Why Is My Radon Still High After Mitigation? (And What Colorado Homeowners Need to Know About Radon in Water)
If you're a Colorado homeowner who tested for radon, had a mitigation system installed, and then retested only to find the number still sitting above 4 pCi/L, you're not imagining things and you're not alone.
Radon mitigation is one of the most effective home safety interventions there is - but "effective" isn't the same as "foolproof," and Colorado has a second radon pathway that most homeowners never think about: the water coming out of the tap.
This post walks through both issues: why a properly-sounding mitigation system can still leave you with high readings, and why Colorado's geology means your well water might be feeding radon into your air supply even after your soil gas system is working perfectly.
First, a quick refresher on what mitigation actually does
The gold-standard fix for soil-gas radon is called active soil depressurization (ASD), most commonly installed as sub-slab depressurization (SSD). A pipe is run through your foundation slab into the soil or gravel beneath it, and a fan pulls radon-laden soil gas out from under the house and vents it above the roofline before it can seep indoors.
The mechanism is two-fold: it reverses the pressure gradient across your slab (so air is being pulled out from under the house rather than radon being pushed in), and it lowers the concentration of radon in the soil gas immediately beneath the slab.
When installed correctly, this method works remarkably well. Engineering literature on sub-slab systems generally reports reductions in the 90–99% range, and the EPA's own design documentation treats a 90 - 95% reduction as the expected outcome for a correctly engineered system.
So why do some homes retest high anyway?
Reason 1: The pressure field never reached every part of the foundation
An SSD system only protects the parts of the sub-slab area where negative pressure actually develops. Studies of sub-slab pressure fields have found that pressure spreads efficiently and uniformly through gravel and other coarse, permeable fill, but drops off quickly with distance in dense native soil, meaning a suction point placed too far from a room, or in a home with compact or poorly draining soil, can leave large sections of the slab essentially untreated even while the system "works" everywhere else.
This is one reason contractors talk about needing a proper diagnostic test (sometimes called a pressure field extension, or PFE, test) before finalizing a design - a home with unusually dense or heterogeneous soil may genuinely need more than one suction point to cover the whole foundation footprint.
Homes with multiple, disconnected foundation sections (a classic example being additions, converted crawl spaces, or homes with more than one foundation "wing") are especially prone to this. If part of the foundation can't be physically accessed or wasn't included in the system design, it simply won't benefit from the depressurization at all.
Reason 2: Underpowered fans, or "bigger fan" fixes that don't fix the real problem
A fan that's too small for the volume of soil gas underneath a large or leaky home won't establish enough negative pressure to counteract radon entry. It's tempting, and common in the industry, to respond to a still-high reading by simply installing a larger fan. Sometimes that helps.
But if the underlying problem is a poorly sealed sump pit, an unsealed crawl space vapor barrier, or missed entry points rather than raw suction power, a bigger fan mostly adds noise and electricity cost without solving anything. The fix in that scenario isn't more suction, it's finding and sealing the pathway that's letting radon bypass the system entirely.
Reason 3: A high water table blocking the air pathway
Sub-slab systems work by moving air through the soil beneath your home and air can't move through saturated soil. If your home sits in an area with a high water table, or floods easily, water in the sub-slab gravel bed can physically choke off the pathway your depressurization system depends on.
In these cases, the mitigation system isn't broken; the site conditions are overwhelming it, and the underlying water issue (drainage, grading, sump pump function) usually needs to be addressed before the radon system can perform as designed.
Reason 4: Mechanical failure or drift over time
Radon mitigation isn't a "set it and forget it" fixture - it's an active mechanical system, and mechanical systems fail. A stalled or failed fan will let radon levels climb right back to baseline, and because these fans typically run continuously for years, motor failure eventually happens to essentially every system. Sump pumps that double as part of the depressurization seal can fail the same way.
On the structural side, even modest soil settling or foundation shifting can open new entry pathways after the fact, and renovations like finishing a basement, adding a room, disturbing soil during landscaping, can change airflow patterns enough to require the system to be reassessed.
This is why the EPA and state radon programs recommend retesting every two years even after a successful mitigation, and it's also why manometers (the little U-tube gauge on your vent pipe) exist - a quick glance tells you whether the fan is still pulling.
What to do if your number is still high
Confirm it's not a testing artifact. Radon fluctuates with weather, season, and barometric pressure, so one high short-term test isn't necessarily proof the system failed - but a pattern across retests is.
Check the fan and manometer. If the gauge shows no vacuum, the fan has likely stopped working.
Ask for a PFE diagnostic. A qualified mitigator can measure how far the negative pressure actually extends and identify sections of the slab that aren't being protected.
Inspect for new gaps or unsealed penetrations - sump pits, crawl space vapor barriers, and slab penetrations are common culprits.
Rule out water in the equation (see below) - this is the piece Colorado homeowners in particular tend to overlook.
The Colorado twist: radon in your water, not just your soil
Most public conversation about radon focuses on soil gas entering through the foundation. But radon is also water-soluble, and where your water comes from matters enormously. Colorado is one of a handful of states - alongside parts of the Appalachians, the Rocky Mountain region generally, and the Basin and Range - that federal water-quality surveys consistently flag for elevated radon in groundwater, largely because of uranium-bearing geology in the Rockies, the Colorado Plateau, and the Great Plains formations that underlie much of the state.
