مدل سازی کمی آبخوان بجنورد با تاکید بر لایه بندی آن

Groundwater is the most important water resource in Iran that provides the majority of agricultural and urban demands. In recent years, natural recharge and extending urban areas caused the water table in alluvial Bojnourd aquifer to be raised. Transferring water from Karstic aquifers and Shirin Dareh dam intensified this problem, especially in the fine grain areas of the city. Supplying water from alluvial aquifer in the natural and urban areas, low thickness and discharge potential of alluvial, as well as development of Bojnourd were caused by the enhancement in the absolute height of water table in the alluvial aquifer. Also, in the plains such as Bojnourd that urban uses are dominant, according to the mentioned problems, water supply is very important. Therefore, it is essential that the current situation and future of Bojnourd alluvial aquifer to be identified and the solutions be managed to eliminate probable problem. Numerical models of groundwater flow can be used as the tools to test the effect of management decisions, but usually lack or incomplete information of the hydraulic characteristics of the aquifer causes limitation in use of these models. Nowadays, in the all of the world many studies are ongoing about of stratigraphy of aquifers in order to more understanding about the complicated conditions in the water flow hydraulics. So, it seems that understand of Bojnourd plain aquifer stratigraphy and a better estimation of the hydrodynamic coefficients can help to make decisions appropriately, to deal with the problems. The aim of this research was to make a model for Bojnourd aquifer with emphasis on hydro-stratigraphy.
According to the variety of Bojnourd aquifer sediments, it cannot be assume as a done-layer aquifer. So, in order to reduce uncertainties, as well as developing and enhancing a conceptual model, natural features that could impact groundwater system (especially stratigraphy of sediments) has been tried to identify and introduced to models. The effective factors include: geological studies, geophysical sections, using of observational wells and determination of the recharge and discharge. In this study, to verify and judgement models with fifteen different lithology cross sections in aquifer were used. Rockworks software was applied to obtain the appropriate layer of the aquifer that eventually lead to more accurate estimation in three-dimensional lithology model. Rockworks software uses lateral Bending Horizontal method to interpolate lithology of wells log data. Then, using three-dimensional model techniques in Rockworks16 software, the three-dimensional model of the aquifer lithology with all available data and information, i.e., logs and geophysical studies was obtained. Three-dimensional models of geology are as subsurface three-dimensional representative that show spatial relations of logs lithology and lenses. Interpolating between the existing logs can fill the undefined gaps between them and increasing the amount of boreholes in aquifer adds the accuracy of these models. The results of three-dimensional models represent three-dimensional groundwater flow model of the aquifer system. The result of lithology modeling of aquifer was composed of four soil textures including of coarse grain, medium-grain, fine grain and very fine grain with four layers and with the thickness of 30m that was important to version1.7.10 of GMS software.
The model was run in steady state for October 2000. In the first step, the initial model coefficients were calculated. After running the model in steady state, the mean square error was 2.77 m. In the next step, the results were better with changes in the soil hydraulic conductivity values and the mean square error was 0.794m after calibration. Heterogeneity and lenses in the aquifer system caused change in hydraulic conductivities of coarse to very fine-grained sediment, so the identification of aggregation and different genders, lithology of the aquifer with higher accuracy was achieved. Consequently, the hydrodynamic coefficients (k,s) could be estimated more precisely. So, considering that the aquifer materials are heterogeneity, the hydraulic conductivity values were obtained as 8.1, 6.7, 1.9 and 0.4 for the soil textures of coarse grain, medium grain, fine grain and very fine grain, respectively. Totally, changes of water level were between 1020 to 1080 meter and in the first layer while the northern and eastern parts of aquifer were dry. According to flow, budget of the aquifer in multi-layer model was obtained with acceptable accuracy, comparing with one-layer models. The calculated flow budget by the model shows that the aquifer layer was in balance with the amount of input and output, approximately.