Testing for radon in water requires collecting a sample and sending it to a certified lab. Unlike air radon tests, which you can do entirely at home with a passive detector, water radon tests need laboratory analysis using specialized equipment. The process is straightforward: you order a sampling kit, collect water from your tap following specific instructions, and mail it back. Results typically come within a few weeks and are measured in picocuries per liter (pCi/L).
Why Radon in Water Matters
Radon is a radioactive gas that dissolves easily in groundwater as it passes through rock and soil containing uranium. When that water enters your home and you run a faucet, shower, or dishwasher, radon escapes into the air you breathe. The bigger health risk is actually this released gas, not the water itself. A National Research Council analysis estimated that radon released from household water causes roughly 160 lung cancer deaths per year in the U.S., while drinking radon-contaminated water causes about 20 stomach cancer deaths annually. In other words, inhalation risk outweighs ingestion risk by about 8 to 1.
This is why experts generally recommend testing your water for radon only if you already know your indoor air radon levels are elevated, or if you rely on a private well in a region with known radon activity. If your home’s air radon level is high and you use well water, the water could be a contributing source.
Who Needs to Test
Private well owners are the primary group that should consider water radon testing. There is no federally enforced drinking water standard for radon, but the EPA has proposed that community water systems keep radon below 4,000 pCi/L (or 300 pCi/L in states without enhanced indoor air programs). Municipal water suppliers that draw from surface sources like rivers and reservoirs generally have very low radon levels because the gas escapes naturally from open water. Groundwater-fed public systems may have higher levels, but these are monitored by the utility.
If you’re on a private well, no one is testing for you. The responsibility falls entirely on you as the homeowner. Wells drilled into granite, shale, or other uranium-bearing rock formations are at the highest risk.
How to Collect a Water Sample
You’ll start by ordering a radon water test kit from a state-certified laboratory. Many state labs offer these kits, and some private labs do as well. The Wisconsin State Laboratory of Hygiene, for example, charges about $101 for a radon-in-water analysis. Kits are typically shipped to you at no cost, though you’ll pay for return shipping.
The sampling process is designed to prevent radon from escaping the water before the lab can measure it. Radon is a gas, so any exposure to air during collection will lower your reading and give a falsely reassuring result. Here’s the general process most labs require:
- Run the tap first. Let cold water run for several minutes before collecting your sample. This purges stagnant water sitting in your pipes and gives you a sample that reflects what’s actually coming from your well.
- Fill the vial underwater. Most kits include a small glass vial. You’ll submerge it in a container of running water or hold it directly under the stream, filling it completely with no air space at the top.
- Cap it while still submerged. The EPA’s standard method specifies capping the sample container while it’s still under water to eliminate any air pocket (called “headspace”) that would let radon escape. Even a small bubble can skew results.
- Ship it quickly. Radon decays with a half-life of about 3.8 days, meaning the concentration drops by half every few days. Most labs want your sample within 24 to 48 hours of collection. Follow the kit’s shipping instructions carefully.
In the lab, your sample is analyzed using a method called liquid scintillation counting. A small portion of your water (usually about 10 mL) is mixed with a special solution in a glass vial. As radon decays, it produces tiny flashes of light in that solution, and a detector counts those flashes to calculate the radon concentration.
Understanding Your Results
Your lab report will show radon concentration in picocuries per liter. Since there’s no final federal standard, the two reference points from the EPA’s proposed rule are the most commonly cited benchmarks:
- 300 pCi/L: The stricter proposed limit, intended for states without enhanced indoor air radon programs. Many health agencies use this as a practical action level for private wells.
- 4,000 pCi/L: The alternative proposed limit for community systems in states with active indoor radon reduction programs. At this level, radon from water contributes roughly 0.4 pCi/L to your indoor air.
Some states have set their own guidelines. If your result is above 4,000 pCi/L, treatment is strongly recommended regardless of where you live. Between 300 and 4,000 pCi/L, the decision depends on your overall indoor air radon levels, your state’s recommendations, and your comfort with the risk. A result below 300 pCi/L is generally considered low.
How Often to Retest
Radon levels in groundwater can shift over time as geological conditions change, water tables rise and fall, and well casings age. Massachusetts’ Department of Environmental Protection recommends testing initially and then at minimum once every ten years. If you install a treatment system, you should retest more frequently to confirm the system is still performing well.
Choosing a Certified Lab
Not every water testing lab is equipped to measure radon. You want a lab certified under your state’s drinking water laboratory certification program. The EPA maintains a directory of state certification officers who can point you to approved labs in your area (available at epa.gov/dwlabcert). Your state health department or environmental agency can also provide a list. Many state university extension programs maintain their own testing labs and offer kits directly to residents.
Professional on-site testing is another option. Some radon mitigation companies will come to your home, collect samples, and handle the lab submission. This typically costs more than a mail-in kit but removes the risk of sampling error. If your first result is borderline or surprisingly high, having a professional collect a confirmation sample is a reasonable next step.
Treatment Options if Levels Are High
Two main approaches can reduce radon in well water: aeration and activated carbon filtration. Both are point-of-entry systems, meaning they treat all water coming into your home, not just one faucet.
Aeration systems work by spraying or bubbling water to release dissolved radon gas, which is then vented safely outside. These are the most effective option. A venturi-type aeration system tested on U.S. water supplies achieved removal rates of 78 to 95%. Aeration doesn’t add chemicals or produce radioactive waste, making it the preferred choice for high radon concentrations.
Granular activated carbon (GAC) filters adsorb radon as water passes through a carbon bed. They’re generally less effective, with removal rates typically ranging from 40 to 80%. GAC filters also accumulate radioactive material over time, which means the filter itself can become a low-level radiation source that needs proper handling and disposal. For this reason, GAC is usually recommended only for moderate radon levels, not for extremely high concentrations.
The cost of a whole-house treatment system varies widely based on your water flow rate and radon level, but most homeowners can expect to spend between $1,000 and $5,000 for equipment and installation. After installation, retesting your water confirms the system is doing its job.