Groundwater modeling in Bojnourd aquifer with emphasis on hydro-stratigraphy

Groundwater is the most important water resources in Iran that provides majority of agricultural and urban demands. In recent years, recharge from natural and extended urban areas caused the water table in alluvial Bojnord aquifer to be increased. Transffering water from Karstic aquifers and Shirin Dareh dam intensified this problem specially in fine grain areas of the city. Supplying water from f alluvial aquifer from natural and urban areas, low thickness and discharge potential of alluvial and development of Bojnourd were caused enhancement in the absolute height of water table in the alluvial aquifer. Also in the plains such as Bojnourd plain that are urban uses, According to the mentioned problems, water supply is very important. Therefore, it is essential as much as possible, the current situation and future of Bojnourd alluvial aquifer are identified and management solutions are pursued to eliminate probable problem. The numerical model of groundwater flow can be used as a tool to test the effect of management decisions but usually lack or incomplete information of the hydraulic characteristics of the aquifer causes limitation of use of these models. Nowadays, in the all of the world many studies are ongoing about stratigraphy of aquifers in order to more understanding about the complicated condition 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 decision appropriately to deal with the problems. The aim of this research was to make model of Bojnourd aquifer with emphasis on hydro-stratigraphy. Discuses: According to the variety of Bojnourd aquifer sediments, it cannot be assumed one-layer aquifer; so in order to reduce uncertainties, as well as developing, the conceptual model has been tried to identify natural effects that impact on groundwater system, especially stratigraphy of sediments are introduced to models. The effective factors include geological studies, geophysical sections, boundary of modeling, using of observational wells and determination of the recharge and discharge. In this study, to verify and judge models with different fifteen lithology cross section during aquifer using Rockworks software to obtain the appropriate layer of aquifer that eventually lead to more accurate estimation of 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 has been obtained. Three-dimensional models of geology are as subsurface three-dimensional representative that those can show spatial relations of logs lithology and lenses. Interpolation between the existing logs is not defined as the distance fillers and increase of boreholes in aquifer adds 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 is composed of four soil textures including of coarse grain, medium-grain, fine grain and very fine grain with four layers and with a thickness of 30m that was imported to version 1.7.10 of GMS software. Conclusion The model was run in steady state for October 1380. 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 changes 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 is achieved and consequently, the hydrodynamic coefficients (k,s) will 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 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; the northern and eastern parts of aquifer were dry. According to flow budget of the aquifer in one layer model was obtained with acceptable accuracy in multi-layer models, to establish flow budget in accordance with the single-layer model. The calculated flow budget by the model showed that the aquifer layer was in balance with the amount of input and output approximately.