Radon is not a housing problem that happens to occur here. It is a geology problem that happens to have houses on top of it — and Hunterdon County sits across three different kinds of rock in the space of about twenty miles.
Radon is a radioactive gas produced by the decay of uranium in rock and soil. It has no smell, no colour and no taste, and the only way to know how much of it is in a building is to measure it. What decides how much a particular house has to deal with is, first and above everything else, what is underneath it.
New Jersey's Department of Environmental Protection classifies every municipality in the state into three tiers of radon potential. Tier 1 is the highest: at least 25 homes tested, with a quarter or more of the results at or above 4 pCi/L. In NJDEP's 2015 Radon Tier Assignment Report, 25 of Hunterdon County's 26 municipalities are Tier 1. Not a handful of them — all but one. That is unusual even in a state the EPA maps as elevated almost end to end.
The Reading Prong, in the north and west
The northwestern corner of the county — Lebanon Township, Bethlehem Township, Union Township, Tewksbury, and the boroughs strung between them — sits on the Reading Prong. This is a belt of Precambrian granite and gneiss that runs out of eastern Pennsylvania, across northwestern New Jersey, and into southern New York. It is old, hard, uranium-bearing rock, and it is the formation that put radon on the national agenda in the first place: the Watras case in Pennsylvania in 1984, where a worker set off the radiation alarms on his way into a nuclear plant, was a house built on this same geological structure.
Homes here are frequently built into rock rather than onto deep soil. That has a practical consequence most homeowners never hear about: the fill under a basement slab can be shallow and coarse, and suction from a single mitigation point may not travel as far through it as a contractor assumes. It is why diagnostic testing before a design is settled earns its keep in this part of the county.
The Newark Basin, through the middle
Flemington, Raritan Township, Readington, Clinton Township and the centre of the county sit on the Passaic Formation — the red shale and mudstone of the Newark Basin. This rock carries less uranium than the Highlands gneiss, but it fractures readily, and it is the fracture network rather than the richness of the rock that moves soil gas toward a foundation. Add several decades of post-war housing with full basements that were later finished and lived in, and you have the county's most common radon situation: a conventional house, a conventional basement, and a reading that surprises the owner.
The Lockatong argillite, in the west and south
Move west and south — Kingwood, Delaware Township, the Delaware river towns — and the rock changes again. The Lockatong Formation is a dark grey to black argillite laid down in deep Triassic lake beds. Organic-rich sediments of this kind concentrate uranium more readily than ordinary red shale, and the rock is hard, thinly bedded and heavily jointed, so gas moves along bedding and joint planes rather than through the body of the stone.
What sits on top of it in this part of the county is often an 18th or 19th century stone farmhouse with a shallow cellar and a floor that was added later, if at all. Uranium-bearing rock underneath and an open stone cellar above is about the least favourable combination in the county, and it is why western Hunterdon produces jobs that need a sealed membrane rather than a standard sub-slab system.
And the diabase, in the south
Sourland Mountain, running through East and West Amwell and on into Somerset and Mercer counties, is a diabase intrusion — hard, dense igneous rock with thin stony soil over it. Diabase moves gas through its fractures, and the thin soil means very little material under a slab for suction to spread through. Multiple suction points are more common here than anywhere else in the county.
What follows from all this
Three things, and they are the reason this site exists.
You cannot infer your reading from your neighbour's. Fractured rock is directional. Two houses on the same road, built the same year by the same builder, can differ by a factor of several because one sits over a joint that runs somewhere useful and the other does not.
Where you are in the county changes what a good system looks like. A single suction point through a modern slab in Raritan Township is a different job from a membrane over an open cellar floor in Delaware Township, which is a different job again from a three-point system on thin soil over diabase in East Amwell.
The bedrock supplies the wells as well as the soil gas. Across most of rural Hunterdon, the water comes out of the same fractured rock that produces the radon. That is a second, separate route into the house, and no air-side system touches it.
None of this tells you anything about your own house. Only a test does that, and in New Jersey a test that counts has to be placed and read by a business certified by NJDEP for radon measurement.
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Get free quotes →General information for Hunterdon County, NJ property owners — not a diagnosis, not a quote, and not a reading for any particular building. Every property is different; a certified professional confirms specifics on site.