The home inspector sets a small canister in the basement, leaves it running for 48 hours, and comes back with a number. If it reads under 4 picocuries per liter, everyone in the transaction exhales. Radon: handled. That is how it works in most of the DMV, and it is how most Great Falls contracts treat it too.
But that canister only measures what is in the air. If the house is on a private well, which a meaningful share of Great Falls properties still are, nobody has yet tested what is in the water coming out of the tap. And in this specific pocket of Fairfax County, that gap matters more than it would in a townhouse a few miles away on public water, because the same bedrock responsible for the county's radon-in-air problem can also put radon directly into the groundwater a well draws from.
This is not a warning about unsafe water. It is a warning about sequencing. The fix for radon in water is different from the fix for radon in air, the timeline to arrange it is different, and if it surfaces after the air test already came back clean, both sides assume the radon question is closed. It is not.
The bedrock doesn't stop at the property line
Great Falls sits in Virginia's Piedmont, the band of terrain between the coastal plain and the Blue Ridge. The Virginia Department of Health describes this region plainly: beneath a thin veneer of soil, the ground is igneous or metamorphic rock, the kind formed under heat and pressure rather than laid down in sediment layers. That rock contains uranium, and uranium decays into radon whether or not anyone is home.
Radon that migrates upward through soil becomes the airborne gas an inspector's canister catches. Radon that dissolves into groundwater flowing through cracks and fractures in that same rock becomes a water issue instead, one that a standard air canister will never detect because it never touches the tap. A house on public water sidesteps this second pathway entirely, since municipal treatment plants typically address it before the water ever reaches a home. A house on a private well does not get that filter. The well draws straight from the rock.
Fairfax already knew this was a problem
Virginia does not require well water testing at the time of sale statewide. But state code carves out a short list of counties allowed to set their own, stricter rules for private well location, construction, and testing, and Fairfax is on that list, alongside places like Loudoun, Prince William, and Fauquier. Fairfax used that authority. The county has its own Private Water Well Ordinance on the books, layered on top of the state's baseline regulations.
That detail is worth sitting with for a second. The county did not add a local well ordinance because private wells here are unusually risky in some alarming way. It added one because enough of the county, including large-lot areas like Great Falls, still relies on private water, and the state's minimum standard, which only mandates a coliform bacteria test at the time a well is first drilled, was not built with resale transactions in mind. After that initial test, further testing is entirely up to the well owner, unless a lender requires it.
Two tests, two different animals
The air test and the water test get lumped together in casual conversation as "the radon thing," but they are not interchangeable, and treating them as one line item on a to-do list is exactly how the water side gets skipped.
| Radon in air | Radon (and quality) in water | |
|---|---|---|
| What it measures | Gas concentration in indoor air, typically basement level | Contaminants dissolved in the well water itself, tested separately for radon |
| Standard in contract | Yes, almost universal in Great Falls inspections | Standard potability panel (bacteria, nitrates, lead) is customary; radon specifically must be requested |
| Typical duration | Short-term test, about 48 hours | Water sample sent to a certified lab, results take days to weeks |
| Who usually orders it | Home inspector, as part of the standard inspection package | Buyer, often required outright by VA lenders when the home is on a private well |
| Cost | Roughly $150 to $250 | Roughly $100 to $400 depending on the contaminant panel |
| If it fails | Mitigation system installed to vent soil gas | Fixes range from simple chlorination for bacteria to, in the rare worst case, a new well entirely |
| Shelf life | Not typically time-limited in a purchase contract | For VA-financed purchases, the result is only valid for 90 days from certification |
The action level for airborne radon is 4 picocuries per liter, the threshold at which the EPA recommends mitigation. Radon dissolved in water is measured and treated differently, usually through aeration systems that bubble the gas out before it reaches household taps, or through carbon filtration. Neither fix is something a buyer wants to be pricing out during the final week before closing.
Septic is running its own clock
While the radon conversation is happening, the septic system needs attention on a separate, longer timeline, and Virginia gives it even less structure than it gives well water. There is no state requirement for a septic inspection before a property changes hands. A lender may ask for one. A buyer's agent should ask for one regardless.
The Virginia Department of Health recommends pumping conventional septic systems every three to five years depending on household use, and advises that any onsite sewage system review happen as early as possible, ideally weeks before closing, because permit work or repairs discovered during that review can take real time to resolve. A buyer who waits until the week of closing to think about the septic system has already lost the ability to negotiate calmly if something comes back needing work.
What the timeline actually looks like
For a private-well, private-septic property in Great Falls, the due diligence period is doing more work than it would for a comparable home on public utilities. A realistic sequence looks like this:
- Order the standard well water potability test (bacteria, nitrates, lead) as soon as the home goes under contract, and ask the lab to add a radon-in-water panel at the same time rather than treating it as a separate errand later.
- Schedule the septic inspection and, if it has been more than three to five years, the pump-out, in the same window. This is the item most likely to reveal something that needs lead time to fix.
- Let the home inspector set the radon-in-air canister during the standard inspection window, as usual.
- If financing with a VA loan, track the 90-day clock on the water test result from the certification date, not from the date the sample was drawn.
- If any result comes back elevated, get pricing on the fix before deciding whether it is a negotiation point or a walk-away point. Chlorination and minor water treatment are inexpensive. A new well is not.
None of this changes because the house is charming or the offer is competitive. It changes because of what is under the lot, and that does not show up on a listing photo.
The point worth keeping
A radon canister passing in the basement tells a buyer one thing is fine. In most of the DMV, that is the whole story. In Great Falls, where private wells are common and the bedrock is the same rock responsible for the area's radon-in-air readings in the first place, it is only half the story. The other half lives in the water test, and it is the half most contracts never mention by name.
A few questions worth asking directly
Does Virginia require a well water test before a home sale can close? No. State law does not mandate it, but it is standard practice in purchase contracts, and VA lenders typically require it when the property is on a private well.
How long is a well water test valid? For VA-financed purchases, the certified result is valid for 90 days from the certification date. Plan the testing window with that clock in mind, especially if financing falls through and a deal has to be restructured.
Is a septic inspection required before closing in Virginia? No state law requires one, but many lenders do, and the Virginia Department of Health advises starting the review weeks ahead of closing since repairs or permits can take time that a tight contract timeline does not have.
If you are buying or selling a well-and-septic property in Great Falls, the sequence matters as much as the results themselves. Cesar Castillo works through this kind of due diligence timeline with clients regularly across Fairfax County. Schedule a free consultation to map out what your specific contract timeline should look like before you are three weeks into it.
