Salinity is quantified indirectly by measuring the Electrical Conductivity of the water in dS/m (1 dS/m in SI units = 1 mmhos/cm in old units). A general conversion that is used: A salt concentration of 640 mg/l (= ppm) = EC of 1 dS/m for most water used in agriculture.
i think , this issue , we have debated number of times on the same platform. There is a definite empirical relationship salinity and TDS of ground water...