Modesto Water Quality at a Glance 2026
serious contamination
Is Modesto Water Safe to Drink in 2026?
Technically Compliant, But Highly Concerning — Modesto water meets federal legal standards but has 8 contaminants above EPA health-based guidelines. Key issues include PFOS and PFOA detected above California’s revised 4 ppt notification level at multiple wells (as of 2024 sampling), the carcinogen 1,2,3-TCP from legacy agricultural fumigants, arsenic, and chromium-6. Independent filtration is strongly recommended — see our filter guide for Modesto for options that address these specific contaminants.
⚠️ Key Concerns for Modesto Residents in 2026
- PFAS “Forever Chemicals”: PFOS at 4.6 ppt and PFOA at 4.1 ppt detected at Well 16 (sampled September 2024) — both above California’s revised October 2025 notification level of 4.0 ppt. Additional wells are under monitoring. Ongoing PFAS lawsuits affect the wider Modesto area.
- 1,2,3-TCP (Agricultural Carcinogen): A potent carcinogen from legacy soil fumigants detected in multiple groundwater wells. California’s MCL is 5 parts per trillion — among the strictest in the world. GAC treatment systems are being installed but not all wells are yet treated.
- Arsenic, Uranium & Chromium-6: Naturally occurring inorganic contaminants present in San Joaquin Valley groundwater. Arsenic levels at the City of Modesto utility have been reported at multiples of the EWG health guideline (though below the EPA legal MCL of 10 ppb).
- 37% of Regional Groundwater Above Safety Benchmarks: A USGS study of the wider Modesto–Turlock–Merced subbasin found 37% of domestic groundwater resources exceed regulatory benchmarks for inorganic contaminants, primarily nitrate, uranium, and arsenic. Private well users in the area face the highest risk.
- 8 Contaminants Above EPA MCLGs: Current data from 17 utilities serving the Modesto area identifies 8 contaminants exceeding EPA health-based guideline values (MCLGs), though all remain below enforceable legal MCLs. See the California state water quality page for statewide context.
Read the full report below for detailed analysis, city-specific data, and actionable recommendations for Modesto residents. Check the live US boil water notices tracker for any current Modesto alerts.
Advertisement — Report continues below
Your tap water just failed.
Here’s the fix.
Contaminants like PFAS, lead, chromium-6, and arsenic can sit within the legal limit but still pose a real health risk. The legal limit and the safe limit are not the same thing.
✅ Removes PFAS, lead & chromium-6 — the contaminants most flagged in failing city reports
✅ NSF/ANSI 42, 58 & 372 certified — independently tested, not just manufacturer claims
✅ Installs in 30 minutes, no plumber — fills a glass in 8 seconds, smart LED filter monitor
Renting or can’t drill? The Waterdrop K19 Countertop RO — plug in, fill the tank, done. No installation, no drilling, no plumber.
Disclaimer: This advert contains affiliate links. If you buy through these links, we may earn a commission at no extra cost to you. Product claims are based on manufacturer certification data and independent SGS lab testing.
Modesto, California — Water Quality Report 2026: PFAS Detections, TCP Contamination & Drinking Water Safety
Modesto’s water system is managed jointly by the City of Modesto and the Modesto Irrigation District (MID), serving the city proper and surrounding communities including Del Rio, Salida, Empire, Grayson, and portions of Ceres and Turlock — approximately 221,000 people in total across 17 utilities. The system uses a dual-source approach: treated surface water from the Tuolumne River (which originates in the Sierra Nevada), delivered through the Modesto Regional Water Treatment Plant (MRWTP, operational since 1995, expanded 2016), and groundwater from 77 active wells tapping the local San Joaquin Valley aquifer. The MRWTP treats and delivers up to 67,000 acre-feet per year. The Water Quality Department takes approximately 892 samples monthly across organic and inorganic chemicals, radioactivity, microbials, and minerals.
In 2026, the most significant concerns facing Modesto residents are PFAS contamination in groundwater wells — with PFOS and PFOA now detected above California’s revised 4 ppt notification levels — and ongoing 1,2,3-TCP (TCP) contamination from legacy agricultural fumigants. The city is installing granular activated carbon (GAC) treatment at affected wells, but treatment is not yet complete across the full system. Private well users in the surrounding area face additional risks, with a USGS study finding 37% of domestic groundwater resources in the Modesto–Turlock–Merced subbasin exceeding regulatory benchmarks. Filtration is strongly recommended for all Modesto tap water users.
