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Tap water quality report

San Marcos, CA

EPA monitoring detected 18 contaminants across the 2 water suppliers serving San Marcos. A detection doesn't automatically mean the water is unsafe — see the level at the main supplier for each below, with its EPA source and period.

2
water suppliers
113,596
people served
18
contaminants found
California rankings Lead in California PFAS in California

What was found in San Marcos's water

The level at the city's main supplier, the highest level anywhere in the city, the EPA limit for each, and where every figure comes from.

ContaminantLevel vs EPA limitSuppliersSource
Nitrate
24800 µg/L 248%
of the EPA limit, at the 95th percentile of samples — 10000 µg/L
Highest in the city: OCOTILLO OASIS, LLC · 96 people · 2019
1/2 SYR4
Fluoride
5111.5 µg/L 128%
of the EPA limit, at the 95th percentile of samples — 4000 µg/L
Highest in the city: OCOTILLO OASIS, LLC · 96 people · 2019
1/2 SYR4
Thallium, Total
71 µg/L 3550%
of the EPA limit, at the 95th percentile of samples — 2 µg/L
Highest in the city: OCOTILLO OASIS, LLC · 96 people · 2016
1/2 SYR4
Lithium
52 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2023
1/2 UCMR-5
TTHM
44.8 µg/L 56%
of the EPA limit 80 µg/L
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Lead
27.86 µg/L 186%
of the EPA limit, at the 95th percentile of samples — 15 µg/L
Main supplier: VALLECITOS WD · 113,500 people · 2017
1/2 SYR4
Total Haloacetic Acids (HAA5)
18 µg/L 30%
of the EPA limit 60 µg/L
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Dibromochloromethane
15 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Bromodichloromethane
14 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Chloroform
13 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Dichloroacetic Acid
9.1 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Bromoform
7.935 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Trichloroacetic Acid
5.085 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Monochloroacetic Acid
4.45 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Dibromoacetic Acid
4.4 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Monobromoacetic Acid
1.9 µg/L
No EPA limit set
Main supplier: VALLECITOS WD · 113,500 people · 2019
1/2 SYR4
Combined Uranium
0.5206 µg/L 1.7%
of the EPA limit 30 µg/L
Highest in the city: OCOTILLO OASIS, LLC · 96 people · 2017
1/2 SYR4
Lead
0.0 mg/L 0%
of the EPA limit 0.0150 mg/L
Main supplier: VALLECITOS WD · 113,500 people · 2026
2/2 LCR
Each bar shows the measured level as a share of the EPA limit: green is well under, amber is close to it, red is over. For lead and copper the limit applies to exactly the 90th-percentile figure shown, so red there means a real exceedance. For everything else the limit applies to a running annual average while the figure is the 95th percentile of the system's samples — over 100% is worth reading, but is not by itself a violation. Formal violations are listed separately. These are the utilities' own EPA-reported results, not a city-wide average. A detection means the lab measured the substance — it isn't the same as exceeding a legal limit. Hover a source tag to see the EPA program and years behind each figure.

Water suppliers serving San Marcos

SupplierPopulation served
VALLECITOS WD 113,500
OCOTILLO OASIS, LLC 96

Frequently asked questions about San Marcos tap water

Is San Marcos tap water safe to drink?

EPA testing detected 18 contaminants in the water serving San Marcos. A detection isn't the same as a violation — most detected levels are within federal limits. Check the level and source for each contaminant above, and your utility's annual Consumer Confidence Report for the official compliance status.

How many water suppliers serve San Marcos?

2 public water suppliers report drinking-water monitoring for San Marcos, California to the EPA. Each is listed above with its population served.

What contaminants are in San Marcos's water?

The contaminants EPA monitoring detected in San Marcos include Nitrate, Fluoride, Thallium, Total, Lithium, TTHM. The full list, with the level at the main supplier and the data source, is in the table above.

Where does this data come from?

Every figure is official EPA drinking-water monitoring — the Lead & Copper Rule, the UCMR-5 nationwide testing program, and the EPA Six-Year Review — not our own testing. Each number is labelled with its source and period.