Geochemical and γ ray characterization of Pennsylvanian black shales: Implications for elevated home radon levels in Vanderburgh County, Indiana.

Journal of Environmental Radioactivity 148 (2015) 154e162

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Geochemical and g ray characterization of Pennsylvanian black shales: Implications for elevated home radon levels in Vanderburgh County, Indiana Kent W. Scheller*, William S. Elliott Jr. Department of Geology and Physics, University of Southern Indiana, 8600 University Blvd, Evansville, IN 47712, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 April 2015 Received in revised form 25 June 2015 Accepted 26 June 2015 Available online 10 July 2015

Radon (222Rn) is a radioactive gas that results from the decay of uranium (238U) in the Earth's crust. This study characterizes the presence and relative quantity of radon precursors in the Pennsylvanian black shales of southwest Indiana. Cores were drilled on the campus of the University of Southern Indiana to a depth of 237.7 m (780 ft) during exploration for coal-bed methane. Gamma ray logs were taken to measure radioactive activity as a function of depth in the bore hole. Activity readings of 270, 467, 555, and 388 GAPI (American Petroleum Institute g ray units) were measured at depths of 124.3 m (408 ft), 154.0 m (505 ft), 187.1 m (614 ft) and 214.0 m (702 ft) in four separate shale layers of the Pennsylvanian stratigraphic column. GAPI units are used in the petroleum industry when drilling to represent the relative intensities of g radiation from 40K, 232Th, and 238U in bore holes (Belknap et al., 1959). For purposes of this study, the high activity readings on the gamma ray logs were used only to identify at which depths further gamma ray spectroscopy of the cores would be completed in the laboratory. Gamma ray spectroscopic studies of these cores were conducted with a large volume NaI crystal detector to observe g rays of specific energies. Characteristic g rays from various isotopes were identified confirming the presence and relative quantity of radon precursors in core samples. Geochemical analysis of cores was also conducted to measure presence and quantity of trace metals and radon precursors. Of 744 homes tested in Vanderburgh County from 2007 to 2013, 169 homes (22.7 percent) had elevated radon levels greater than 148 mBq L1 (4.0 pCi L1). Additionally, 246 homes (33.1 percent) had measured radon levels of 74e145 mBq L1 (2.0e3.9 pCi L1). About 80 percent of elevated radon levels greater than 148 mBq L1 (4.0 pCi L1) are located in proximity to depositional contacts between the Dugger and Shelburn formations, or the Shelburn and Patoka formations. These formational contacts are stratigraphically associated with Pennsylvanian black shales, which are interpreted to be the ultimate source of radon in Vanderburgh County, Indiana. Moreover, high radon levels also occurred in homes built on alluvium, terrace deposits, and outwash adjacent to the Ohio River. These elevated levels are probably due to transmission of radon through soil gas in highly permeable sands and gravels sourced from buried bedrock exposures of Pennsylvanian black shales. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Radon Isotopes Gamma ray log Radioactive decay Trace element analysis

1. Introduction The U.S. Environmental Protection Agency (EPA, 1993) characterizes most southwest Indiana counties to be Zone 2 with Moderate Potential for radon, meaning each county has a predicted average indoor screening level from 74 to 148 mBq L1

* Corresponding author. University of Southern Indiana, 8600 University Blvd, SC 2223, Evansville, IN 47712, USA. E-mail address: [email protected] (K.W. Scheller). http://dx.doi.org/10.1016/j.jenvrad.2015.06.023 0265-931X/© 2015 Elsevier Ltd. All rights reserved.

