Abstract Measurement of short lived radium isotopes (²²⁴Ra and ²²³Ra) in seawater samples collected from Mumbai Harbour Bay was carried out using MnO₂ based in situ pre-concentration technique coupled with delayed coincidence counting. Standardisation and validation of various parameters required for quantification was carried out. Longitudinal variation of ²²⁴Ra and ²²³Ra found in the range 17.74–98.66 and 0.56–2.26 dpm 100 L^-1 respectively. Elevated concentration of ²²⁴Ra and ²²³Rawas found near shore sampling points where reused seawater and surface water from land intrudes in creek. Higher concentration in creek compared to offshore locations indicates presence of submarine groundwater discharge.