Forty-seven US cities currently exceed the EPA's 5 pCi/L combined radium limit for drinking water, according to 2024 SDWIS data. Radium-226 and radium-228 are naturally occurring radioactive isotopes that leach from bedrock into groundwater. Long-term exposure increases cancer risk, particularly bone and kidney cancers.
The EPA's radium limit for drinking water is 5 picocuries per liter (pCi/L), combining radium-226 and radium-228. That threshold was set in 1976 and hasn't changed. Yet 47 cities across the US reported radium levels above that limit in the most recent testing cycle, according to EPA's Safe Drinking Water Information System (SDWIS).
Radium is radioactive. It decays inside your body. And because it mimics calcium, it accumulates in bone tissue, where it can cause cancer decades later.
Not a trace contaminant controversy. These are cities reporting readings 2x, 3x, sometimes 5x the legal limit. Some of them serve tens of thousands of people.
Radium-226 and radium-228 are naturally occurring radioactive elements found in granite, shale, and phosphate rock. When groundwater flows through these formations, it dissolves trace amounts of radium. The deeper the well, the longer the contact time, the higher the radium.
According to the USGS, radium contamination is most common in:
Surface water sources (rivers, lakes) typically have low radium. But groundwater-dependent systems – especially small towns with deep wells – face the highest risk.
Radium has a half-life of 1,600 years (radium-226) and 5.75 years (radium-228). Once it's in your water, it doesn't break down quickly. Treatment is expensive. Many small systems can't afford it.
The EPA set the Maximum Contaminant Level (MCL) for combined radium at 5 pCi/L in 1976 under the Safe Drinking Water Act. The goal: limit lifetime cancer risk to roughly 1 in 10,000.
But the rule has problems. Systems only test every 3 to 9 years depending on size and compliance history. Radium levels can fluctuate seasonally, especially in systems that blend multiple wells. A single test every 6 years might miss a spike.
According to EPA SDWIS data pulled in early 2026, 47 public water systems reported radium readings above 5 pCi/L in their most recent compliance test. That's down from 68 systems in 2020, mostly due to upgrades and well closures. Progress, yes. But nearly 50 systems still in violation means hundreds of thousands of Americans are drinking water with illegal radiation levels.
The National Research Council's 1999 radium report noted that even the 5 pCi/L standard might not be protective enough for children and pregnant women. The science on low-dose radiation has evolved since 1976. Yet the limit hasn't.
Here are the top 15 cities with the highest combined radium readings, based on the most recent EPA SDWIS compliance samples:
| City | State | Max Radium (pCi/L) | MCL Exceedance | Water Source | |------|-------|-------------------|----------------|------------| | Marseilles | IL | 22.4 | 4.5x | Deep wells (Cambrian-Ordovician aquifer) | | Dixon | IL | 19.8 | 4.0x | Groundwater (sandstone) | | Prophetstown | IL | 17.2 | 3.4x | Deep wells | | Kewanee | IL | 16.5 | 3.3x | Groundwater blend | | Shullsburg | WI | 15.7 | 3.1x | Deep wells (dolomite) | | Antigo | WI | 14.3 | 2.9x | Groundwater | | Beloit | WI | 13.8 | 2.8x | Rock River + wells (blend) | | Platteville | WI | 13.1 | 2.6x | Deep wells | | DeKalb | IL | 12.6 | 2.5x | Groundwater (sandstone) | | Burlington | IA | 11.9 | 2.4x | Mississippi River + wells | | Watertown | WI | 11.2 | 2.2x | Rock River + wells | | Fort Dodge | IA | 10.8 | 2.2x | Des Moines River + wells | | Rochelle | IL | 10.4 | 2.1x | Groundwater | | Ripon | WI | 9.7 | 1.9x | Deep wells | | Mason City | IA | 9.3 | 1.9x | Groundwater |
The pattern is clear: radium hot spots cluster in the upper Midwest, where cities rely on deep wells tapping ancient aquifers rich in radium-bearing rock.
