# San Mateo, CA — Drinking Water Quality (2024)

> Contaminant levels for the San Mateo, CA public water system from its 2024 Consumer Confidence Report, compared to federal limits.

- Page: https://www.thewatermap.com/water/ca/san-mateo/2024
- JSON API: https://www.thewatermap.com/api/water/ca/san-mateo/2024
- Source: the utility's annual Consumer Confidence Report (CCR)
- Verification: transcribed by a model, cross-checked by a second model, approved before publishing
- Reporting year: 2024
- Contaminants measured: 30
- Contaminants with a federal limit: 19
- Contaminants at or above the federal limit: 0
- Part of The Water Map — https://www.thewatermap.com

## Contaminants measured

| Contaminant | Category | Measured level | Sampling context | Federal limit | Status |
|---|---|---|---|---|---|
| Chloramine | Disinfectants | 2.8 mg/L (Average) | System-wide | 4 mg/L (MRDL) | Within the limit |
| Chlorate | Disinfection byproducts | 144 ug/L (Average) | SFPUC Water | 800 ug/L (NL) | Within the limit |
| HAA5 | Disinfection byproducts | 40 ug/L (Average) | Annual Average | 60 ug/L (MCL) | Within the limit |
| TTHM | Disinfection byproducts | 41 ug/L (Average) | Annual Average | 80 ug/L (MCL) | Within the limit |
| Bromide | Inorganic chemicals | Not detected ug/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Chloride | Inorganic chemicals | 9.3 mg/L (Average) | SFPUC Water | 500 mg/L (MCL) | Within the limit |
| Fluoride | Inorganic chemicals | 0.7 mg/L (Average) | System-wide | 2 mg/L (MCL) | Within the limit |
| Nitrate | Inorganic chemicals | Not detected mg/L (Average) | System-wide | 10 mg/L (MCL) | Within the limit |
| Sulfate | Inorganic chemicals | 18 mg/L (Average) | SFPUC Water | 500 mg/L (MCL) | Within the limit |
| Aluminum | Metals | Not detected ug/L (Average) | SFPUC Water | 200 ug/L (MCL) | Within the limit |
| Boron | Metals | 0.04 mg/L (Average) | SFPUC Water | 1 mg/L (NL) | Within the limit |
| Calcium | Metals | 15 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Chromium, Hexavalent | Metals | 0.1 ug/L (Average) | System-wide | 10 ug/L (MCL) | Within the limit |
| Copper | Metals | Not detected mg/L (90th percentile) | At the tap | 1.3 mg/L (Action level) | None detected |
| Iron | Metals | 14 ug/L (Average) | SFPUC Water | 300 ug/L (MCL) | Within the limit |
| Lead | Metals | Not detected ug/L (90th percentile) | At the tap | 15 ug/L (Action level) | None detected |
| Lithium | Metals | Not detected mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Magnesium | Metals | 5.7 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Manganese | Metals | Not detected ug/L (Average) | SFPUC Water | 50 ug/L (MCL) | Within the limit |
| Silica | Metals | 7.5 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Sodium | Metals | 16 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Escherichia coli (E. coli) | Microbial | 0 (Maximum) | Monthly | 0 (MCL) | None detected |
| Giardia lamblia | Microbial | 0.02 (Average) | SFPUC Water | 0 (Public health goal) | Detected — no federal limit |
| Alkalinity | Physical & aggregate | 60 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Hardness | Physical & aggregate | 60 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| pH | Physical & aggregate | 8.9 (Average) | Distribution System-Wide | No federal limit | Detected — no federal limit |
| Specific Conductance | Physical & aggregate | 193 (Average) | SFPUC Water | 1600 (MCL) | Within the limit |
| TOC | Physical & aggregate | 1.5 mg/L (Average) | SFPUC Water | No federal limit | Detected — no federal limit |
| Total Dissolved Solids | Physical & aggregate | 102 mg/L (Average) | SFPUC Water | 1000 mg/L (MCL) | Within the limit |
| Turbidity | Physical & aggregate | 0.4 NTU (Maximum) | SFPUC Water | No federal limit | Detected — no federal limit |

