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Community Water System · PWS CA5010039

Western Hills Water District/Diablo Gran

Patterson, California 95363 - drinking water served from surface water sources to 2,000 people, tracked in EPA SDWIS and UCMR5.

2,000
People served
87
EPA violations
62
Health-based
Untested
UCMR5 result

The verdict

EPA records 87 drinking-water violations at Western Hills Water District/Diablo Gran, 71% of them health-based - above the California per-system average.

87
Total EPA violations on record
71%
Health-based (MCL / treatment failure)
2,000
People served by this system
N/A
PFAS compounds detected (UCMR5)

Water Quality Snapshot: Western Hills Water District/Diablo Gran

Western Hills Water District/Diablo Gran is a local-owned community water system that delivers drinking water to 2,000 residents in Patterson, California (Stanislaus County) through 596 service connections. Its water is drawn from surface water sources. EPA's Safe Drinking Water Information System records 87 total violations for this system , of which 62 (71%) are health-based, meaning a contaminant exceeded an EPA Maximum Contaminant Level or a required treatment technique failed. A further 19 monitoring and reporting violations are on file. The most recent violation on record dates to 2018.

The most frequently cited contaminant at this system is TTHM, recorded in 46 violations (MCL, health-based). This system has not yet been sampled under EPA's UCMR5 PFAS monitoring program, so no PFAS detection data is available here.

Across California, EPA tracks 7,249 public water systems serving 42,404,883 people, with 153,308 cumulative violations and 63,983 health-based violations on record. About 89% of systems in the state carry at least one violation, and state-wide the average per system is 21.1 violations. Statewide, 447 of 694 UCMR5-tested systems have reported PFAS detections (64.4%).

Western Hills Water District/Diablo Gran's 87 violations sit above the California average.

All figures above are sourced directly from EPA SDWIS and UCMR5 public data releases and are updated as EPA publishes new reporting cycles.

Population Served
2,000
Total Violations
87
Health-Based Violations
62
Water Source
Surface Water

System Details

System Type
Community
Owner Type
Local
Connections
596
County
Stanislaus
School/Daycare
No
MCL Violations
60
Monitoring Violations
19
Treatment Tech Violations
2

Violation History

Contaminant violations recorded by EPA.

Contaminant Category Count Latest
TTHM MCL 46 2017
Total Haloacetic Acids (HAA5) MCL 14 2006
Coliform (TCR) MR 7 2008
Lead and Copper Rule MR 6 2018
Surface Water Treatment Rule MR 4 2008
Consumer Confidence Rule Other 3 2009
CARBON, TOTAL MR 2 2009
Surface Water Treatment Rule TT 2 2006

Verify This Water System

The figures above are aggregated from EPA's public databases. To verify the underlying records, or to file a complaint, request a Consumer Confidence Report, or check current monitoring status, go directly to the federal and state regulators that enforce the Safe Drinking Water Act for Western Hills Water District/Diablo Gran.

Federal Source of Truth

EPA SDWIS, Federal Reports

EPA's Safe Drinking Water Information System (SDWIS) holds the federal compliance record for every regulated public water system. Open the system-level report by PWS ID:

View PWS ID CA5010039 on SDWIS

Source: EPA SDWIS Federal Reports Search

State Regulator

California Drinking Water Authority

California State Water Resources Control Board, Division of Drinking Water is the primacy agency that licenses and inspects Western Hills Water District/Diablo Gran under EPA-delegated authority.

Open CA regulator portal

Source: California State Water Resources Control Board, Division of Drinking Water

Violation Timeline

Each row links to the EPA SDWIS public record for verification. Cross-reference the contaminant code on EPA's federal report to see violation dates, return-to-compliance status, and enforcement actions.

