Radon and Michigan's Geology: What the Ground Beneath Us Says

Every conversation about radon eventually arrives at the same underlying reality: the risk begins in the ground. The geology beneath Michigan — the specific composition of the soil, rock, and glacial deposits that form the foundation of the landscape — is the source of the radon that enters homes across the state. Understanding what Michigan's geology actually looks like, why it produces radon at the levels it does, and why certain parts of the state carry higher risk than others provides the scientific foundation for everything else in the radon conversation.

This post goes deeper into the geological dimension than any earlier post in this series has gone. Radon's geological origins were mentioned in our foundational post on what radon is, and the Zone 1 and Zone 2 designations for specific Michigan counties have been discussed in the county-specific posts throughout this series. This post owns the geology deep dive — the specific minerals, the glacial history, and the soil science that explain why Michigan is a high-radon state and why West Michigan in particular sits at the elevated end of the risk spectrum.

The Uranium Connection

Radon does not exist independently in the earth. It is a decay product — a gas produced when uranium-238, one of the naturally occurring radioactive elements present in soil and rock, undergoes a multi-step decay sequence. The sequence runs from uranium-238 through a series of intermediate decay products — including radium-226 — before producing radon-222, the isotope of radon that is relevant to indoor air quality. Radon-222 is the radon measured in home tests and referenced in EPA guidelines.

The practical implication of this decay chain is straightforward: the more uranium present in the soil and rock beneath a home, the more radon is produced, and the more that can potentially migrate into the home above. Uranium concentrations in Michigan soils and bedrock vary across the state, but several geological features of Michigan's landscape contribute to concentrations that are elevated relative to the national average in substantial portions of the state.

Michigan's Bedrock Geology

Michigan's bedrock — the solid rock that underlies the glacial deposits across the state — consists largely of sedimentary formations laid down over hundreds of millions of years when shallow seas covered what is now the Great Lakes region. These formations include shales, limestones, sandstones, and dolomites that accumulated in sequence over geological time and were later buried beneath the glacial deposits that define Michigan's surface landscape.

Several of Michigan's bedrock formations contain naturally elevated uranium concentrations relative to average crustal abundances. Black shales — a rock type present in several Michigan bedrock formations — are particularly relevant. Black shales form in oxygen-poor environments that favor the concentration of organic material and associated trace elements including uranium. The uranium that was concentrated in these shales during their formation millions of years ago is still present today, still decaying, and still producing radon that migrates upward through the overlying geological layers toward the surface.

The Antrim Shale, a black shale formation that underlies much of northern and central Michigan, is among the most uranium-rich bedrock formations in the state. Other shale formations present in Michigan's sedimentary sequence also contribute to elevated uranium concentrations in bedrock across various parts of the state. Where these formations are present at depth, the radon produced by their uranium content has a pathway through overlying soils toward the surface and ultimately toward the foundations of homes above.

The Glacial Legacy

If Michigan's bedrock geology were the only factor in the radon picture, the distribution of radon risk would follow the bedrock geology relatively closely. But Michigan's surface geology — the soils and unconsolidated deposits that directly underlie most homes — is dominated by glacial deposits, and those deposits add another layer of complexity to the radon story.

Michigan was covered by multiple advances of the Laurentide Ice Sheet during the Pleistocene epoch, with the last glacial retreat occurring roughly 10,000 to 12,000 years ago. Those glacial advances ground bedrock into fine particles, transported material from across a wide area, and deposited it across Michigan in a complex mosaic of glacial landforms. Glacial till — the unsorted mixture of clay, silt, sand, and gravel that glaciers deposit directly — blankets much of Michigan. Glacial outwash — the sorted gravels and sands deposited by meltwater streams flowing from glacier margins — forms the permeable layers that underlie many Michigan communities.

These glacial deposits are relevant to radon in two distinct ways. First, they contain the ground-up mineral material derived from the bedrock the glaciers traversed — including the uranium-bearing minerals from uranium-rich bedrock formations. Glacial till and outwash derived from uranium-rich bedrock carry elevated uranium concentrations into the soils directly beneath Michigan homes. Second, the physical characteristics of glacial deposits — particularly the permeable gravels and sands in glacial outwash — provide efficient pathways for radon migration from depth toward the surface. Radon produced at depth can move through permeable glacial outwash layers relatively easily, reaching the shallow subsurface where it can enter home foundations.

