Radon potential in the upper Midwest is controlled by three primary factors. Bedrock geology (Fig. 1, 43kb) provides the source material for the overlying glacial deposits, and in areas with no glacial cover, directly provides the parent material for the soils. Glacial geology (Fig. 2, 15kb) is an important factor because glaciers redistributed the bedrock and glacially-derived soils have different soil characteristics from soils developed on bedrock. Climate, particularly precipitation and temperature, control soil moisture, the extent of soil development and weathering, and, in concert with the soil's parent material, the types of weathering products that form in the soils. The following is a brief, generalized discussion of the bedrock and glacial geology of the upper Midwestern states as they pertain to indoor radon.
Much of North and South Dakota, western and southern Minnesota, and northern Iowa are underlain by deposits of the Des Moines, James, and Red River glacial lobes. Included within this region are clay and silt deposits of glacial lakes Agassiz, Souris, Dakota, and Devil's Lake, which generate some of the highest radon levels in the area. Des Moines lobe tills are silty clays and clays derived from the Pierre Shale and from Tertiary sandstones and shales which have relatively high concentrations of uranium and high radon emanating power. The lower part of the Pierre Shale has an overall higher uranium content than the upper part, and locally contains black shales such as the Sharon Springs Member. The Pierre Shale as a whole contains higher-than-average amounts of uranium (average crustal abundance is 2.5 parts per million ), in part because it was deposited under reducing conditions under which uranium is relatively immobile and thus more likely to concentrate at the site of deposition, and because it contains an abundance of clay minerals that form weak bonds with metals, including uranium. Glacial deposits of the Des Moines and James lobes generate high (> 4 pCi/L) average indoor radon concentrations (Fig. 3, 21kb), have high proportions of homes with elevated indoor radon levels (Fig. 4, 21kb), and have high radon potential (Fig. 5, 18kb). Southern Iowa is covered by pre-Wisconsinan glacial deposits and loess overlying Paleozoic and Mesozoic limestone and dolomite, shale, and sandstone. Radon levels in southern Iowa are disproportionately high compared to areas underlain by similar deposits in adjacent Illinois (Fig. 3, Fig. 4) and northern Missouri, but may reflect different bedrock sources and movement directions of the glacial lobes that covered each area (Fig. 1, Fig. 2).
Southwestern North Dakota and western South Dakota are underlain by unglaciated Tertiary and Cretaceous sandstones, siltstones, and shales, some of which include uraniferous coals and carbonaceous shales. Carbonaceous shales, uranium-bearing coals, and ash clay beds in Tertiary sedimentary units of southwestern North Dakota and western South Dakota, including the Fort Union Formation (and its equivalents) and the White River Group, generate locally very high radon levels, and have overall moderate to high radon potential (Fig. 5). Uranium deposits occur locally in Paleozoic and Mesozoic marine sedimentary rocks surrounding the Black Hills, particularly in sandstones of the Lower Cretaceous Inyan Kara Group in the southern Black Hills. Granites in the core of the Black Hills have moderate radon potential.
Northern Wisconsin, the western part of the Upper Peninsula of Michigan, and part of northern Minnesota are underlain by glacial deposits of the Lake Superior lobe. Parts of northern Minnesota are also underlain by deposits of the Rainy and Wadena lobes (Fig. 2). The underlying source rocks for these tills are volcanic rocks, metasedimentary and metavolcanic rocks, and plutonic (granitic) rocks of the Canadian Shield (Fig. 1). The volcanic, metasedimentary, and metavolcanic rocks have relatively low uranium contents, and the granitic rocks have variable, mostly moderate to high, uranium contents. The sandy tills derived from these rocks have relatively high permeability, but because of their lower uranium content and lower emanating power, they have mostly moderate to locally high radon potential (Fig. 5). Granites and granite gneisses, black slates and graphitic schists, and iron formation are associated with anomalous uranium concentrations and locally high radon in northern Wisconsin and adjacent northwestern Michigan. In central Wisconsin, uraniferous granites of the Wolf River and Wausau plutons are exposed at the surface or covered by a thin layer of glacial deposits and cause some of the highest indoor radon concentrations in the State. An area covering mostly southwestern Wisconsin, but including adjacent parts of Minnesota, Iowa, and Illinois, is called the "driftless area" (Fig. 2). It is not covered by glacial deposits but was likely overrun by glaciers at least once. The driftless area is underlain mostly by limestone, dolomite, and sandstone with moderate to high radon potential.