(2e4 pCi L1). Since radon is considered to be the second leading cause of lung cancer in the United States (National Academy of Sciences, 1999), identifying its presence and its precursors is paramount to understanding potential health risks and the need for remediation. In southwest Indiana, Hasenmueller (1988) indicated that brownish-black shale associated with Middle Pennsylvanian strata may influence radon potential in very limited areas in southwest Indiana. In regards to sedimentary strata, g ray spectroscopy of sediments has been used to determine relationships of specific

K.W. Scheller, W.S. Elliott Jr. / Journal of Environmental Radioactivity 148 (2015) 154e162

geological material underlying dwellings and the transmission of the radon through soils with high permeability (Appleton, 2007; Gundersen et al., 1992; Scheib et al., 2013). In the Midwest United States, several studies have identified elevated radon concentrations in groundwater associated with black shales (Hasenmueller, 1988; Schumann, 1993). In southern Illinois and southwest Indiana, uranium is enriched in black shales associated with Middle Pennsylvanian deposits (Hasenmueller, 1988). Although, Spencer (2000) conducted a radon study in Kansas City, Missouri and concluded that the proximity of a dwelling to black shale is not an indicator alone of elevated radon gas. This is because the source of 222Rn in a dwelling is related to transport of the radionuclides of the 238U decay chain in groundwater or through soils rather than release of radon gas directly from the black shale. With a half-life of only 3.82 days for 222Rn from the 238 U decay chain, proximity to the radon yielding geological material is important. Thus, dwellings in areas near black shales enriched in 238U are at the greatest risk for localized elevated concentrations of radon gas (Gundersen and Wanty, 1991; Gundersen, 1992). The objectives of this study are:

radionuclides associated with organic constituents and/or clay minerals (Bohacs and Schwalbach, 1994; Hesselbo, 1996; Schwalbach and Bohacs, 1996; Luning and Kolonic, 2003; Doveton and Merriam, 2004; Perry, 2011) and also to identify important stratigraphic contacts in carbonates (e.g. Ehrenberg and Svana, 2001). Specifically, Luning and Kolonic (2003) used spectral g ray analysis to determine the organic richness of source rocks for petroleum production. Similarly, Hesselbo (1996) proposed that spectral g ray analyses may be used to differentiate clay minerals in Cenozoic sediments in offshore New Jersey. The occurrence of 238U and 40K are specifically related to the abundance of organic matter or illite clay minerals respectively (Schwalbach and Bohacs, 1996). In Kansas, Doveton and Merriam (2004) concluded that increased uranium concentrations are related to elevated organic concentrations in shales overlying coals in Pennsylvanian deposits. In general, marine black shales in Pennsylvanian deposits across the mid-continent of the United States are a source of uranium, and therefore a precursor to 222Rn. In addition to g rays from 238U and 40 K decay, the 232Th decay series accounts for g activity when present in sedimentary strata. Of note is that one of the decay daughters in the series is another radon isotope, 220Rn. It is unlikely that 220Rn is a significant isotope in our study. First, 232Th is not abundant in bedrock of southern Indiana and our study indicates that 232Th is not present in the black shales. Second, the very short half-life of 220Rn (56 s) reduces its likelihood to be found in elevated concentrations within homes (rarely above 12 mBq L1). Ultimately, because of the short half-life of 220Rn, it is not considered to be a significant contributor to elevated radon in homes (Bodansky, 1987; Ettlinger, 1987; U.S. Department of Health and Human Services, 2012). Thus, our study will focus on 222Rn and its precursors. The occurrence of 222Rn has been identified as a significant health risk and a cause of lung cancer (Hall et al., 1987; Tilsley, 1992; Hopke et al., 2000; EPA, 2015). The source of 222Rn is from the radioactive decay of 238

238

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(1) To characterize the geochemistry and g ray spectroscopy of Pennsylvanian black shales from core samples collected in southwest Indiana; (2) To report residential radon data collected from 2007 to 2013; and (3) To interpret the significance of Pennsylvanian black shales to elevated radon in the substructures of residential homes in Vanderburgh County, Indiana.

