The groundwater aquifer in the coastal region of the northwest of Oman has been used extensively since the early 1980s for agricultural, industrial and municipal purposes. The over pumping of this reservoir has led to the intrusion of seawater and therefore to the deterioration of the groundwater quality. In this study, an investigation was carried out in the southern part of this region to identify the quality of groundwater, to understand the main sources of groundwater mineralisation, and to check the suitability of groundwater for drinking and irrigation. The spatial distributions and temporal variations of groundwater level and electrical conductivity were studied for the period from 1982 to 2005 using data collected from 225 wells. In addition, groundwater samples were collected recently in 2012 from eight wells and analysed for pH, EC, and major ions to understand the sources of dissolved ions and assess the chemical quality of the groundwater. The study area was divided into two strips parallel to the coastline, A and B, located in the discharge and recharge parts of the aquifer, respectively. Results showed a significant increase in the degree of water mineralisation in the direction of south to north following the regional flow direction. Results showed also that the groundwater in the last area could be used for irrigation with little danger of exchangeable sodium while this aquifer is unsuitable for irrigation in the discharge area because it presents a very high salinity hazard.