Tap Water Quality Report
Tap water quality in SAN MARCOS, California
Tap water in SAN MARCOS, California receives a grade of F (55/100) 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.
EPA compliance record
Official U.S. EPA records for VALLECITOS WD (system ID CA3710002) for the last 3 years. Checked 2026-10-07.
| Rule | Type | Since | Status |
|---|---|---|---|
| Lead and Copper Rule Revisions | Missed reporting | 07/02/2025 | Open |
Source: U.S. EPA Enforcement and Compliance History Online (ECHO), from the Safe Drinking Water Information System. See EPA's full report.
Recommended filters for SAN MARCOS
Filters matched to the specific contaminants in SAN MARCOS's water supply.
Filter certified for PFOA/PFOS (NSF/ANSI 53 or 58)
$180-$750Reverse osmosis and some carbon block filters reduce PFAS. Look for certification to NSF/ANSI 53 or 58 for PFOA/PFOS.
Reverse osmosis certified for arsenic (NSF/ANSI 58)
$429-$599Reverse osmosis certified under NSF/ANSI 58 for arsenic is the most reliable home option.
Carbon filter certified for VOCs (NSF/ANSI 53)
$180-$599Activated carbon is effective for disinfection byproducts. Look for NSF/ANSI 53 VOC reduction.
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.