Tap Water Safety Report for San Marcos, California
San Marcos, California tap water scored 45/100 (F) based on EPA data. Multiple contaminants are at concerning levels. A water filter is recommended, especially for children and pregnant women.
Overall Score
45/100
Grade: F
Contaminants Tested
8
All within limits
EPA Violations
8
1 active
Water Source
groundwater
Mixed sources
Population Served
100,051
residents
Water Systems
1
serving this area
1 contaminant(s) are approaching EPA limits and should be monitored.
Detected level
52.3439 ug/L
65% of EPA limit
Health effects: Disinfection byproducts linked to increased risk of bladder cancer and reproductive issues with long-term exposure.
San Marcos's water system has 1 unresolved violation(s) with the EPA:
Based on San Marcos's water quality data, here are our specific recommendations:
San Marcos's water has multiple contaminants at concerning levels. An under-sink reverse osmosis system or a certified multi-stage filter is strongly recommended for drinking and cooking water.
Babies and young children are especially vulnerable to water contaminants. Use filtered or bottled water for mixing formula and for drinking water until filtration is installed.
How contaminant levels compare to EPA limits (50% of overall score)
EPA violation history and enforcement actions (30% of overall score)
Water system infrastructure and source quality (20% of overall score)
With a score of 45/100, San Marcos's tap water has significant quality concerns. We strongly recommend against using unfiltered tap water for babies or young children. Use bottled water or invest in a reverse osmosis filter for any water consumed by children. Infants are especially vulnerable to contaminants like lead and nitrates, which can cause serious developmental and health issues.
San Marcos's water has been tested for 8 contaminants. None currently exceed EPA maximum contaminant levels. The most notable detections include: Total Trihalomethanes (52.3439 ug/L, EPA limit: 80 ug/L — 65% of limit); Nitrate (4.1231 mg/L, EPA limit: 10 mg/L — 41% of limit); Haloacetic Acids (22.9834 ug/L, EPA limit: 60 ug/L — 38% of limit); Chromium (0.0383 mg/L, EPA limit: 0.1 mg/L — 38% of limit); Copper (0.4285 mg/L, EPA limit: 1.3 mg/L — 33% of limit). The water system also has 1 active EPA violation(s). Water quality can vary seasonally and by neighborhood, so home testing provides the most accurate picture for your specific tap.
Absolutely. With a water quality score of 45/100, filtering is strongly recommended in San Marcos. A reverse osmosis (RO) system is the most effective option, removing up to 99% of contaminants. At minimum, use a certified pitcher filter rated for the specific contaminants found in your water. Multiple contaminants at concerning levels make filtration an important health measure.
San Marcos's water quality score of 45/100 (F) is 27 points below the national average of 72. The city's water comes from a mix of groundwater and surface water sources. This indicates significant water quality challenges that the city should work to address. You can see how San Marcos compares to nearby cities in our state rankings.
Detailed contaminant data, violation history, and water system information for San Marcos.
Find the best water filter for San Marcos based on detected contaminants and your budget.
See how San Marcos compares to other cities across California.
See how San Marcos ranks among all cities in the US for water quality.
Data Disclaimer
This report is based on publicly available EPA data and water quality test results. Water quality can vary by neighborhood, building age, and season. This information is for educational purposes and should not replace professional water testing. If you have specific health concerns about your water, contact your local water utility or a certified water testing laboratory. Last data update: February 2, 2026.