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County water report

San Joaquin County Water Report

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

Water grade

D

Water score

45.0

State rank

#31

of 58

Health violations

179

EPA SDWIS, 5-year lookback

Watershed impaired

47.7%

109 water bodies assessed

Monitoring sites

203

97,925 recent measurements

Live streamflow

35%

SAN JOAQUIN R NR VERNALIS CA

Water at a glance

Key Water Indicators for San Joaquin County

EPA SDWIS

Safety Grade

D

Score: 45.0 / 100

EPA SDWIS

Active Violations

179

5-year health-based lookback

EPA ATTAINS

Watershed Health

48% impaired

109 bodies assessed

USGS NWIS

Streamflow Snapshot

35% of mean

SAN JOAQUIN R NR VERNALIS CA

EPA WQP

Monitoring Sites

203

97,925 recent readings

Source: EPA SDWIS · Safe Drinking Water Information System

Drinking Water Compliance

Compliance grade

D

Based on EPA SDWIS compliance history.

Water score

Higher scores indicate cleaner recent compliance records.

45.0/100

Health violations

179

Health-based violations

Violations per 100K served

24.1

Population-normalized SDWIS rate

Editorial analysis

Understanding San Joaquin County’s Water

Drinking Water Quality Overview

EPA SDWIS

San Joaquin County's drinking water received a D grade, scoring 45.0 out of 100. Over the past five years, EPA SDWIS records 179 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 substantial 47.7% of assessed waterways are impaired (52 of 109 water bodies) across San Joaquin County's watersheds. The leading impairment causes are dissolved oxygen and pathogens. 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 2026-05-14T11:45:00.000-07:00) puts SAN JOAQUIN R at 1.6k cfs — well below its long-term average at 35% 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 Joaquin County has extensive coverage with 203 active monitoring sites with 97,925 recent measurements on record. Predominant monitoring categories include organics, pesticide and physical. 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 Joaquin County

Water Verdict

San Joaquin County, California receives a below-average water quality assessment with a grade of D and a score of 45.0 out of 100. Residents should review their utility's Consumer Confidence Report and may want to consider additional water filtration for drinking. Separately, EPA ATTAINS assessments cover 109 water bodies in the county's watershed, with 47.7% 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 35.4% — worth watching in drier seasons.

Violation Context

EPA SDWIS records for San Joaquin County, California show 179 health-based violations, indicating multiple instances where federal contaminant limits or treatment requirements were not met. At 24.1 violations per 100,000 people served, this rate is high and signals significant water quality management issues. This compliance record sits alongside an unusually deep monitoring archive: 97,925 water-quality results have been logged across the county in the EPA Water Quality Portal over the past five years.

Consumer Guidance

San Joaquin County's drinking-water compliance is below average with a Grade D, indicating repeated or unresolved violations in the recent record. San Joaquin County's drinking-water compliance score is 45.0 out of 100. The violation rate for San Joaquin County is 24.1 per 100,000 people served. Residents are encouraged to use an NSF 53 or NSF 58-certified filter for drinking and cooking water until the underlying violations are resolved. Running tap water for 30 seconds before use and avoiding older lead-pipe connections can also reduce exposure risk. The current Consumer Confidence Report from your utility will specify the contaminants of concern. Dissolved Oxygen is the leading impairment cause in San Joaquin County's watershed. With 203 active water-quality monitoring sites in San Joaquin County, data coverage is strong. A pipeline streamflow snapshot from the SAN JOAQUIN R gauge is also available on this page.

Regional Context

San Joaquin County has water quality close to the average county in California. Its water score is within 0.4 points of the state average, meaning its overall water system performance is broadly representative of California as a whole.

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 Joaquin County's water environment

Watershed Impairment Causes (EPA ATTAINS)

  • 1

    Low dissolved oxygen

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

  • 2

    Pathogens

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

  • 3

    Chlorpyrifos

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

Source: EPA ATTAINS · Reporting cycle 2022

Official EPA Resources for San Joaquin County

EPA How's My Waterway

Interactive watershed health data for San Joaquin County

EPA ECHO: Drinking Water Systems

Search water utilities and violation records for San Joaquin County

Consumer Confidence Reports (CA)

Annual water quality reports from utilities in California

Clean Water Act §303(d)

Watershed Health

Impaired Water Bodies

47.7%

52 of 109 assessed

Moderate concern

Top Impairment Causes

  • 1

    DISSOLVED OXYGEN

  • 2

    PATHOGENS

  • 3

    CHLORPYRIFOS

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

203

Active in the past 5 years

Measurements Recorded

98K

97,925 total readings

Most Measured

  • Organics, Pesticide
  • Physical
  • Nutrient

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

1,550cfs

May 14, 6:45 PM UTC

vs Long-Term Average

35%

Well below typical

Primary Streamgage

SAN JOAQUIN R NR VERNALIS CA

USGS site
11303500
Drainage area
13,539 sq mi
Long-term mean
4,382 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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Estimate Your Water Costs

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 Joaquin 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 Joaquin County, California?
San Joaquin County, California has a drinking-water quality grade of D with a score of 45.0/100, based on EPA SDWIS compliance data. The county has 179 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 Joaquin County?
San Joaquin County has 179 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 Joaquin County?
EPA ATTAINS assessments under Clean Water Act §303(d) indicate 47.7% of San Joaquin County's 109 assessed water bodies are classified as impaired (52 impaired). The top reported causes are DISSOLVED OXYGEN, PATHOGENS, CHLORPYRIFOS. 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 Joaquin County?
EPA's Water Quality Portal records 97,925 measurements from 203 monitoring sites in San Joaquin County over the past five years. The most frequently measured characteristic groups are Organics, Pesticide, Physical, Nutrient. 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 Joaquin County right now?
San Joaquin County's primary USGS streamgage on the SAN JOAQUIN R has a pipeline snapshot of 1,550 cubic feet per second — 35% of the long-term mean of 4,382.04 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 Joaquin County water compare to the California average?
San Joaquin County's SDWIS water quality score of 45.0/100 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 Joaquin County?
Based on EPA SDWIS data, San Joaquin County has a water quality grade of D (45.0/100). 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 San Joaquin County have so many water violations?
San Joaquin County has 179 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 Joaquin County rank for water quality in California?
San Joaquin County ranks #31 out of 58 counties in California by SDWIS water quality score (1 = best). With a score of 45.0/100, it falls in the middle 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

Sonoma County

California

D

45.1

Water score

Yolo County

California

D

45.6

Water score

Merced County

California

D

43.2

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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