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County water report

San Mateo County Water Report

Drinking-water compliance, watershed health, monitoring records, and river conditions for San Mateo County, California.

Water grade

B

Water score

67.1

State rank

#7

of 58

Health violations

19

EPA SDWIS, 5-year lookback

Watershed impaired

23.8%

105 water bodies assessed

Monitoring sites

71

32,510 recent measurements

Live streamflow

38%

SAN GREGORIO C A SAN GREGORIO CA

Water at a glance

Key Water Indicators for San Mateo County

EPA SDWIS

Safety Grade

B

Score: 67.1 / 100

EPA SDWIS

Active Violations

19

5-year health-based lookback

EPA ATTAINS

Watershed Health

24% impaired

105 bodies assessed

USGS NWIS

Streamflow Snapshot

38% of mean

SAN GREGORIO C A SAN GREGORIO CA

EPA WQP

Monitoring Sites

71

32,510 recent readings

Source: EPA SDWIS · Safe Drinking Water Information System

Drinking Water Compliance

Compliance grade

B

Based on EPA SDWIS compliance history.

Water score

Higher scores indicate cleaner recent compliance records.

67.1/100

Health violations

19

Health-based violations

Violations per 100K served

2.4

Population-normalized SDWIS rate

Editorial analysis

Understanding San Mateo County’s Water

Drinking Water Quality Overview

EPA SDWIS

San Mateo County earns a B grade for drinking water quality, scoring 67.1 out of 100. Over the past five years, EPA SDWIS records 19 health-based violations — a pattern that public water utilities are required to disclose and correct.

Watershed Conditions

EPA ATTAINS

Under the Clean Water Act §303(d), EPA ATTAINS tracks whether waterways meet quality standards for drinking, recreation, and aquatic life (reporting cycle: 2022). A notable 23.8% of assessed waterways carry an impairment designation (25 of 105 water bodies) across San Mateo County's watersheds. The leading impairment causes are pathogens and mercury. Impairment does not mean tap water is unsafe — it measures ambient waterway conditions upstream of treatment, not finished drinking water.

River & Streamflow Status

USGS NWIS

USGS NWIS gauge data (as of 2026-05-14T11:15:00.000-07:00) puts SAN GREGORIO C A SAN GREGORIO CA at 13.1 cfs — well below its long-term average at 38% of mean — low-flow conditions worth noting for water-dependent ecosystems. Streamflow is a leading indicator of drought stress, sediment load, and dilution capacity: low flows concentrate pollutants and warm water temperatures, stressing aquatic life and, in surface-water-dependent systems, the source water quality for treatment plants.

Monitoring Network

EPA WQP

EPA's Water Quality Portal (WQP) aggregates monitoring data from federal, state, and tribal agencies. San Mateo County has extensive coverage with 71 active monitoring sites with 32,510 recent measurements on record. Predominant monitoring categories include microbiological and nutrient. More monitoring sites generally indicate greater scientific attention to local water conditions — and provide the baseline data that regulators use to set future impairment listings.

Editorial advisory

What the data suggests for San Mateo County

Water Verdict

San Mateo County, California receives a fair water quality assessment with a grade of B and a score of 67.1 out of 100. The water supply meets baseline federal standards, but there may be periods of elevated contaminant levels or infrastructure concerns worth monitoring. Separately, EPA ATTAINS assessments cover 105 water bodies in the county's watershed, with 23.8% listed as impaired under the Clean Water Act — a source-water measure that is tracked independently of tap-water compliance. At the most recent USGS reading, local streamflow was below its historical mean at 38.1% — worth watching in drier seasons.

Violation Context

EPA SDWIS records for San Mateo County, California show 19 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 2.4 violations per 100,000 people served, this rate is relatively low compared to many U.S. counties. This compliance record sits alongside an unusually deep monitoring archive: 32,510 water-quality results have been logged across the county in the EPA Water Quality Portal over the past five years.

