Arsenic is one of the most common naturally occurring contaminants in U.S. groundwater, and the federal limit is a single hard number: 10 parts per billion (ppb), or 0.010 milligrams per liter, set in 40 CFR 141.62. Cross it once and you have a health-based violation with a public notice attached. Because arsenic has no taste, color, or odor, the only way to know you are over the line is to sample — and a system that has not sampled on its full schedule, or that sampled years ago and stopped watching, is carrying an answer it does not have. That "unknown" is the most expensive status to be in when the state asks.

What the 10 ppb number actually is

The arsenic maximum contaminant level of 0.010 mg/L has been the federal standard since 2001, with a January 23, 2006 compliance date under the Arsenic Rule. It is an MCL, not an action level — which means it is a hard legal ceiling, not a treatment trigger. A single confirmed sample at or above the MCL is a health-based violation under the federal rule, and your state primacy agency enforces it alongside its own requirements. See the difference between an MCL and an action level.

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Why the health risk is a compliance risk

Arsenic is a known human carcinogen. The chronic, low-dose exposure the drinking water limit is meant to control — skin, bladder, and lung cancer, plus cardiovascular and other effects — is exactly why the number is measured in parts per billion rather than parts per million. That is also why an arsenic exceedance is a Tier 2 public notice: not the 24-hour acute clock of nitrate or E. coli, but a 30-day notice to every customer, filed with your state. The water may look perfect while the notice clock runs.

Where arsenic shows up in your source

Arsenic is mostly geology, not industry. It dissolves out of bedrock and aquifer sediments, so it concentrates in the same regions over and over: the Southwest, the Great Basin, parts of the Plains, and the granite-bedrock groundwater of New England. A groundwater system over the wrong formation can run above 10 ppb without any upstream polluter within a hundred miles. That is why sampling history — not proximity to industry — is the real predictor.

What an exceedance forces you to do

Once a confirmed sample hits 10 ppb, the obligations stack quickly:

  • Public notice — a Tier 2 notice to customers within 30 days, with required educational language.
  • Quarterly monitoring — you move to quarterly sampling until you are reliably back under the MCL.
  • Treatment or a new source — the durable fix is one of a short list of proven technologies, and you are on a corrective-action clock the state sets.

The treatment options are well established for small systems: adsorption onto iron-based or activated-alumina media, ion exchange, reverse osmosis, and coagulation-assisted filtration. For very small systems, point-of-use and point-of-entry treatment is a recognized compliance path under the Arsenic Rule. The cost varies with water chemistry and flow, but every option is cheaper when you have a current baseline to plan from.

Why a current sampling record is the cheapest fix

Most systems that get caught flat-footed by arsenic are not systems that refused to treat — they are systems that did not know they had a problem. The gap between a missed sampling period and a public notice is where a small system's compliance story goes wrong. See what triggered source-water monitoring requires and how to read your own compliance record.

Check your water system's arsenic status — free at orevant.com, then run the scan for the full action plan.

FAQ

Is the arsenic MCL still 10 ppb, or has it changed?

10 ppb (0.010 mg/L) is the current federal MCL under 40 CFR 141.62. A few states set their own lower or equal standard, but the federal baseline has held since the 2006 compliance date.

My system serves under 1,000 people. Does the 10 ppb MCL still apply?

Yes. The arsenic MCL applies to community and non-transient non-community water systems regardless of size, and small systems have access to the point-of-use/point-of-entry and other compliance options written into the Arsenic Rule.

We have never sampled for arsenic. What should we do first?

Confirm your monitoring schedule with your primacy agency and pull your current SDWIS record. A $199 scan maps your arsenic sampling status and any open violation onto a prioritized action plan so you are not caught without a baseline.