A NOVEL APPROACH USING SPECIFIC CONDUCTANCE AND WATER TYPE TO ESTIMATE SALINITY

The total concentration of dissolved constituents in water is routinely quantified by measurements of salinity or total dissolved solids (TDS). Despite being used interchangeably in some studies, salinity and TDS are operationally defined by their analytical methods and are not equivalent for most waters. The primary difference between salinity and TDS is the accounting of carbonate. Salinity is the mass of dissolved salts in a given mass of solution and is typically determined by summing the concentrations of dissolved major ions. TDS is determined by residue on evaporation (ROE), which is the mass of anhydrous residue remaining from a filtered water sample in a vessel after oven drying at a defined temperature. TDS can also be estimated by summing the dissolved major ions (∑ROE), except for alkalinity. To account for the evaporative loss of CO₂(g) during ROE measurements, the alkalinity value (as HCO₃^-) is divided by two and then added to ∑ROE. In addition, specific conductance (SC) has been used to estimate salinity and TDS; however, such estimates are affected by variations in ionic composition. Thus, we compared several methods to calculate salinity and TDS for 6,391 surface water samples and conclude the following: ROE or TDS measurements are best suited for studies of anhydrous residue (e.g., scaling); salinity determined by summing the speciated ion concentrations, termed S, is the most comprehensive method to represent the concentration of dissolved constituents in natural waters; and salinity computed by a new proxy method using SC and major-ion water type, termed S_{SC_WT}, is nearly equivalent to the speciated S. The S_{SC_WT} proxy method utilizes unique S-SC correlations for each water type. For the surface waters used in this study, the median difference between S_{SC_WT} and S was -2.4%. The median difference between TDS and S was -19% to -24%, depending on method, owing to the partial exclusion of carbonate from TDS. The S_{SC_WT} approach does not require speciation computations and is cost effective, rapid, and capable of providing reliable real-time salinity determinations at surface water sites where SC data are available and water type is known. Furthermore, S_{SC_WT} is likely applicable to a wide range of groundwater compositions and is the focus of current studies.