Radon in Williamsport, PA: The Zone 1 City Where Radon Is in the Air and the Water

Williamsport is known nationally as the home of the Little League World Series. Within Pennsylvania's radon landscape, it should be known for something else entirely: it is one of the few cities in the state where radon is a documented problem in both indoor air and drinking water — a dual-pathway exposure that most Pennsylvania homeowners never consider.

Every other city in the PA Radon Hub dataset deals with airborne radon: gas migrating from bedrock through soil into building foundations. Williamsport has that problem too — Lycoming County is EPA Zone 1, and an estimated 41.5% of tested homes exceed the action level. But USGS testing in 2014 revealed that two-thirds of sampled private wells in Lycoming County contained radon-222 concentrations exceeding the proposed federal drinking water standard of 300 picocuries per liter. Three samples exceeded the less protective proposed standard of 4,000 pCi/L.

For Williamsport homeowners on private wells — which includes much of the rural and suburban housing outside the city's municipal water service area — radon enters the home through two independent pathways: up through the foundation and out of the tap. A home with an ASD system addressing airborne radon may still have elevated radon exposure if its well water is untreated.

The Geology Beneath Williamsport

The Susquehanna West Branch Valley

Williamsport sits in the valley of the West Branch Susquehanna River, carved through a sequence of Paleozoic sedimentary rocks. The bedrock includes Devonian-age shale and siltstone (approximately 360–420 million years old) and underlying Silurian-age limestone and shale (approximately 420–440 million years old). Both rock types contain naturally occurring uranium-238, with concentrations sufficient to sustain elevated indoor radon levels across the valley.

The geology here differs from the other major radon systems in Pennsylvania. Williamsport is not on the Reading Prong (crystalline granite), not on developed karst limestone (like State College), and not in an anthracite mining zone (like Scranton). The Susquehanna West Branch valley represents a distinct geological setting: Appalachian Valley and Ridge province geology, where folded and faulted sedimentary rocks create alternating zones of higher and lower radon potential based on the specific formation exposed at the surface.

Ridge and Valley Structure

The terrain surrounding Williamsport is classic Ridge and Valley topography — long, parallel ridges of resistant sandstone alternating with valleys carved in softer shale and limestone. The ridges (Bald Eagle Mountain to the south, various smaller ridges to the north) are generally lower in radon potential. The valleys — where Williamsport and most of the county's population resides — are underlain by the more uranium-rich shale and limestone formations that produce the elevated radon readings.

This ridge-valley structure means that radon risk in Lycoming County is strongly correlated with topographic position. Valley-floor communities face higher risk than ridgetop properties. Williamsport, sitting on the valley floor of the West Branch corridor, is in the highest-risk topographic position in the county.

The Marcellus Shale Connection

The Marcellus Shale — the same Devonian black shale that drives Pittsburgh's radon baseline — is present in the deep subsurface beneath Lycoming County. Lycoming County has been a significant area for Marcellus Shale natural gas development since the late 2000s. While the relationship between hydraulic fracturing and indoor radon remains scientifically debated, the Marcellus itself is an established radon source formation. Its presence beneath the valley adds to the uranium budget of the local geological column.

For the full analysis of how the Marcellus Shale, Appalachian Valley geology, and other Pennsylvania formations produce indoor radon, see our geology pillar post.

The Well Water Radon Problem

What the USGS Found

In 2014, the USGS Pennsylvania Water Science Center tested 75 private drinking water wells across Lycoming County. The results for radon-222 were striking: 50 out of 75 samples — two-thirds — contained radon concentrations exceeding the proposed federal drinking water standard of 300 picocuries per liter. Three samples exceeded 4,000 pCi/L, a level at which waterborne radon contributes meaningfully to indoor airborne concentrations when the water is used for showering, laundering, and dishwashing.

The USGS noted that these radon levels are consistent with statewide patterns in groundwater drawn from fractured bedrock aquifers — the same rock types that produce elevated airborne radon. Lycoming County's radon problem is not limited to the air; it permeates the geological column and enters homes through every available pathway.

