Selenium
An essential nutrient with a narrow margin. Too little is a dietary problem; too much in water is a real one.
- Safe range
- 0 – 0.05 mg/L (US EPA primary standard, with the health goal set at the same figure.)
- Measured in
- mg/L
What it is
Selenium occurs naturally in sedimentary rock and is concentrated in some arid-region soils, where irrigation drainage can raise it further. It reaches groundwater by ordinary weathering.
Why it matters
Selenium is genuinely necessary in the diet, and the gap between necessary and harmful is narrower than for most elements. Water is not where anybody should be getting it, so the drinking water limit is about avoiding an unintended addition to what food already supplies. Sustained excess causes selenosis — hair loss, brittle nails, numbness in fingers and toes.
How to measure it
A laboratory metals panel, which normally includes it.
Certified laboratory, mail-in kit
- How trustworthy
- Reference grade
- Cost
- Most expensive
- Effort
- Careful sampling, then days or weeks of waiting
You are posted bottles and instructions, you fill them exactly as told, and you post them back. Weeks later a report arrives with figures nobody has to interpret by eye. This is what a result you intend to act on should be built from.
Standard on multi-metal panels. Worth ordering deliberately if you are in an arid, irrigated region where selenium is a known local issue.
Where to find it: State-certified drinking water laboratories; your state's certification officer keeps the list, and many public health departments will point you at one.
Your utility's annual water quality report
- How trustworthy
- Reference grade
- Cost
- Free
- Effort
- A few minutes of reading
Every community water system in the US has to publish one each year, free, listing what it found and what the limits are. It is real laboratory data about the water leaving the plant and travelling the mains. What it cannot describe is your own plumbing.
Regulated and published annually by public systems.
Where to find it: Your water bill, your supplier's website, or the EPA's index of reports. It is also called a Consumer Confidence Report or CCR.
Further reading: US EPA: National Primary Drinking Water Regulations, the enforceable health-based limits
If it is too low
Nothing to act on. Dietary selenium comes from food, and a low water figure is not a deficiency.
- • No action needed.
If it is too high
Sustained consumption can cause hair and nail loss and peripheral numbness.
- • Reverse osmosis, activated alumina and ion exchange all remove it.
- • Common in arid agricultural regions where irrigation drainage concentrates it; a neighbour's well result is a useful comparison.
Common questions
Isn't selenium a supplement people take?
It is, which is exactly why the water limit exists. Food and supplements already provide what the body needs, and the margin between adequate and excessive is small — so selenium arriving unnoticed in the water supply is an addition nobody accounted for.
Does a water softener remove selenium?
No. Selenium is present as a negatively charged ion and a softener exchanges positive ones, so it passes through untouched.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.