Western Hills Water District/Diablo Gran
What EPA’s and California’s records show in this utility’s water: nitrate, arsenic and other regulated chemicals; lead and copper; and violations since 2016, with the certification that handles each result.
- Serves
- 2,000 people
- Connections
- 596
- Water source
- Surface water
- Based in
- Patterson, CA
- County
- Stanislaus
- Owner
- Local government
- Hardness
- Moderately hard
The ZIP code it serves
The short answer
1 result is above the health goal. The worst: Arsenic, above the health goal. 4 health-based violations since 2016.
What handles itWhat was found
Arsenic, total trihalomethanes (TTHM), haloacetic acids (HAA5), nitrate, fluoride and chromium (total): California’s own results, samples taken from Jan. 2, 2020 to July 8, 2026, with EPA’s 2012 to 2019 file for earlier years.
Above the health goal1
Arsenic
Comes from the source
2.5ppb
Highest yearly average, 2020 to 2025
2025 average: 1.2 ppb
Averaged by year, the highest was 2.5 ppb, in 2020: below EPA’s legal limit of 10 ppb, but above its health goal of zero. Its highest single sample was 2.7 ppb. In 2025, the latest year on file, its two samples averaged 1.2 ppb. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 3.5 ppb, in 2014.
What handles it
- NSF/ANSI 58 look for “Arsenic (Pentavalent)<=50 ppb Reduction”
- NSF/ANSI 53 look for “Arsenic (Pentavalent)<=50 ppb Reduction”
Below EPA’s limits4
Total trihalomethanes (TTHM)
Forms or is added in treatment
60.3ppb
Highest yearly average, 2020 to 2026
2026 average: 24.8 ppb
Averaged by year, the highest was 60.3 ppb, in 2022, below EPA’s legal limit of 80 ppb. Its highest single sample was 140 ppb. In 2026, the latest year on file, its six samples averaged 24.8 ppb. EPA recorded no violation for total trihalomethanes (TTHM) at this utility since 2020. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 123 ppb, in 2015, and EPA recorded violations in 2014, 2015, 2016 and 2017.
What handles it
- NSF/ANSI 53 look for “Trihalomethanes (TTHM) Reduction”
- NSF/ANSI 58 look for “Trihalomethanes (TTHM) Reduction”
Haloacetic acids (HAA5)
Forms or is added in treatment
41.5ppb
Highest yearly average, 2020 to 2026
2026 average: 10.3 ppb
Averaged by year, the highest was 41.5 ppb, in 2023, below EPA’s legal limit of 60 ppb. Its highest single sample was 79 ppb. In 2026, the latest year on file, its six samples averaged 10.3 ppb. EPA recorded no violation for haloacetic acids (HAA5) at this utility since 2020. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 34.3 ppb, in 2016.
What handles it
No certification claim covers this compound yet.
Nitrate
Comes from the source
2.2ppm
Highest sample; found in 176 of 335
2026 average: 0.41 ppm
The highest sample, in 2023, was 2.2 ppm, below EPA’s legal limit of 10 ppm. In 2026, the latest year on file, its 26 samples averaged 0.41 ppm. Earlier, from 2012 to 2019, EPA’s file shows a highest sample of 0.81 ppm, in 2012.
What handles it
- NSF/ANSI 58 look for “Nitrate/Nitrite Reduction”
- NSF/ANSI 53 look for “Nitrate/Nitrite (30 ppm challenge)”
Fluoride
Forms or is added in treatment
0.11ppm
Highest yearly average, 2020 to 2025
2025: not found
Averaged by year, the highest was 0.11 ppm, in 2021, below EPA’s legal limit of 4.0 ppm. Its highest single sample was 0.12 ppm. In 2025, the latest year on file, it wasn’t found in any of its two samples. Earlier, from 2012 to 2019, EPA’s file shows a highest yearly average of 0.12 ppm, in 2012.
What handles it
- NSF/ANSI 58 look for “Fluoride Reduction”
- NSF/ANSI 53 look for “Fluoride”
2 compounds tested and not foundBelow the reporting level in every sample. Not found is not the same as zero.
- Chromium (total)
- Lead
What handles it
A certification covers specific claims, not a whole standard. Look for these claims on the certifier’s listing, not the box. Wording varies a little: NSF writes “Lead Reduction,” WQA just “Lead.”
- NSF/ANSI 53
Claim: Arsenic (Pentavalent)<=50 ppb Reduction
For: Arsenic
Other certifiers’ wording: Arsenic (III) (50 ppb influent level)
- NSF/ANSI 58
Claim: Arsenic (Pentavalent)<=50 ppb Reduction
For: Arsenic
Other certifiers’ wording: Arsenic (Pentavalent)<=300 ppb Reduction; Arsenic (V) (300 ppb influent level)
At the tap, or for the whole house?
These come with the water itself, so a whole-house system certified for the same claims treats every tap and shower. A filter at the tap treats drinking water for much less. How whole-house systems compare.
