TDS

USGS puts it plainly: whether water “comes out of your kitchen faucet, is in a swimming pool or dog dish, comes out of the ground or falls from the sky, the water will contain significant amounts of dissolved substances, minerals, and chemicals.” The most common dissolved substances, the same page says, are salts.

USGS's page on hardness describes one way groundwater picks up part of that load: as water moves through soil and rock, it dissolves small amounts of naturally occurring minerals and carries them into the groundwater supply. TDS is the total of whatever a given sample carries. It counts how much is dissolved, not which substances make up the total.

NGA writes that water quality “can be monitored with a handheld TDS or conductivity meter.” Conductivity is a routine measurement well beyond glass cleaning: USGS scientists almost always measure it when they collect water samples, using a probe the agency calls a conductivity cell.

The ions are what tie the two readings together. USGS explains that even a small amount of ions in a water solution makes it able to conduct electricity, and that pure water, such as distilled or deionized water, is an excellent insulator, although even water of that purity can contain ions. The conversion from one reading to the other is NGA's, not USGS's: NGA states that a reading of 40 microsiemens per centimeter represents a TDS level of 20 PPM. The USGS page states no conversion of its own.

TDS shows up by name in exterior glass cleaning because of what happens after a rinse. In the pure water technique NGA's paper records, the rinse water is generally allowed to evaporate from the glass, so the water used in both the wash and the rinse must have a TDS of 20 PPM or less to prevent spotting and streaking. NGA adds that the use of tap water is not acceptable.

The 20 PPM figure belongs to that one technique: NGA's paper prints it in its pure water paragraph and nowhere else. Why a window cleaner checks the number is worked through in what TDS is and why window cleaners measure it, and the spotting itself in why pure water dries without leaving spots. The NGA passage is quoted in context in the window cleaning page's pure water section. The dissolved calcium and magnesium that USGS says make water hard have their own entry at hard water.