Consumer Guidance

Tap water in San Mateo County meets baseline safety standards, though the compliance record shows some violations worth watching. San Mateo County's drinking-water compliance score is 67.1 out of 100. The violation rate for San Mateo County is 2.4 per 100,000 people served. Running tap water for 30 seconds before drinking can reduce any localized lead exposure from household plumbing. Requesting your utility's most recent Consumer Confidence Report is the fastest way to identify which specific contaminants were flagged. Pathogens is the leading impairment cause in San Mateo County's watershed. With 71 active water-quality monitoring sites in San Mateo County, data coverage is strong. A pipeline streamflow snapshot from the SAN GREGORIO C A SAN GREGORIO CA gauge is also available on this page.

Regional Context

San Mateo County has better water quality than the average county in California. Its water score is 22.5 points higher than the state average, indicating stronger water system performance relative to neighboring counties.

Advisory text summarizes county-level public records and is not a replacement for your utility's current Consumer Confidence Report or direct local notices.

Contaminants & Resources

Key issues flagged in San Mateo County's water environment

Watershed Impairment Causes (EPA ATTAINS)

  • 1

    Pathogens

    Impairment cause per EPA Clean Water Act §303(d) assessment

  • 2

    Mercury

    Impairment cause per EPA Clean Water Act §303(d) assessment

  • 3

    Trash

    Impairment cause per EPA Clean Water Act §303(d) assessment

Source: EPA ATTAINS · Reporting cycle 2022

Official EPA Resources for San Mateo County

EPA How's My Waterway

Interactive watershed health data for San Mateo County

EPA ECHO: Drinking Water Systems

Search water utilities and violation records for San Mateo County

Consumer Confidence Reports (CA)

Annual water quality reports from utilities in California

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

23.8%

25 of 105 assessed

Some impairment

Top Impairment Causes

  • 1

    PATHOGENS

  • 2

    MERCURY

  • 3

    TRASH

Source: EPA ATTAINS · Reporting cycle 2022

Impairment is determined under the Clean Water Act §303(d): a water body is impaired when it fails to meet state-defined quality standards for designated uses (drinking, recreation, aquatic life). Assessment coverage varies by state; counties without assessed water bodies are not shown.

Past 5 years

Water Quality Monitoring

Monitoring Sites

71

Active in the past 5 years

Measurements Recorded

33K

32,510 total readings

Most Measured

  • Microbiological
  • Nutrient
  • Physical

Categories measured most frequently

Data from the EPA Water Quality Portal (WQP), aggregating monitoring records from federal, state, and tribal sources. Each measurement represents a single sample analyzed for a specific characteristic (e.g., E. coli, pH, dissolved oxygen, nitrogen).

Live USGS Streamgage

River & Stream Conditions

Current Discharge

13.1cfs

May 14, 6:15 PM UTC

vs Long-Term Average

38%

Well below typical

Primary Streamgage

SAN GREGORIO C A SAN GREGORIO CA

USGS site
11162570
Drainage area
50.9 sq mi
Long-term mean
34.4 cfs

One representative streamgage (the one with the largest drainage area in the county). Many counties have multiple gauges; this view summarizes the primary one. The long-term mean is the full-record annual average; the percent-of-typical value compares the latest reading against that average.

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Water Cost Estimate

3

3 people  ·  ~225 gal/day

Annual Total

$558

Monthly

$47

Water Bill

$558/yr

Filter Cost

$0/yr

Safety Grade for San Mateo County:C — Moderate

Some violations or watershed impairment detected.

Estimates use the national average residential water rate ($0.0068/gal, EPA/AWWA 2023) and EPA WaterSense per-person consumption baseline (75 gal/person/day). Actual bills vary by utility, usage tier, and local infrastructure fees. For informational purposes only.

