The salinity refers to all ionic salts in the solution, not sodium.
Actually it refers to all the dissolved matter with the organics oxidized, the carbonate and bicarbonate converted to oxide the bromide and iodide replaced by equivalent amounts of chloride. IOW it's pretty complicated and difficult to measure directly and that's why it is almost universally measured (estimated) through conductivity. Wherein lies the rub.
I'm pretty sure that AJ knows that and was just dozing.
No, not dozing this time. As I said in the earlier post, I suspect that the TDS and salinity numbers were derived from the given conductivity measurement as salinity is almost never measured any other way and it is common practice to "measure" TDS with a conductivity meter. I have one in my RO unit. It measures conductivity and displays it as the mg/L sodium chloride which has that same conductivity.
The OP has a conductivity of 914 uS/cm. The art here is in determining how to process that number. If you are an oceanographer you want salinity and so divide the conductivity reading by the conductivity of a standard potassium chloride solution and insert the ratio into a fairly elaborate polynomial. That process is only valid down to S =2 but there is a correction which lets you go lower - another polynomial.
If you aren't an oceanographer you have lots of options, one of which is to use the extended polynomial or to do what most do and that is take half the conductivity and call it the TDS ; here 457. Whether this is an accurate measurement of the TDS depends on the water ion profile. Conductivity is proportional to the amount of ions dissolved but also to their mobilities.
Note OP's posted value of 429 which is pretty close but not equal to half his posted conductivity. Thus suggests that the lab may use a sligthly different factor based on familiarity with their water.
Another approach is to convert the conductivity reading to the amount of sodium chloride that has the same conductivity as is done in the meter on my RO unit. A 1000 uS/cm NaCl solution contains 491 mg/L NaCl so that a 914 uS/cm solution would contain about 448 mg/L NaCl. Again pretty close to the reported TDS with the disparity possibly explained by the fact that the three ways of processing the conductivity reading are by no means the only ways. Most conductivity meters give the user the option of inserting his own calibration constants and have programmed within them one or more of these methods in which the user can change the coefficients dependent on his particular application.
Now what apparently does not compute here is the salinity listed as 0.2%. But though OP wrote 0.2 % (percent) he meant to write 0.2 permil. The permil sign looks like a percent sign except that there are 2 0's to the right of the slash (0/00 not 0/0) (
Per mil - Wikipedia, the free encyclopedia). If I stick my conductivity probe in 1000 uS/cm standard and put it in salinity mode it reads 0.5 per mil consistent with the common practice of calling TDS 0.5 times the conductivity - unless I change the meter's factor. If I stick the 914 uS/cm number into the practical salinity formula (the one that compares conductivity to a standard KCl solution) I can only get down to 0.2 permil if I use a temperature close to 0 °C. At 20 C I get 0.41 per mil - close to the value assumed by taking one half.
It is because of all this variability that I suggested that a conversation with the lab might be of some help.
The sodium value is an estimate since there is not enough data on which to base an accurate assessment.
As the TDS and salinity numbers (which latter is inconsistent with the conductivity) depend only on the conductivity and some unknown model connecting conductivity to TDS I don't see how any conclusion, other than a crude bound, could be drawn concerning the sodium level.
AJ: I believe you didn't notice that Zac was from Peru ..
True, I was dozing there.