The AquaGap™: Why Legal Isn’t Always Clean

The question I couldn't answer

My dad was a professor. When he retired, he moved to a new house in a small town. About a year later, after 72 years of relative health, he was diagnosed with diabetes. His diet hadn't changed. His routine hadn't changed. The only thing that had changed was his ZIP code.

Then I came across a report linking high levels of arsenic in drinking water with diabetes. By then his health had turned, and he passed away from kidney complications in June 2017.

I'm a data scientist, so I went looking for answers the way I look for anything: in the record. What I found wasn't a villain. It was a design.

For arsenic, the legal limit in American drinking water is 10 parts per billion. The health goal, the level at which no known or anticipated health effects are expected, is zero. Both numbers are public. Both are correct. They just answer different questions.

I never learned where my dad's water sat between those two numbers. Nobody had ever put them side by side for him. That's the job we took on.

What "legal" actually means

Under the Safe Drinking Water Act of 1974, the U.S. Environmental Protection Agency sets national rules for drinking water. Those rules now cover more than 90 contaminants, from bacteria to metals to industrial chemicals.

States with "primacy" enforce those rules. In Georgia, that's the Environmental Protection Division. States can set stricter rules of their own. Some do, especially for PFAS; Georgia generally follows the federal standard.

Utilities test on federal and state schedules and publish the results once a year in a Water Quality Report (the federal name is Consumer Confidence Report). Starting in 2027, systems serving more than 10,000 people will publish twice a year.

There are more than 148,000 public water systems in the United States, and together they serve about 90% of Americans. They all answer to the same national standard.

So when a city's water is within every legal limit, that means something. It means operators, lab technicians and engineers did the work, around the clock, to meet a national standard. We say so on every score we publish.

Two numbers for every contaminant

Here's the part most people never get told.

The health goal. EPA's name for it is the Maximum Contaminant Level Goal. The law defines it as the level at which "no known or anticipated adverse effects on the health of persons occur," with a margin of safety. It's set on health alone. Cost doesn't enter into it. For contaminants that can cause cancer, and for disease-causing microbes, EPA usually sets the goal at zero.

The legal limit. EPA's name for it is the Maximum Contaminant Level, and it's what a utility must meet. The law generally tells EPA to set it as close to the health goal as is "feasible," and defines feasible as what the best available treatment technology can achieve, "taking cost into consideration." (In some cases the law also lets EPA weigh costs against benefits.)

So, by design, the two numbers can differ. Here's what that looks like for four contaminants:

Two notes. Lead doesn't have a legal limit in the usual sense. It has an action level that triggers steps like corrosion control and public education. And nitrate is a reminder that the gap isn't everywhere: when the science and the technology line up, the two numbers match.

States can add a third number. California publishes its own public health goals, often stricter. For lead, California's goal is 0.2 parts per billion.

The AquaGap™ is built in, not broken

Because legal limits are set by what's feasible and health goals are set by health alone, water can be fully legal and still sit above a health goal. That's true in Atlanta, and it can happen in any city in America, because that's how the national standards are built.

It isn't a sign that anyone cut corners. A utility that's within every legal limit has done exactly what the law asks of it. The gap belongs to the system we all share, not to the people running a treatment plant.

The legal line also moves. In April 2024, EPA set the first national limits for six PFAS chemicals. On May 20, 2026, EPA proposed rescinding the limits for four of them, and in a separate proposal, letting systems that are granted an exemption take until 2031, instead of 2029, to meet the remaining two. Whatever you think of that, it shows why one number isn't enough. The AquaScore™ reads the record against both, and publishes which benchmark it used and the date.

Who the Health view is for

Most people will look at "within every legal limit" and move on with their day. That's fine.

The Health view is for the people who need more than that:

• parents mixing infant formula;
• people who are pregnant;
• people with weakened immune systems;
• anyone managing a health condition with their doctor.

For them, "legal" answers the wrong question. The Health view answers theirs. It's information, not an accusation, and it isn't a safety call. Every score carries this line, word for word:

AquaScore describes what was measured or modeled against health benchmarks. It is not a determination that water is safe or unsafe to drink.

What this looks like in Atlanta

The City of Atlanta's water system serves about 1.09 million people. On our homepage, it shows two ways:

• Legal view: 96. Every reading in the city's 2025 Water Quality Report and its 2024 home-tap lead testing is within its federal limit.
• Health view: 62 · ELEVATED. At least one reading sits above its health goal.

What drives the Health view is lead at home taps. In 2024, 90% of samples were at or below 1.4 parts per billion. That's well within the 15 ppb federal action level. EPA's health goal for lead is zero, and California's is 0.2 ppb, because no level of lead exposure is known to be without risk, especially for children.

