English abstract
Seawater intrusion is a worldwide phenomenon in coastal aquifers where groundwater abstraction exceeds groundwater replenishments especially in arid and semiarid areas. Sultanate of Oman, which is considered as arid and semiarid country, suffers from shortage in water resources with an increasing water demand. This situation especially characterizes Batinah coastal aquifer at northern part of the Sultanate where extensive pumping takes place to cover rapid development in all sectors; in particular agriculture activities. This situation induced seawater to encroach freshwater. Seawater intrusion in the Batinah coastal plain has been identified by the government in the early 1980 and started to monitor the situation and implement appropriate strategies such as recharge dam construction. The present study aims at determining seawater/freshwater interface in Al-Khaburah area which is part of the Batinah region. In the study area, Hawasinah dam was built in 1995 to replenish groundwater and counter seawater intrusion that affect all the Batinah coastal plain. This study also aims to estimating temporal and spatial variation of seawater intrusion in the area. This was based on a comparative study with previous work, conducted in 2002. In addition, it also assesses the contribution of the dam in groundwater replenishment.
Hydrogeochemical and geophysical methods were used during this study. The hydrogeochemical method involved groundwater sampling of 40 wells and analyses of chemical parameters such as major cations, major anion, trace elements, EC and TDS. The results show general trend of increasing concentration of major cations and anion seawards. The groundwater of the study area is classified into three groups; group one; Na - Cl" water type, group two; Cat2 + Mg2+- Cl" + SO42- water type and group three; Cat4 + Mg-* - HCO3. The first group represents seawater chemistry, whereas the third group represents freshwater. The second group represents mixing seawater and freshwater. Source of Cl" in most of water samples has been determined by Na"/ Cl" ratio that suggests seawater as the source of the solute. Meanwhile, carbonate dissolution took place in the study area, where all samples have high CaCO3 which is indicated by HCO3/SiO2 ratio. Similarly, Cat2 /S04 2 ratio indicated that the source of Ca*is from dissolution of carbonates.
Time domain electromagnetic (TDEM) survey, which is the main geophysical method, was carried out for 16 stations along three parallel lines perpendicular to the coast. TDEM outputs show low resistivities value close to the coast that represent saline water while the high values 5 km away reflect freshwater. These values were best fits to those of direct resistivity measured by vertical electrical sounding (VES). In the TDEM line one, the interface shifted seawards compared to 2002. Similarly line 2 but with less variation. The interface varies temporally and spatially with depth. Recession of seawater is interpreted as due to positive contribution of the Hawasinah dam in groundwater replenishment.
