State Well Water Guide
California Private Well Water Guide
Approximately 10% of Californians — over 4 million people — rely on private wells for drinking water. California's diverse geology creates region-specific contamination risks: naturally occurring arsenic and uranium in the Central Valley and foothills, nitrate contamination from intensive agriculture, PFAS near military installations, and hexavalent chromium (chromium-6) in several inland regions. The State Water Resources Control Board oversees small water systems, but private domestic wells are the responsibility of the homeowner.
Testing Guidance
California recommends testing private wells at least annually for coliform bacteria and nitrates. Additional testing is strongly advised for arsenic and uranium (Central Valley and Sierra foothills), chromium-6 (Mojave Desert, Hinkley area, parts of the Central Valley), PFAS (within 1 mile of military bases, airports, or industrial sites), and manganese (agricultural regions). After significant rainfall, flooding, or nearby construction, bacterial re-testing is recommended regardless of your regular schedule.
What to Test For in California
Total coliform bacteria and E. coli — annual minimum for all wells
Nitrates — annual for wells near agriculture or septic systems
Arsenic — all Central Valley, Sierra foothills, and Mojave Desert wells
Uranium — Central Valley wells, especially Tulare, Kings, Fresno, and San Joaquin counties
Hexavalent chromium (chromium-6) — San Bernardino County, Central Valley, and areas near chrome plating operations
PFAS — wells within 2 miles of military bases (Vandenberg, Edwards, Camp Pendleton, March ARB) and major airports
Manganese — agricultural Central Valley wells; can cause neurological effects at high levels
1,2,3-Trichloropropane (1,2,3-TCP) — Central Valley; industrial solvent contaminant
pH, hardness, and total dissolved solids — baseline water chemistry
Common Contamination Risks in California
Arsenic from natural geological sources — among the highest natural arsenic groundwater concentrations in the U.S., particularly in the eastern San Joaquin Valley
Nitrate contamination from agricultural fertilizers — the Central Valley has some of the highest nitrate concentrations in the nation's groundwater
PFAS from military and industrial sites — multiple Superfund sites and military installations across Southern California and the Central Valley
Hexavalent chromium (chromium-6) — naturally occurring in serpentinite rock in the Coast Ranges and from industrial contamination in the Mojave Desert
Uranium from natural sources — granitic bedrock in the Sierra Nevada foothills releases uranium into groundwater
Saltwater intrusion in coastal areas — overdraft of aquifers near the coast allows seawater to infiltrate groundwater in Santa Barbara, Ventura, and parts of the Bay Area
1,2,3-TCP industrial contamination — widespread in the southern San Joaquin Valley from historical pesticide manufacturing byproducts
Contaminant Guides Relevant to CA Wells
PFAS
Synthetic Chemicals
EPA limit: 4 ppt
Lead
Heavy Metals
EPA limit: 15 ppb (action level)
Nitrates
Agricultural Chemicals
EPA limit: 10 mg/L
DBPs
Disinfection Byproducts
EPA limit: 80 µg/L (TTHMs) / 60 µg/L (HAA5)
Arsenic
Heavy Metals
EPA limit: 10 ppb
Hard Water
Minerals
EPA limit: No federal limit
Chlorine
Disinfection Chemicals
EPA limit: 4 mg/L (MRDL)
Microplastics
Emerging Contaminants
EPA limit: No federal limit
Bacteria
Microbial Contamination
EPA limit: Zero E. coli / < 1 coliform per 100 mL
Fluoride
Minerals
EPA limit: 4 mg/L (MCL) / 2 mg/L (Secondary MCL)
Chromium-6
Heavy Metals
EPA limit: 100 ppb (total chromium)
Copper
Heavy Metals
EPA limit: 1.3 mg/L (action level)
Radon
Radioactive Contaminants
EPA limit: No finalized MCL
Iron & Manganese
Minerals
EPA limit: 0.3 mg/L iron / 0.05 mg/L manganese (aesthetic SMCLs)
Atrazine
Agricultural Chemicals
EPA limit: 3 ppb (0.003 mg/L)
Uranium
Radioactive Contaminants
EPA limit: 30 µg/L (30 ppb)
Perchlorate
Industrial Chemicals
EPA limit: 0.056 mg/L (56 ppb)
VOCs
Industrial Chemicals
EPA limit: Varies by compound: benzene 5 ppb; TCE 5 ppb; PCE 5 ppb; vinyl chloride 2 ppb
Radium
Radioactive Contaminants
EPA limit: 5 pCi/L (combined Ra-226 + Ra-228)
Crypto & Giardia
Microbial Contamination
EPA limit: Zero (treatment technique standard)
Hydrogen Sulfide
Naturally Occurring Compounds
EPA limit: No MCL; Secondary MCL (aesthetic) of 0.05 mg/L
Selenium
Minerals
EPA limit: 50 ppb (0.05 mg/L)
Cadmium
Heavy Metals
EPA limit: 5 ppb (0.005 mg/L)
Mercury
Heavy Metals
EPA limit: 2 ppb (0.002 mg/L)
Barium
Heavy Metals
EPA limit: 2 mg/L
pH
Water Quality Indicators
EPA limit: 6.5–8.5 (secondary standard)
Find a Certified Lab in California
Use the California state-certified laboratory program to find accredited labs for private well testing. Always verify current certification before submitting samples.
CA Certified Lab Directory ↗California Well Water FAQs
Related Pages
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
Quick Reference
State program
CA Certified Labs ↗EPA guidance
EPA Private Wells Program ↗State lab directory
CA Certified Labs ↗Minimum Annual Tests
Plus state-specific contaminants listed above
Other State Guides