Impact of Seawater Intrusion on Coastal Aquifers of Karachi, Pakistan

Groundwater is an important resource for domestic and agricultural use. In several countries coastal regions are facing groundwater salinization due to seawater intrusion. The coastal aquifers of Karachi are continuously at risk of salinization. The deeper aquifers in the study area are mainly hosted in Nari (Oligocene) and Gaj (Miocene) formations, while shallow aquifers are situated in Holocene and Pleistocene unconsolidated deposits. In order to evaluate the impact of seawater intrusion on groundwater quality of Karachi region, 25 groundwater samples were collected from different wells (ranging from 12 to 46 m) of Chanesar Gotth, Karachi, Sindh for determining their physical and chemical characteristics. The analytical results of physical (turbidity, pH, TDS, EC, hardness, odor, and chemical (sodium, potassium, magnesium, calcium, chloride, nitrates, bicarbonate, sulfate, zinc, nickle, cobalt, copper and iron) constituents showed that EC and TDS range between1828- 5190 μS/cm, and from 1550 to 4920 mg/L, respectively. The results indicate that groundwater EC and TDS are above the guidelines set by WHO and did not satisfy the requirement for various purposes like domestic and agricultural use. Data revealed that groundwater is dominated by sodium and chloride, and dominant water types in the area are NaCl>NaHCO3>CaCl. Seemingly, low Na/Cl ratio suggests that groundwater is severely affected by seawater intrusion. High Na/Cl ratio in few water samples reflect impact of infiltration of polluted surface water. In addition Gibbs plot showed evolution of groundwater from seawater intrusion to evaporative process. Durov diagram revealed the mixing of fresh water with seawater. Furthermore, sodium absorption ratio (SAR) and Wilcox diagrams showed that water is highly saline and is unsuitable for irrigation purpose. The natural cause of seawater intrusion is sea-level rise, which has significantly impacted the groundwater salinization dynamics in the study area. Keywords: Seawater intrusion, groundwater salinity, hydro-geochemistry, Karachi.