2. Geological background Southwest Indiana is underlain by gently, westward dipping (2

234 230 226 222 218 214 210 206 U! Th!234 Pa!234 U! Th! Ra! Rn! Po! Pb!214 Bi!214 Po! Pb!210 Bi!210 Po! Pb a a a a a a a a b

b

Elevated uranium concentrations occur in several types of geologic materials, including black shale (e.g. Gundersen and Wanty, 1991; Harrell et al., 1991; Gundersen, 1992; Doveton and Merriam, 2004), crystalline rocks (e.g. Hall et al., 1987; Sundal et al., 2004), and glacial sediments derived from uraniumenriched bedrock sources (e.g. Morrow, 2001). The 238U and other isotopes in its decay series may be mobilized from these geological materials by groundwater, given the right weathering conditions, € m and Olofsson, fracturing, water chemistry, and pH (Skeppstro 2007). This may result in elevated concentrations of 238U, 226Ra, and 222Rn in aquifers (Michel and Jordana, 1987; Fenelon and Moore, 1996; Morrow, 2001; Ayotte et al., 2007). Moreover, transport of 222Rn by groundwater and its subsequent radon transmission from this water has been cited as a source of radon in € strom and Lenhard, dwellings, posing a health risk to humans (Oo 1996). Hall et al. (1987) concluded that elevated 222Rn in groundwater was related to underlying bedrock containing an enrichment of 238U and related isotopes in its decay series. Additionally, a more significant contribution of radon in residential substructures results from soil gas derived from geologic materials enriched in 238U rather than directly from groundwater. Regardless, elevated concentrations of radon gas may be predicted based upon the type of

b

b

b

b

to 3 ) Middle to Upper Pennsylvanian strata on the southern margin of the Illinois Basin consisting of rhythmic deposits of limestone, sandstone, shale, and coal (Weir, 1965; Heckel, 1986; Heckel et al., 1998; Nelson et al., 2013). USI 1-32, an exploratory well for coalbed methane drilled in 2009, is located at 37.951 N and 87.670 W south of the campus of the University of Southern Indiana in Vanderburgh County, Indiana (Fig. 1). The total depth of the well is 237.7 m (780 ft), with core intervals from: (1) 23.2e30.5 m (76e100 ft); (2) 120.4e126.5 m (395e415 ft); (3) 150.9e157.0 m (495e515 ft); (4) 164.6e170.7 m (540e560 ft); and (5) 199.6e205.7 m (655e675 ft). These recovered cores respectively sampled (Fig. 2): (1) West Franklin Limestone of the Shelburn Formation (2) Springfield Coal of the Petersburg Formation (3) Houchin Creek Coal and overlying Excello Shale of the Petersburg Formation (4) Survant Coal of the Linton Formation, and (5) Seelyville Coal of the Staunton Formation. From the g probe study of the well, four significant g ray “spikes” were observed in USI 1-32 that correspond to thin marine black

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(2) sand-rich outwash plains (3) mud-rich slackwater lacustrine sediments deposited as valley fills, especially in the eastern part of Vanderburgh County, and (4) loess deposits that mantle hill tops across the study area (Moore et al., 2009). Soils that develop on loess have moderate hydraulic conductivity (1.5e5.0 cm per hour) and are often located in upland areas with low water tables (Hasenmueller, 1988; Moore et al., 2009). Soils developed on alluvium and outwash have low to high hydraulic conductivity (0.2e15.2 cm per hour) and occur adjacent to the Ohio River (Hasenmueller, 1988; Moore et al., 2009). Finally, soils developed on bedrock in the Middle to Upper Pennsylvanian bedrock typically have low to moderate hydraulic conductivity (less than 0.2e5.0 cm per hour) and occur primarily in the western part of the study area (Hasenmueller, 1988; Moore et al., 2009). Finally, hydraulic conductivity in soils developed in sandstone bedrock range from 0.2 to 15.2 cm per hour (Hasenmueller, 1988). In general, because gas molecular diffusion coefficients are approximately four orders of magnitude greater than water, the rate of diffusion of gas through these soils will be much greater than that of water (Scanlon et al., 2002). Thus, rates of gas movement through unsaturated soils will be ~1000 times greater than the rates for hydraulic conductivity in these soils. 3. Methods

Fig. 1. Geologic map of the study area with the location of the USI 1-32 well as indicated by a star. In addition, Series of Pennsylvanian units are used to produce this geologic map (modified from King and Beikman, 1974). Finally, a dotted line demarcates the southern extent of Wisconsinan glaciation in this area, although significant loess drapes hilltops north of the Ohio River and south of the glacial limit.

