Extension of the Practical Salinity Scale to Estimate Major Ion Concentrations: Application to the San Francisco Estuary

This paper presents a novel approach to estimate major ion concentrations in estuaries from a known specific conductance. Assuming two-source conservative mixing at steady state, we propose an extension of the Practical Salinity Scale 1978 to estimate concentrations of major ions and total dissolved solids as functions of the conductivity ratio, which is the ratio of sample conductivity and seawater conductivity. We employ an extensive salinity data set that includes measurements of specific conductance and concentrations of major ions and total dissolved solids to demonstrate its validity and limitations for the San Francisco estuary. We show the proposed extension to be valid in waters influenced by seawater intrusion at specific conductance values greater than or equal to 0.25 mS/cm and propose adjusted model constants to account for ionic composition under less saline conditions. We observed evidence of seasonal bias in model residuals and hypothesize that this bias is related to the simplified assumption of fixed upstream end member concentrations. This finding of seasonal bias suggests that upstream end member concentrations are more reflective of high alkalinity freshwater contributions from December through June, compared to other months.