Arsenic in Well Water: The One That Comes From the Rock Itself
Most well-water problems announce themselves. Iron stains the sink orange, sulfur smells like eggs, hard water crusts up the faucet. Arsenic does none of that. It's colorless, odorless, and tasteless, and a glass of water with too much of it looks and tastes identical to a glass with none. That's what makes it the contaminant worth testing for on purpose, because it will never tell you it's there.
Where it comes from
The surprising part for a lot of well owners is that arsenic in a private well usually isn't somebody's pollution. It's natural. Arsenic is a common element in the earth's crust, and in a lot of regions it sits in the bedrock and sediments that your aquifer runs through. Groundwater moving slowly through that rock dissolves a little of it and carries it up your well. So two houses on the same road can have very different arsenic levels depending on the geology under each well and how deep each one is drilled.
Because it's geological, arsenic doesn't clear up on its own and it isn't seasonal the way surface contamination can be. If your rock has it, your water has it, year after year, until you treat for it.
What the number means
The EPA's enforceable limit for arsenic in public drinking water — the Maximum Contaminant Level — is 0.010 mg/L (10 parts per billion), set to that level back in 2006. Public utilities have to meet it. Your private well is not regulated by anyone, so nobody is checking it for you; that standard is simply the health-based line to measure your own result against. If your test comes back at or above 0.010 mg/L, that's the point health agencies say to act. The health concern with arsenic is long-term — years of exposure is linked to several cancers and other effects — so it's about the water you and your kids drink day after day, not a single glass.
How you actually find out
You test. A certified drinking-water lab runs an arsenic panel on a sample you collect — it's not part of a basic bacteria test, so you have to ask for it specifically. It's worth doing at least once for any well you drink from, and again if you drill deeper, replace the well, or move into a house whose well history you don't know. Ask your county health department first; many keep records of what's common in local groundwater and some help with testing.
| Your arsenic result | What it means |
|---|---|
| Below 0.010 mg/L | Under the EPA limit — retest every few years |
| At or above 0.010 mg/L | Above the health-based limit — plan treatment for your drinking water |
| Never tested | Unknown — you can't rule it out by taste or look; get one panel done |
If it's high, what fixes it
The good news is arsenic is treatable at the tap. The common approaches are a point-of-use reverse-osmosis unit under the kitchen sink (which handles the water you actually drink and cook with) or a whole-house adsorptive media / anion-exchange system sized to your water chemistry. Which one fits depends on your arsenic level, whether it's the type known as arsenic III or arsenic V, and the rest of your water's makeup — which is another reason the lab test matters, because it tells the installer what they're actually treating. Boiling does not remove arsenic; it concentrates it, so that's the one thing not to do.
Joe — the septic-ignoring, well-neglecting legend himself — has bragged for twenty years that his well water is the best on the road: cold, clear, no taste, never made anybody sick. He's right about all of that. He's also never once tested it for arsenic, because it tastes fine, and "tastes fine" is the exact standard arsenic is designed to pass. Clear and cold tells you nothing about the one thing you can't taste.
The short version
Arsenic is the well contaminant that hides in plain sight — natural, invisible, tasteless, and tied to the rock your water runs through. The EPA health limit is 0.010 mg/L, your private well isn't checked by anyone, and the only way to know your number is a certified test. For the plain-English rundown of what to test a private well for and why, the EPA's own guide is a solid start: EPA National Primary Drinking Water Regulations.