Radium in Ground Water from Public-Supply Aquifers in Northern Ilinois

The U.S. Geological Survey, in cooperation with the Illinois Environmental Protection Agency, has prepared a Fact Sheet that was published in September, 1999. The information presented below is an expanded version of the original fact sheet which is available for viewing from the Publications Database Web Page.

Radium-226 and radium-228 concentration data in public-water supply systems obtained during previous studies were combined with data obtained in March 1999 to determine the extent of elevated radium in aquifers used for public supply in northern Illinois. With a few exceptions, radium concentrations in water from public supplies in northern Illinois exceed the USEPA standard of 5 picoCuries per liter (pCi/L) only in the Cambrian-Ordovician (figs. 1, 2) and Mt. Simon (fig. 3) aquifers. The area where elevated radium concentrations may be present in northern Illinois, also referred to as the area of concern, extends from Kankakee, Livingston, Woodford, Tazewell, Fulton, McDonough, and Hancock Counties north to the Wisconsin State line.

Figure 1. Area in northern Illinois where radium concentrations in the Ironton-Galesville Sandstone unit of the Cambrian-Ordovician aquifer may exceed the USEPA standard of 5 picoCuries per liter.

Figure 2. Area in northern Illinois where radium concentrations in the St. Peter Sandstone unit of the Cambrian-Ordovician aquifer may exceed the USEPA standard of 5 picoCuries per liter.

Figure 3. Area in northern Illinois where radium concentrations in the Mt. Simon aquifer may exceed the USEPA standard of 5 picoCuries per liter.

The Cambrian-Ordovician aquifer is the most highly developed aquifer in northern Illinois and consists of the Galena-Platteville Dolomite, Glenwood-St. Peter Sandstone, Prairie du Chien Group, Trempealeau Dolomite, Franconia Formation, and Ironton-Galesville Sandstone (fig. 4). The Eau Claire Formation and the shales of the Maquoketa Group or the Pennsylvanian System confine the aquifer under artesian pressure where it has not been overutilized. The most productive units in this aquifer are the Glenwood-St. Peter and Ironton-Galesville Sandstones, with the Ironton-Galesville being more productive. The Mt. Simon aquifer is the deepest aquifer in northern Illinois and consists of the Mt. Simon Sandstone and the Elmhurst Member of the basal Eau Claire Formation.

Figure 4. Stratigraphic column for Illinois.

Many of the public-water supply wells that pump water from the Cambrian-Ordovician aquifer in the northeastern part of Illinois have open boreholes from the Galena-Platteville Dolomite to the Ironton-Galesville Sandstone and pump water from both the St. Peter and Ironton-Galesville Sandstones. Many of the public-water supply wells in the southern and southwestern parts of northern Illinois that pump water from the Cambrian-Ordovician aquifer have open boreholes from the Galena-Platteville Dolomite through the Glenwood-St. Peter Sandstone and draw water primarily from the Glenwood-St. Peter Sandstone. In the north-central and northwestern parts of the area of concern, and at a few locations in northeastern Illinois, many public-water supply wells are open from the Galena-Platteville Dolomite through the Mt. Simon Sandstone and draw water primarily from the Glenwood-St. Peter, Ironton-Galesville, and Mt. Simon Sandstones. The Mt. Simon aquifer is not substantially utilized for public water supply in the southern part of northern Illinois because of the great depth of the aquifer and the high mineral content of the water in the lower part of the aquifer.

The general pattern of ground-water flow in northern Illinois is to the south, east, and west, away from a ground-water divide defined by a zone of high water levels that is present in western McHenry County, continues south through western DeKalb County, eastern Lee County, and through LaSalle County west of Ottawa (Sasman and others, 1982)(fig. 5). Recharge to the Cambrian-Ordovician aquifer occurs in north-central and northwest Illinois. The recharge area corresponds to that part of northern Illinois where the shale deposits of the Maquoketa Group and the Devonian, Mississippian, or Pennsylvanian System have been removed by erosion and the deposits of the Cambrian-Ordovician aquifer are at the bedrock surface (fig. 5).

