The Determination of Radon Activities in Ground Water from Wisconsin Tills in Southwestern Ohio and Southeastern Indiana

TWO hypotheses have been suggested by previous workers to explain the relatively elevated radon activities of ground water in certain areas of southwestern Ohio and southeastern Indiana. First, radon may be produced close to or at the Ordovician-Silurian unconformity by the concentration of uranium and radium on iron and manganese oxides and hydroxides and on clay minerals at this zone of weathering. Second, radon may be formed from the radioactive decay of radium which is concentrated on iron-oxides in zones of higher hydraulic conductivity in the lower carbonate section of the Silurian System. In both cases, it has been proposed that the elevated radon activities result from either the application of radium-bearing phosphate fertilizers or from the inclusion of radium-bearing fragments of Devonian black shale in the till. The present study attempted to determine if ground water from different Wisconsin till units in southwestern Ohio contain significantly distinct radon activities due to the differing amounts of Devonian black shale contained in the drift. Duplicate samples were collected from 47 private wells and springs that produce water from different glacial units. The samples were analyzed for their radon activity using liquid scintillation methodology. Data indicate that it is not possible to differentiate statistically between radon activities in water from wells that penetrate the Upper and Lower Shelbyville or the Crawfordsville tills. Instead, those wells and springs with high radon activities appear to be associated with production of water from horizons of unusually high hydraulic conductivity. Wells that produce water from glacial drift close to areas where carbonate rocks of the lower Silurian System subcrop also have a higher radon signature. These higher radon activities are believed to be associated with flow of ground water up into the drift from zones of high hydraulic conductivity in the lower Silurian System. OHIO J. SCI. 95 (3): 24S-253, 1995 INTRODUCTION Radon-222 (hereafter radon) is a colorless, odorless, radioactive gas that is a daughter element in the decay chain of uranium-238. The immediate parent of radon is radium-226. Radon occurs naturally in elevated concentrations in the air of some homes and beneath the Earth's surface in soil gas and ground water. The alpha radiation emitted by radon and its polonium progeny is considered a significant health hazard by the United States Environmental Protection Agency (USEPA) because at elevated levels it causes lung cancer. Radon activities in the air of at least 40% of the homes in the Dayton, OH, area frequently exceed the four picocuries per liter environmental-action level set by the USEPA (Paul and Lindstrum 1987). Few studies, however, have sought to define the geologic source of this radon. Possible Origin of Radon in Southwestern Ohio Harrell et al. (1993) postulated that the source of indoor radon in Ohio may be the till that blankets the upland regions in the glaciated regions of southwestern Ohio. They theorized that the Wisconsin glaciers eroded and transported large quantities of uraniferous Devonian black shale to the southwest as it flowed across outcrops of the easily eroded black shale near Bellefontaine in Manuscript received 17 March 1994 and in revised form 10 April 1995 (#94-06). west-central Ohio. These black shales have an arithmetic mean of 26.7 ppm uranium (Harrell and Kumar 1988). Harrell et al. (1993) further suggested that the extensive weathering of the carbonate clasts that comprise a high percentage of the till may be another source of the elevated indoor radon activities. The weathering of these limestone clasts leaves behind a concentration of insoluble residues (clays, iron oxides, and hydroxides) in the resulting soils. These weathering residuals, with their capacity to adsorb uranium and radium, could be a source of the elevated radon activities recorded in many homes within the glaciated region. The importance of glacial drift as a generator of radon was further suggested in work by Duval (1982). His aerial radiometric contour map, showing variations in uranium concentrations in soils in Ohio, mimics closely the lobate nature of the recessional moraines that cover the glaciated portion of the state. In an attempt to define more precisely the geologic source of elevated indoor radon activities in southwestern Ohio, Baldwin and Treick (1991) collected ground water from 157 private wells for radon analysis. Their work suggested that there is a distinct zone of elevated radon production in the lower Brassfield Formation, a carbonate unit that lies immediately above the Ordovician-Silurian contact. Gall et al. (1995), however, found that elevated radon activities occurred in ground water not only in the lower Brassfield Formation but also in the Dayton