Science of the Total Environment 468–469 (2014) 764–773

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Science of the Total Environment

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Determining a pre-mining radiological baseline from historic airbornegamma surveys: A case study

Andreas Bollhöfer ⁎, Annamarie Beraldo 1, Kirrilly Pfitzner, Andrew Esparon, Che DoeringEnvironmental Research Institute of the Supervising Scientist (ERISS), Australian Government, Department of Sustainability, Environment, Water, Population and Communities, PO Box 461,Darwin, NT 0801, Australia

H I G H L I G H T S

• Data from a historic airborne gamma survey at a uranium mine were groundtruthed.• Algorithms were developed to upscale the ground gamma data in ArcGIS.• Correlation models allow estimation of pre-mining radiological conditions.• Pre-mining γ-dose rates, Rn flux, and soil 226Ra activity concentrations determined• Approach can be used at mines where historical airborne gamma survey data exist.

⁎ Corresponding author at: Environmental ResearchScientist (ERISS), Department of Sustainability, EnvironCommunities, PO Box 461, Darwin, NT 0801, Australia. Te

E-mail addresses: Andreas.bollhoefer@environment.goABeraldo@globalskm.com (A. Beraldo), Kirrilly.pfitzner@eAndrew.esparon@environment.gov.au (A. Esparon), Che.d(C. Doering).

1 Present address: Sinclair Knight Merz Pty Ltd, PO Box

0048-9697/$ – see front matter. Crown Copyright © 2013http://dx.doi.org/10.1016/j.scitotenv.2013.09.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 May 2013Received in revised form 27 August 2013Accepted 1 September 2013Available online 26 September 2013

Editor: Mae Sexauer Gustin

Keywords:Uranium miningRadium-226Radon exhalationRadiological baseline assessment

Knowing the baseline level of radioactivity in areas naturally enriched in radionuclides is important in theuraniummining context to assess radiation doses to humans and the environment both during and after mining.This information is particularly useful in rehabilitation planning and developing closure criteria for uraniummines as only radiation doses additional to the natural background are usually considered ‘controllable’ forradiation protection purposes. In this case study we have tested whether the method of contemporarygroundtruthing of a historic airborne gamma survey could be used to determine the pre-mining radiologicalconditions at the Ranger mine in northern Australia. The airborne gamma survey was flown in 1976 beforemining started and groundtruthed using ground gamma dose rate measurements made between 2007 and2009 at an undisturbed area naturally enriched in uranium (Anomaly 2) located nearby the Ranger mine.Measurements of 226Ra soil activity concentration and 222Rn exhalation flux density at Anomaly 2 were madeconcurrent with the ground gamma dose rate measurements. Algorithms were developed to upscale the groundgamma data to the same spatial resolution as the historic airborne gamma survey data using a geographic infor-mation system, allowing comparison of the datasets. Linear correlation models were developed to estimate thepre-mining gamma dose rates, 226Ra soil activity concentrations, and 222Rn exhalation flux densities at selectedareas in the greater Ranger region. Themodelled levels agreedwithmeasurementsmade at theRangerOrebodies1 and 3 before mining started, and at environmental sites in the region. The conclusion is that our approach canbe used to determine baseline radiation levels, and provide a benchmark for rehabilitation of uranium mines orindustrial sites where historical airborne gamma survey data are available and an undisturbed radiologicalanalogue exists to groundtruth the data.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

Institute of the Supervisingment, Water, Population andl.: +61 8 89201142.v.au (A. Bollhöfer),nvironment.gov.au (K. Pfitzner),oering@environment.gov.au

2145, Parap NT 0820, Australia.

Published by Elsevier B.V. All rights

1. Introduction

Australia is the third largest uranium producer worldwide, withRanger uranium mine in the Northern Territory of Australia being thelargest producer in Australia (OECD, 2012). Mining at Ranger startedin 1980. The mining of Ranger Orebodies 1 and 3 is now complete,and site closure is anticipated in 2026. Rehabilitation planning of theRanger Project Area, which includes the operational mine site, landapplication areas, Jabiru East and its airstrip (Fig. 1) and several areasof exploration, is of high importance to ensure the integration of therehabilitated site into the surrounding environment of Kakadu National

reserved.