The City has also reported that it hasn't found a single lead service line in its inventory, and it's working with EPA and Georgia EPD to confirm it. That's real progress.

So where does lead at a tap come from? Where there's no lead service line, as the City reports for Atlanta, it usually comes from what sits between the street and your glass: older solder, fixtures and faucets inside the building. That stretch, curb to tap, is what I call the Last Mile. It's the subject of the next paper in this series.

What we do when something changes

Water changes. A main breaks. Something spills upstream. A storm overwhelms a system. A new reading comes in.

Think of us like ADT for water. ADT doesn't tell you your neighborhood is dangerous. It watches, and if something changes, you're the first to know.

That's AquaAlert™. It's built to notice a change and notify residents fast, and to put officials' updates in the same place, so people aren't stitching it together from social media. We alert. Officials declare.

An open door

To every utility leader and city official reading this: we read the same public record you publish. Every number we show carries its source and its date.

Before we publish a score for your system, we'll show it to you and welcome a technical review of the underlying data. If the record has an error, we'll fix it. The score stays independent; the facts get checked. We'd much rather build this with you than about you.

Write to us any time: hello@aquagenuity.com.

The bottom line

Legal isn't always clean. But legal is real, legal is earned, and the people who deliver it deserve our respect. Both things are true at once, and the people who drink the water deserve to see both.

Water is not a political issue. It's a human issue.

Check your water at aquagenuity.com. Become a Guardian. It's free.

Next in this series: Who tests for what: EPA vs. state vs. local, and why there's a gap.

Sources

1. Safe Drinking Water Act §1412(b)(4), definitions of the health goal (MCLG), the legal limit (MCL) and "feasible." Summarized in Congressional Research Service, Safe Drinking Water Act (SDWA): A Summary of the Act and Its Major Requirements, R46652, Mar. 3, 2021. https://www.everycrsreport.com/reports/R46652.html
2. Over 90 regulated contaminants; health goal usually zero for carcinogens and microbes: same CRS report, pp. 1, 12.
3. Arsenic: health goal 0 (40 CFR 141.51), legal limit 0.010 mg/L (40 CFR 141.62). eCFR, accessed Oct. 15, 2026.
4. Lead: health goal 0; action level 15 ppb, lowered to 10 ppb under the Lead and Copper Rule Improvements (final Oct. 8, 2024; compliance Nov. 1, 2027). EPA, https://www.epa.gov/ground-water-and-drinking-water/lead-and-copper-rule-improvements, accessed Oct. 15, 2026. EPA is defending the rule in the D.C. Circuit (notice Feb. 20, 2026; final brief Apr. 17, 2026): NRDC case page, https://www.nrdc.org/court-battles/american-water-works-association-et-v-epa-lead-and-copper-rule-improvements
5. PFOA: health goal 0, legal limit 4.0 ppt (EPA final rule, Apr. 2024). Proposed rescission of PFHxS, PFNA, HFPO-DA and the hazard index, 91 FR 29413, May 20, 2026 (proposed, not final). https://www.federalregister.gov/documents/2026/05/20/2026-10085 Separate proposal (FR Doc. 2026-10086, May 20, 2026): exemptions extending PFOA/PFOS compliance to Apr. 26, 2031, with interim controls for systems at or above 12 ppt. Summary: Jones Day, May 2026.
6. Nitrate: health goal and legal limit both 10 mg/L as nitrogen (40 CFR 141.51, 141.62).
7. California public health goal for lead, 0.2 ppb: Office of Environmental Health Hazard Assessment, April 2009. https://oehha.ca.gov/water/public-health-goal/public-health-goals-lead-oxamyl-and-pentachlorophenol
8. More than 148,000 public water systems; about 90% of Americans served: EPA, "Information about Public Water Systems" (archived Jan. 19, 2025).
9. Twice-a-year Water Quality Reports for systems serving more than 10,000 people, first compliance date Jan. 1, 2027: EPA Consumer Confidence Report Rule Revisions (May 2024), via AWWA.
10. City of Atlanta population served, 1,089,893: EPA SDWIS via mytapwater.org, accessed Oct. 14, 2026.
11. Atlanta lead results (90th percentile 1.4 ppb; max 10.0 ppb) and service line inventory: City of Atlanta Department of Watershed Management, 2025 Water Quality Report.
12. "Has not found a single lead service line": City of Atlanta DWM news release, Dec. 3, 2025. https://atlantawatershed.org/dwmnewsrelease-dec-3-2025/
13. Atlanta AquaScore™ (Legal 96; Health 62 · ELEVATED): Aquagenuity AquaScore™ method v2.0.0, computed Oct. 15, 2026.

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