How do you read a water quality report? A line-by-line Sonoma County guide

By The Downspout · Posted Oct 19, 2026 · 12 min read

Go row by row through the main table. Check that the violation column says no, that the amount is under the legal limit (MCL), and how far it is above the health goal (PHG). Petaluma's 2025 report met every legal limit, yet its arsenic result was about 770 times California's health goal.

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Start with the main table, the list of substances found in your water. For each row, ask three questions. Did the supplier break a rule? Is the amount under the legal limit? And how far is it from the health goal?

That third question is the one most people skip. It is also the most interesting one.

That row is real. It comes from the City of Petaluma's 2025 report.^[1] Look at the small number in row 3: 0.004. That is California's public health goal for arsenic.^[1]^[2] Petaluma's 3.08 was about 770 times that goal, and still well inside the legal limit of 10.

Both are true at once. Above the goal is still legal. Below, we explain why the two numbers are so far apart, and what you can do about it.

Every city and water district that serves a community has to send one to its customers each year by July 1.^[3] It is free testing your water bill already paid for. Here is how to read yours in a few minutes, with Petaluma's as the worked example.

First, get your report

The official name is the Consumer Confidence Report, or CCR. Many suppliers just call it the water quality report.^[3]

Search your supplier's website for "water quality report". Petaluma, for example, posts all its reports on one page.^[4]
Or use EPA's online search tool, or call the number on your bill and ask for a copy.^[3]
Not sure who your supplier is? Type your address into our water map. More ways to find it are in our guide to what is in your tap water.

In your report, read the part about where your water comes from first. It explains the numbers. Petaluma buys its water from Sonoma Water, the county's wholesale water agency, which draws it from beside the Russian River. Petaluma also has its own wells.^[1] That is why its report shows two sets of results: one for Sonoma Water's water and one for Petaluma's own samples. For the whole trip from lake to tap, see where Sonoma County tap water comes from.

The words you need, in plain English

A report uses the same short codes in every table. Learn these and the rest is easy.

MCL, maximum contaminant level. The legal limit. It is the most of a substance allowed in drinking water.^[1]
PHG, public health goal. The level with no known or expected health risk, set by California's environmental health office, OEHHA.^[1]^[5] It is a goal, not a rule.
AL, action level. Used for lead and copper. Go over it, and the supplier must take steps such as treating the water to keep pipes from corroding.^[1]^[6]
ND, not detected. The lab did not find it at the level its test can measure.^[1] That is not always zero. More on that below.
ppm, ppb and ppt. Parts per million is milligrams per liter. Parts per billion is micrograms per liter, 1,000 times smaller.^[1] Parts per trillion is 1,000 times smaller again. Check the unit on each row before you compare numbers.
pCi/L, picocuries per liter. A measure of radioactivity. In Petaluma's report it comes from natural deposits.^[1]

Read each row in three checks

Do these in order, row by row, in the main table.

Look at the violation column. It should say no. If it says yes, the report must explain what happened and what the supplier is doing about it.^[1]^[3]
Compare the amount with the MCL. If the amount is smaller, the water met that legal standard.^[1]
Compare the amount with the PHG. This tells you how much room there is between legal and ideal. Most rows will be under it. A few may not.

Then glance at two more columns. Range is the lowest and highest single test.^[1] Year sampled matters too. Some things are tested less than once a year because they rarely change, so the newest result may be a few years old.^[1]

How close did Petaluma come to the limits?

We took every substance detected in Petaluma's own samples in its 2025 report and divided the amount by its legal limit. Look for how short the bars are. The longest reaches just over a third of the way.

Trihalomethanes and haloacetic acids, the first and third bars, are disinfection byproducts. They form when chlorine, added to kill germs in the pipes, reacts with natural organic matter in the water.^[1] Petaluma's own samples had about twice as much trihalomethane as Sonoma Water's: 29.84 ppb against 13.9.^[1]

If you want less of them, Petaluma suggests letting the tap run briefly before use, using a certified carbon filter, or keeping a pitcher of water in the fridge so some of them escape.^[1] For more on chlorine and its byproducts, see our tap water guide.

Below the limit, above the goal

Now the third check. Four of Petaluma's results sit above their health goal while staying under the law. This table shows how far above for three of them.

Substance	Amount	Health goal (PHG)	Times the goal
Arsenic	3.08 ppb	0.004 ppb	770
Radium 228	0.41 pCi/L	0.019 pCi/L	about 22
Lead (2023)	2.4 ppb	0.2 ppb	12

Source: City of Petaluma 2025 report, our math. Gross alpha has a goal of zero, so any amount is above it.

Why can water be legal and still miss the goal by that much? Because the two numbers answer different questions.

The goal looks only at health. For a chemical that can cause cancer, OEHHA usually sets the goal where no more than one person in a million would get cancer from drinking that water every day for 70 years.^[5] The arsenic goal is set that way, based on lung and bladder cancer risk.^[2]

The limit also weighs cost and technology. State law says the limit must be as close to the goal as is technically and economically feasible.^[5] Petaluma's report says the arsenic standard balances what is known about health effects against the cost of removing arsenic.^[1]

People read that gap differently.

The case for tightening. California's drinking water regulators are studying whether the arsenic limit can be lowered closer to the goal. No rule has been proposed yet.^[7]^[2]
The case for caution about the goal. When the goal was proposed in 2003, NSF, a group that tests water products, said the goal comes from a theoretical risk calculation, and that a small, exact number like this gets extra weight from the media and the public.^[8]

Our take. Use the MCL to judge whether your supplier is following the law. Use the PHG to decide what you want for your own glass. If one row sits far above its goal and that bothers you, first ask your supplier what it is doing about it. A home filter certified to reduce that substance is a modest step of your own. The label lists what it reduces. Our guide to water filters walks through the choices. This is our view, not medical advice.

