Salinity and total dissolved solid determinations using PHREEQCI

The total concentration of dissolved constituents in water is routinely quantified by measurements of salinity or total dissolved solids (TDS). However, salinity and TDS are operationally defined by their analytical methods and are not equivalent for most waters. Furthermore, multiple methods are available to determine salinity and TDS, and these methods have inherent differences. TDS is defined as the mass of anhydrous residue remaining in a sample vessel after evaporation and subsequent oven drying at a defined temperature. Salinity is a measure of the mass of dissolved salts in a given mass of solution. In addition, there are approaches that quantify the total solute (TS) concentration, including gases.

PHREECI was utilized to rapidly calculate salinity, TDS, and TS of 6,391 surface water samples from 523 sites using eight different approaches. PHREEQCI is a widely used geochemical computer program that can be used to calculate chemical speciation and specific conductance of a natural water sample from its chemical composition (Charlton and Parkhurst, 2002; Parkhurst and Appelo, 1999; McCleskey, 2018). The PHREECI input files and water-quality data utilized in this study are presented here.

PHREEQCI can be obtained at https://www.usgs.gov/software/phreeqc-version-3

File information:

PHREEQCI_Salinity.txt contains a script used to calculate salinity, TDS, and TS using eight different approaches. The file contains a script that generates a text file containing all the calculated parameters (Salinity.out) including calculated specific conductance (McCleskey and others, 2012; McCleskey, 2018), salinity (4 methods), TDS (1 method), and TS (1 method), and the Practical Salinity Scale (2methods). In addition, a QAQC graph is created (McCleskey, 2018).

Notes for creating input files and running the PHREEQCI input script: (1) The field measured specific conductance (in microSiemens per centimeter at 25 °C) must be listed under the “Description” character field because PHREEQCI does not have a “Defined Heading” for specific conductance in the SOLUTION_SPREAD block. (2) The Bounds.TSV file must be in the same file folder as the input file for the QAQC bounds to be included on the specific conductance imbalance versus charge balance graph. (3) PHREEQCI was run using the wateq4f.dat thermodynamic database.

Bounds.TSV is a tab separated file used to create the QAQC bounds in the specific conductance imbalance versus charge balance graph (McCleskey and others, 2012a). The file must be in the same file folder as the input file for the QAQC bounds to be included on the specific conductance imbalance versus charge balance graph. The bounds are set at ±15% for both specific conductance imbalance and charge balance.

PHREEQCI_SalinityInput.pqi is an example PHREEQCI input file. The solutions are 6,391 surface water samples where the water-quality data were obtained from the Water Quality Portal (https://www.waterqualitydata.us/).

PHREEQCI_SalinityOutput.pqo is the PHREEQCI output file created using PHREEQCI_SalinityInput.pqi.

Salinity.txt is a text file that is generated by PHREEQCI that contains the calculated electrical conductivity (EC_Calc); the specific conductance using the non-linear (SC_nlf) temperature compensation factors (McCleskey, 2018); milliequivalents (meq) of Ca, Mg Na, K Cl, SO4, NO3, F, Al, Sr, Ba, Fe, Mn, Zn Br, Li, and inorganic carbon (C); total meq of the cations (Cat) and anions (An); the percent of the of the total meq due to Ca, Mg Na, K Cl, SO4, NO3, F, Al, Sr, Ba, Fe, Mn, Zn, Br, Li, and C; water type (WT) based on major ions (0 = mixed, 1 = Cl rich, 2 = carbonate rich, 3 = CaSO4 rich, and 4 = Na2SO4 rich), salinity (S) in mg/L based on a specific conductance-water type method (S_SCWT_mg/L); S in mg/L based on speciated solute concentrations (S_spec_mg/L); S in mg/L based on unspeciated solute concentrations (S_unspec_mg/L); S in mg/L based on speciated silica (S_Si_mg/L); total solutes, including CO2(g), in mg/L (TS_mg/L); TDS determined using methods (U.S. Geological Survey, 1989; USGS parameter code 70301) (TDS70301_mg/L), Practical Salinity Scale determinations in parts per thousand (PSS_ppt), and the low-level Practical Salinity Scale determinations in parts per thousand (PSS_low_ppt).

WaterQuality.csv is a comma-separated values (csv) file containing water-quality data for 6,391 samples utilized in this study.

SampleSite.csv is a csv file containing station name and latitude and longitude.

SCI_CB.JPG is a QAQC graph created using the PHREEQCI_SalinityInput.pqi file.

ReadMe.txt contains general information about the PHREEEQCI scprit and files.

References Cited

Charlton, S.R., and Parkhurst, D.L., 2002, PhreeqcI--A graphical user interface to the geochemical model PHREEQC: U.S. Geological Survey Fact Sheet FS-031-02, 2 p.

McCleskey, R.B., Nordstrom, D.K., Ryan, J.N., and Ball, J.W., 2012a, A New Method of Calculating Electrical Conductivity With Applications to Natural Waters: Geochimica et Cosmochimica Acta, v. 77, p. 369-382. [http://www.sciencedirect.com/science/article/pii/S0016703711006181]

McCleskey, R.B., 2018, Calculated specific conductance using PHREEQCI: U.S. Geological Survey software release, https://doi.org/10.5066/F7M907VD.

Parkhurst, D.L., and Appelo, C.A.J., 1999, User's guide to PHREEQC (Version 2)--a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Water- Resources Investigations Report 99-4259, 312 p.

U.S. Geological Survey, 1989. Methods for determination of inorganic substances in water and fluvial sediments. U.S. Geological Survey Techniques of Water-Resources Investigations 05-A1.