I need to calculate total dissolved solids (TDS) of various groundwater samples from chemical analyzes. I have concentrations of ions/molecules in mg/L. I’ve found various formulas to calculate TDS but the ions/molecules I have never match. I don’t have a lot knowledge in chemistry.
Here are the three equations I have found. (As i said, I found many different equations and I don't know which one I should use):
Eq. 1 : TDS=Na(+) +K(+) +Ca(2+) +Mg(2+) + Fe^(2+) +HCO3(-) +Cl(-) +SO4(2-) +NO3(-)
Eq.2 : TDS=0.6*Alkalinity (as CaCO3) +Na(+) +K(+) +Ca(2+) +Mg(2+) +Fe(2+) +Cl(-) +SO4(2-) +NO3(-) 
Eq.3 : TDS=cations +anions +SiO2
Problems/questions I have:
•I don’t have the concentration of HCO3(-).
•I have the concentration of CaCO3. In natural waters, alkalinity (mg/L CaCO3) is HCO3(-)+CO3(2-) +OH(-) +H(+). At pH under 8.3, could I simply convert the concentration of CaCO3 in HCO3(-)? If yes and if the pH is over 8.3, is there a way to calculate the concentration of both HCO3- and CO3(2-)?
•I don’t have the concentration for NO3-. I only have the concentration for NO3-+NO2. Is there a way to isolate the concentration of NO3(-)?
•I don’t have the concentration for SiO2. I only have the concentration for Si. Could I simply use the concentration of SiO2?
•The pH of my water samples varies from 6 to 9.
•Also, I don't have the samples so I cannot titrate the water or measure electrical conductivity.
 Federation, W. E., & American Public Health Association. (2005). Standard methods for the examination of water and wastewater. American Public Health Association (APHA): Washington, DC, USA.