How Waterborne Radon Becomes Airborne

Radon dissolved in water is released into indoor air whenever the water is agitated or heated. The highest-release activities are showering (hot water, aerosolization), washing machines (agitation), and dishwashers (heat and spray). The EPA estimates that for every 10,000 pCi/L of radon in water, approximately 1 pCi/L is added to the indoor air.

For a Lycoming County well producing water at 3,000 pCi/L — well within the range found by the USGS — the waterborne contribution to indoor air radon is approximately 0.3 pCi/L. This is a modest addition to the airborne radon entering through the foundation, but it is additive. A home testing at 3.7 pCi/L from the foundation pathway alone could be pushed above the 4.0 pCi/L action level by the waterborne contribution.

Who Is Affected

Municipal water systems — including the Williamsport Municipal Water Authority — treat and aerate water before distribution, which removes most dissolved radon before it reaches homes. Homeowners served by municipal water are generally not affected by the waterborne pathway.

The risk is concentrated in homes on private wells, which includes much of the suburban and rural housing in Lycoming County outside the city's service area. These homes receive untreated groundwater directly from the bedrock aquifer. If the well draws from fractured Devonian or Silurian rock with elevated radium concentrations, the water can carry radon directly into the home's plumbing system.

What to do: Homeowners on private wells in Lycoming County should test both indoor air and well water for radon. Airborne radon testing follows standard ANSI-AARST protocols. Water testing requires a dedicated water sample collected at the point of entry before any treatment. If water radon exceeds 4,000 pCi/L, granular activated carbon (GAC) filters or aeration systems can reduce waterborne radon before it enters the home's plumbing.

Radon Risk Across Williamsport and Lycoming County

Downtown Williamsport and the Millionaires' Row District (Elevated Risk)

Downtown Williamsport and the historic West Fourth Street corridor — known as "Millionaires' Row" for its concentration of Victorian-era mansions built by lumber barons in the 1860s–1890s — sits on the valley floor directly over the most uranium-productive formations in the county. The Victorian housing stock is the architectural equivalent of Lancaster's stone farmhouses in terms of radon vulnerability: deep basements with stone foundations, unsealed mortar joints, no vapor barriers, and in some cases direct contact between occupied basement spaces and the underlying soil.

These homes are architecturally magnificent and structurally challenging to mitigate. Stone foundation walls require extensive sealing before ASD can establish an effective pressure field. Multi-point suction systems are common due to the compartmentalized foundation geometry of large Victorian homes with additions built across different decades.

South Williamsport and the Little League World Series Complex

South Williamsport — across the West Branch from downtown — sits on the same valley-floor geology. The residential neighborhoods surrounding the Little League World Series complex are predominantly mid-century construction (1940–1970) with concrete-block foundations. These homes respond well to standard ASD with block wall depressurization where needed. The combination of valley-floor geology and older block construction puts South Williamsport firmly in the elevated-risk category.

Montoursville and Eastern Lycoming County (Elevated Risk)

Montoursville, immediately east of Williamsport, sits on similar valley-floor geology with the added variable of alluvial deposits from the West Branch. The alluvial material can have variable gas permeability — gravel lenses transmit radon efficiently while clay layers retard it — creating neighborhood-level variability similar to Erie's glacial till patchwork.

Northern Lycoming County and the Ridgetops (Lower-Moderate Risk)

Communities north of the valley — including the ridgetop areas approaching Tioga County — sit on more resistant sandstone formations with lower uranium content. Radon risk is generally lower on the ridges than in the valley, though DEP recommends testing all homes in Zone 1 counties regardless of topographic position.

Rural Properties on Private Wells (Dual-Pathway Risk)

Rural Lycoming County properties outside municipal water service face the unique dual-pathway exposure. These homes should be tested for both airborne radon (standard ASD addresses this) and waterborne radon (requires separate water treatment). The combined exposure from both pathways can exceed what either pathway alone would produce.