Modesto Water Quality: Current Status 2025–2026
Latest Testing Results and Compliance
- Compliance Status: Modesto’s water meets all federal and state enforceable drinking water standards. However, 8 contaminants exceed non-enforceable EPA health-based guideline values (MCLGs), and PFOS and PFOA have been detected above California’s revised notification levels at multiple wells.
- PFAS Detections (2024): Sampling at Well 16 on 12 September 2024 detected PFOS at 4.6 ppt and PFOA at 4.1 ppt — both above California’s October 2025 revised notification level of 4.0 ppt for both compounds. California notification levels trigger a requirement for utilities to inform customers; response levels (which trigger source removal from service) are higher at 40 ppt for PFOS and 10 ppt for PFOA. The city’s PFAS monitoring is ongoing across all wells under California’s General Order DW-2025-0002-DDW.
- 1,2,3-TCP (TCP): This legacy agricultural carcinogen remains present in portions of Modesto’s groundwater supply. California enforces one of the strictest TCP maximum contaminant levels in the world at 5 parts per trillion. Granular activated carbon (GAC) treatment is being rolled out to affected wells but installation is ongoing.
- PCE Superfund Site: The EPA-designated Modesto groundwater contamination Superfund site involving tetrachloroethylene (PCE) remains under active management with groundwater treatment and soil vapour extraction systems continuing to operate.
Dual Water Sources
- Tuolumne River (Surface Water): Originating in the Sierra Nevada Mountains, this source undergoes comprehensive multi-stage treatment at the MRWTP. It is generally lower in contaminant risk than the local groundwater but subject to drought-related supply reduction.
- Groundwater Wells (77 Active): These wells tap the San Joaquin Valley aquifer, which is heavily impacted by decades of intensive agriculture. They provide a significant share of supply, especially during drought, but are the primary source of TCP, PFAS, arsenic, nitrate, and uranium concerns.
- Regional Groundwater Quality: A USGS assessment of the Modesto–Turlock–Merced subbasin found inorganic contaminants exceeding regulatory MCL benchmarks in 37% of domestic groundwater resources, with nitrate, uranium, and arsenic the most prevalent. PFAS were detected in 23% of domestic wells in the study area. Private well users face the highest unfiltered risk and are not covered by city treatment systems.
Treatment Processes
- Surface Water Treatment (MRWTP): Multi-stage conventional treatment including coagulation, flocculation, sedimentation, filtration, and disinfection. Capable of treating up to 67,000 acre-feet per year from the Tuolumne River.
- Groundwater Treatment: Specialised treatment varies by well. GAC filtration for TCP removal is being installed at affected wells. Aeration is used for volatile organic compounds. Chlorine disinfection is applied across the groundwater supply before distribution.
- PFAS Treatment Gap: As of 2026, not all Modesto wells with PFAS detections have dedicated treatment in place. The EPA’s PFAS MCLs (4 ppt PFOA/PFOS, effective 2024) give utilities until 2029 to achieve full compliance — but California’s notification levels already exceed this standard in terms of triggering customer notification requirements.
Drought and Supply Pressure
Modesto’s recurring exposure to California drought conditions places ongoing pressure on both supply and water quality. During severe drought years, surface water deliveries from MID have fallen from an average of 30 million gallons per day to approximately 11.5 million gallons per day, forcing increased reliance on groundwater wells — the same wells carrying the highest contamination risk. The dual-source system provides resilience, but greater groundwater dependence during droughts intensifies PFAS, TCP, and nitrate exposure risk. Water conservation programmes are actively promoted and staged use restrictions may be implemented during drought declarations. Follow the water alert news page for any California drought advisories affecting Modesto’s supply.
Recommendations for Modesto Residents in 2026
Install a Reverse Osmosis Filter
Given PFAS detections above California’s notification levels and the presence of TCP, arsenic, and chromium-6 in Modesto’s water supply, an NSF Standard 58-certified reverse osmosis (RO) system is the most effective option for home use. RO removes PFAS, TCP, arsenic, nitrates, uranium, and chromium-6 — the full range of Modesto’s highest-concern contaminants. See our recommended filters for certified RO options suited to Modesto water.
Review the Annual Consumer Confidence Report
The City of Modesto publishes an annual Consumer Confidence Report (CCR) detailing testing results across all monitored contaminants. Review your CCR at modestogov.com or call (209) 577-5395. Cross-reference with the national water quality database to see how Modesto compares to other California cities. Note that CCR results reflect legal MCL compliance — they do not indicate whether contaminants are below health guideline values.