shales (Fig. 2). In addition to the two “spikes” that were sampled in the cored intervals, there are two other g ray “spikes” at 187.5 m (615 ft) and 213.4 m (700 ft) depth, in the Mecca Shale and the Logan Quarry Shale respectively. A comprehensive paleoecological study of the Mecca and Logan Quarry revealed these shales contain abundant organic matter (Zangerl and Richardson, 1963). Furthermore, a previous study of the Mecca Shale in Indiana revealed a significant enrichment of uranium with concentrations of 140e165 ppm (Coveney et al., 1987). Additionally, this study (as well as previous studies) concluded that there is a direct correlation between uranium concentration to elevated organic content, along with increased concentrations of Cr, Mo, and V (e.g. Grauch and Huyck, 1990). Thus, g ray responses in USI 1-32 are the result of increased uranium concentrations in organic-rich black shales (Luning and Kolonic, 2003; Doveton and Meriiam, 2004; Algeo and Maynard, 2008). Geochemical data is provided on the concentrations of uranium in these black shales, and spectral g ray analyses provide insights into specific radionuclides in these rocks. This in turn provides valuable insights into the precursors of radon that originate from these Pennsylvanian black shales in southwest Indiana. Overlying the bedrock in Vanderburgh County are numerous Quaternary units, including: (1) floodplain and terrace deposits of the Ohio River

Geophysical data from USI 1-32 were collected by Geolog Well Services, Inc. in June 2009. Samples were collected at 30 cm intervals in the cored sections of the well. The selected core samples were trimmed on a water lubricated rock saw, followed by breakage in a Chipmunk Crusher into ~1 cm diameter fragments in the geology preparation laboratory at University of Southern Indiana. Samples were then pulverized into a powder using a SPEX CertiPrep ball mill with hardened steel vials. For g ray spectroscopic analysis, 2.5 cm thick disks of drilled cores (7.5 cm in diameter) or 1 L grab bags of sample material were placed in contact with the face of a large volume NaI detector (7.5 7.5 cm). Samples and detector were housed within lead shielding to reduce contribution of events from background radiation. Gamma events from the decay of radon precursors and daughters were processed through a Tennelec TC-145 pre-amplifier and Ortec 460 delay line amplifier, then sent to a Perkin Elmer Trump PCI-8k multichannel analyzer for pulse height analysis (Leo, 1994). The system was calibrated to record g rays of energies from 0 to 1400 KeV. Known gamma rays from 137Cs (662 KeV), 57Co (122.1 and 136.5 KeV), and 60Co (1173 and 1332 KeV) (Firestone et al., 1999) were measured to calibrate spectra. Since g decay energies are specific to the isotope that is decaying (Knoll, 2010), g rays were used to uniquely identify the radioactive isotopes present in the core. For radon daughters present in the 238U decay series, of specific interest are g rays from the decay of 214Bi and 214Pb. The decay of 214Bi yields a characteristic g ray at 609 KeV, while the decay of 214Pb yields three unique g rays at energies of 242, 295, and 352 KeV. Additionally, the immediate precursor to 222Rn, 226Ra, emits g rays at 186 KeV. The presence of any of these g rays in a core spectrum unambiguously confirms the presence of these radon daughters or its precursor. Quantitative studies of the aforementioned radioisotopes were not made for this work. Geochemical analyses of black shales were conducted following procedures outlined in Jarvis (1988), Jenner et al. (1990), and Longerich et al. (1990). For trace metal analyses, approximately 0.2 g of sample was mixed with 0.6 g of LiBO2, and then added to a graphite crucible. The sample was then placed in a muffle furnace at

Patoka

0

(feet) 0

157

USI 1-32 Gamma Ray Log 0

150

GAPI

300

450

Cored Intervals

Inglefied Sandstone

50

unnamed shale

Ditney Coal West Franklin Limestone

Shelburn

DEPTH

(meters)

