عنوان مقاله [English]
Groundwater is the largest available freshwater resource in the world. Aquifers provide drinking water to at least 50% of the global population, and account for 43% of all water used for irrigation. Groundwater resources can be expected to be increasingly relied upon, in the near future, as a consequence of rapid population growth and global environmental change. Cost-effective and efficient techniques for groundwater exploration, especially in karstic regions, can be used to as an appropriate tool to recognition of karst hydrogeological potential.
This paper provides a method based on the RS/GIS for the recognition of high groundwater potential areas and geoelectrical tomography for precise determination of the water well drilling location. Groundwater mapping has been defined as a tool for systematic development and planning of water resources (Elbeih, 2015). Hydrogeological maps provide spatially distributed information about aquifers, including their geological, hydrogeological and hydrochemical characteristics.
In this study, a hydro-tectonic model include effective layers on karst hydrogeology applied for the recognition of the high groundwater potential in karstic areas of Izeh, northeast Khuzestan. The combination of remote sensing and GIS used to overlay the major layers, i.e. distance from discharge point, elevation difference, fracture density, slope, and fracture intersection density. Generally, high altitude regions have a low groundwater potential and more groundwater can be found at lower altitudes; therefore, the altitude map generated from the DEM represents difference to known elevation of the discharge points. The areas away from the discharge point generally have lower probability of groundwater occurrence. The distance analysis in GIS was used to determine the map of distance from discharge point. Slope angle can be considered as a surrogate of surface runoff velocity and vertical percolation which affects recharge processes. However, in this study, the slope angle was considered as a positive factors on groundwater potential in the karstic areas. Geological fractures can have a significant effect on storage and flow of groundwater reservoirs. Especially in areas with shallow bedrock fractures, water infiltration can be enhanced due to increased porosity and hydraulic conductivity (Rao et al. 2001). The fracture locations in the study area were determined from the remote sensing techniques. The parameter are weighted from 1 to 5 based on their importance in karst hydrogeology.
For the exact determination of the water well drilling locations in high groundwater potential areas, the geoelectrical operation is done in two profiles using Dipole-Dipole array followed by electrical resistivity tomography. Over 20 boreholes have been drilled in karstic aquifer of Izeh for supplying the residence with drinking water. Despite the common use of geology for improving the siting of boreholes, some of the drilled holes does not deliver enough water to be equipped. The ERT method is used to determine the electrical resistivity distribution of the subsurface. Resistivity of the limestone rocks is linked to several parameters including type of limestone, cavity, water content, marl layer, electrical conductivity of water and the layer thickness. Because of different respective electrical resistivities in karstic areas, the ERT method provides useful results on the geometry of bedrock and aquifer. In an ERT survey, after inversion of the field data, the method provided a two-dimensional (2D) resistivity model of a section of the underground. Field data processing was performed with RES2DINV software. The parameters used in the inversion were the same for both of profiles, and topography was taken to normalize profile elevations to the actual ground surface. A robust algorithm was chosen for the inversion, because it provides more net changes in resistivity between different parts of the section. However, care must be taken when studying the final sections, because the geometry and boundaries of the structures are not always clearly identified and may be influenced by changes in resistivity due to rocks outside the plane of the section. The interpreted sections must be understood as an indication of the approximate location of the lithological boundaries, and not as its true geometry. The interpretation of the resistivity sections for all the ERT profiles has been drawn with the help of the correlation between the resistivity and the lithology along with the hydrogeologic data, and taking into account the continuity of the resistivity values at the crossing of the profiles. Overall, a very complicated structure is interpreted with the presence of dry and wet limestones, cavities, and marly layers interbeded with carbonates. Finally, two locations were proposed for drilling of water wells in the Izeh karstic area.
The drilling of a high yield water well (discharge of 61 L/S) and the low drawdown (0.48 m) in the karst of west Izeh at autumn 2019 indicates the effectiveness of the integration of the applied exploration methods. This work shows the power of geoelectrical method in poorly understood and tectonically complex areas in addition to the RS/GIS groundwater potential mapping to evaluate karst hydrogeology.