Isotope hydrochemical investigation of saline intrusion in the coastal aquifer of Karachi, Pakistan

Environmental stable isotope parameters δ 18 O and δ 2 H in water molecules, δ 13 C in Total Dissolved Inorganic Carbon (TDIC) and δ 34 S in SO 4 , have been used in conjunction with physico-chemical tools to study the extent and origin of saline intrusion in the coastal aquifer system of Karachi. Physico-chemical data show that the shallow groundwater is moderately saline. Shallow wells in close proximity of Karachi coast have much higher values of electrical conductivity, salinity, contents of aqueous chloride and sulfate as compared to all other locations relatively far away from the coast. The mean stable isotope contents of 18 O and 2 H indicate that the shallow aquifer system is recharged by a mixture of fresh water of mainly Indus River origin and the polluted waters of the Layari and Malir Rivers and their tributaries, both under natural infiltration conditions and artificially induced infiltration conditions. Much depleted values of δ 13 C (less than −6‰ V-PDB) indicate the impact of pollution from the Layari and Malir Rivers into the shallow groundwater environment. Relatively deep groundwater is mostly saline and has high electrical conductivity and salinity as compared to shallow groundwater. Physico-chemical data of deep groundwater show that the deep wells have relatively higher values of electrical conductivity and salinity as compared to the shallow wells. The hydrochemical and stable isotope results indicate that the confined aquifer hosts a mixture of rainwater from the hinterlands and surrounding regions around coastal Karachi, as well as sea trapped water/seawater through intrusion under natural infiltration conditions or under induced recharge conditions. The present investigations prove seawater intrusion and existence of trapped seawater salinity and build-up of salt-water up-coning in the shallow and deep confined aquifer in coastal Karachi.