West Michigan's Specific Geological Context

The West Michigan region — Kent, Ottawa, Allegan, Muskegon, Ionia, and Barry counties — sits on a geological landscape shaped by the advance and retreat of the Lake Michigan lobe of the Laurentide Ice Sheet. This glacial lobe deposited a specific suite of materials across West Michigan as it advanced and retreated, including till derived from the bedrock formations it traversed in what is now the Great Lakes basin.

The soils across much of West Michigan are dominated by loamy glacial till and glacial outwash deposits that provide both the uranium-bearing mineral content and the physical permeability that favor radon production and migration. The combination of these factors contributes to the Zone 1 designation that Kent County and several surrounding counties carry — a designation that reflects measured radon data from homes across the region as well as the geological prediction models that the EPA uses to assign zone designations.

The variability within West Michigan is also geologically explicable. Areas underlain by coarser, more permeable outwash deposits may have different radon characteristics than adjacent areas underlain by finer-grained till. River valleys — where alluvial deposits overlie glacial materials — may have local soil conditions that differ from upland areas. The lakeshore dune systems along Lake Michigan represent a distinct geological environment with different soil characteristics than inland areas. This spatial variability is why individual home testing is always more informative than zone-level generalizations — the geology beneath two homes a mile apart may be meaningfully different in ways that translate to different radon levels.

Soil Moisture and Radon Migration

The moisture content of the soil is a geological variable that affects radon migration in ways that have practical implications for testing and seasonal variation. Radon moves through soil primarily through the interconnected pore spaces between soil particles. When those pore spaces are filled with water — during periods of heavy rainfall, snowmelt, or high water table conditions — radon migration through the soil is impeded. When pore spaces are air-filled — during dry conditions — radon migrates more freely.

This moisture dynamic contributes to some of the seasonal and weather-related variability in radon levels that we covered in our posts on Michigan winters and radon and on weather effects on radon testing. Periods of sustained dry weather allow radon to migrate more freely through the soil, potentially increasing the rate of radon entry into homes. Extended wet periods temporarily suppress migration. These are not dramatic effects that dominate the radon picture, but they are real and measurable contributions to the variability that makes any single short-term test a snapshot rather than a permanent characterization.

Glacial Lake Deposits and Local Variability

One additional geological feature of Michigan's landscape worth noting is the presence of glacial lake deposits — the fine-grained clays and silts that settled to the bottom of the large glacial lakes that existed in Michigan's lake basins as the ice retreated. These fine-grained lake deposits have very low permeability and tend to impede radon migration from depth rather than facilitating it. Areas of West Michigan underlain by significant glacial lake clay deposits may have locally lower radon levels than adjacent areas with more permeable soils, even if the underlying bedrock uranium concentrations are similar.

This is one of the geological reasons why radon levels can vary significantly at the neighborhood or even the lot level in ways that are not predictable from zone designations or even from the general soil map of an area. The specific thickness and continuity of clay layers beneath an individual home's foundation can meaningfully affect how much radon from depth reaches the shallow subsurface — and ultimately the home above.

What the Geology Tells Us About Testing

The geological picture of Michigan — uranium-bearing bedrock overlain by permeable glacial deposits with significant local variability — supports a clear practical conclusion: there is no substitute for individual home testing. County-level zone designations reflect aggregate geological risk and provide useful context for understanding why Michigan has the radon profile it does. But they do not predict the radon level beneath any individual home with precision, because the geological variability at the local and lot level is too significant to be captured in county-level summaries.

Understanding the geology helps explain why the risk is real and why it is distributed across the state the way it is. It does not replace the measurement that tells a specific homeowner what is actually happening beneath their specific foundation on their specific piece of Michigan ground.

At All Michigan Mitigation, we bring a deep understanding of West Michigan's geological context to every home we test and mitigate. Knowing why the risk exists and where it comes from informs better diagnostic judgment and more effective mitigation design — which is why radon specialization, rather than general home services experience, produces better outcomes for the homeowners we serve.

Visit us at allmichiganmitigation.com to schedule your test or to reach out with any questions about radon risk in your area of Michigan.

All Michigan Mitigation is a West Michigan radon testing and mitigation company dedicated exclusively to helping homeowners understand and eliminate radon risk in their homes.