MEMBER

LITHOLOGY

GROUP

FORMATION

McLeansboro

MISSOURIAN

SERIES


100

50

Well Cuttings

1

150

200

unnamed shale

250

Danville Coal

300 Herrin Coal

100 350

Carbondale Group

DESMOINESIAN

Dugger

unnamed shale

400

unnamed shale

Springfield Coal 450 Petersburg Excello Shale Houchin Creek Coal

150

Survant Coal

Linton

500

3

550

4

600

Mecca Shale Colchester Coal 200

Raccoon Creek

2

650 5

Seelyville Coal Staunton

Logan Quarry Shale

700

750

Fig. 2. Stratigraphic column, age designation, and g-ray log for USI 1-32 used in this study. Identification of stratigraphic units and nomenclature based primarily from Shaver et al. (1986). Additional sources were used to resolve shale units, such as Mecca and Logan Quarry, and the various named coal seams (Zangerl and Richardson, 1963; Heckel et al., 1998; Droste and Horowitz, 1998). The depth of the five cored intervals and well cuttings collected from USI 1-32 are identified.

950 C for 20 min. The melted sample was then added to 50 g of a 5 percent solution of HNO3 in a 100 mL Nalgene bottle. After the melted sample is added to the solution, the bottle was capped and shaken until all the sample was dissolved into solution. These prepared solutions were then analyzed by an Inductively Coupled

Plasma e Mass Spectrometry by Activation Laboratories in Houston, Texas. Trace metal concentrations are reported in parts per million (ppm). Residential radon data used in this study were provided by the Indiana State Department of Health (ISDH) for home tests

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conducted by radon testing professionals from 2007 to 2013. The data were collected using a short-term test with a passive radon diffusion chamber with alpha spectrometry by certified radon testing professionals (Indiana State Department of Health, 2015). Short-term testing methods are designed to provide a quick radon value and may be measured from 48 h to 90 days; although most short-term radon tests conducted in Vanderburgh County had a duration of two weeks. About 65 percent of the radon data from Vanderburgh County was measured in the basement of the residence, with ~30 percent measured on the main/lowermost floor of the residence. Another ~5 percent of radon values were measured in the crawlspace of the residence. The instrument for short term

testing was placed at least 25 cm from the nearest wall, at least 50 cm above the floor, and 150 cm from the nearest door, window, or ventilation. Long-term testing methods are designed to provide an annual average of radon gas and the instrument may be in a residence for up to two years. Less than 2 percent of residences (14 homes) in Vanderburgh County were tested using the long-term method. All radon data are reported in mBq L1 units.

4. Results Gamma probe data from the well yielded maximum GAPI (American Petroleum Institute gamma ray units) (Belknap et al.,

Fig. 3. Gamma ray spectra of sample material from USI 1-32 well at depths of 154 and 187 m. Present are the characteristic g rays from the radon precursor 226Ra and subsequent radioactive decay daughters of 222Rn: 214Pb and 214Bi.


1959) readings of 270, 467, 555, and 388 at depths of 124.4 m (408 ft), 153.9 m (505 ft), 187.1 m (614 ft), and 214.0 m (702 ft) respectively. With normal baseline g activities of 75-150 GAPI (Hallenburg, 1998) it was evident that elevated radioactivity was present at these depths. While these elevated g ray responses indicate the presence of radioactive decay in the subsurface, gross counting of g rays with a bore hole g probe will not identify their isotopic source. Most g activity associated with sedimentary materials is from 40K, the radioactive daughters of 238U, and the radioactive daughters of 232Th (Hallenburg, 1998). By performing spectroscopic analysis of samples taken at depths indicating high g ray response, we have identified the elements whose decay contributed to the g ray activity. Coinciding with the highest activities measured by g ray response from the geophysical well logs, spectroscopic study of samples from depths of 153.9 m (505 ft) and 187.1 m (614 ft) recorded high g activity (Fig. 3) from the decay of 226 Ra, 214Pb, and 214Bi. Radium-226 is the direct precursor to 222Rn, while 214Pb, and 214Bi are subsequent g decay daughter products of 222 Rn. These unambiguously identify the presence of radon whose ultimate source is 238U from these black shales. Trace element analysis found the maximum uranium concentration in the black shale above the Springfield Coal to be 47.8 ppm. In addition, elevated concentrations of Mo (>100 ppm), Cr (210 ppm), and V (796 ppm) were also measured for the black shale above the Springfield Coal. Likewise, uranium concentrations in the Excello Shale ranged from 17 to 80 ppm, with corresponding elevated concentrations of Mo (>100 ppm), Cr (90e150 ppm) and V

