Nearly 35% of Mumbai's area is highly vulnerable to seawater intrusion into its freshwater aquifers, a Central Ground Water Board (CGWB) study has found.
The developmen has raised concerns over the quality and future usability of the city's groundwater as parts of Maharashtra face drought-like conditions.
The study identified saline groundwater in Fort, Shivaji Park in Dadar, Mulund East and Mahim, The Indian Express reported.
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It warned that excessive pumping can lower freshwater pressure in aquifers and allow seawater to move in, making groundwater unusable even in areas where extraction levels are considered safe.
The findings are part of the CGWB's 2025 Aquifer Management Plan, presented earlier this year by the Union Ministry of Jal Shakti's Department of Water Resources, River Development and Ganga Rejuvenation.
The assessment mapped aquifers across 603 sq km, covering Mumbai city and its suburban districts.
Shallow aquifers along the western and southern coasts were found to be particularly vulnerable to saline intrusion. In the eastern zone, deeper aquifers near creeks face a higher risk. The board observed saline intrusion especially during the pre-monsoon period.
Groundwater extraction stands at 46% in Mumbai city district and 26% in the suburban district. These levels are not considered high overall, but the report cautioned that they do not rule out localised seawater intrusion.
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In the most saline zones, including Fort, Mulund East and Shivaji Park, electrical conductivity, an indicator of the concentration of dissolved salts in water, was recorded above 10,000 microsiemens per centimetre.
Mumbai depends mainly on seven surface reservoirs for drinking water, replenished by the monsoon between June and September. Groundwater, once drawn from wells and other local sources, is now largely used to supplement the city's supply.
The report recommends restricting groundwater development in Fort to depths of up to 60 metres. It calls for a complete ban in Shivaji Park, where highly saline water was found at shallow depths.
To check further intrusion, it suggests recharge structures such as pits with filters, recharge shafts, and percolation tanks or ponds, particularly in parks and gardens in vulnerable areas. These could help replenish freshwater aquifers and maintain the pressure needed to resist the movement of saline water.
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The CGWB also recommends regular monitoring of electrical conductivity in coastal observation wells. New deep borewells tapping fractured rock zones should be permitted only after detailed hydrogeological and geophysical investigations, it said.
Separately, the Brihanmumbai Municipal Corporation (BMC) has commissioned the All India Institute of Local Self Government to study salinity levels in the city's groundwater.
The agency will collect water samples from wells across Mumbai to assess the extent of salinisation.
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