Monitoring of sea water intrusion in coastal aquifers, is an essential issue for any long-term integrated water resources management plan. In Salalah at the Southern part of Oman, a tertiary coastal aquifer, of 60 m thick and 40 km wide, bounded laterally by brackish water from limestone rock is considered the main source of fresh water supply and all water demands for other purposes. Over the last three decades, Salalah city had undergone accelerating developments activities and consequently, increased water demand for agriculture, domestic, industrial and tourism purposes. Accordingly, the coastal aquifer is subjected to over exploitation and deterioration of it’s water quality in some parts. Therefore, monitoring of any water quality deteriorations and water level declines is essential in applying any sustainability approach to use such groundwater resources. The aim of this research is twofold: to specify the current (2010) groundwater distribution of the Salalah plain aquifer; and to monitor and to detect the groundwater deterioration during the period 1995-2010. The study area is limited to the area covered by 692.4 km2 of Salalah plain. In 2010, a 302 groundwater samples were collected from productive wells distributed over the Salalah Plain; in both recharge and discharge areas. The results were compiled and compared to the previous salinity records for each well since 1995. GIS techniques were used to produce salinity distribution maps for the years 1995, 2000, 2005 and 2010 in order to detect the salinity variations in both space and time. The results showed that groundwater quality deterioration is detected at some parts of the aquifer particularly along the coast at both eastern and western side of the aquifer, as brackish water from carbonates formation laterally moved to the central part of the aquifer causing salinisation of it’s water. On the other hand, all the recharge areas and the central part of the aquifer shows no evidence for any long term deterioration which could effect the drinking water supplies sources. In general, the Salalah plain aquifer is suffering from deficit between water available and water use. Any further excessive groundwater exploitation will seriously affect the groundwater quality through seawater intrusion at the coastal areas and deterioration of groundwater by the more saline water from east and west carbonate rocks. Therefore, it’s important to reduce groundwater abstractions through transferring of Rhodes grass to the Nejd area, shut down the current wellfield and replaced by the new desalination plant to provide 20 Mm3/yr and continuing recharging the coastal area with the 5 Mm3/yr of treated sewage water through the 40 tube wells. Key Words Salalah Plain - Coastal aquifers - Groundwater monitoring - GIS techniques – Groundwater management - Wastewater

Monitoring of sea water intrusion in coastal aquifers, is an essential issue for any long-term integrated water resources management plan. In Salalah at the Southern part of Oman, a tertiary coastal aquifer, of 60 m thick and 40 km wide, bounded laterally by brackish water from limestone rock is ...