Gaithersburg Tap Water: 5 Things to Know (2026)

Gaithersburg's tap water passes every federal test, and that is only the start of the story.

The water is legal, with no violations on file for the utility that supplies it. But the Environmental Working Group (EWG), which sets stricter, health-only guidelines, flags four contaminants in that supply. Two get most of the attention. The other two, arsenic and chromium-6, rarely come up in conversation, and they are worth a closer look. Here are five things every Gaithersburg household should know.

1. The city doesn't run its own water system

Unlike neighboring Rockville, which treats its own water, Gaithersburg does not run a treatment plant. The city is supplied by WSSC Water, the Washington Suburban Sanitary Commission, which serves about 1.9 million people across Montgomery County and Prince George's County.

On the EPA's record, WSSC Water supplies its customers through 438,639 service connections. It draws surface water from the Potomac River and the Patuxent River, and treats it with powdered activated carbon, algae control, coagulation, flocculation, rapid sand filtration and fluoridation before disinfecting it.

Practically, that means questions about Gaithersburg's water are questions about WSSC Water. Our report covers ZIP codes 20877, 20878, 20879 and 20886, plus surrounding Gaithersburg areas.

2. Disinfection byproducts lead the list

The two largest figures in EWG's analysis are both disinfection byproducts:

• Haloacetic acids (HAA5): 49 ppb on average, 490× EWG's guideline of 0.1 ppb. EWG links them to cancer.
• Total trihalomethanes (TTHMs): 66 ppb on average, 440× EWG's guideline of 0.15 ppb. EWG links them to cancer and to effects on fetal development.

These are a byproduct of keeping water safe. Surface water needs heavy disinfection, and chlorine reacting with organic matter from the river creates them. They form in the pipes after treatment, not at the plant. In a system this large, water spends days in the distribution network before it reaches a Gaithersburg tap, which gives them time to accumulate.

3. Arsenic and chromium-6 are the quieter flags

Because the byproduct multiples are so large, the other two contaminants on EWG's list are easy to skip past. Here they are on their own:

Contaminant	Found	EWG guideline	Over by
Chromium-6	0.13 ppb	0.02 ppb	6×
Arsenic	Detected	0.004 ppb	10×

Source: WSSC Water data via EWG. "Over by" compares the level with EWG's health guideline, not the federal legal limit.

Chromium-6, also called hexavalent chromium, averages 0.13 ppb, six times EWG's guideline of 0.02 ppb. EWG links it to stomach cancer.

Arsenic was detected in the supply, at 10 times EWG's guideline of 0.004 ppb. EWG links arsenic to cancer, cardiovascular effects and developmental effects.

Neither result breaks a federal rule. Federal limits weigh health against the cost and technical feasibility of treatment. EWG's guidelines, in its own words, are "based only on protecting health and does not consider technical feasibility or the cost of water treatment." EWG last updated them in February 2025. The gap between the two is what these multiples measure.

One more data point from the same supply: the EPA's UCMR5 round (2023 to 2025) found 2 PFAS compounds, PFPeA at 6.7 ppt and PFHxA at 4.8 ppt. Both were under EWG's health standard, with no federal limit for either. EWG links them to harm to fetal growth and child development.

4. Lead is low, and the federal bar is dropping

Lead is sampled at taps inside homes, and utilities report the 90th percentile. WSSC Water's was 1.05 ppb. Nine in ten homes tested came in below that, and one in ten came in above it.

The federal benchmark is also changing. The EPA's lead action level is being lowered from 15 ppb to 10 ppb, in force November 2027. WSSC Water's figure sits well under both the old and new levels. Measured against EWG's guideline of 0.2 ppb, it is about 5.3 times higher, and EWG links lead to harm to the brain and nervous system.

The bigger variable is your building. Lead mostly comes from service lines and household plumbing, so the age of your home can matter more than the utility average.

5. You can narrow the gap at your own tap

Since the water already meets federal limits, any step here is a personal choice. If you would like lower levels in the water you drink, these are the usual options:

1. Get a baseline. A free water test shows what is coming out of your faucet, which matters most for lead. We can mail a kit anywhere in the U.S.
2. Try a carbon filter. Pitchers and faucet-mount filters are inexpensive and mainly improve taste and odor. Results depend on the product and on changing the filter on time.
3. Consider reverse osmosis (RO). An under-sink RO system is a common way to reduce dissolved contaminants such as disinfection byproducts, and it covers your drinking and cooking water.

The Sanavia Hydra is the tankless under-sink RO system we sell and install. In independent accredited lab testing of an Arlington, VA home's tap water, total trihalomethanes fell from 58 ppb to 2.79 ppb after the Hydra, a 95% reduction. The full lab results are on our proof page.

Every figure in this article, in one place, is in our Gaithersburg, MD water quality report.