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LOCAL WATER & HOME

Folsom Lake Water Averaged 53 ppb of Chlorine Byproducts in Granite Bay Last Year. Here's What Trihalomethanes Are, and Why Scientists Are Counting Hundreds More.

Every utility in our area adds chlorine to the water for a good reason: it kills the bacteria and viruses that used to make people sick. But chlorine doesn't stop working once the germs are gone. It keeps reacting with the natural leftovers of leaves, soil and algae in the water, and that...

Family First Water November 12, 2026

Folsom Lake Water Averaged 53 ppb of Chlorine Byproducts in Granite Bay Last Year. Here's What Trihalomethanes Are, and Why Scientists Are Counting Hundreds More.

Every utility in our area adds chlorine to the water for a good reason: it kills the bacteria and viruses that used to make people sick. But chlorine doesn't stop working once the germs are gone. It keeps reacting with the natural leftovers of leaves, soil and algae in the water, and that reaction creates a family of compounds called disinfection byproducts. The best known are trihalomethanes, or THMs, and the latest San Juan Water District report shows exactly how much of them Folsom Lake water carries into homes around Granite Bay, Orangevale, Fair Oaks and Citrus Heights.

What the local numbers say

San Juan Water District's 2025 Consumer Confidence Report, which covers the water it treats from Folsom Lake and shares with Citrus Heights Water District, Fair Oaks Water District and Orange Vale Water Company, lists total trihalomethanes in the distribution system at an average of 53.3 parts per billion for 2025, with samples ranging from 38 to 61 ppb. Haloacetic acids, the second major byproduct family, averaged 44 ppb and ranged from 19 to 62. The federal limits are 80 ppb for THMs and 60 ppb for haloacetic acids, so the system is within them, but it is running closer to the line than much of the state. EWG's Tap Water Database puts the California average for THMs at about 27 ppb.

That gap mostly reflects the source. Folsom Lake is surface water from a forested Sierra watershed, and surface water carries far more dissolved organic matter than deep groundwater does. More organic matter plus chlorine equals more byproducts. That's also why the numbers move with the seasons and with storms, and why Napa had to send notices to one part of its service area in April 2025 when running averages for THMs crept up.

What trihalomethanes are, in plain English

When chlorine meets organic carbon, it swaps chlorine atoms onto small carbon molecules. The result is a group of four compounds, the biggest of which is chloroform. Haloacetic acids form the same way but don't evaporate as easily. The EPA has regulated these two groups since its Stage 1 rule in 1998 and tightened monitoring with the Stage 2 rule in 2006, which is why your utility reports them every year.

The reason regulators care is that long-term studies have linked THMs and haloacetic acids to bladder cancer and to problems with fetal development. That's about decades of exposure, not a glass of water, and the research team behind the newest work on these compounds is clear that American tap water is safe to drink. But they also think the 11 byproducts the EPA tracks are only a small part of the picture.

Why scientists say there are hundreds more

In January 2026, engineers at Stevens Institute of Technology published a study in Environmental Science & Technology Letters that used machine learning to estimate the toxicity of disinfection byproducts nobody has had the time or money to test in a lab. Lead researcher Tao Ye noted that "there are 11 such byproducts regulated by the EPA," while research has identified several hundred more that remain poorly understood. His team found real toxicity data for only 227 known compounds, then trained a model on those results to predict the rest. It scored 1,163 byproducts in all.

Some of the unregulated compounds came out two to ten times more toxic than chemicals the EPA already limits. A September 2026 Earth.com report on the study singled out three families, iodinated acetonitriles, halogenated diones and halobenzoquinones, as the ones that rose to the top of the rankings. Ye describes the model as a screening tool: a high score is a reason to test a compound in the lab, not proof of harm. Still, it's the clearest evidence yet that the regulated byproducts stand in for a much longer list.

How to check your own water, and what filtration does about it

Start with your utility's annual report. Look for the rows labeled "TTHM" or "Total Trihalomethanes" and "HAA5" and note both the average and the range; the range tells you how high the water gets in the worst months. Then type your zip code into EWG's Tap Water Database, which pulls the same test results and compares them with the state average. If your home is on a well, byproducts aren't a concern unless you chlorinate the well yourself.

Two household steps address these compounds, and both come from the Stevens team. Boiling drives off the volatile ones like chloroform. Carbon filtration captures chlorine, chloramine and many of the byproducts they create, which is why activated carbon is the heart of a whole-home filtration system. Treating all of the water, not just the kitchen tap, matters here because chlorine is also what leaves hair dry and skin itchy after a shower. Reverse osmosis at the kitchen sink takes dissolved contaminants a step further, pulling out nitrates, PFAS, lead and other compounds a carbon filter can't catch, so the water your family actually drinks is the cleanest in the house.

Curious what's in your water? Book a complimentary water test — about 30 minutes, no obligation.

Healthier water, healthier family.

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