Water quality testing lab representing Southern California water analysis

Original data · 2026

Southern California Water Quality by County

Chromium-6, hardness, perchlorate, and arsenic across seven counties. Sourced from utility Consumer Confidence Reports, EPA SDWIS, USGS monitoring, and OEHHA health goals.

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Southern California delivers tap water to roughly 19 million residents across seven counties from four source waters: the Colorado River, the Sacramento-San Joaquin Delta via the State Water Project, the Eastern Sierra aqueduct, and local groundwater basins. Most households receive one annual summary from their utility, a contaminant value measured against an EPA maximum contaminant level. That rarely tells the full story. California sets health goals that are far stricter than federal law. For chromium-6, the state Office of Environmental Health Hazard Assessment set a public health goal of 0.02 ppb; several SoCal utilities have documented the metal at five to thirty times that level in recent reporting periods. This index synthesizes 2023 to 2026 utility Consumer Confidence Reports, EPA SDWIS data, USGS hardness monitoring, and OEHHA health goals to map what each county's water actually contains and what treatment removes it.

How we compiled this index

Data in this index comes from publicly available sources: utility annual Consumer Confidence Reports (CCRs) filed with the California State Water Resources Control Board for reporting years 2023 to 2025, the EPA Safe Drinking Water Information System (SDWIS) national database, USGS water hardness mapping at water.usgs.gov/edu/hardness.html, and OEHHA's Proposition 65 and Public Health Goal documents at oehha.ca.gov. Contaminant ranges reflect the minimum and maximum values reported across monitoring periods within each county, not a single snapshot. A county's listed range may include both treated surface water and groundwater sources within the same utility service area.

Water quality data by county

Sources: utility CCRs 2023-2025, EPA SDWIS, USGS hardness data, OEHHA PHGs. Chromium-6 levels are below California's 10 ppb MCL (reinstated October 2024) in all counties shown; they are compared against OEHHA's 0.02 ppb public health goal for context.

County / Area	Primary utility	Chromium-6 range	Hardness (mg/L)	Key contaminants	Sources
Los Angeles	LADWP	0.09-0.42 ppb	280-430	TTHMs, HAA5s, hardness	EWG Tap Water Database; LADWP 2023-2024 CCR
Los Angeles (San Gabriel Valley)	Basin Water Agency / MWD	0.15-0.60 ppb	320-450	Arsenic (groundwater wells), perchlorate	EPA UCMR3 results; SGVMWD 2023 CCR
Inland Empire (Riverside + San Bernardino)	IEUA + Western MWD	0.14-0.30 ppb	380-510	Perchlorate (aerospace legacy), arsenic, hardness	IEUA 2023-2024 CCR; EPA CERCLA records
Orange County	OCWD / MWDOC	0.04-0.18 ppb	230-380	Nitrate, TTHMs	OCWD 2023 Water Quality Report; EWG database
San Diego	SDCWA + municipal distributors	0.04-0.14 ppb	300-450	TDS (Colorado River blend), TTHMs	SDCWA 2024 Water Quality Report
Ventura	Calleguas MWD	0.07-0.16 ppb	290-420	Nitrate, TDS	Calleguas MWD 2023 CCR; MWD 2024 Water Quality Report
Imperial Valley	IID (Imperial Irrigation District)	below 0.10 ppb	470-640	TDS (800-950 ppm), arsenic, sulfate	IID 2024 CCR; USGS Colorado River salinity monitoring

OEHHA public health goal for chromium-6: 0.02 ppb. California MCL (total chromium): 10 ppb effective October 2024. WQA classifies water above 180 mg/L as very hard. All ranges are below the enforceable MCL.

Key findings

Chromium-6 above the state health goal in every county

California's OEHHA public health goal for hexavalent chromium is 0.02 ppb. Every county in this index has documented detections above that level in at least one utility reporting period. The San Gabriel Valley has the highest range at 0.15 to 0.60 ppb, linked to four EPA Superfund sites in the basin.

Hard to very hard water across all seven counties

Every county in Southern California delivers water classified as hard (above 120 mg/L) or very hard (above 180 mg/L) by the Water Quality Association scale. The Inland Empire and Imperial Valley have the highest readings, driven by Colorado River supply through alkaline desert geology. High hardness shortens appliance lifespan and increases detergent consumption.

Perchlorate remains elevated in Inland Empire groundwater

Perchlorate is a byproduct of solid rocket propellant that leached into groundwater from aerospace manufacturing sites active from the 1950s through the 1980s. IEUA and local Riverside County utilities continue to blend and treat for perchlorate. The California MCL is 6 ppb; some monitoring wells in the region have documented levels above that threshold in recent EPA CERCLA records.

Imperial Valley leads in total dissolved solids

Imperial Valley draws almost exclusively from the Colorado River, which USGS monitoring has documented at 800 to 950 ppm TDS at the delivery point to Imperial County. That is nearly double the EPA secondary standard of 500 ppm and far above the WHO guideline of 600 ppm. High TDS accelerates scale buildup in pipes, water heaters, and appliances.

