Studies On Key Hydrogeological Problems In The Western Alxa, China

The Western Alxa, China, is characterized by the Badain Jaran Desert(BJD) which extends in a large area and is surrounded by mountains of granite rocks. The existing studies on regional hydrogeological conditions in this area were very limited. The formation and evolution of lakes in the BJD were controlled by groundwater because no surface runoff exists. However, the features of groundwater flow in the BJD has not been clearly revealed in previous investigations. In the last decade, there were heated debates on the relationships between recharge and discharge as well as between the BJD and its surrounding mountain and plain areas, especially on the viewpoint of that the recharge of groundwater in the BJD is originated from Qilian Mountain or even from the Tibetan plateau.In this study the new data of drilled boreholes and the previous investigation data were integrated to obtain new answers on three key hydrogeological problems in the Western Alxa, China. It was indicated that the BJD is a semi-closed basin of groundwater. The fractured rock aquifers and pore media aquifers were developed well, the thicknesses of them are larger than 100 m and 500 m, respectively. The pore media aquifers have high permeability and provide a moderate volume as groundwater flow domain. Moreover, they were directly linked with the precipitation infiltration and evaporation processes. It is certain that the pore media aquifers control the flow of shallow groundwater. The thickness of fractured rock aquifers is huge so they can also influence the regional groundwater flow, although the permeability of these aquifers is relatively small. Most of the faults, which have cut the fractured rock aquifers, role as preferential passageway for groundwater flow. However, the impacts of these faults were limited in local area and in a discontinuous pattern according to the natural characteristics of the faults. On the whole, distribution of the aquifers dominants the regional groundwater flow while the faults make a local disturbance. There were significant errors existed in placing the locations and directions of faults in some previous study.More than 400 control points of shallow groundwater were obtained through field work, satellite imagery and previous data. Based on the points from field work, the flow direction and hydraulic gradient of groundwater were determined using triangular mesh method. The hydraulic gradient varies between 0.6‰ and 12.9‰, which is large in the east and little in the west regions of the studied area. At regional scale, groundwater flows from southeast to northwest. Based on the whole points, the map of one order approximate contours of groundwater level was obtained. Accordingly, the study area could be divided into two relatively independent groundwater systems: the Zongnai Mountain groundwater system and the Beida Mountain-Yabulai Mountain groundwater system. Four discharge areas, such as the lakes group in the southeast of the BJD, Shugui-Yingejing areas, Gurinai lake and Guaizi lake, could be recognized.Based on numerical simulation of regional groundwater flow, the impact of lakes evaporation on groundwater flow was discussed both at lakes group scale and at an individual lake scale. Groundwater around the lakes group tends to converge, but the macro trends of that groundwater flows from southeast to northwest is not changed. The hydraulic gradient turns less when groundwater flows through the lakes group because of the evaporation of lakes, and in this area, the evaporation of phreatic water in no-lake depressions is approximately equal to that of lakes. In general, a lake is recharged by the shallow groundwater in the sand dunes around it since it locates in the lowest point in a local watershed. The lakes are also recharged by upward leaky of deep groundwater which is recharged by lateral flow at the regional scale. The hydraulic gradient inside the watershed of a lake turns large from the divide to the lake.Using the “influence radius” model to interpret the specific capacity(q) of a pumping well in previous studies is not reasonable. New interpret equations of specific capacity in different pumping conditions were developed in this study. The status of pumping tests could be judged by the curve of 1/q~Q, where q and Q are the specific yield and pumping rate of the pumping well, respectively. The hydraulic conductivity could be calculated more reasonable through the new equations. This method was applied to analysis the pumping tests of boreholes Y1~Y10 in the Yabulai Mountain and plain area.An weather station was installed inside the desert to monitoring the meteorological items such as precipitation and evaporation for water balance analysis. Considering the impact of domination orientation of tectonic stress, the vertical decay of hydraulic conductivity and the multistory structure of aquifers, a numerical model of regional groundwater flow in the Western Alxa was built. Three scenes were designed in considering of the uncertainty in the values of main parameters and in the recharge and discharge conditions. The results showed that the discharge in the Western Alxa could be balanced by the recharge occurred in the this area. It is not necessary to introduce the remote sources of groundwater by the deduced faults to balance the evaporation loss of water in lakes in the BJD.