By SCWS Team
Published February 17, 2026 · 10 min read
Radon is a colorless, odorless radioactive gas that can dissolve in groundwater and enter your home every time you turn on the tap. While most people know about radon seeping through basement cracks, fewer realize that well water can be a significant source of indoor radon exposure.
When radon-containing water is used for showers, laundry, and cooking, the gas releases into your home's air where it can be inhaled. Radon is the second leading cause of lung cancer after smoking, making it a serious health concern for well owners in high-radon areas.
Understanding Radon
Radon (Rn-222) is a naturally occurring radioactive gas produced by the radioactive decay of uranium, which is found in varying amounts in soil and rock worldwide. As uranium breaks down over millions of years, it produces radium, which then decays into radon gas.
How Radon Gets Into Groundwater
When groundwater flows through rocks and soil containing uranium or radium, radon dissolves directly into the water. Because radon is a noble gas, it doesn't react chemically with other substances—it simply dissolves and travels with the water until it's released.
Radon levels in groundwater vary enormously based on local geology:
- Granite formations: Often contain elevated uranium; common in mountainous regions
- Shale deposits: Can have significant uranium content
- Phosphate-rich areas: Phosphate rock often contains uranium
- Glacial deposits: Variable, depending on source rock
California Radon Zones
While California generally has lower radon levels than the northeastern U.S., significant variations exist. Mountainous areas with granite geology, including parts of the Sierra Nevada foothills and some Southern California mountain communities, can have elevated radon. The EPA's radon zone map provides general guidance, but local conditions vary—testing is the only way to know your specific situation.
Health Risks of Radon Exposure
Inhalation Risk (Primary Concern)
The primary health risk from waterborne radon comes from breathing radon-contaminated air, not from drinking the water. When radon-containing water is used throughout your home, the gas escapes into indoor air:
- Showers and baths: Hot water and agitation release radon rapidly
- Dishwashers and washing machines: Contribute to indoor radon
- Running faucets: Any water use releases some radon
The EPA estimates that approximately 1-2% of radon in water transfers to indoor air. While this sounds small, high water concentrations can significantly impact indoor air quality. As a rule of thumb, every 10,000 pCi/L (picocuries per liter) of radon in water adds about 1 pCi/L to indoor air radon levels.
Inhaled radon decays in the lungs, releasing alpha particles that can damage lung tissue and increase lung cancer risk. The EPA estimates radon causes about 21,000 lung cancer deaths annually in the United States.
Ingestion Risk (Lower but Present)
Drinking radon-containing water exposes the stomach and digestive tract to radiation. While the risk is lower than inhalation, studies suggest possible associations with stomach cancer. The EPA estimates that drinking water with 4,000 pCi/L of radon over a lifetime could cause 1-2 stomach cancer deaths per 10,000 exposed people.
Testing for Radon in Well Water
Radon testing requires special procedures because the gas escapes quickly from water:
When to Test
- If indoor air radon tests show elevated levels (above 4 pCi/L)
- If you live in an area with known granite or uranium-bearing geology
- If neighbors have found radon in their water
- When buying property with a well
- If you're concerned about comprehensive water quality
Testing Procedure
- Use a certified laboratory experienced in radon water testing
- Follow lab instructions exactly—sample containers and procedures matter
- Run water for several minutes before sampling to clear standing water
- Minimize air exposure during sample collection
- Keep samples cold and deliver quickly (usually within 24-48 hours)
Testing costs range from $25-100 depending on the laboratory. Some state radon programs offer subsidized testing.
