Disentangling salinity sources in underground oil storage: evidence for produced water inflows over seawater intrusion

This study examines the geochemical variability of seepage water in a coastal underground crude oil storage cavern in Korea, with a focus on differentiating produced water inflows from seawater intrusion.A multi-year dataset (2016 -2024) combining electrical conductivity (EC), major ion chemistry, and stable isotopes ( 18 O, 2 H) reveals that, although geochemical indices such as Gibbs plots and Cl/HCO 3 ratios often indicate Na-Cl facies, isotopic compositions consistently exclude seawater as a significant source.Instead, episodic Na-Cl surges were temporally coincident with crude-oil import operations and supported by Stiff diagram evidence.Three lines of evidence converge: (i) no persistent long-term salinity increase was detected; (ii) isotopic signatures were incompatible with seawater mixing; and (iii) transient EC and ion spikes coincided with oil imports.We conclude that produced water inflows, rather than seawater intrusion, are the dominant driver of salinity variability.This finding underscores the importance of monitoring and managing produced water influx during oil imports as the key operational factor influencing cavern geochemical stability.