What Radon Mitigation Costs in Williamsport

Active sub-slab depressurization systems in the Williamsport area typically cost $900 to $2,300:

Standard basement ASD (post-1960 poured concrete or concrete block). $900–$1,400. Single or double suction point with a standard inline fan. Effective for the majority of Williamsport's mid-century and newer housing stock. Block wall depressurization may supplement sub-slab suction in homes with hollow-block foundations.

Victorian-era homes (pre-1920 stone foundations). $1,500–$2,300. The Millionaires' Row district and surrounding Victorian neighborhoods require the most complex installations. Stone foundation sealing, multi-point suction, and careful pipe routing through architecturally significant interiors add cost and coordination. These installations require DEP-certified contractors experienced with historic building constraints.

Crawlspace properties. $1,200–$1,800. Properties in the flood plain and older rural homes with dirt-floor crawlspaces require sub-membrane depressurization. The Susquehanna West Branch flood plain produces seasonally high water tables that can affect system performance — the same pressure coupling challenge documented for Erie and Pittsburgh.

Water treatment for well radon (separate from ASD). $1,000–$3,500 for granular activated carbon (GAC) systems or aeration units installed at the point of entry. This is in addition to ASD for airborne radon. Homes on private wells with confirmed elevated water radon may need both systems.

For technical details on system design, stone foundation mitigation, and pressure field extension, see our ASD engineering standards guide.

SB 760 and Lycoming County Schools

The Williamsport Area School District and South Williamsport Area School District both operate buildings in the valley corridor on Zone 1 geology. SB 760 mandates testing in every building by the 2026-2027 school year.

Given Lycoming County's Zone 1 classification and the estimated 41.5% residential exceedance rate, a significant percentage of school buildings can be expected to test above 4.0 pCi/L — particularly older buildings with below-grade spaces on the valley floor. The historic school buildings in the Williamsport district, some dating to the early 1900s, are the highest priority for testing.

SB 760 addresses airborne radon in school buildings. It does not address waterborne radon in school drinking water. However, school buildings served by private wells rather than municipal water should independently assess water radon levels — the USGS data suggests this is a real concern in Lycoming County.

Mitigation must be completed within six months of confirmatory testing. For full SB 760 compliance details, see our Pennsylvania Radon Compliance 2026 guide.

Real Estate and Radon in Williamsport

At a median home price of $165,000, Williamsport offers affordable housing in a Zone 1 county where radon testing should be non-negotiable in every transaction.

For buyers: Test every property for airborne radon during the inspection period. If the property is on a private well, test the water separately. The dual-pathway exposure risk is unique to the Williamsport/Lycoming County market and should be addressed as part of standard due diligence. If airborne results exceed 4.0 pCi/L, negotiate ASD mitigation ($900–$2,300). If water radon exceeds 4,000 pCi/L, budget for water treatment ($1,000–$3,500) as well.

For sellers: Proactive testing — both air and water if on a private well — removes the most common environmental contingencies from the transaction. At Williamsport's price points, combined air and water mitigation costs can represent a significant percentage of transaction value. Addressing these before listing prevents deal disruption.

For buyers of Victorian properties: The Millionaires' Row and historic district homes are architecturally significant and radon-vulnerable. Budget for mitigation at the upper end of the cost range ($1,500–$2,300) and select a DEP-certified contractor with documented experience in stone foundation mitigation. A poorly designed system in a historic home can damage architectural features; an experienced contractor designs around them.

Nearby Cities: Regional Radon Context

Williamsport connects the north-central PA geological corridor:

• Lock Haven — Clinton County, Zone 1. Ordovician limestone in the Susquehanna West Branch valley, 44.8% exceedance. Thirty miles west — same river valley, related but different geology.
• State College — Centre County, Zone 1. Ordovician karst limestone, 68.5% exceedance. Sixty miles southwest — different geological system (karst vs. valley shale).
• Scranton — Lackawanna County, Zone 2. Anthracite basin, 31.8% exceedance. Eighty miles east — mining-legacy geology, no well water radon angle.
• Harrisburg — Dauphin County, Zone 2. Triassic Lowland/Blue Ridge transition, 33.4% exceedance. Eighty miles south — different geological setting.