Uranium and Gross Alpha Radioactivity in San Fernando Valley Tap Water: What Federal Limits and LADWP Data Show

What are uranium and gross alpha radioactivity, and why are they in water?

Uranium is a naturally occurring heavy metal and radioactive element found in varying concentrations in soil and rock throughout the western United States. It dissolves into groundwater as water moves through uranium-bearing geologic formations. In Southern California, the granitic and sedimentary formations that underlie the region contribute naturally occurring uranium to the groundwater supply. According to USGS National Water Quality Assessment (NAWQA) data, uranium detections in Southern California groundwater are not unusual. The presence of uranium at any concentration does not automatically mean the water exceeds the MCL; that determination requires comparison to the 30 ppb federal limit.

Gross alpha is not a specific contaminant. It is a screening measurement: a laboratory test that detects the total alpha particle radioactivity from all alpha-emitting isotopes present in the water sample, expressed in picocuries per liter. Uranium, radium-226, radium-228 (which decays to radium-226), and other naturally occurring isotopes all contribute to a gross alpha reading. Because uranium is regulated separately and emits alpha particles, utilities typically report uranium and gross alpha as separate line items in their Consumer Confidence Reports. A gross alpha result above the 15 pCi/L MCL triggers further speciation testing to identify which isotopes are driving the result.

Where does San Fernando Valley tap water come from, and does the source affect radionuclide levels?

LADWP supplies water to most of the San Fernando Valley from two categories of sources: local groundwater drawn from the San Fernando Valley Basin and the Raymond Basin, and imported water from the State Water Project (via the California Aqueduct) and the Colorado River Aqueduct. The blend ratio changes seasonally and by supply conditions. In dry years, LADWP draws more heavily on imported water; in wet years, local groundwater recovery is higher.

Local SFV groundwater has a different mineral and radionuclide profile than imported water. Groundwater in contact with local geology for longer periods accumulates more dissolved solids, including naturally occurring radioactive materials, compared to surface-derived imported supplies. Our earlier article on SFV tap water quality by source covers the blending ratios and seasonal variation in more detail. For radionuclides specifically, the key point is that both source categories can carry naturally occurring radioactive material at different concentrations, and the blended delivery is what LADWP measures against the MCL in its annual Consumer Confidence Report.

Las Virgenes Municipal Water District serves Calabasas and Westlake Village primarily with imported water from the Metropolitan Water District of Southern California, which in turn draws from the State Water Project and the Colorado River Aqueduct. LVMWD publishes radionuclide test results in its annual Consumer Confidence Report at lvmwd.com/your-water/water-quality-reports.

How does the EPA regulate uranium in drinking water?

The EPA finalized the Radionuclides Rule in December 2000, with compliance required by December 2003. The rule is codified at 40 CFR Part 141, Subpart B. Before 2003, uranium was not separately regulated; the prior gross alpha MCL of 15 pCi/L had been in place since 1976 but uranium was counted against that total. The 2003 rule established a standalone uranium MCL of 30 micrograms per liter and clarified that uranium is excluded from the gross alpha measurement for compliance purposes, preventing double-counting.

Unlike perchlorate (where California adopted a stricter standard of 1 ppb compared to the federal 18 ppb) or chromium-6 (where California has a 10 ppb state MCL compared to the federal 100 ppb total chromium standard), California has not set a stricter state MCL for uranium. The federal 30 ppb limit is the operative standard. Homeowners comparing California and federal standards across contaminants will find this is one area where the state deferred to the federal rule. Our article on perchlorate in SFV tap water illustrates how different that comparison looks for a contaminant where California did adopt a stricter standard.

The health basis for the uranium MCL differs from most other drinking water contaminants. For most contaminants, the MCL is set primarily on cancer risk from long-term ingestion. For uranium, the MCL is also based on kidney toxicity: uranium is a nephrotoxin, and the 30 ppb limit reflects the dose level at which kidney damage risk is assessed. The radiological risk from uranium in drinking water is generally considered secondary to its chemical toxicity at concentrations near the MCL (verify current EPA risk assessment documents for updates, as agency risk positions can change).

What does LADWP report for radionuclides in its Consumer Confidence Report?

LADWP is required under the Safe Drinking Water Act to test for uranium, gross alpha, radium-226, and radium-228 and report the results in its annual Consumer Confidence Report (CCR). The CCR shows the range of detected values across monitoring locations, the highest detected value during the monitoring period, the federal MCL, and whether the system was in compliance.

The most current CCR is available at ladwp.com/account-services/water-quality/water-quality-reports. When reading the radionuclide table, note the distinction between the gross alpha result reported as a total (which excludes uranium under the 2003 rule) and the uranium result reported separately in micrograms per liter rather than in pCi/L. These are different units for different regulatory purposes. The EPA also maintains the Safe Drinking Water Information System (SDWIS), which logs utility violations; a utility that exceeded an MCL would appear there with a violation record.

Hardness and radionuclide levels are different measurements that sometimes get conflated. The LADWP hard water article covers hardness separately. Dissolved calcium and magnesium (hardness) and dissolved radionuclides are independent variables measured in different units with different health significance.

Does reverse osmosis remove uranium from tap water?

Reverse osmosis is the best-documented point-of-use treatment for uranium reduction. The RO membrane physically rejects dissolved ionic species including the uranyl ion (UO2), the predominant dissolved form of uranium in drinking water at typical pH levels. Systems certified under NSF/ANSI 58 with a specific contaminant reduction claim for uranium have been independently tested to confirm that reduction. Not every NSF/ANSI 58 certified RO system includes uranium on its contaminant reduction certificate: the certification is system-specific, not category-wide. The correct procedure is to look at the specific model's certificate of performance, not just the NSF/ANSI 58 listing.

Ion exchange water softeners certified under NSF/ANSI 44 are not verified to reliably remove uranium. A cation exchange softener removes calcium, magnesium, and some other divalent cations by exchanging them for sodium. The uranyl ion does not behave consistently on standard strong acid cation resin, and NSF/ANSI 44 does not certify uranium reduction. A softener that does an excellent job on hardness (as covered in our article on water softeners vs. descalers in the SFV) does not substitute for an NSF/ANSI 58 RO system for uranium reduction purposes.

For SFV homeowners who want point-of-use uranium reduction alongside other contaminant concerns (PFAS, lead, nitrates, disinfection byproducts), a single certified RO system addresses multiple categories. Our comparison of reverse osmosis vs. whole-house filtration covers how to match system type to the contaminant profile you are trying to address.

What about radon in SFV drinking water?

Radon is a radioactive gas that forms from the decay of radium in soil and rock. In drinking water, it is primarily a concern in groundwater systems; surface water loses radon to the atmosphere before delivery. The EPA proposed a radon MCL of 300 pCi/L for water systems with multimedia mitigation programs and 4,000 pCi/L for those without, but that proposed rule was never finalized. There is no federally enforceable MCL for radon in drinking water.

California adopted a Maximum Contaminant Level of 4 pCi/L for radon in drinking water under the California Safe Drinking Water Act. This California standard applies to public water systems. The primary exposure concern with radon is indoor air: radon released from soil beneath a structure is a far greater source of radon inhalation exposure than radon in drinking water for most households. LADWP's blended supply, which includes surface-derived imported water, typically has lower dissolved radon than systems relying entirely on local groundwater.

Sources used in this article: EPA Radionuclides in Drinking Water (40 CFR 141.66); EPA Safe Drinking Water Information System (SDWIS); USGS National Water Quality Assessment (NAWQA); LADWP Consumer Confidence Reports; Las Virgenes MWD Consumer Confidence Reports; NSF International: Certified Water Filters.