Conserve Water — Especially During Drought
Modesto’s water supply faces sustained pressure from California’s recurring droughts. Greater drought means greater groundwater reliance — and greater exposure to well contaminants. Follow city conservation guidelines, fix leaks promptly, install water-efficient fixtures, and reduce outdoor irrigation during dry periods. Staged water use restrictions may be activated during drought emergencies.
Flush After Extended Non-Use
If water has been sitting in pipes for an extended period — after returning from vacation or a long absence — run cold water taps for 2–3 minutes before drinking or cooking. This is especially important in homes with older plumbing or in areas drawing from affected groundwater wells. Never use hot water directly from the tap for drinking or cooking, as it can concentrate contaminants leached from pipes and fittings.
Report Water Quality Issues
If you notice unusual taste, odour, or appearance in your water, contact the City of Modesto Water Services at (209) 577-5395. For water waste concerns, use the city’s dedicated reporting system. You can also track current boil water notices across the US and follow national water safety developments on the water alert news page.
Quality News About Your Water
Get the comprehensive water quality news coverage you need with our dedicated US Water News Service. From coast to coast, we deliver in-depth reporting and expert analysis on PFAS contamination, EPA regulatory changes, infrastructure developments, and emerging water safety issues affecting communities nationwide. While mainstream media only covers the biggest stories, we provide the detailed, ongoing coverage that helps you understand the full scope of America’s water challenges.
What’s actually in your tap water? Enter your ZIP code for a full breakdown of contaminants detected in your local supply
Drinking water from a well? Check our directory here for more information
Frequently Asked Questions — Modesto Water Quality 2026
Is Modesto tap water safe to drink in 2026?
Modesto water meets all enforceable federal and state drinking water standards and is legally considered safe to drink. However, this does not mean it is without concern. As of 2026, Modesto’s water supply contains 8 contaminants above EPA non-enforceable health guideline values (MCLGs), and PFOS and PFOA have been detected above California’s 4 ppt notification levels at multiple wells.
The primary risks are: PFAS (PFOS and PFOA above California’s revised notification threshold); 1,2,3-TCP, a potent carcinogen from legacy agriculture; arsenic and chromium-6 in groundwater; and nitrates from agricultural runoff. Treatment is in progress at affected wells, but not all wells have GAC or dedicated PFAS treatment installed yet.
For drinking and cooking, an NSF 58-certified reverse osmosis system is the most effective protection. Alternatively, an NSF 53-certified carbon block filter will reduce TCP, DBPs, and some PFAS. See our filter recommendations for certified options. The EWG Tap Water Database also lists Modesto contaminants against their health advisory values — note these are not legal limits but represent more conservative health-based thresholds.
What is 1,2,3-TCP and why is it in Modesto’s water?
1,2,3-Trichloropropane (TCP) is a synthetic chemical detected in multiple Modesto groundwater wells. It originated as an impurity in soil fumigants used widely in California agriculture from the 1940s through the 1980s, manufactured by companies including Shell and Dow Chemical.
1. Why it persists: TCP has a half-life of hundreds of years in groundwater. It is denser than water and highly mobile, meaning it sinks deep into aquifers and spreads over wide areas — making it exceptionally difficult to remediate.
2. Health risk: California classifies TCP as a potent carcinogen and has set one of the most stringent maximum contaminant levels in the world at 5 parts per trillion — more than 1,000 times lower than the federal arsenic MCL of 10 ppb.
3. Treatment: Granular activated carbon (GAC) filtration is the primary treatment technology being deployed at affected Modesto wells. Installation is ongoing. Until all affected wells are treated, some portion of the blended supply may still carry trace TCP.
4. At home: NSF Standard 53-certified carbon block filters can reduce TCP, as can NSF 58-certified reverse osmosis systems. See our filter guide for options certified against these contaminants.
What are California’s new PFAS rules and where does Modesto stand?
California has two layers of PFAS oversight for drinking water: the EPA’s national enforceable MCLs (effective 2024, with a 2029 compliance deadline) and California’s own non-regulatory notification and response levels, which were revised in October 2025.
California’s current notification and response levels (as of October 2025) are: PFOA — notification 4.0 ppt, response 10 ppt; PFOS — notification 4.0 ppt, response 40 ppt; PFHxS — notification 3.0 ppt, response 10 ppt. When a notification level is exceeded, utilities must inform customers. When a response level is exceeded, the source should be taken out of service.
