Freshening of seawater in the Mahim Bay, Mumbai, India: Insight from an environmental isotope study

ABSTRACT An environmental isotope study was conducted to assess the cause for the freshening of seawater observed in the Mahim Bay, Mumbai, India after a storm event during the southwest monsoon period. Water samples were collected from the various locations of the coastal water and the suspected inland water sources such as rain, river and groundwater and analysed for major ion species and stable isotopes (δ18O and δ2H). Dissolved radon (222Rn) in the coastal water was monitored in-situ. Field monitoring survey in the coastal water indicated lower electrical conductivity (1,730 μS/cm) near Mahim Mosque compared to the surrounding shelf waters. Relatively high excess 222Rn activities (up to 55 Bq/m3) were observed in the Mahim Bay, even after 13 days of seawater freshening event. Based on the hydrochemical and isotope results, various prevailing hypotheses on the occurrence of low salinity water in the Mahim Bay were tested. It is inferred that the low salinity coastal water was associated with groundwater discharge occurring in the Mahim Bay and in the Mithi River and were unlikely due to the overflow of Vihar and Powai Lakes in the catchment of Mithi River and surface runoff because of the rain/storm events. Temporal variations of electrical conductivity and stable isotopic composition of coastal water in the Mahim Bay showed that the groundwater inputs were decreasing after the storm event. 222Rn was found to be a useful tracer for distinguishing the subsurface flow of water to the coastal system.