Tap Water Quality Report
Tap water quality in SAN MARCOS, California
Tap water in SAN MARCOS, California receives a grade of F (53/100) from TapWaterSafety.org. It's served by VALLECITOS WD. The most significant water quality concerns are: PFAS contamination; Arsenic* significantly above health guideline; Haloacetic acids (HAA5) significantly above health guideline; Haloacetic acids (HAA9) significantly above health guideline. Data sources: EPA SDWIS, EPA ECHO, EWG Tap Water Database.
What's in SAN MARCOS's tap water
Top water quality concerns identified by the EPA and Environmental Working Group across the utility serving SAN MARCOS.
PFAS contamination
Severe concernPerfluorooctane sulfonate (PFOS)* detected. PFAS are persistent chemicals linked to cancer and immune effects with no known safe exposure threshold.
Arsenic* significantly above health guideline
Moderate concernArsenic* detected at 1.49 ppb, 373x above the EWG health guideline of 0.004 ppb.
Haloacetic acids (HAA5) significantly above health guideline
Moderate concernHaloacetic acids (HAA5) detected at 36.2 ppb, 362x above the EWG health guideline of 0.1 ppb.
Haloacetic acids (HAA9) significantly above health guideline
Moderate concernHaloacetic acids (HAA9) detected at 10.3 ppb, 171x above the EWG health guideline of 0.06 ppb.
Recommended filters for SAN MARCOS
Filters matched to the specific contaminants in SAN MARCOS's water supply.
Reverse Osmosis or NSF P473 certified carbon
$80-$750Activated carbon and RO are the only technologies proven to remove PFAS at certified levels.
Reverse Osmosis
$249-$750Arsenic is most effectively removed by reverse osmosis. Some specialized adsorptive filters also work.
Carbon block (NSF/ANSI 42 + 53)
$40-$750Activated carbon is highly effective for disinfection byproducts like TTHM and HAA5.
Disclosure: TapWaterSafety earns a commission from purchases made through these links. This does not influence our scoring methodology or filter selection.
All contaminants detected in SAN MARCOS's tap water
Every contaminant identified, compared side-by-side against US EPA legal limits, the EU Drinking Water Directive (2020/2184), WHO Guidelines, and California's Public Health Goal. Why we show multiple standards →
| Contaminant | Detected | EWG US health-based |
EPA US legal |
EU DWD Europe |
WHO global |
CA PHG strictest US |
Tested sample year |
Status |
|---|---|---|---|---|---|---|---|---|
| Arsenic* Heavy metal |
1.49 ppb | 0.004 | 10.0 | 10 | 10 | 0.004 | — | 373× over |
| Haloacetic acids (HAA5) Disinfection byproduct |
36.2 ppb | 0.1 | 60.0 | 60 | — | — | — | 362× over |
| Haloacetic acids (HAA9) Disinfection byproduct |
10.3 ppb | 0.06 | — | — | — | — | — | 171× over |
| Total trihalomethanes (TTHMs) Disinfection byproduct |
21.9 ppb | 0.15 | 80.0 | 100 | — | — | — | 146× over |
| Bromodichloromethane Disinfection byproduct |
6.51 ppb | 0.06 | — | — | — | — | — | 108× over |
| Dibromoacetic acid Disinfection byproduct |
1.89 ppb | 0.03 | — | — | — | — | — | 63× over |
| Dibromochloromethane Disinfection byproduct |
6.07 ppb | 0.1 | — | — | — | — | — | 61× over |
| Bromate* Disinfection byproduct |
2.39 ppb | 0.1 | 10.0 | 10 | 10 | 0.1 | — | 24× over |
| Chloroform Disinfection byproduct |
6.46 ppb | 0.4 | — | — | — | — | — | 16× over |
| Dichloroacetic acid Disinfection byproduct |
3.09 ppb | 0.2 | — | — | — | — | — | 15× over |
| Trichloroacetic acid Disinfection byproduct |
1.49 ppb | 0.1 | — | — | — | — | — | 15× over |
| Bromoform Disinfection byproduct |
2.79 ppb | 0.5 | — | — | — | — | — | 5.6× over |
| Uranium* Radiological |
1.68 pCi/L | 0.43 | 20.0 | 30 | 30 | 0.43 | — | 3.9× over |
| Chromium (hexavalent) Heavy metal |
0.0638 ppb | 0.02 | — | — | — | — | — | 3.2× over |
| Nitrate* Inorganic |
0.145 ppm | 0.14 | 10.0 | 11.3 | 11.3 | 10 | — | 1.0× over |
| Perfluorooctane sulfonate (PFOS)* PFAS |
0.25 ppt | 0.3 | 4.0 | 100 | 100 | 1 | — | Below guideline |
| Chlorate Disinfection byproduct |
106.1 ppb | 210.0 | — | — | — | — | — | Below guideline |
| Nitrate and nitrite* Inorganic |
0.04 ppm | 0.14 | 10.0 | 11.3 | 11.3 | 10 | — | Below guideline |
| Manganese Heavy metal |
13.5 ppb | 100.0 | — | 50 | 80 | — | — | Below guideline |
| Molybdenum Other |
4.14 ppb | 40.0 | — | — | — | — | — | Below guideline |
| Barium* Heavy metal |
60.6 ppb | 700.0 | 2000.0 | — | 1300 | 2000 | — | Below guideline |
| Aluminum* Heavy metal |
37.0 ppb | 600.0 | — | 200 | — | 600 | — | Below guideline |
| Vanadium Other |
0.188 ppb | 21.0 | — | — | — | — | — | Below guideline |
| Monobromoacetic acid Disinfection byproduct |
0.0406 ppb | 25.0 | — | — | — | — | — | Below guideline |
| Monochloroacetic acid Disinfection byproduct |
0.05 ppb | 53.0 | — | — | — | — | — | Below guideline |
| Strontium Radiological |
0.994 ppb | 1500.0 | — | — | — | — | — | Below guideline |
| Perfluorohexanoic Acid (PFHxA)* PFAS |
0.275 ppt | 1000.0 | — | — | — | — | — | Below guideline |
| Bromide* Other |
160.7 ppb | — | — | — | — | — | — | Below guideline |
| Fluoride* Inorganic |
0.578 ppm | — | 4.0 | 1.5 | 1.5 | — | — | Below guideline |
| Lithium Other |
13.3 ppb | — | — | — | — | — | — | Below guideline |
All values in the unit of the detected level. Red cells indicate the detected level exceeds that standard. Some contaminants have limits in some jurisdictions but not others (shown as —). The "Tested" column shows the year each contaminant sample was collected.
Sources: US EPA, EU Drinking Water Directive 2020/2184, WHO Guidelines, California OEHHA PHGs, EWG Tap Water Database.
Treatment plant contact info
For service issues, water quality concerns, or to request a Consumer Confidence Report from VALLECITOS WD.