Dolomite, a carbonate unit approximately 18 m (59 ft) higher in the Silurian section in the OHIO JOURNAL OF SCIENCE T. L. HAIR, JR. AND A. D. BALDWIN, JR. 249 Dayton area. They found that the zones of highest radon activities in ground water are also horizons of highest hydraulic conductivity and uranium concentrations in the bedrock. Thus they postulated that the flow of large volumes of water containing low concentrations of uranium and radium has resulted in the adsorption of these radionuclides on iron and manganese oxides and clays in these horizons. MATERIALS AND METHODS The study area lies in southwestern Ohio and southeastern Indiana (Fig. 1). Water was collected from 41 private wells and six springs in an attempt to determine if different till sheets were the source of elevated radon values in ground water in this region. Where possible, water was collected from wells which penetrate different till units deposited by ice which entered southwestern Ohio from either the northeast or northwest. It was postulated that the sediment deposited from ice flowing from the northeast would have a higher percentage of black shale fragments and thus produce more radon because it would have crossed the uranium and radium-rich, Devonian, black-shale outlier near Bellefontaine. No nearby source of Devonian shale inclusions exists for tills deposited by ice that flowed into southwestern Ohio from the northwest. Well logs were utilized where available to determine subsurface conditions and water-producing horizons. Water was collected from springs wherever they were found. Geology of Study Area Thin limestone and shale units possessing low hydraulic conductivity and belonging to the Cincinnatian Series of the Ordovician System underlie much of the FIGURE 1. Location of study area. study area (Fig. 2). Wells cased to bedrock seldom yield more than 19 1 per minute (5 gals per minute). Where usable quantities of water are produced, the water is derived from bedrock fractures or from a weathered zone at the top of the Ordovician section. Limited outcrops of the Brassfield Formation, a lower Silurian carbonate unit, overlie unconformably the upper Ordovician rocks along the northern edge of the study region. Overlying the Ordovician-Silurian bedrock is nonindurated Pleistocene drift. As reported by Stewart and Miller (1987), the drift is predominantly of Wisconsin age (Fig. 3). The Fayette Till, the Connersville sands and gravel, the Shelbyville Tills, the sands and gravel deposited during the Shelbyville Interphase and the Crawfordsville Till lie unconformably on top of the Ordovician and Silurian section under much of the study area. The Fayette Till is a compact, dark gray, pebbly to sandy clay material. Hydraulic conductivity tests on the Fayette Till yielded a mean value of 4.5 X 10 cm/s. This low value limits the downward movement of ground water. Till fabric analysis suggests that the unit was deposited by ice which flowed into the area from the northeast. The overlying Connersville sand and gravel deposit is a discontinuous, poorly sorted sand, silt, and gravel unit. Apart from the valley-train deposits of sand and gravel that fill the larger valleys in the area, the Connersville is the most important water-producing unit in the study area. It has a mean hydraulic conductivity of 4.9 X 10" cm/s. The Shelbyville Till lies above the Connersville sand and gravel and is divided into two subunits (Goldthwait et al. 1981). The Lower Shelbyville is a blue-gray to oxidized brown, poorly sorted clayey to silty sand with a moderate hydraulic conductivity of 1.5 X 10 3 cm/sec. This unit thus allows ground water to move vertically through it until the water reaches an underlying relatively impermeable layer such as the Fayette Till or bedrock. The till has a northwest till fabric because it was deposited on the east flank of the White River Ice Lobe (Goldthwait et al. 1981). The Lower Shelbyville crops out as the surface till in the western portion of the study area (Fig. 4) and most likely lies beneath the Upper Shelbyville in the eastern and northeastern sections of the study area. The Upper Shelbyville Till was deposited at the western edge of the Miami Lobe (Goldthwait et al. 1991). It has a northeast fabric and thus potentially a higher uranium/radium radon concentration. The Fayette Till also has a northeast till fabric; however, it is discontinuous through the study area and, as mentioned previously, is relatively impermeable. The Upper Shelbyville is a medium brown, loosely compacted, and sandy textured till (Goldthwait et al. 1981). The Camden Moraine and the Crawfordsville Till lie above the Upper Shelbyville Till in the northeastern portion of the study area. During Crawfordsville time, the ice advanced from the northeast to a terminal position marked by the Camden Moraine. The till deposited during this phase is yellow brown to gray brown and 250 AQUEOUS RADON IN TILLS OF OHIO VOL. 95