TDS is a total, not an ingredient list.
What it is
Total dissolved solids is the combined weight of everything dissolved in a water, reported in milligrams per liter. A handheld meter does not actually weigh it; it measures electrical conductivity and converts, which is why two waters with the same TDS reading can be made of entirely different things. The EPA secondary standard sits at 500 milligrams per liter, set on taste.
What it usually does
A high total usually comes with a flat, mineral, or salty taste, and usually tracks with hardness in Michigan since calcium and magnesium are a large part of the dissolved load. A sudden rise after a softener was installed usually points at sodium from the brine rinse rather than at anything new in the well.
What it does not prove
This is the one most often read as a verdict, and it is not one. TDS does not identify a single substance. Arsenic, lead, nitrate, and bacteria are all invisible to it, and a low reading usually says nothing about any of them. A pitcher filter that drops TDS has not thereby removed anything in particular.
When to test
Useful as a cheap before-and-after check on a reverse osmosis membrane, and useful for tracking change over time on the same tap. Usually not worth paying a lab for on its own, because the panel it sits in is what actually names things.
See what this usually means for your water.
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