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Frequently Asked Questions

What is the water quality in San Mateo County, California?
San Mateo County, California has a drinking-water quality grade of B with a score of 67.1/100, based on EPA SDWIS compliance data. The county has 19 health-based drinking water violations over the past 5 years. Watershed health, monitoring records, and streamflow snapshots are reported separately on this page.
Are there any water violations in San Mateo County?
San Mateo County has 19 health-based drinking water violations recorded by the EPA over the past 5 years. Health-based violations indicate instances where contaminant levels exceeded EPA Maximum Contaminant Levels (MCLs). Violations may have been resolved — check with your local water utility for current status.
How healthy are the watersheds in San Mateo County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 23.8% of San Mateo County's 105 assessed water bodies are classified as impaired (25 impaired). The top reported causes are PATHOGENS, MERCURY, TRASH. Impairment means the water body fails to meet state quality standards for at least one designated use — drinking water source, recreation, aquatic life, or fish consumption. Note: watershed impairment doesn't always translate to tap-water issues; treatment plants can remove most regulated contaminants.
How much water-quality monitoring happens in San Mateo County?
EPA's Water Quality Portal records 32,510 measurements from 71 monitoring sites in San Mateo County over the past five years. The most frequently measured characteristic groups are Microbiological, Nutrient, Physical. Each measurement is a single sample analyzed for one characteristic (E. coli, pH, dissolved oxygen, etc.). High monitoring density means more scientific evidence behind any reported signal — it does not by itself indicate water quality.
What's happening with rivers in San Mateo County right now?
San Mateo County's primary USGS streamgage on the SAN GREGORIO C A SAN GREGORIO CA has a pipeline snapshot of 13.1 cubic feet per second — 38% of the long-term mean of 34.43 cfs. This is well below typical — often a signal of drought stress on source water. For the latest gauge feed, visit waterdata.usgs.gov.
How does San Mateo County water compare to the California average?
San Mateo County's SDWIS water quality score of 67.1/100 is higher than the California state average of 44.6. The average water quality grade across California is D, based on data from 58 counties with available SDWIS data.
Is tap water safe to drink in San Mateo County?
Based on EPA SDWIS data, San Mateo County has a water quality grade of B (67.1/100). This indicates good to excellent water quality with strong SDWIS compliance. The grade speaks to the public water system, not the watershed — for watershed-level concerns, see the Watershed Health zone. For the most up-to-date information, contact your local water utility or review your Consumer Confidence Report (CCR).
Why does San Mateo County have so many water violations?
San Mateo County has 19 health-based drinking water violations on record from the EPA SDWIS database. A higher violation count can result from aging infrastructure, underfunded water utilities, agricultural runoff contamination, or industrial pollution. Counties with more water systems may also see more violations simply due to scale. Residents concerned about water quality should consider independent water testing and home filtration systems.
How does San Mateo County rank for water quality in California?
San Mateo County ranks #7 out of 58 counties in California by SDWIS water quality score (1 = best). With a score of 67.1/100, it falls in the top third of counties statewide. The ranking reflects EPA SDWIS compliance only — not watershed impairment, monitoring density, or streamflow, which are tracked separately on this page.

Counties with Similar Water Quality

Marin County

California

B

66.8

Water score

Santa Clara County

California

B

67.5

Water score

Los Angeles County

California

B

66.5

Water score

Data Sources

Drinking-water compliance data from the EPA Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database. Scores reflect compliance history and health-based violation counts.

Watershed health and impaired-waterway data from the EPA ATTAINS Clean Water Act §303(d) assessments, state-reported and EPA-finalized.

Water-quality monitoring counts from the EPA Water Quality Portal (WQP), federated USGS, EPA, and state agency sampling records over a rolling 5-year window.

Live streamflow from the USGS National Water Information System (NWIS), continuous discharge measurements from the largest-drainage gauge in each county, compared against the full-record long-term annual mean.

Disclaimer: This data is informational only. It is not health, legal, or professional advice. For concerns about your specific water supply, contact your local water utility.

By Evan Brooks, Data EditorUpdated Reviewed by Evan Brooks, Data Editor

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