual mean status was 1.67 g/L,which was very close to the observed results(1.64 g/L).This agreement verified the efficiency of the model.The three-dimensional(3D)groundwater flow filed of the lake-group area in the BJD was numerically investigated,using the 3D finite-difference modeling method with MODFLOW.Based on the simulation,the GCZs were identified and the interactions of the GCZs between lakes were discussed.The lakes produce a radial from flow of groundwater at the regional scale.A lake is generally surrounded by a U-shape divide line of shallow groundwater and its GCZ is constrained by the U-shape divide lines of the othe lakes in the upper stream area.Corridor-shape ribbon collection zone will be created in this way.On profiles,the special divide lines will be exhibited below grouped lakes to separate shallow and deep groundwater.The ensemble of groundwater streamlines toward different lakes exhibit 3D overlay structure.Several branches of streamlines can develop as lakes in the lower stream area.The GCZs control the coming sources of groundwater that contribute to the lakes and,indirectly,dominate the ratio of imported salts in the lakes that originated from shallow and deep groundwater.