Abstract. While the thermodynamic properties of Standard Seawater are very well known, the quantitative effect of sea salt composition anomalies on various properties is difficult to estimate since comprehensive lab experiments with the various natural waters are scarce. Coastal and estuarine waters exhibit significant anomalies which also influence to an unknown amount the routine salinity calculation from conductivity measurements.

Recent numerical models of multi-component aqueous electrolytes permit the simulation of physical chemical properties of seawater with variable solute composition. In this paper, the FREZCHEM model is used to derive a Gibbs function for Baltic seawater, and the LSEA_DELS model to provide estimates for the conductivity anomaly relative to Standard Seawater. From additional information such as direct density measurements or empirical salinity anomaly parameterisation, the quantitative deviations of properties between Baltic and Standard Seawater are calculated as functions of salinity and temperature. While several quantities show anomalies that are comparable with their measurement uncertainties and do not demand special improvement, others exhibit more significant deviations from Standard Seawater properties. In particular density and sound speed turn out to be significantly sensitive to the presence of anomalous solute. Suitable general correction methods are suggested to be applied to Baltic Sea samples with known Practical Salinity and, optionally, directly determined density.