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County water report

Mariposa County Water Report

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

Water grade

F

Water score

25.0

State rank

#51

of 58

Health violations

6

EPA SDWIS, 5-year lookback

Watershed impaired

15.0%

40 water bodies assessed

Monitoring sites

8

2,206 recent measurements

Live streamflow

312%

MERCED R A POHONO BRIDGE NR YOSEMITE CA

Water at a glance

Key Water Indicators for Mariposa County

EPA SDWIS

Safety Grade

F

Score: 25.0 / 100

EPA SDWIS

Active Violations

6

5-year health-based lookback

EPA ATTAINS

Watershed Health

15% impaired

40 bodies assessed

USGS NWIS

Streamflow Snapshot

312% of mean

MERCED R A POHONO BRIDGE NR YOSEMITE CA

EPA WQP

Monitoring Sites

8

2,206 recent readings

Source: EPA SDWIS · Safe Drinking Water Information System

Drinking Water Compliance

Compliance grade

F

Based on EPA SDWIS compliance history.

Water score

Higher scores indicate cleaner recent compliance records.

25.0/100

Health violations

6

Health-based violations

Violations per 100K served

90.7

Population-normalized SDWIS rate

Editorial analysis

Understanding Mariposa County’s Water

Drinking Water Quality Overview

EPA SDWIS

Mariposa County's water systems carry a failing grade, scoring 25.0 out of 100. Over the past five years, EPA SDWIS records 6 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 15.0% of assessed waterways carry an impairment designation (6 of 40 water bodies) across Mariposa County's watersheds. The leading impairment causes are mercury and alkalinity. 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 2026-05-14T12:00:00.000-07:00) puts MERCED R A POHONO BRIDGE at 2.0k cfs — running significantly above its long-term average at 312% of mean flow. 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. Mariposa County has limited coverage with 8 active monitoring sites with 2,206 recent measurements on record. Predominant monitoring categories include physical 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 Mariposa County

Water Verdict

Mariposa County, California receives a poor water quality assessment with a grade of F and a score of 25.0 out of 100. The water supply has documented quality issues. Residents are strongly encouraged to use filtered or bottled water for drinking and to stay informed about utility improvement plans. Separately, EPA ATTAINS assessments cover 40 water bodies in the county's watershed, with 15.0% 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 running above its historical mean at 312.0%.

Violation Context

EPA SDWIS records for Mariposa County, California show 6 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 90.7 violations per 100,000 people served, this rate is high and signals significant water quality management issues. For independent context, 2,206 water-quality monitoring results have been recorded across the county in the EPA Water Quality Portal over the past five years.

Consumer Guidance

Drinking-water compliance in Mariposa County is rated Grade F, reflecting significant health-based violations in the recent reporting period. Mariposa County's drinking-water compliance score is 25.0 out of 100. The violation rate for Mariposa County is 90.7 per 100,000 people served. An NSF 53 or NSF 58-certified filter is recommended for drinking and cooking water. Check the Consumer Confidence Report from your utility to identify the specific contaminants and required corrective actions — utilities are legally required to notify customers of violations. Mercury is the leading impairment cause in Mariposa County's watershed. There are 8 active water-quality monitoring sites in Mariposa County. A pipeline streamflow snapshot from the MERCED R A POHONO BRIDGE gauge is also available on this page.

Regional Context

Mariposa County has poorer water quality than the average county in California. Its water score is 19.6 points lower than the state average, suggesting more challenges with contamination control or infrastructure than 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 Mariposa County's water environment

Watershed Impairment Causes (EPA ATTAINS)

  • 1

    Mercury

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

  • 2

    Alkalinity

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

  • 3

    Toxicity

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

Source: EPA ATTAINS · Reporting cycle 2022

Official EPA Resources for Mariposa County

EPA How's My Waterway

Interactive watershed health data for Mariposa County

EPA ECHO: Drinking Water Systems

Search water utilities and violation records for Mariposa County

Consumer Confidence Reports (CA)

Annual water quality reports from utilities in California

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

15.0%

6 of 40 assessed

Some impairment

Top Impairment Causes

  • 1

    MERCURY

  • 2

    ALKALINITY

  • 3

    TOXICITY

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

8

Active in the past 5 years

Measurements Recorded

2.2K

2,206 total readings

Most Measured

  • Physical
  • Nutrient
  • Inorganics, Major, Metals

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

1,970cfs

May 14, 7:00 PM UTC

vs Long-Term Average

312%

Well above typical

Primary Streamgage

MERCED R A POHONO BRIDGE NR YOSEMITE CA

USGS site
11266500
Drainage area
321 sq mi
Long-term mean
631 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 Mariposa 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 Mariposa County, California?
Mariposa County, California has a drinking-water quality grade of F with a score of 25.0/100, based on EPA SDWIS compliance data. The county has 6 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 Mariposa County?
Mariposa County has 6 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 Mariposa County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 15.0% of Mariposa County's 40 assessed water bodies are classified as impaired (6 impaired). The top reported causes are MERCURY, ALKALINITY, TOXICITY. 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 Mariposa County?
EPA's Water Quality Portal records 2,206 measurements from 8 monitoring sites in Mariposa County over the past five years. The most frequently measured characteristic groups are Physical, Nutrient, Inorganics, Major, Metals. 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 Mariposa County right now?
Mariposa County's primary USGS streamgage on the MERCED R A POHONO BRIDGE has a pipeline snapshot of 1,970 cubic feet per second — 312% of the long-term mean of 631.33 cfs. This is well above typical — often a signal of recent precipitation or storm runoff. For the latest gauge feed, visit waterdata.usgs.gov.
How does Mariposa County water compare to the California average?
Mariposa County's SDWIS water quality score of 25.0/100 is lower 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 Mariposa County?
Based on EPA SDWIS data, Mariposa County has a water quality grade of F (25.0/100). This indicates below-average compliance with significant violations. Residents may want to consider home water filtration or independent testing. 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 Mariposa County have so many water violations?
Mariposa County has 6 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 Mariposa County rank for water quality in California?
Mariposa County ranks #51 out of 58 counties in California by SDWIS water quality score (1 = best). With a score of 25.0/100, it falls in the bottom 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

Sierra County

California

F

26.1

Water score

Tulare County

California

F

23.4

Water score

Inyo County

California

F

27.5

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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