Groundwater Circulation Patterns Of Typical Lake Area In Northern Ordos Cretaceous Basin

Such resources as coal, gas, oil, salt and minerals were contained in Ordos Basin, which lies in the eastern arid or semiarid region of northwest China. This region has been a new energy & chemical base, and an important energy-developing zone in 21st century, so it holds strategic significance to national economical and social development. Especially, the Cretaceous basin north of Baiyu Mountain, where plenty of energy and minerals deposited, is becoming a new energy source and chemical base. Because of it, prosperous economy, social progress, and new opportunities for developments have been bought to this region. However, rational development and utilization of local water resources requires a thorough knowledge of the geological and hydrogeological conditions. Desert-plateau region of north Ordos Basin, distributed with numerous lakes, is the main discharge area of surface water and groundwater. Evaporation is the major groundwater consuming pathway in this region. The interaction between water in lakes and groundwater is the core factor of groundwater balance. It determines the size, time-space distribution, shape, hydrological and chemical features of lakes to a certain extent, and also reflectes local groundwater dynamics since it is closely related with groundwater recharge, runoff & drainage conditions and cycling pattern as well. Therefore, while groundwater being pumped, analysis on the variety of lakes is of great value to find out the recharge scope, water cycle and its effects on the environment. Of the many lakes distributed in the desert plateau region, we focus on representative large and medium-sized lakes in the inner flow area surrounded by groundwater divide and modern erosion basis, and study the groundwater cycling characteristics and patterns in the north lake region of Ordos Cretaceous basin.Considering geographical location and recharge sources, three typical lakes were chosen. They are Dakebo, Subeinao and Yanhaizi, which represent the characteristics of the lakes controlled by partial, intermediate, and regional groundwater system respectively. Then, hydrodynamic analysis, water chemical and isotopic methods were combined to determine their groundwater cycling patterns.Since the characteristics of the groundwater movement and the hydraulic link between surface water and groundwater are reflected by hydrodynamic field of surface water and groundwater, the interaction between lake water and groundwater can be derived by flow characteristics of the dynamic field. Besides, hydrochemical and isotopic methods were further introduced. The groundwater age was calculated by tritium and CFCs methods.The main conclusions of this paper were drawn as follows:Cretaceous Ordos basin are widely distributed by numerous lakes with various sizes, which mainly are affected by the combination of local topography, groundwater flow and such factors. The space features of lake distribution in the study area are mainly manifested in two aspects. First, lakes can be found all over the region, with a majority of small sized but few large and medium-sized lakes. Second, the distribution, scale and size of local lakes depend on the topography, while the characteristic of groundwater flow is such an important factor that influences the distribution of large and medium-sized lake. Specifically, most large and medium-sized lake are found in groundwater discharge zone.Local lakes are the main concentrated discharge area of surface runoff and groundwater flow. According to the spatial distribution of lakes and their impact on the groundwater cycle, they are classified under three categories: small sized lakes controlling partial discharge area, middle sized ones of moderate area, and large sized ones of region.The lake which dominated by local groundwater cycle always is small-sized lake, that located on low-lying salt marsh land, the lithology of salt marsh land surface is silt sand with high clay content, which causes the precipitation can hardly infiltration but to form surface flow to recharge lake, therefore, the character of this lake is intensive seasonal change, in rainy season, the lake serve as a water logged soil, and expands its water area during a heavy rain; in dry season, the lake dry up and shrink away as a salt marsh land with intensive evaporation; the small-sized lake—Dakebo region which controlled by local groundwater cycle receive the recharge of precipitation in the around high-topography area, and discharge to the lake from the surface watershed. The groundwater cycle has a rapid rate and a short path, the groundwater with a relative deep depth downward move and then turn to lateral runoff in east and west in the region of surface watershed. The lake water level has a stable state, and the groundwater is about several decades of age and 100m circulation depth.The lake which dominated by middle groundwater cycle always is middle-sized lake, it has a big water area and some seasonal character, in rainy season, the lake receive the recharge of precipitation and runoff, and expands its water area; in dry season, the lake dry up and shrink its water area. The middle-sized lake—Subeinao area which dominated by middle groundwater cycle model is a discharge area of middle groundwater system with wider extension and deeper depth than small-sized lake, it has a circulation depth 400m. The groundwater receives recharge in the area of watershed and moves downward, forms runoff in the sink and lake along topography, and moves upward in the deeper layer, discharges to bigger sink and forms middle-sized lake.The lake which dominated by regional groundwater cycle always is large-sized lake, it has a largest water area and can perennial exist water, the lake which dominated by middle-sized and small-sized groundwater cycle always located on the low-lying of sand-girder, and it serve as a datum plane of groundwater in the large area, the lithology of sand-girder land surface is medium-fine sand, fine sand, and the precipitation can easily infiltration, the lake mainly receive the recharge of around groundwater in most condition except a precipitation of storm; the large-sized lake—Yanhaizi which controlled by regional groundwater cycle is a discharge area of regional groundwater system with wider recharge extension and deeper circulation depth than middle-sized lake, it has a circulation depth 600m and reach the bottom of cretaceous. The groundwater receive recharge in the area of watershed, then move vertical and discharge to the low-lying along topography, and forms the large-sized lake with a great scale.