Filters with those claims on a certifier’s listing
Disclosure: TapBuff earns a commission if you buy through some links on this page. It does not change which standard or filter we name. As an Amazon Associate, TapBuff earns from qualifying purchases. How TapBuff makes money
Countertop reverse osmosis
- IAPMO listing:NSF/ANSI 42NSF/ANSI 53NSF/ANSI 58NSF/ANSI 401checked Oct. 4, 2026
Certified for: Arsenic
Aquasana SmartFlow Reverse Osmosis (AQ-SFRO2)
Under-sink reverse osmosis
- WQA listing:NSF/ANSI 42NSF/ANSI 53NSF/ANSI 58NSF/ANSI 401checked Oct. 9, 2026
Certified for: Arsenic
Brondell Capella RC250 Reverse Osmosis System
Under-sink reverse osmosis
- WQA listing:NSF/ANSI 42NSF/ANSI 53NSF/ANSI 58NSF/ANSI 401checked Oct. 9, 2026
Certified for: Arsenic
Hardness
Moderately hard, typically 105 mg/L (6.1 grains per gallon), across 2 sampling points, 2019 to 2025.
Hardness is calcium and magnesium. EPA sets no limit for it: it shows up as scale on fixtures and in water heaters, and as soap that won’t lather. USGS calls water above 60 mg/L moderately hard, above 120 hard and above 180 very hard.
Violations since 2016
| Contaminant or rule | Type | Period | Measured | Status |
|---|---|---|---|---|
| TTHM | The running average was above the legal limit.EPA: Maximum Contaminant Level Violation, Average | Jan. 1, 2017 to March 31, 2017 | 0.086 MG/L | Resolved |
| TTHM | The running average was above the legal limit.EPA: Maximum Contaminant Level Violation, Average | Oct. 1, 2016 to Dec. 31, 2016 | 0.11 MG/L | Resolved |
| TTHM | The running average was above the legal limit.EPA: Maximum Contaminant Level Violation, Average | July 1, 2016 to Sept. 30, 2016 | 0.12 MG/L | Resolved |
| TTHM | The running average was above the legal limit.EPA: Maximum Contaminant Level Violation, Average | April 1, 2016 to June 30, 2016 | 0.14 MG/L | Resolved |
| Health-based violations only. Resolved and archived mean the utility returned to compliance or the state closed the record. 2 monitoring or reporting violations not shown. | ||||
What this report can’t tell you
Everything else. Arsenic, total trihalomethanes (TTHM), haloacetic acids (HAA5), nitrate, fluoride and chromium (total) results here are California’s own from 2020 on, with EPA’s national file for 2012 to 2019. Western Hills Water District/Diablo Gran’s own annual report, which it must send you every year, has the contaminants this page does not show.
Your own plumbing. Lead results are the 90th percentile across homes the utility samples. Your home’s service line, solder and fixtures decide yours. A certified lab test is the only way to know.
What it means for your health. This page reports what the records show and EPA’s limits and goals; it is not medical advice. If you are worried about a health effect, talk to a doctor or your local health department.
Anything after the latest data. SDWIS data is from 2026 Q3; UCMR 5 didn’t sample this utility; California’s results run from Jan. 2, 2020 to July 8, 2026.
Testing your own tap
The 3 labs California certifies closest to the area Western Hills Water District/Diablo Gran serves, in a straight line. Call first: certification is test by test, and not every lab takes samples from homeowners.
Certified Laboratories - Turlock
Turlock, CA · about 19 miles
Certified for bacteria
Institute of Environmental Health, Inc. dba IEH-JL Analytical
Modesto, CA · about 19 miles
Certified for bacteria, nitrate, minerals and other inorganics
Microbac Laboratories, Inc. - Turlock
Turlock, CA · about 20 miles
Certified for bacteria, nitrate, lead and copper, metals, minerals and other inorganics, organic chemicals, disinfection byproducts, radioactivity
Sources
- EPA UCMR 5 occurrence data, file of Oct. 15, 2026
- EPA SDWA bulk download (SDWIS federal data), 2026 Q3
- California State Water Resources Control Board, Division of Drinking Water, results from Jan. 1, 2020 to July 17, 2026, read Oct. 8, 2026
- EPA Six-Year Review 4 compliance monitoring data, 2012 to 2019, file of Feb. 5, 2024
- Certified labs: California State Water Resources Control Board, ELAP accredited laboratories and their fields of accreditation, read Oct. 17, 2026; distances from the Census 2020 ZIP code tabulation area gazetteer
- Hardness: California State Water Resources Control Board, Division of Drinking Water, results from June 5, 2019 to Nov. 13, 2025, read Oct. 10, 2026
- Limits and goals: each contaminant’s page, with its EPA source.
- How this page is built: methodology.
PWSID CA5010039. Rebuilt from EPA data on Oct. 18, 2026.
What to do next
- Read up on ArsenicWhat each one does to health, where it comes from, and the certification claim that addresses it.About Arsenic
- Get the utility’s own reportWestern Hills Water District/Diablo Gran must send you a water quality report every year. It covers contaminants this page does not show.How to read it
- Test your own tap for leadYour service line, solder and fixtures decide the lead at your faucet, not the utility’s average.Lead in your water: what to do
- Compare certified filtersWhat the filters certified for these results cost over five years, cartridges included.Compare five-year costs