## What these contaminants are

- **Chloramine** — A longer-lasting disinfectant made by combining chlorine with ammonia. Holds disinfection further into the pipe network, but is regulated under the same residual-disinfectant cap as chlorine.
- **Chlorate** — A byproduct that can form during disinfection, especially when hypochlorite solutions degrade. Has no enforceable federal limit but is on the EPA contaminant candidate list; high levels can affect the thyroid.
- **HAA5** — Haloacetic acids — a group of five disinfection byproducts formed when disinfectants react with organic matter. Long-term exposure above the federal limit is associated with an increased cancer risk.
- **TTHM** — Total trihalomethanes — a group of four chemicals (including chloroform) formed when chlorine reacts with natural organic matter. Long-term exposure above the federal limit is linked to liver, kidney, and central-nervous-system effects and increased cancer risk.
- **Bromide** — A naturally occurring salt found in source water. Not directly regulated, but a precursor that increases formation of brominated disinfection byproducts.
- **Chloride** — A naturally occurring salt compound. Regulated only as a secondary (cosmetic) standard; high levels cause a salty taste and can corrode pipes.
- **Fluoride** — A mineral often added to drinking water to help prevent tooth decay. Beneficial at low levels, but long-term exposure above the federal limit can cause bone disease and tooth mottling.
- **Nitrate** — A compound from fertilizer runoff, septic systems, and erosion of natural deposits. Levels above the federal limit can cause 'blue baby syndrome,' a serious oxygen-transport condition in infants.
- **Sulfate** — A naturally occurring mineral from rock and soil. No health-based federal limit; high levels can have a laxative effect and a bitter taste.
- **Aluminum** — A common element sometimes used as a treatment coagulant. Regulated only as a secondary (cosmetic) standard; high levels can discolor water.
- **Boron** — A naturally occurring element from rock and soil. No enforceable federal limit; the EPA has issued a health advisory level.
- **Calcium** — A naturally occurring mineral that contributes to water hardness. Not federally regulated for health; affects scaling and taste.
- **Chromium, Hexavalent** — Hexavalent chromium ('chromium-6') — the more toxic form of chromium. A known carcinogen by inhalation; regulated nationally only within the total-chromium limit, with stricter limits in some states.
- **Copper** — A metal that enters water from corroding household plumbing. Short-term exposure causes stomach distress; long-term exposure can damage the liver and kidneys.
- **Iron** — A naturally occurring metal common in groundwater. Regulated only as a secondary (cosmetic) standard; causes rusty color, staining, and metallic taste.
- **Lead** — A toxic metal that leaches into water from old service lines, solder, and plumbing fixtures. There is no safe level of lead; it harms brain development in children and raises blood pressure in adults. The EPA sets an action level, not a health goal above zero.
- **Lithium** — A naturally occurring element found in some groundwater. No enforceable federal limit; on the EPA contaminant candidate list for further study.
- **Magnesium** — A naturally occurring mineral that contributes to water hardness. Not federally regulated for health; affects scaling and taste.
- **Manganese** — A naturally occurring metal from soil and rock. No enforceable federal limit; high levels stain fixtures and laundry and can affect taste, with a health advisory for infants.
- **Silica** — A naturally occurring compound from sand and rock. Not federally regulated for health; relevant mainly for industrial water use.
- **Sodium** — A naturally occurring salt component. Not federally regulated for health; relevant for people on sodium-restricted diets.
- **Escherichia coli (E. coli)** — Escherichia coli — bacteria found in the gut of humans and animals. Its presence in drinking water indicates fecal contamination and a real risk of waterborne illness.
- **Alkalinity** — A measure of the water's capacity to neutralize acids. Not federally regulated for health; relevant to corrosion control and treatment.
- **Hardness** — A measure of dissolved calcium and magnesium minerals. Not federally regulated for health; affects scaling, soap use, and taste.
- **pH** — A measure of how acidic or basic the water is. Regulated only as a secondary standard; very low or high pH can corrode pipes or affect taste.
- **Specific Conductance** — A measure of how well water conducts electricity, which tracks dissolved mineral content. Not federally regulated for health; used as a proxy for total dissolved solids.
- **TOC** — Total organic carbon — a measure of organic material dissolved in the water. Not harmful itself, but it is the raw material that forms disinfection byproducts; removal is a treatment requirement.
- **Total Dissolved Solids** — Total dissolved solids — the combined content of all dissolved minerals and salts. Regulated only as a secondary (cosmetic) standard; high levels affect taste and hardness.
- **Turbidity** — A measure of cloudiness from suspended particles in the water. High turbidity can shelter microbes from disinfection; the EPA enforces it through a treatment-technique standard.

## How to read this

- A water-quality report covers an entire service area, not a single address.
- 'Federal limit' is the EPA standard (MCL, action level, treatment technique, etc.) that the measured level is compared against.
- 'At or above the federal limit' means the utility's own reported figure met or exceeded that standard.

_Figures are the utility's own published numbers. Generated 2026-05-25 from thewatermap.com._