Glacial deposits in southern Wisconsin, northern and central Illinois, and western Indiana are primarily from the Green Bay and Lake Michigan lobes. The Green Bay and Lake Michigan lobes advanced from their source in the Hudson Bay region and moved southward into Illinois. These tills range from sandy to clayey and are derived mostly from shales, sandstones, and carbonate rocks of southern Wisconsin, the western Michigan Basin, and the northern Illinois Basin. A small part of eastern Illinois and much of western Indiana are covered by deposits of the Huron-Erie lobe, and west-central Illinois is covered by pre-Wisconsinan, mostly Illinoian, deposits. The Huron-Erie lobe entered Illinois from the east and moved primarily westward into the state. Huron-Erie lobe and pre-Wisconsinan glacial deposits are derived from shales, sandstones, siltstones, carbonate rocks, and coal of the Illinois Basin, and they are commonly calcareous due to the addition of limestones and dolomites of northern Indiana and Ohio and southern Ontario. In contrast, Lake Michigan lobe deposits contain significant amounts of dark gray to black Devonian and Mississippian shales of the Michigan Basin, accounting for the high clay content of Lake Michigan lobe tills. Unglaciated southern Illinois is part of the Mississippi Embayment of the Gulf Coastal Plain and has low radon potential.
Wisconsinan-age glacial deposits in Indiana were deposited by three main glacial lobes—the Lake Michigan lobe, which advanced southward as far as central Indiana, the Huron-Erie lobe, and the Saginaw sublobe of the Huron lobe (labeled Huron lobe on Fig. 2), which advanced from the northeast across northern Ohio and southern Michigan, respectively. Michigan lobe deposits are clayey near Lake Michigan, sandy and gravelly in an outwash and morainal area in northwestern Indiana, and clayey to loamy in west-central Indiana. Saginaw sublobe deposits are loamy and calcareous and are derived primarily from carbonate rocks and shale. The Huron-Erie lobe advanced from the northeast and covered much of northern and central Indiana at its maximum extent. Eastern Indiana and western Ohio are underlain by tills of the Huron-Erie lobe that are derived in part from the Ohio and New Albany black shales, but also including limestone, dolomite, sandstone, siltstone, and gray shale. Black shales underlie and provide source material for glacial deposits in a roughly north-south pattern through central Ohio, including the Columbus area, and extend south of the glacial limit, forming an arcuate pattern in northern Kentucky that curves northward into southern Indiana and underlies glacial deposits in east-central Indiana. The overall radon potential of this area is high.
The Michigan Basin covers all of the Southern Peninsula and the eastern half of the Northern Peninsula of Michigan, as well as parts of eastern Wisconsin and northeastern Illinois, northern Indiana, and northwestern Ohio. Glacial deposits include silty and clayey tills of the Lake Michigan, Huron, and Huron-Erie lobes (Fig. 2). Huron lobe tills are sandy to gravelly and calcareous, containing pebbles and cobbles of limestone, dolomite, and some sandstone and shale, with boulders of igneous and metamorphic rocks and quartzite. Tills of the Erie and Lake Michigan lobes are derived from similar source rocks but are more silty and clayey in texture. Source rocks for these tills are sandstones, gray shales, and carbonate rocks of the Michigan Basin, which are generally poor radon sources. In the Southern Peninsula, the Devonian Bell Shale, Devonian-Mississippian Antrim Shale, and Mississippian Sunbury and Ellsworth Shales locally contain organic rich (black shale) layers with higher-than-average amounts of uranium, except for the Antrim Shale which is organic-rich throughout. These shales underlie and constitute source rock for glacial deposits in the northern, southeastern, and southwestern parts of the Southern Peninsula, and are locally exposed at the surface in the northern part of the Southern Peninsula. Because of generally moist soils , soils developed on tills derived from black shales in Michigan generate moderate to locally high radon, with higher values more common in the southern part of the state (Fig. 3, Fig. 4).