Water Quality in San Marcos, CA: 112 Violations — Grade C
Grade C · Score 69/100 · 2 water systems · Updated 2026-05-03
San Marcos's water has moderate compliance issues. Residents in affected areas should consider filtration.
Data: EPA Safe Drinking Water Information System (SDWIS) Last verified: 2026-05-03
Compliance records maintained by the EPA for San Marcos, CA log 112 violations — accumulated reportable events drawn from every local utility in the SDWIS national tracking system.
Water Quality by ZIP Code in San Marcos
Water quality varies across San Marcos's 4 ZIP codes. Check your specific ZIP for detailed contaminant data.
| ZIP Code | Grade | Score | Violations | Health | |
|---|---|---|---|---|---|
| 92069 | C | 66 | 28 | 6 | View report → |
| 92079 | C | 69 | 28 | 6 | View report → |
| 92096 | C | 69 | 28 | 6 | View report → |
| 92078 | B | 71 | 28 | 6 | View report → |
Violation Trend
EPA violations recorded for water systems serving San Marcos.
Water Quality Overview
San Marcos, California receives a water quality grade of C with an overall score of 69 out of 100, based on EPA compliance data from the Safe Drinking Water Information System (SDWIS).
The 2 water systems serving San Marcos have accumulated 112 EPA violations, including 24 health-based violations. These violations are tracked across 4 ZIP codes serving approximately 113,596 people.
What Grade C Means
A grade of C indicates moderate compliance issues. San Marcos's water systems have some violations that warrant attention. While the water may still be safe for most people, residents in affected ZIP codes should consider additional testing or filtration.
Lead Levels
The average 90th percentile lead level across San Marcos water systems is 0.0011 mg/L — within EPA limits. No ZIP codes exceed the EPA lead action level.
With 41% of homes built before 1986, San Marcos has a higher-than-average risk of lead from older plumbing. Lead solder was banned in 1986, and homes built before this date may have lead in pipes, solder joints, or fixtures.
Water Systems Serving San Marcos
San Marcos is served by 2 community water systems regulated under the Safe Drinking Water Act. These systems collectively serve approximately 113,596 people across 4 ZIP codes.
Each water system is identified by a Public Water System ID (PWSID) and is required to:
- Test for over 90 regulated contaminants on a regular schedule
- Report results to the EPA and state regulators
- Issue an annual Consumer Confidence Report (CCR) to all customers
- Take corrective action when violations occur
The EPA has taken 8 enforcement actions against water systems serving San Marcos. Enforcement actions range from informal compliance assistance to formal orders and penalties.
How to Check Your Water in San Marcos
Enter your ZIP code on ZipCheckup to see your specific water system data, including contaminant levels, violation history, and safety scores
Request your CCR — your water utility must provide an annual Consumer Confidence Report with detailed test results for all monitored contaminants
Get a home test — certified kits cost $20–$50 and test for lead, bacteria, and common contaminants. Professional lab tests ($150–$400) cover a broader panel
Consider filtration — NSF-certified filters can remove specific contaminants. Reverse osmosis removes the broadest range; activated carbon is effective for chlorine and many organics
Check for advisories — monitor your local utility website and local news for boil water advisories or system alerts
Contaminants Detected in San Marcos Water Systems
The following contaminants have been detected or caused violations across San Marcos's water systems:
| Contaminant | Category | MCL | Violations | ZIPs Affected | Health-Based |
|---|---|---|---|---|---|
| Nickel | Inorganic | 0.1 mg/L | 96 | 4 | Yes |
| Lead | Inorganic | 0.015 mg/L | 8 | 4 | No |
| Stage 2 DBP Rule | Treatment Technique | N/A | 4 | 4 | No |
| Stage 1 DBP Rule | Treatment Technique | N/A | 4 | 4 | No |
MCL = Maximum Contaminant Level (EPA enforceable standard). Health-based violations indicate levels that may pose direct health risks.
Violation Trend Over Time
Year-by-year EPA violations recorded for water systems serving San Marcos:
| Year | Total Violations | Health-Based |
|---|---|---|
| 2021 | 12 | 12 |
| 2023 | 8 | 8 |
Violations have decreased from 12 in 2021 to 8 in 2023, indicating improving compliance.
EPA Enforcement History
The EPA and state regulators have taken 2 enforcement actions against water systems serving San Marcos. Recent actions:
| Date | Action Type |
|---|---|
| 2024-10-31 | State Filed Suit |
| 2024-06-12 | State Filed Suit |
Enforcement actions range from informal compliance assistance to formal orders and penalties. The most recent action was on 2024-10-31.
San Marcos vs. Nearby California Cities
How San Marcos's water quality compares to similar cities in California:
| City | Grade | Violations | Systems | Population |
|---|---|---|---|---|
| San Marcos | C | 112 | 2 | 113,596 |
| Ventura | B | 88 | 5 | 114,606 |
| Antioch | B | 0 | 1 | 115,074 |
| Santa Barbara | C | 17 | 5 | 116,466 |
Water Quality by ZIP Code in San Marcos
Water quality varies across San Marcos's ZIP codes. The table below shows the worst-scoring and best-scoring areas:
| ZIP Code | Score | Grade | Violations | Health-Based |
|---|---|---|---|---|
| 92069 | 66 | C | 28 | 6 |
| 92079 | 69 | C | 28 | 6 |
| 92096 | 69 | C | 28 | 6 |
| 92078 | 71 | B | 28 | 6 |
Common Questions About San Marcos Water
These are the most common questions residents search for about water quality in San Marcos, California:
Is San Marcos water hard or soft? Water hardness varies by source. San Marcos's 2 water systems use multiple sources. Check your ZIP code report for specific hardness data.
Can I drink San Marcos tap water? San Marcos's water receives a grade of C (69/100). There are 24 health-based violations — consider filtration.
What is the best water filter for San Marcos? A NSF/ANSI 53-certified filter for lead removal is a good general choice. See our Water Filter Matcher for personalized recommendations.
San Marcos vs. California Average
Understanding how San Marcos compares to the broader California average helps contextualize your local water quality. Factors like water source, treatment methods, and infrastructure age all contribute to differences between cities in the same state.
To see how individual neighborhoods compare, check the ZIP code reports below — water quality can vary significantly even within San Marcos.
ZIP Codes in San Marcos
San Marcos spans 4 ZIP codes. The ZIP with the lowest water quality score is 92069. Each ZIP code has its own water quality profile based on the specific water system serving that area. Check each ZIP code for detailed contaminant data, violation history, and system information.
Data Sources
- Water quality: EPA Safe Drinking Water Information System (SDWIS)
- Lead/copper: EPA Lead and Copper Rule sampling data
- Radon: EPA Map of Radon Zones
- Enforcement: EPA ECHO database
- Filter recommendations: NSF/ANSI certified products
Updated daily.