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(150e1830 ppm). Additionally, the Excello black shale is enriched in organic matter (Smith et al., 2013), and thus, is consistent with previous studies linking elevated uranium concentrations to enriched organic matter in black shales (Luning and Kolonic, 2003; Doveton and Merriam, 2004; Algeo and Maynard, 2008). Of the 744 homes tested in Vanderburgh County from 2007 to 2013 (Fig. 4), 169 homes (22.7 percent) had elevated radon levels (greater than 148 mBq L1 (4.0 pCi L1)). Additionally, 246 homes (33.1 percent) had measured radon levels of 74e145 mBq L1 (2.0e3.9 pCi L1). The average radon level in homes for Vanderburgh County is 108 mBq L1 (2.9 pCi L1), and the average for Indiana is 178 mBq L1 (4.8 pCi L1), using data from 2007 to 2013 (ISDH, 2015). About 70 percent of these elevated radon levels (greater than 148 mBq L1 (4.0 pCi L1)) are located in proximity to depositional contacts between the Dugger and Shelburn formations, or the Shelburn and Patoka formations, or the Patoka and Bond formations. These formational contacts are stratigraphically associated with black shales, which are interpreted to be the ultimate source of radon in Vanderburgh County, Indiana (Fig. 5). Moreover, high radon levels also occurred in homes built on alluvium, terrace deposits, and outwash adjacent to the Ohio River (Fig. 5). These alluvial, terrace, and outwash deposits are not enriched in uranium or radium. Thus, the elevated levels are probably due to transmission of radon through soil gas in highly permeable sands and gravels sourced from buried bedrock exposures of Pennsylvanian black shales.

Fig. 4. Histogram showing distribution of the 744 radon levels measured in residential structures from 2007 to 2013. For example, a value of 148 on the histogram equates to all measured values of radon from 148 to 185 mBq L1 (4e5 pCi L1) measured in Vanderburgh County homes.

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5. Discussion The ultimate source for radon in Vanderburgh County, Indiana results from the radioactive decay of 238U in Pennsylvanian black shale. From this source, radon is transmitted through various sediments and soils, impeded by impermeable layers, resulting in varied radon levels in the substructures of residential homes. Approximately 70 percent of homes with elevated radon levels (greater than 148 mBq L1) occur in close proximity to surface or near surface exposures of Pennsylvanian black shales. There is also a clustering of elevated radon levels (~15 percent of elevated radon levels in Vanderburgh County) in homes just to the east of Evansville, probably the result of rapid transmission of radon through highly permeable Quaternary alluvium, outwash, and their respective soils. Again, the radon is ultimately sourced from the Pennsylvanian black shales, but its transmission through permeable sediments produces a more complicated spatial distribution of elevated radon levels. Likewise, variations in this Quaternary alluvium, outwash, and their soils may locally act as a barrier to radon migration, making it difficult to predict radon levels in substructures of residential structures erected on these deposits. The remaining ~15 percent of elevated radon levels (greater than 148 mBq L1 (4.0 pCi L1)) occurs in areas underlain by the Patoka Formation. Most likely, areas of elevated radon in the Patoka Formation are the result of transmission through the Inglefield Sandstone originating from the black shale capping the Ditney Coal at the base of the Patoka Formation. Heckel et al. (1998) concluded that the Inglefield Sandstone erosionally truncates the Ditney Coal, and in places rests unconformably on the coal. This erosional truncation results in the juxtaposition of the Inglefield Sandstone with the Pennsylvanian black shale overlying the Ditney Coal (Shaver et al., 1986). Given the complicated nature of this erosional basal contact of the Inglefield Sandstone, this would result in a wider distribution of elevated radon levels for homes constructed within the Patoka Formation. Furthermore, upland areas in the study area are overlain by loess deposits that range in thickness from a few cm to 10 m. Given the decreased hydraulic conductivity of these wind-blown deposits, areas underlain by thick loess (greater than 5 m) may act as a barrier to the migration of radon into residential structures. These upland areas occur to the north of the Ohio River near the northern and western boundaries of Vanderburgh County. Finally, radon levels in residential homes are also controlled by other factors, which explains why homes next to one another can have substantially different radon levels (Tanner, 1991a, 1991b; Tilsley, 1992; EPA, 2015). These factors include, but are not limited to the type of substructure (basement or slab), construction materials (stone, concrete block, concrete), and ventilation of the substructure (old versus new homes; basement vents, radon remediation). Additionally, because transmission of radon occurs at a higher rate through soil gas, areas with deeper water tables and high soil permeability are at greater risk for elevated radon levels.