Figure 5. Bedrock surficial geology in northern Illinois.

Combined concentrations of radium-226 and radium-228 in water from wells open to the Mt. Simon aquifer were typically less than or near the USEPA standard of 5 pCi/L, except in northeast Illinois where combined radium concentrations may exceed 20 pCi/L (fig. 3). The highest combined radium concentrations in the Mt. Simon aquifer were typically associated with brackish or saline water with a high concentration of total dissolved solids (TDS)(Gilkeson and others, 1983).

Combined concentrations of radium-226 and radium-228 in water from wells open to the Ironton-Galesville Sandstone unit of the Cambrian-Ordovician aquifer were typically less than USEPA standard in the north-central and northwestern parts of Illinois (fig. 1). Combined radium concentrations generally increased to the east, west and south in the direction of ground-water flow where the Cambrian-Ordovician aquifer is confined by shale deposits. Combined radium concentrations in the Ironton-Galesville Sandstone are typically greater than 5 to 10 pCi/L in the southwestern part of the area of concern, exceed 10 pCi/L in the northeastern Illinois, and exceed 20 pCi/L in parts of southern Cook and Will Counties.

Combined concentrations of radium-226 and radium-228 in water from wells open to the St. Peter Sandstone unit of the Cambrian-Ordovician aquifer were typically less than USEPA standard in the north-central and northwestern parts of Illinois. As is the case in the Ironton-Galesville Sandstone, combined radium concentrations generally increased to the east, west and south. Combined radium concentrations in the St. Peter Sandstone are typically greater than 10 pCi/L in the southwestern and northeastern parts of the area of concern, and exceed 20 pCi/L in the far southwestern part of the area of concern where the water from this sandstone contained elevated TDS. Combined radium concentrations in the St. Peter Sandstone tend to be higher than in the Ironton-Galesville Sandstone in the southwestern part of the area of concern. There is no clear difference in combined radium concentrations in the sandstones of the Cambrian-Ordovician aquifer in northeast Illinois.

The presence of elevated concentrations of radium-226 in ground water in the Cambrian-Ordovician aquifer appears to be related to the accumulation of its parent elements, particularly uranium-238, uranium-234, and thorium-230 (fig. 6a) on the surface of the sandstone matrix (Gilkeson and others, 1983). The parent elements are transported in ground water from source areas in the rocks along shallower parts of the flow path and will precipitate onto the sand grains that form the aquifer matrix in response to changes in chemical conditions along the flow path. Radium-226 is less strongly sorbed to the silicate surface than its thorium or uranium parents and has the potential for desorption from the aquifer matrix to ground water when the thorium-230 decays. Thorium-230 decays by alpha particle emission. Therefore, there also is the potential for the newly formed radium-226 ions to be physically ejected into solution from the aquifer matrix upon their creation (alpha recoil). In addition, the bond to the matrix can be weakened by the emission energy, increasing the potential for the radium-226 to leach to ground water because of increased competition for cation exchange sites on the aquifer matrix from other ions in solution. Leaching of radium-226 from the aquifer matrix may explain the observed correlation between the concentration of radium-226 and TDS in the aquifer.

Figure 6a. Radioactive decay sequence for Uranium 238.

Figure 6b. Radioactive decay sequence for Thorium 232.

The concentration of radium-228 in ground water from the Cambrian-Ordovician and Mt. Simon aquifers appears to be affected primarily by the concentration and distribution of the thorium-232 parent in the minerals in the sandstone bedrock, including feldspar bearing sediments (fig. 6b)(Gilkeson and others, 1983). This appears to be particularly true for the Mt. Simon aquifer. The affects of alpha recoil may contribute to desorption of radium-228 from the aquifer matrix to ground water. Competition for cation exchange sites on the aquifer matrix may have an effect on radium-228 concentrations in the aquifers

References

Sasman, R.T., Benson, C.R., Ludwigs, R.S., and Williams, T.L., 1982, Water-level trends, pumpages, and chemical quality in the Cambrian Ordovician aquifer in Ilinois: Illinois State Water Survey Circular 154, 64 p.