Modesto’s position: PFOS at 4.6 ppt and PFOA at 4.1 ppt were detected at Well 16 in September 2024 — both above the revised 4.0 ppt notification level. This triggers a customer notification requirement. Neither compound reached the California response level (40 ppt for PFOS, 10 ppt for PFOA). All utilities must comply with EPA federal MCLs of 4 ppt for PFOA and PFOS by 2029. General Order DW-2025-0002-DDW requires all California community water systems to conduct initial PFAS monitoring — results will continue to be published via the State’s GeoTracker mapping tool. See the California water quality overview for statewide PFAS status.
How does drought affect Modesto’s water quality and supply?
Drought has a direct and compounding effect on Modesto’s water quality risk. When surface water allocations from MID fall — as they did from an average of 30 million gallons per day to around 11.5 million gallons per day in recent drought years — the city draws more heavily on groundwater wells. Those wells carry the highest contamination burden: TCP, PFAS, arsenic, nitrate, and uranium.
During drought conditions:
• Greater groundwater reliance increases average exposure to well contaminants across the blended supply
• Reduced aquifer recharge can concentrate contaminants in shallower wells
• Staged water use restrictions may be activated — check modestogov.com or call (209) 577-5395 for current status
• Increased pumping pressure on the aquifer can mobilise contaminants not previously drawn into active supply zones
Modesto’s dual-source design provides meaningful resilience, but drought periods are precisely when home filtration delivers the most value. Monitor water alert news for California drought advisories and any Modesto-specific notices, and use the live boil water notices tracker for current alerts.
Contaminants of Concern — Modesto 2026
1,2,3-Trichloropropane (TCP)
Source: Legacy agricultural soil fumigants used across the San Joaquin Valley from the 1940s to the 1980s. TCP was an unintentional impurity in these products that leached into and persists in the regional aquifer. Active EPA Superfund remediation covers PCE contamination at a separate site in Modesto.
Health Effects: Classified as a potent carcinogen by the State of California. Laboratory studies demonstrate increased cancer risk with long-term exposure, as well as potential liver, kidney, and central nervous system effects. California’s MCL of 5 ppt is among the strictest drinking water standards in the world for any contaminant.
Current Status (2026): Detected in multiple Modesto groundwater wells. GAC treatment systems are being installed at affected wells to bring TCP below the 5 ppt MCL prior to distribution. Installation is ongoing. Residents drawing from untreated well zones should use certified home filtration. An NSF Standard 53-certified carbon block filter or NSF 58 reverse osmosis system will reduce TCP effectively. See our filter recommendations.
PFAS, Arsenic, Nitrates & Other Groundwater Contaminants
PFAS: PFOS at 4.6 ppt and PFOA at 4.1 ppt were detected at Well 16 in September 2024 — above California’s revised notification level of 4.0 ppt for both compounds. PFAS were detected in 23% of domestic groundwater resources across the wider Modesto–Turlock–Merced subbasin in a USGS study, with 4% exceeding California notification or response levels. The EPA’s enforceable 4 ppt MCLs for PFOA and PFOS apply from 2024 with a 2029 compliance deadline. Note: EWG PFAS health guidelines (sometimes below 0.07 ppt) are advisory and not legal standards.
Arsenic, Uranium & Chromium-6: Naturally present in San Joaquin Valley groundwater geology. Arsenic at the City of Modesto utility has been reported at levels significantly above the EWG health guideline, though below the EPA legal MCL of 10 ppb. Uranium and nitrate exceed regulatory MCL benchmarks in 37% of domestic groundwater resources in the subbasin. Chromium-6 (hexavalent chromium) — a known carcinogen — is also present in California Central Valley groundwater. The EPA MCL for total chromium is 100 ppb; California’s stricter MCL for chromium-6 specifically is 10 ppb. An NSF 58-certified reverse osmosis system is the most effective residential solution for this full suite of inorganic contaminants. See the California water quality overview for the statewide contaminant picture.
Please read – our information
The information presented on cleanairandwater.net is compiled from official water quality reports, trusted news sources, government websites, and public health resources. While we strive for accuracy and thoroughness in our presentations, we are not scientists, engineers, or qualified water quality professionals.
Our mission is to present water quality information in an accessible, real-world format that helps people understand what’s in their water and make informed decisions about their health and safety. We believe that complex environmental information should be available to everyone in a format that’s easy to understand.
We make every effort to ensure our content is current and accurate, but we cannot guarantee that all information is complete or error-free. This website should not replace official communications from your local water utility or health department. We always recommend consulting official sources for the most up-to-date information regarding your specific water system.
Clean Air and Water is not liable for any unintentional errors, omissions, or outdated information. The content on this site is provided for informational purposes only and should not be considered professional advice.