This isn't a minor academic point.
A peer-reviewed environmental health study of 27 private wells in a rural Colorado mountain community found that 100% of the sampled wells exceeded the EPA's proposed maximum contaminant level for radon in drinking water, and more than a third exceeded the higher "alternate" limit as well - far above the average level in the local municipal supply.
Separately, USGS research near Conifer, Colorado found that radon concentrations in groundwater there were driven by highly localized geologic factors — pockets of uranium-bearing rock, faults, and folds — rather than anything predictable from the water's general chemistry, which is part of why radon levels can vary enormously between two wells just a short distance apart.
Why this matters for your indoor air
Here's the mechanism: when water containing radon is used for showering, running the dishwasher, or doing laundry, the radon degasses out of the water into your indoor air, similarly to how carbon dioxide fizzes out of a soda when you open the bottle. The EPA's general rule of thumb is that about 10,000 pCi/L of radon in water adds roughly 1 pCi/L to your indoor air radon reading. That means a well delivering a few thousand pCi/L (not unusual in Colorado's fractured granite terrain) can be a meaningful, ongoing contributor to indoor air levels, one that a soil-gas mitigation system does nothing to address, because it's not coming from the soil at all.
This is precisely the scenario that trips people up: a homeowner installs a textbook-perfect sub-slab depressurization system, watches soil-gas entry points get sealed, and still can't get the number down - because the source was never solely the soil. If you're on a private well and haven't specifically tested your water for radon (a separate test from standard air testing), it's worth asking whether it explains a stubborn air reading.
What to do about radon in water
If well-water testing confirms elevated radon, two treatment approaches are established and effective:
Granular activated carbon (GAC) filtration: water passes through activated carbon that adsorbs radon. It tends to cost less up front, but because radioactive decay products accumulate on the filter media over time, spent filters can require special handling and disposal.
Aeration systems: air is bubbled or sprayed through the water in a sealed chamber, stripping the radon out and venting it outdoors via an exhaust fan, similar in spirit to how a soil-gas system vents radon from beneath your slab. Aeration methods are generally considered more effective at very high water-radon concentrations and avoid the radioactive buildup issue that GAC has, though the vented radon does need to be exhausted safely outdoors rather than into an enclosed space.
Either system is typically installed as a point-of-entry treatment (treating all water entering the house, not just the kitchen tap) since the exposure pathway (showering, laundry) matters as much as drinking water itself.
Red flags that suggest you should investigate further
Your air mitigation system tests fine at the vent pipe (manometer shows vacuum, fan is audibly running) but indoor levels are still elevated.
You're on a private well, especially in the Front Range, the mountains west of Denver, or other areas with granite or gneiss bedrock.
Your neighbors' test results vary wildly from yours despite similar homes - a signature of the highly localized geology that drives water-radon variability in Colorado.
It's been more than two years since your last test, mitigated or not - the EPA and Colorado's radon program both recommend periodic retesting regardless of past results.
The bottom line
A high reading after mitigation doesn't necessarily mean you were sold a bad system or a bad installer, though it can mean that. Just as often it means the diagnosis was incomplete: a suction point that didn't reach the whole foundation, a fan that's since failed, a new crack from a renovation, or - especially relevant if you're on Colorado well water - a radon source that was never in the soil to begin with. Getting to the bottom of it usually means retesting carefully, having a certified mitigator check the mechanical system, and, if you're on a private well, testing the water itself rather than assuming the soil-gas system should have handled everything.
This article is for general educational purposes and isn't a substitute for a site-specific evaluation by a certified radon professional. If you suspect elevated radon in your home or water, testing is inexpensive and the first step regardless of what you ultimately find.
Sources
U.S. EPA, Handbook: Sub-Slab Depressurization for Low Permeability Fill Material (design and performance data for SSD systems)
Lawrence Berkeley National Laboratory, Relative Effectiveness of Sub-Slab Pressurization and Depressurization Systems for Indoor Radon Mitigation
ScienceDirect, A full-scale experimental study of sub-slab pressure fields induced by underground perforated pipes as a soil depressurisation technique in radon mitigation
CLU-IN/EPA technical report, Design, Effectiveness, and Reliability of Sub-Slab Depressurization Systems
National Academies Press / NCBI Bookshelf, Risk Assessment of Radon in Drinking Water — chapters on baseline radon-in-water data and water mitigation techniques
Reese, et al., Radon-Contaminated Drinking Water From Private Wells: An Environmental Health Assessment Examining a Rural Colorado Mountain Community's Exposure, Journal of Environmental Health, 76(4), 2013
U.S. Geological Survey, Geohydrologic, geochemical, and geologic controls on the occurrence of radon in ground water near Conifer, Colorado
U.S. Geological Survey, Quality of Groundwater in the Denver Basin Aquifer System, Colorado, 2003–5
U.S. Geological Survey, Water Quality in the Rio Grande Valley, Colorado, New Mexico, and Texas, 1992–95
U.S. EPA, Basic Information About Radon in Drinking Water and Radon in Drinking Water: Questions and Answers
Colorado Geological Survey, Radon program overview
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