Interpreting Results
There's no enforceable federal standard for radon in private well water. However, these guidelines help interpret results:
| Radon Level (pCi/L) | Recommendation |
|---|---|
| Less than 300 | Generally considered low; EPA's proposed MCL for public systems |
| 300-4,000 | Moderate; consider treatment, especially with elevated indoor air radon |
| 4,000-10,000 | Elevated; treatment recommended |
| Above 10,000 | High; treatment strongly recommended |
Treatment Options
Two main technologies effectively remove radon from water:
Aeration Systems
Aeration is the most effective treatment for high radon levels:
- How it works: Air bubbles through the water, allowing radon to escape; the gas is vented safely outdoors
- Effectiveness: 95-99% radon removal
- Best for: High radon levels (above 10,000 pCi/L)
- Cost: $3,000-5,000+ for residential systems
- Maintenance: Regular cleaning; air blower maintenance
- Pros: Very effective; no radioactive waste; works for any concentration
- Cons: Higher cost; needs proper outdoor venting; noise from blowers
Granular Activated Carbon (GAC)
GAC filters adsorb radon onto activated carbon:
- How it works: Water passes through carbon filter; radon adheres to carbon surface
- Effectiveness: 90-99% removal, depending on contact time
- Best for: Moderate radon levels (under 10,000 pCi/L)
- Cost: $1,000-2,500 for residential systems
- Maintenance: Carbon replacement every 3-5 years
- Important consideration: Carbon becomes radioactive over time
- Pros: Lower cost; simpler installation
- Cons: Radioactive waste disposal; radiation buildup near unit
Important: Radioactive Carbon
When GAC filters remove radon, the radon decays on the carbon, leaving behind radioactive decay products. This can create a radiation hazard around the filter unit. GAC systems should be located away from living areas, and spent carbon may require disposal as low-level radioactive waste. For high radon levels, aeration is preferred to avoid this issue.
Point-of-Entry vs. Point-of-Use
For radon, point-of-entry (whole house) treatment is essential. Unlike contaminants where you might only treat drinking water, radon releases from all water use throughout the home. A point-of-use system at the kitchen tap wouldn't address radon released during showers, laundry, or dishwashing—which are actually major sources of airborne radon.
Radon in Water vs. Air
If you have elevated indoor radon, it's worth understanding whether your water is contributing:
- Soil is usually the main source: Most indoor radon comes from soil gas entering through foundations
- Water can be significant: Very high water radon can measurably increase air levels
- Test both: If air radon is elevated, test water to understand the source
- Address both if needed: Water treatment alone won't solve a soil-gas problem
The relationship: approximately 10,000 pCi/L of radon in water contributes about 1 pCi/L to indoor air. If your water contains 20,000 pCi/L and your indoor air reads 6 pCi/L, water is contributing about 2 pCi/L of that total.
Frequently Asked Questions
How does radon get into well water?
Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. When groundwater flows through rock formations containing uranium, radon dissolves into the water. This is particularly common in areas with granite, shale, or phosphate-rich rocks. Radon levels in water vary greatly by location—some areas have minimal radon while others have very high concentrations.
Is radon in well water dangerous?
Yes, radon in well water poses health risks, though the primary danger is from inhalation rather than drinking. When water containing radon is used for showers, washing, and cooking, radon gas escapes into indoor air where it can be inhaled. Inhalation of radon is the second leading cause of lung cancer after smoking. The EPA estimates that 1-2% of radon in water transfers to indoor air.
What is a safe level of radon in well water?
The EPA proposed a Maximum Contaminant Level of 300 pCi/L for public water systems, with an Alternative MCL of 4,000 pCi/L for systems implementing radon mitigation programs. There's no enforceable federal standard for private wells. Many experts recommend treatment when radon exceeds 4,000 pCi/L in water.
How do I test my well water for radon?
Radon water testing requires special sample collection because radon escapes rapidly from water when exposed to air. Use a certified laboratory and follow their specific instructions carefully. Samples must be kept cold and delivered to the lab quickly. Testing costs $25-100.
How do you remove radon from well water?
Two main treatment methods effectively remove radon: aeration systems bubble air through water allowing radon to escape (95-99% removal), and granular activated carbon filters adsorb radon (90-99% removal). Aeration is preferred for high levels above 10,000 pCi/L. Both require point-of-entry installation to treat all household water.
Concerned About Radon in Your Well?
Southern California Well Service can help coordinate water testing and connect you with water quality solutions. Call us for expert guidance.
Call (760) 463-0493