What this means for Southern California homeowners

The utility CCR your household receives each year reflects averages across an entire service area and an entire calendar year. It does not reflect conditions at your specific tap, which can vary based on how old your building's interior plumbing is, whether your home uses a private well in addition to the municipal supply, and where you sit in the distribution system relative to treatment plants and storage tanks.

For chromium-6, the gap between the legal maximum (10 ppb) and the health-protective goal (0.02 ppb) is substantial enough that many households in affected counties install an additional treatment barrier at the kitchen tap. A reverse osmosis system certified under NSF/ANSI Standard 58 is the most practical residential approach; certified membranes typically reduce hexavalent chromium by 90 percent or more at concentrations found in municipal supplies, per NSF International testing protocols.

For hardness, the practical concern is cumulative: scale deposits from hard water reduce water heater efficiency and shorten appliance life over years, not months. A 2009 Pacific Northwest National Laboratory study (PNNL-18925) found tankless water heaters on untreated hard water failed at 1.6 years in hard-water conditions, versus full service life with a water softener in place. Southern California water, at 200 to 500 mg/L depending on county and supply blend, is in the range where that damage accumulates measurably.

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What treatment addresses what

NSF International certifications listed below verify independent laboratory testing against the named standard. Systems without certification for the specific contaminant make no verified reduction claim.

Contaminant concern	Recommended treatment	Most relevant counties
Chromium-6	Reverse osmosis (NSF/ANSI 58 certified)	All counties; most critical in San Gabriel Valley and Inland Empire
Hard water (scale, appliance damage)	Salt-based water softener (NSF/ANSI 44 certified)	All counties; most pressing in Inland Empire and Imperial Valley
Perchlorate	Strong-base anion exchange or reverse osmosis	Inland Empire (Riverside, San Bernardino), parts of LA County
Arsenic	Reverse osmosis or oxidation-filtration for well water	San Gabriel Valley (groundwater wells), Imperial Valley, rural Riverside
High TDS (taste, scale, corrosion)	Whole-home reverse osmosis or RO at point of use	Imperial Valley, Inland Empire, coastal San Diego (Colorado River blend years)
PFAS / forever chemicals	Activated carbon block (NSF/ANSI 53 for PFOA/PFOS) or reverse osmosis	Riverside County (documented detections per CA PFAS monitoring), parts of LA County

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Frequently asked questions

Which Southern California county has the worst water quality?▼

No single county has the worst water quality across every contaminant. The San Gabriel Valley portion of Los Angeles County has the highest documented chromium-6 concentrations (up to 0.60 ppb in some monitoring periods) due to Superfund industrial contamination. Imperial Valley has the highest hardness and total dissolved solids, driven by Colorado River supply at 800 to 950 ppm TDS. Inland Empire carries elevated perchlorate from aerospace legacy sites. The right answer depends on which contaminant your household is most concerned about.

Is Southern California tap water safe to drink?▼

Water utilities in Southern California are required to meet federal and California maximum contaminant levels (MCLs), and most do. However, California public health goals (PHGs) set by OEHHA are often far stricter than the enforceable MCLs. For chromium-6, the state PHG is 0.02 ppb while most SoCal supplies run 0.04 to 0.60 ppb. Compliance with the legal limit does not mean the water meets every health-protective goal. Households with infants, people who are pregnant, or those with specific health concerns often add an NSF-certified treatment layer beyond what the utility provides.

Does Orange County have better water quality than Los Angeles?▼

Orange County tap water generally has lower chromium-6 concentrations (0.04 to 0.18 ppb versus up to 0.60 ppb in the San Gabriel Valley) and lower hardness in coastal service areas. However, Orange County utilities have documented higher nitrate levels in some groundwater zones, driven by historical agricultural runoff. Both counties blend imported Metropolitan Water District water with local groundwater, so quality varies significantly by specific city and utility.

What is a Consumer Confidence Report and where do I find mine?▼

A Consumer Confidence Report (CCR), also called an Annual Water Quality Report, is a document your water utility must publish each year under the EPA Safe Drinking Water Act. It lists contaminants detected during the reporting period, the levels found, and the legal limits. You can find it on your utility's website under water quality or annual report, or search the EPA's CCR database at epa.gov/ccr. California utilities also submit data to the State Water Resources Control Board at waterboards.ca.gov.

Should I test my well water separately from my utility report?▼

Yes. Private wells are not regulated by the Safe Drinking Water Act and are not covered by utility CCR reporting. Contaminant levels in private wells reflect local geology and nearby land use, which can differ substantially from the municipal supply even in the same neighborhood. Arsenic, nitrate, hardness, and volatile organic compounds are the most common concerns in SoCal private wells. A test from a California ELAP-certified laboratory is the only way to know your well's current condition.

How often does Southern California water quality change?▼

Water quality in blended supply systems shifts with the seasonal mix of imported water sources. Metropolitan Water District draws from the Colorado River and the State Water Project (Sacramento-San Joaquin Delta). In dry years when Colorado River allocation increases, hardness and TDS rise across the region. In wet years with more State Water Project water, hardness typically falls. Groundwater wells show slower seasonal variation but can shift year over year depending on pumping rates, recharge, and upstream land use. Utilities publish annual CCRs reflecting these changes.

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