Year (latest) Contaminant Category Count EPA Record
2018 Lead and Copper Rule MR 6 SDWIS / CA5010039 / 5000
2017 TTHM MCL 46 SDWIS / CA5010039 / 2950
2009 Consumer Confidence Rule Other 3 SDWIS / CA5010039 / 7000
2009 CARBON, TOTAL MR 2 SDWIS / CA5010039 / 2920
2008 Coliform (TCR) MR 7 SDWIS / CA5010039 / 3100
2008 Surface Water Treatment Rule MR 4 SDWIS / CA5010039 / 0200
2006 Total Haloacetic Acids (HAA5) MCL 14 SDWIS / CA5010039 / 2456
2006 Surface Water Treatment Rule TT 2 SDWIS / CA5010039 / 0200

How Western Hills Water District/Diablo Gran Compares

Cross-reference this system's record against state averages and the federal MCL framework for context.

Metric Western Hills Water District/Diablo Gran California avg Federal benchmark
Total violations 87 21.1 SDWA compliance, any non-zero count is a recorded breach
Health-based violations 62 8.8 Indicates a contaminant exceeded a federal MCL
PFAS detection None 64.4% EPA final rule (2024): PFOA/PFOS MCL = 4.0 ppt
Population served 2,000 5,850 Sizing context for compliance burden

Sources: EPA SDWIS and EPA National Primary Drinking Water Regulations (40 CFR Part 141). State averages computed across 7,249 regulated public water systems in California.

Federal MCL reference, Safe Drinking Water Act thresholds ▼
Contaminant Federal MCL / Action Level Note
Lead 0 mg/L (Action Level: 0.015 mg/L) Lead and Copper Rule treatment technique
Arsenic 0.010 mg/L (10 ppb) Health-based MCL since 2006
Total Coliform Treatment technique (RTCR) Indicator organism, monitoring trigger
PFOA / PFOS (PFAS) 4.0 ppt each (final 2024 rule) Compliance deadline 2029 (EPA proposed extending to 2031, not yet final as of May 2026)
Nitrate (as N) 10 mg/L Acute health risk for infants

Frequently Asked Questions

Is Western Hills Water District/Diablo Gran water safe to drink? ▼
Western Hills Water District/Diablo Gran (PWS ID: CA5010039) has 87 recorded violations in the EPA SDWIS database. No PFAS contamination was detected in UCMR5 testing. This system serves 2,000 people using Surface Water sources.
How many people does Western Hills Water District/Diablo Gran serve? ▼
Western Hills Water District/Diablo Gran serves 2,000 people in Patterson, California. It is a Local-owned system using Surface Water water sources with 596 service connections.
What type of violations does Western Hills Water District/Diablo Gran have? ▼
Western Hills Water District/Diablo Gran has 87 total violations: 62 health-based violations (MCL exceedances or treatment failures), 19 monitoring/reporting violations, and 2 treatment technique violations. Health-based violations indicate contaminant levels exceeded EPA safe limits.
Has PFAS been detected in Western Hills Water District/Diablo Gran water? ▼
No PFAS testing data is available for Western Hills Water District/Diablo Gran under the EPA's UCMR5 monitoring program.
What water source does Western Hills Water District/Diablo Gran use? ▼
Western Hills Water District/Diablo Gran uses Surface Water as its primary water source. It is classified as a Community Water System (CWS), serving residential populations year-round.
Where does this data come from? ▼
All data comes from the EPA Safe Drinking Water Information System (SDWIS) and the UCMR5 PFAS monitoring program. SDWIS tracks compliance for all public water systems regulated under the Safe Drinking Water Act.
Data Sources: EPA Safe Drinking Water Information System (SDWIS), Q4 2025. This data is provided for informational purposes only.

Read our methodology - how this data is sourced, computed, and verified.

Data sourced from EPA SDWIS and UCMR5 PFAS monitoring data. See our methodology for details. Retrieved and formatted by PlainWater Editorial

Every figure on PlainWater is rendered directly from EPA SDWIS drinking-water data, no number is typed in by an editor. This system's figures are rendered directly from EPA SDWIS data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.