Approximately 70 percent of these elevated radon levels are located in proximity to the stratigraphic position of Pennsylvanian black shales, which are interpreted to be the ultimate source of radon in Vanderburgh County, Indiana. Moreover, high radon levels also occurred in homes built on alluvium, terrace deposits, and outwash adjacent to the Ohio River. These elevated levels are probably due to transmission of radon through soil gas in highly permeable sands and gravels. Furthermore, elevated radon levels also occur in homes underlain by the Patoka Formation. Most likely, areas of elevated radon in the Patoka Formation are the result of transmission of radon through the permeable Inglefield Sandstone. Although predicting elevated radon in homes is difficult (thus, the need to test all substructures for radon), this study indicates that areas of higher risk may be identified using detailed bedrock and surficial geologic maps, resolving the geologic source of radon,

6. Conclusions This study characterized the geochemistry and g ray spectroscopy of Pennsylvanian black shales from cores recovered from a well drilled in Vanderburgh County, Indiana. Gamma ray spectroscopy identified the presence of radon (222Rn) daughters, 214Pb and 214Bi, as well as its precursor, 226Ra, in core samples at multiple depths. Geochemical analyses revealed elevated concentrations of 238 U in these same black shale intervals. Thus, the source of radon from Pennsylvania black shale in southwest Indiana is confirmed. In Vanderburgh County, 22.7 percent of residential homes had elevated radon levels (greater than 148 mBq L1 (4.0 pCi L1)).

Fig. 5. Levels of radon measured in residential settings from 2007 to 2013 in Vanderburgh County, Indiana. This radon data is plotted on a bedrock geologic map; the black shale overlying the Danville coal corresponds to the contact between the Dugger and Shelburn formations. Likewise, the black shale above the Ditney Coal is exposed stratigraphically just above the boundary between the Shelburn and Patoka formations. Additionally, the boundary of Ohio River floodplain and terrace deposits, as well as the northern margin of outwash deposits are highlighted on this map. Geologic contacts from Cable and Wolf (1977); Hatch and Affolter (2002) and Moore et al. (2009).


