The Qassim province is one of the most arid regions of Saudi Arabia, however heavy rainfall events have led to water logging problems in the populated centers like Buraidah mainly as a consequence of construction in low lying areas and valleys. Urban rain storm water logging problem were reported at 4 localities in the Buraidah Municipal area after heavy rainfall in November 2008 and March 2009. The logged water from these sites were directed to 4 manmade lakes however the stagnant water in these lakes posed a serious environmental threat mainly in the form of water borne disease. Keeping this problem in mind, a detailed geophysical investigation in the form of Electrical Resistivity Tomography was carried out using the SYSCAL Pro Unit at these 4 locations with a dipole-dipole configuration. The survey was helpful in delineating the wet zones from the dry ones and based on the interpretations the optimum depth and sites of 4 injections bore-wells were determined at each locality. These injection bore-well would serve the dual propose of getting rid of the stagnant water in these lakes and also recharging the underlying aquifers.
Qassim; Arid Regions; Water Logging; ERT Survey; Aquifer Recharge1. Introduction
Central Saudi Arabia experiences an arid type of climate with mean annual rainfall rarely exceeding 150 mm. The low rainfall has resulted in scanty vegetation in the region except for the wadis where farms and date palm plantations can be seen due to the availability of groundwater. However the chance of flash floods increases to a great extent due to lack of vegetation cover in the events of heavy rainfall.
The city of Buraidah which is the administrative capital of the Qassim province has undergone rapid urbanization in the recent years and due to lack of proper urban planning a lot of low lying areas and wadi beds have been allotted to housing colonies. During the heavy rainfall in the month of November 2008 and March 2009, many such housing colonies in the low lying areas were inundated by flood water.
The municipal corporation of Qassim decided to divert this water to man-made lakes in Buaraidah City. The stagnant water in the lakes in turn became a threat for the environment by providing the breeding ground for mosquitoes and other water borne disease. Thus it was decided to drill a few bore wells in these manmade lakes with the dual purpose of recharging the aquifer and getting rid of the stagnant surface water thereby preventing environmental degradation.
Keeping this objective in mind Electrical Resistivity Survey was carried out at 4 locations (Figure 1) in Buraidah for investigating the depth of the wet zone in these localities for estimating the approximate depths of injection bore wells to be drilled to get rid of the logged water collected during heavy rainfall events.
2. Methodology
Electrical Resistivity methods have been widely used for surveys where the sub-surface resistivity is heterogeneous and can provide valuable information regarding bed rock geometry [1]. Electrical Resistivity is a function of the water content in the subsurface strata, the porosity of the geologic material, the electrical conductivity of water, the rock type an temperature, [2-4].
The electrical potential difference created by an electrical current passing through a material is measured which helps in calculating the resistance of the material. The variations in resistivity with depths helps in the construction of a 2D profile and is considered to be the most
non-destructive techniques for studying the subsurface geological variations [5-7].
The surveys at all the 4 locations were carried out using the Dipole-Dipole configuration with the unit electrode spacing ranging from 2.5 meters to 5 meters depending upon the ground clearance using the SYSCALPro 72 unit. Dipole-Dipole configuration was selected for the survey as it is more sensitive to lateral variations in electrical resistivity.
RES2DINV Software [8] was used for inverting the apparent resistivity values to a resistivity model section. The least square fitting technique [9] was used for getting the best fit for the resistivity model by iterations.
3. Results and Discussions3.1. Site 1
The first site M1 was in the Buhairatil Khaleej (Figure 2) which is situated in the South of Burayda City. The total line length of the survey was 360 meters with the unit electrode spacing of 5 meters using dipole-dipole configuration. The depth of investigation at this site was around 72 meters.
3.1.1. Result
Figure 3 shows the result of the resistivity survey. The resistivity values range from 7.07 ohm·m to 36.9 ohm·m.
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