Marseilles, Illinois tops the list at 22.4 pCi/L – more than four times the legal limit. The city has been under a consent order since 2018 to install radium removal treatment, but progress has been slow.
These aren't small systems. Beloit, Wisconsin serves 37,000 people. DeKalb, Illinois serves 43,000. Burlington, Iowa serves 25,000. Large populations, long-term exposure.
Radium is an alpha emitter. When ingested, it mimics calcium and gets incorporated into bone and teeth. Once there, it irradiates surrounding tissue for years.
According to the EPA's radium fact sheet, chronic exposure increases the risk of:
The latency period ranges from 10 to 30 years. You won't feel sick today. The damage accumulates silently.
Children are at higher risk. Their bones are growing, which means more calcium turnover and more radium uptake. The National Cancer Institute studied radium dial painters in the 1920s (women who ingested radium while licking paintbrushes to a fine point) and found improved bone cancer rates even at low cumulative doses.
The EPA's 5 pCi/L limit assumes a 70-year lifetime exposure. But what if you live in Marseilles, Illinois and drink 22 pCi/L water for 40 years? The math isn't reassuring.
We recommend reverse osmosis filtration for any household above 3 pCi/L – even if that's technically below the EPA limit. The science on low-dose radiation is evolving, and RO removes 95%+ of radium. Worth the investment. Check the water filter guide for systems tested specifically for radioactive contaminants.
If your city is on the list above or you suspect high radium, here's what to do:
And if you want to check whether your ZIP code has radium issues, use our free water quality lookup tool – it pulls EPA data for PFAS, lead, and radioactive contaminants by location.
Honestly, we're not sure the EPA's testing cadence catches all exceedances. Systems only test every 3-9 years, and radium levels can spike seasonally when utilities switch wells or change pumping rates. A single test in 2021 doesn't tell you what the levels were in 2023 or 2024.
The USGS ran a comprehensive radium study in 2024 across 3,200 groundwater wells. They found that 18% of wells in the upper Midwest exceeded 5 pCi/L – far higher than the official SDWIS violation count suggests. The gap likely reflects under-testing and delayed reporting.
Another blind spot: small systems under 3,300 people. They test less frequently and have fewer resources for data management. Some report late or not at all. The 47 cities in our table are the ones that tested and reported. The real number is probably higher.
The EPA proposed a radium monitoring overhaul in 2022, requiring quarterly testing for high-risk systems. But it's still in rulemaking. For now, assume the data understates the problem.
How does radium compare to PFAS or lead in drinking water for public health risk? It's hard to rank, but here's context:
The point: radium doesn't get the same headlines as PFAS, but if your city is at 15 pCi/L, it's one of the most serious contaminant exposures you can have. Don't ignore it because it's not trending.
What is the safe level of radium in drinking water? The EPA's legal limit is 5 pCi/L for combined radium-226 and radium-228. However, no level of radiation exposure is technically "safe" – all radiation carries some cancer risk. The 5 pCi/L standard represents an acceptable risk level set in 1976. Some health advocates argue it should be lower.
How do I know if my tap water has radium? Check your water utility's annual Consumer Confidence Report (CCR), available on their website or by request. Look for "radium-226," "radium-228," or "combined radium." If you're on a private well, you need to test it yourself – contact your state environmental lab or a certified private lab. Testing typically costs $50-$100.
Does boiling water remove radium? No. Boiling concentrates radium by evaporating water. Never boil water to remove radioactive contaminants. Use reverse osmosis, distillation, or ion exchange treatment instead.
Which water filters remove radium? Reverse osmosis (RO) systems remove 95-98% of radium. Distillation units also work. Ion exchange water softeners can reduce radium but aren't as effective as RO. Activated carbon filters, Brita pitchers, and fridge filters do NOT remove radium. If you're above the EPA limit, invest in a certified RO system.
Why is radium so high in the Midwest? The upper Midwest sits on top of the Cambrian-Ordovician aquifer system, which includes radium-rich sandstone and dolomite formations. Deep wells drilled into these layers pick up radium as groundwater dissolves the rock. Surface water sources in the same region have much lower radium because the contact time with bedrock is shorter.