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The coastal plain aquifer down gradient of Wad Essaquia Elhamra (WEE), is the main source of groundwater in the arid region of Laayoune Essaquia El Hamra located in south of Morocco. The over-exploitation of this aquifer over the last decade for water supply, agriculture and industry led to deterioration of groundwater quality, including seawater intrusion. The objective of this study is to investigate the spatial variation of groundwater quality, and to assess the influence of sea water intrusion on the groundwater quality using hydro-chemical tools. Several measurement campaigns of physico-chemical parameters of the groundwater were performed, which shows a very high mineralization ratios versus chloride, confirming the double influence of mixing mechanisms of fresh and saline waters and the water-rock interactions. Computed seawater fraction for sampled water shows that the average mixing rate of seawater intrusion reached 10.5%, confirming the marine intrusion in the aquifer. The highest values were registered in coastal wells. In the upstream around Laayoune and the spreading area, nitrate concentrations exceed 50 mg/L which is the threshold set by the World Health Organization, revealing a high level of contamination by domestic sewage. The combination of ionic ratios and seawater fraction is a useful tool to assess marine intrusion. This study reveals that the phenomenon of marine intrusion is not the only process that dominates hydrochemistry of ground water. The high groundwater mineralization is also due to rockwater interaction associated with reverse ion exchange with clay material and anthropogenic pollutants. [ABSTRACT FROM AUTHOR]