and recognizing transmission pathways for radon. Currently, radon testing is not required by the Indiana State Department of Health for real estate transactions, thus this study may be used as a guide to homeowners to identify areas of risk of elevated radon concentrations in homes located in Vanderburgh County, Indiana. Funding Support for this work was provided by the Pott College of Science, Engineering and Education at the University of Southern Indiana. Christopher Smith of Weatherford Labs in Houston, along with Clinton Broach, provided geochemical data from Activation Laboratories on the black shales. Acknowledgments The authors benefited from discussions with our colleagues Paul K. Doss and Norman King. Many undergraduate students at University of Southern Indiana were involved in aspects of this project, including Clinton Broach, Ryan Voegrel, and Aaron Feldhaus. Christopher Smith of Weatherford Labs in Houston, along with Clinton Broach, provided geochemical data from Activation Laboratories on the black shales. Many thanks to Jeff Turner of the Indiana State Department of Health for providing residential radon data for Vanderburgh County from 2007 to 2013. Finally, the manuscript benefited from thoughtful criticism by R. Randall Schumann and two additional anonymous reviewers. References Algeo, T.J., Maynard, J.B., 2008. Trace-metal covariation as a guide to water-mass conditions in ancient anoxic marine environments. Geosphere 4 (5), 872e887. Appleton, J.D., 2007. Radon: sources, health risks, and hazard mapping. R. Swed. Acad. Sci. 36 (1), 85e88. Ayotte, J.D., Flanagan, S.M., Morrow, W.S., 2007. Occurrence of Uranium and 222 Radon in Glacial and Bedrock Aquifers in the Northern United States, 1993e2003. U.S. Geological Survey. Scientific Investigations Report 2007e5037. Belknap, W.B., Dewan, J.T., Kirkpatrick, C.V., Mott, W.E., Pearson, A.J., Rabson, W.R., 1959. API Calibration Facility for Nuclear Logs: American Petroleum Institute, Drilling and Production Practice, pp. 289e317 (summary of RP33; reprinted in SPWLA Reprint Volume on Gamma Ray, Neutron, and Density Logging, Mar. 1978). Bodansky, D., 1987. Overview of the indoor radon problem. In: Bodansky, D., Robkin, M., Stadler, D. (Eds.), Indoor Radon and its Hazards. University of Washington Press, Seattle, Washington, pp. 3e16. Bohacs, K.M., Schwalbach, J.R., 1994. Natural gamma-ray spectrometry of the Monterey formation at Naples Beach, California: insights into lithology, stratigraphy, and source-rock quality. In: Field Guide to the Monterey Formation between Santa Barbara and Gaviota, California, pp. 85e94. Cable, L.W., Wolf, R.J., 1977. Ground-water resources of Vanderburgh County, Indiana. Indiana Dep. Nat. Resour. Bull. 38. Coveney Jr., R.M., Leventhal, J.S., Glascock, M.D., Hatch, J.R., 1987. Origins of metals and organic matter in the Mecca Quarry Shale member and stratigraphically equivalent beds across the midwest. Econ. Geol. 82, 915e933. Doveton, J.H., Merriam, D.F., 2004. Borehole petrophysical chemostratigraphy of Pennsylvanian black shales in the Kansas subsurface. Chem. Geol. 206, 249e258. Droste, J.B., Horowitz, A.S., 1998. The Raccoon creek group (Pennsylvanian) in the subsurface of the Illinois Basin. Proc. Indiana Acad. Sci. 107, 71e78. Ehrenberg, S.N., Svana, T.A., 2001. Use of spectral gamma-ray signature to interpret stratigraphic surfaces in carbonate strata: an example from the finnmark carbonate platform (CarboniferousePermian), Barents Sea. AAPG Bull. 85 (2), 295e308. Environmental Protection Agency, 1993. EPA's Map of Radon Zones. Document EPA402-R-93e071. U.S. Environmental Protection Agency, Washington, D.C. Environmental Protection Agency, 2015. Radon. http://www.epa.gov/radon (accessed 29.03.15.). Ettlinger, L., Sayala, D., Smith, B., 1987. Predicting High Radon Areas: a Study in Northern Virginia. Indoor Radon II, pp. 5e9. Pittsburgh, Pennsylvania. Fenelon, J.M., Moore, R.C., 1996. Radon in the Fluvial Aquifers of the White River Basin, Indiana, 1995, p. 2. U.S. Geological Survey Fact Sheet 124-96. Firestone, R.B., Shirley, V.S., Baglin, C.M., Chu, S.Y.F., Zipkin, J., 1996, 1998, 1999. Table of Isotopes, eighth ed. John Wiley & Sons, Inc., New York. Metalliferous black shales and related ore deposits. In: Grauch, R.I., Huyck, H.L.O. (Eds.), 1990. Proceedings, 1989 United States Working Group Meeting,

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