California Water Quality Guide
Understanding water quality across the Golden State, from major metropolitan systems to rural communities affected by drought and agricultural runoff.
Environmental Health Research
Expert Reviewed
This content has been reviewed by Dr. Sarah Chen, PhD, Environmental Science Advisor, to ensure accuracy and alignment with current environmental science standards.
Last reviewed: April 2026
California Water System Overview
California has one of the most complex water systems in the world, serving nearly 40 million residents across diverse geography. The state relies on an intricate network of reservoirs, aqueducts, and groundwater basins to deliver water from the Sierra Nevada mountains and the Colorado River to communities throughout the state.
Major Water Systems
- State Water Project (SWP): Delivers water from Northern California to Southern California, serving 27 million people through 700 miles of canals and pipelines.
- Central Valley Project: Federal system providing water to agricultural and urban users in the Central Valley.
- Los Angeles Aqueduct: Brings water from the Owens Valley and Mono Basin to Los Angeles.
- Colorado River Aqueduct: Supplies water from the Colorado River to the Metropolitan Water District of Southern California.
- Hetch Hetchy System: Provides pristine mountain water to San Francisco and surrounding Bay Area communities.
Water Sources by Region
Northern California communities often enjoy higher-quality surface water from protected watersheds. Southern California depends heavily on imported water and increasingly on desalination and recycled water. Central Valley communities frequently rely on groundwater, which faces contamination challenges from agricultural activities.
Drought Impacts on Water Quality
California's recurring droughts significantly affect water quality. When reservoir levels drop and groundwater is pumped more heavily, contaminant concentrations can increase. Understanding these dynamics helps residents prepare and protect their families.
How Drought Affects Water Quality:
- Concentrated contaminants: Less water means higher concentrations of naturally occurring and man-made contaminants
- Increased groundwater pumping: Deeper wells may tap into arsenic-rich or nitrate-contaminated aquifers
- Algae blooms: Warmer, stagnant reservoir water promotes harmful algal growth
- Infrastructure stress: Changing water levels can disturb sediments and aging pipes
- Treatment challenges: Higher turbidity requires more intensive treatment
Drought Response Measures
Water utilities implement various measures during drought, including increased monitoring, adjusted treatment protocols, and sometimes blending water from multiple sources. The State Water Resources Control Board provides drought-related guidance and enforces conservation measures that also help maintain water quality.
Common Contaminants in California
California faces unique water quality challenges due to its geology, agricultural industry, and industrial history. Several contaminants are of particular concern across the state.
Chromium-6 (Hexavalent Chromium)
California was the first state to set a specific limit for chromium-6, a probable carcinogen made famous by the Erin Brockovich case. While the state's 10 ppb standard was later withdrawn due to legal challenges, California continues to monitor this contaminant closely. Many communities, particularly in the Central Valley, have elevated chromium-6 levels from both natural sources and industrial contamination.
Nitrates
Agricultural Runoff Concern:
The Central Valley has some of the highest nitrate levels in the nation due to intensive agriculture. Over one million Californians drink water from systems that have had nitrate violations. Nitrates are particularly dangerous for infants, causing "blue baby syndrome."
Arsenic
Natural arsenic deposits in California's geology result in elevated arsenic levels in many groundwater sources. Communities in the Central Valley, Mojave Desert, and parts of the Sierra foothills are most affected. Long-term arsenic exposure is linked to cancer and cardiovascular disease.
1,2,3-Trichloropropane (TCP)
This industrial solvent and pesticide component has contaminated groundwater in agricultural areas throughout California. The state has set an MCL of 5 parts per trillion, making it one of the strictest standards in the nation.
PFAS (Forever Chemicals)
PFAS contamination has been detected near military bases, airports, and industrial sites across California. Communities near Travis Air Force Base, George Air Force Base, and numerous other locations have documented PFAS in their water supplies. California is developing some of the nation's strictest PFAS standards.
Regional Water Quality Snapshots
Greater Los Angeles
Los Angeles receives water from multiple sources, creating variable quality. The Metropolitan Water District blends imported water from the Colorado River and State Water Project with local groundwater. Lead from aging infrastructure and perchlorate from aerospace industry contamination are ongoing concerns.
View Los Angeles water quality details
San Francisco Bay Area
San Francisco enjoys exceptional water quality from the Hetch Hetchy Reservoir in Yosemite National Park. This protected watershed produces water so clean it doesn't require filtration. East Bay communities receive water from the Mokelumne River system, also highly rated.
View San Francisco water quality details
San Diego County
San Diego historically imported most of its water but has invested heavily in desalination and recycled water. The Carlsbad Desalination Plant and Pure Water San Diego program are reducing reliance on imported water while maintaining quality standards.
View San Diego water quality details
Central Valley
Agricultural communities in the Central Valley face the greatest water quality challenges. Nitrate contamination, arsenic, and pesticide residues affect many small water systems. The state's SAFER program is working to address these issues in disadvantaged communities.
Resources for California Residents
State and Federal Resources:
Testing Your Water
California residents can request their water utility's Consumer Confidence Report (CCR) annually. For additional testing, state-certified laboratories can analyze for contaminants not routinely monitored. The State Water Board maintains a list of certified labs.
Filtration Recommendations
Given California's common contaminants, residents may benefit from certified water filtration:
- For chromium-6: Reverse osmosis or strong base anion exchange systems
- For nitrates: Reverse osmosis or ion exchange systems (NSF 58 certified)
- For arsenic: Reverse osmosis or adsorptive media filters (NSF 53 certified for arsenic)
- For PFAS: Granular activated carbon or reverse osmosis systems
SAFER Program:
California's Safe and Affordable Funding for Equity and Resilience (SAFER) program helps communities with contaminated water access safe drinking water. If your community lacks safe water, contact the State Water Board for assistance.