7 Assessing groundwater quality of the shallow alluvial aquifer system in the Midyan Basin, northwestern Saudi Arabia 1 Department of Geology and Geophysics, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia 2 Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan A. Batayneh *1 , A. Laboun 1 , S.Qaisy 1 , H. Ghrefat 1 , T. Zumlot 1 , H. Zaman 1 , E. Elawadi 1 , S. Mogren 1 , K. Al-Qudah 2 ABSTRACT KEYWORDS The Midyan Basin is characterized by arid climate with rare surface water flow. Groundwater is the main water resources in this region. In order to evaluate their suitability for drinking, domestic and irrigation purposes, chemical characteristics of groundwater in the Midyan Basin (northwestern Saudi Arabia) have been investigated and evaluated. A total of 72 water samples were collected from different wells and analyzed for hydrogen ion concentration, total dissolved solids, electrical conductivity, total hardness, Ca2+, Na+, Mg2+, K+, Cl-, HCO3-, NO3- and SO42-. To understand quality of groundwater and their suitability, chemical indices like sodium percentage, sodium adsorption ratio and salinity values have been calculated using analytical techniques. From traditional Piper diagram for water classification, the water quality is placed at Na+-Ca2+-SO42--Cl- type. According to the results of electrical conductivity and sodium adsorption ratio, 37% of the studied samples are regarded as highly saline, while 63% of them are classified as very highly saline. As per the Arab Gulf and international standards, such waters are not suitable for irrigation under normal condition and further action is required to remediate such problem by salinity control. Sodium content in 24% of the studied samples is regarded as high that can’t be used for irrigation purposes for any soils. Thus high level of salinity, sodium adsorption ratio and sodium percentage in most water samples have made them unsuitable Introduction Groundwater is the only reliable water resource for domestic, agriculture and industrial uses in the arid country like Saudi Arabia. Rapidly depleting groundwater aquifers as a consequence of high population growth and rapid industrialization are threatening the quality of water resources in Saudi Arabia. To evaluate the suitability of groundwater for different purposes, understanding about the chemical composition of these important resources is necessary. It is possible to investigate changes in groundwater quality due to rock-water interaction (weathering) or due to anthropogenic activities (Babiker et al., 2007; Batayneh et al., 2008; Batayneh et al., 2012). Such improved knowledge can contribute to effective management and utilization of this vital resource. For this purpose, monitoring of the groundwater quality (chemical and physical constituents) is as important as assessing its quantity. The quality of water in any area is very much dependent on the type of uses. Major chemical elements, such as Na+, K+, Ca2+, Mg2+, HCO3-, Cl- and SO42-, play a significant role in classifying and assessing the groundwater quality. In this study hydrochemical assessment of the shallow water aquifer from Midyan Basin has been carried out for the above given elements. The alluvial groundwater aquifer in Midyan Basin, which provides a source of water for different consumptions (mainly for industrial and agricultural purposes), is experiencing serious depletion and quality degradation due to population growth, Received 13.03.2012 Revised 10.04.2012 Accepted 23.04.2012 rapid industrialization and enhanced agricultural activities. Objective of the present study on the Midyan Basin is to assess the suitability of groundwater in the shallow aquifer for different uses. Materials and Methods The study area The Midyan Basin is located in the west of Tabouk province, northwestern Saudi Arabia. The basin is bounded to the north, east and northeast by mountains of the Proterozoic rocks, to the west by sedimentary and Proterozoic rocks, and to the south by the Red Sea (Figure 1). This basin is situated from 28º 00ʹ to 28º 40ʹ N latitude and 34º 40ʹ to 35º 20ʹ E longitude, which covers an area of about 1500 km2 with a population of about 5000 (Central Department of Statistics & Information, 2012, personal communication). The climate of the region is characterized by hot dry summer and cold dry winter with an annual average precipitation of 60 mm (Saudi Presidency of Meteorology and Environment, 2012, personal communication). The surface elevation ranges between 0 and 300 meters above mean sea level. The dominant industries in the area are machine brick, ceramic, and asphalt manufacturing. Other main activities going on in this area are breeding sheeps, goats and agriculture crop production. The main crops produced in the area are clover, fruits and vegetables farms. *Corresponding author: A. Batayneh E-mail: awni@ksu.edu.sa groundwater quality, major ions, arid climate, Midyan Basin, Saudi Arabia AGJSR 30 (1) 2012: 7-13 A. Batayneh et al.