Why one test can top the limit with no violation

Look at Petaluma's arsenic range again: ND to 11. One test came in above the limit of 10, and the violation column still says no.^[1]

That is not a mistake. For arsenic, the rule looks at the average of a year of tests at each spot where water is sampled. One high test is not a violation. A high average is.^[9]

The flip side: "ND" does not always mean none. Labs report arsenic only down to 2 ppb, the state's reporting level.^[7] The goal is 0.004 ppb.^[7] So a not detected arsenic result can still be above the goal.

Lead: the row about your own pipes

Lead is different from everything else in the table. It mostly does not come from the river or the wells. It comes from service lines, the pipes from the street to homes, and from home plumbing.^[1] So the supplier tests at people's taps.

The number shown is the 90th percentile. That means 9 out of 10 homes tested at or below it.^[1] In Petaluma's 2023 round, that was 2.4 ppb, and none of the 30 homes went over the action level.^[1]

The lead action level is changing. Under a 2024 federal rule, it drops from 15 ppb to 10 ppb on November 1, 2027.^[10]^[6] California is working to adopt the same change.^[6] Petaluma's 2.4 would still be about a quarter of the new level.

The rule is in court. The American Water Works Association, a water utility group, says it shares the goal of replacing lead pipes but calls the rule not feasible. It points to cost, which it says will land mostly on water bills, and to a duty to replace lines on private property.^[11] EPA and environmental groups are defending the rule. The court heard arguments on September 30, 2026.^[12]

A low number for the city is not a number for your house. Lead levels vary over time and from home to home.^[1] What to do, from Petaluma's own advice:^[1]

Run the tap for several minutes before using water for drinking, cooking or formula. A shower, laundry or dishes does it too.
Use only cold water for drinking, cooking and baby formula. Boiling does not remove lead.
Use a filter certified to reduce lead, and follow its instructions.
Ask your supplier about testing, and check its service line inventory, a list of what each service line is made of. Petaluma posts its inventory online.

Plumbing pipes, spigots, and shutoff valves mounted to a wooden post beside a stucco building foundation. — Pipes, spigots and shutoff valves beside a house. Past the water meter, the plumbing belongs to the owner.

The other tables: taste, hardness and PFAS

Below the main table are smaller ones. They matter less for health, but they explain a lot about how your water looks and tastes.

Secondary standards cover taste, smell and looks, not health.^[1] Iron and manganese show up here. They can change the taste, clarity and color of water.^[1]
Hardness is mostly calcium and magnesium. It leaves scale on fixtures. Petaluma's report calls it a quality issue, not a safety one.^[1] Petaluma's own samples averaged 170.6 ppm, compared with 109.8 for Sonoma Water's.^[1]
Unregulated substances are things EPA is studying to decide whether they need a limit.^[1] In 2025 this is where PFAS, the long-lasting "forever chemicals", appeared. Sonoma Water's highest result for any of them was 1.8 parts per trillion. One, PFNA, is printed in ppm, likely a typo for ppb.^[1] For scale, EPA's limit for each of the two best-known PFAS, PFOA and PFOS, is 4.0 parts per trillion.^[13] Water systems have until 2029 to meet it, and EPA has proposed allowing up to 2031.^[14] The limit is not in force yet, and the report still lists them here. Petaluma says it will begin its own PFAS testing in 2026.^[1]

What is changing in 2027

Reports are getting easier to read. Starting in 2027, every report must open with a short summary that names any violations, and any time lead went over the action level.^[15] Systems serving 10,000 people or more will send reports twice a year.^[15]

When the report is not enough

A report covers the water your supplier delivers, up to your meter. Past the meter, the plumbing is yours. Santa Rosa, for example, says this and points owners to a private lab.^[16]

Home test kits are a first look only. A 2025 UMass Amherst study found their accuracy varies a lot. Kits that test one substance did better than kits that test many.^[17]

Test your own tap if your home's plumbing has not been updated in about 20 years, or after a flood or wildfire.^[17]
For metals like lead and copper, take the first water of the morning, after it has sat in the pipes overnight. If metal is coming from your pipes, that first water will have the most.^[17]
Use a state accredited lab for results you can trust. Our well testing guide lists the labs on Sonoma County's list, and our directory of local water help has their phone numbers.

On a private well, there is no report at all. No one tests it but you. Start with our well test planner.

What it means for the river

Petaluma's report also has a reminder that is easy to skip. Its storm drains run to creeks and the Petaluma River with no treatment, so what goes into the street can reach local waterways and drinking water sources downstream.^[1] Our guide to keeping the storm drain clean has the simple fixes.

Wooden outdoor staircase descending a steep overgrown bank toward a wide river surrounded by trees. — Stairs down a steep bank to a river in Sonoma County. What washes into a storm drain can end up in water like this.

What it means for you

Find your report this week. Your supplier's website, the number on your bill, or our water map will get you there.
Read the main table in three checks: violation, legal limit, health goal.
If a row is far above its goal, call the number in your report and ask. Then decide whether a certified filter is worth it to you.
If your plumbing is older, test your own tap at a lab, with the first water of the morning.

Next: see what your supplier is doing about chlorine, fluoride and filters in our guide to your Sonoma County tap water.

Photos by Devan Hemmings unless noted.

Sources

Checked 2026-10-19. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.