Fig. 1. The Alligator Rivers Region and Ranger uraniummine. RP 1 and 2: Retention Ponds; TSF: Tailings Storage Facility.

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766 A. Bollhöfer et al. / Science of the Total Environment 468–469 (2014) 764–773

Park. A set of Environmental Requirements with which the operatormust comply set out the Commonwealth of Australia's environmentalprotection objectives for the rehabilitation of the Ranger Project Area(Commonwealth of Australia, 1999). In relation to radiological matters,the rehabilitation objective is for “stable radiological conditions on areasimpacted by mining so that, the health risk to members of the public,including traditional owners, is as low as reasonably achievable”. TheEnvironmental Requirements also state that “radiation doses receivedfrom natural background sources… are not to be included in the calcu-lation of radiation doses.” Because of the latter, knowledge of the pre-mining radiological conditions is important for the development ofradiological closure criteria for the rehabilitated site (i.e. soil radionu-clide activity concentrations, terrestrial gamma dose rates, radon-222(222Rn) exhalation flux densities), and to guide decisions on the accept-ability of potential doses to the public for different land use scenarios.

Before mining started at the Ranger uranium mine, Orebodies 1and 3 and several other radiation anomalies were known to exist inthe area (e.g. Eupene et al., 1975; Hegge et al., 1980). Soil uraniumand radium activity concentrations, terrestrial gamma dose rates and222Rn exhalation flux densities on and around the Ranger Project Areawere elevated compared to the typical environmental backgroundnearby. Although extensive pre-mining environmental studies wereconducted (e.g. Conway et al., 1974), they provide relatively little quan-titative radiological data that are appropriately spatially referenced toassess the baseline radiological conditions at Ranger mine. Some earlystudies exist that assessed the uranium mineralisation occurring atand around theorebodies, but these studieswere focussed on determin-ing the radiation levels and the extent of the outcropping anomaly ororebody (Ryan, 1972; Eupene et al., 1975), rather than determiningaverage gamma dose rates or soil radionuclide activity concentrationsover the whole area. Only Kvasnicka and Auty (1994) have reportedoutcomes from a pre-mining assessment for the Ranger uraniummine. The results from their assessment are reproduced in Table 1.

A novel approach to determine the pre-mining radiological conditionsfor a uraniummine site is to groundtruth historic airborne gamma surveydata for the site. Although it was not until the late 1990s that airbornegamma surveys were routinely used to assess the radiological statusof areas contaminated by naturally occurring radioactive material(NORM) residues (Winkelmann et al., 2001; Pfitzner et al., 2001;Coetzee et al., 2006; Martin et al., 2006; Bollhöfer et al., 2008) airbornestudies were used for decades in uranium exploration, starting in 1949in the Northwest Territories of Canada (Darnley, 1972). The Pine CreekGeosyncline in remote Northern Australia is an area of extensiveuranium mineralisation and airborne gamma surveys have also beenused there as a tool for exploration (Tucker et al., 1980). The Rangerorebodies were discovered during a survey flown in 1969 by Noranda(Australia) (Ryan, 1972) and large anomalies were also detected overNabarlek (Tipper and Lawrence, 1972) and Koongarra in 1970 (Foyand Pedersen, 1975). Eupene et al. (1975) published radiation contoursfrom the 1969 survey, which show the total counts acquired overRanger Orebodies 1 and 3. This survey also pointed to a radiationanomaly, Anomaly 2, to the south of Orebody 1, that was exposed at

Table 1U3O8 concentrations, terrestrial gamma dose rates, 226Ra activity concentrations and 222Rn flux(1994).

Area[ha]

U3O8

[%]

Orebody 1 44 0.061 ± 0.063a

Orebody 3 66 0.037 ± 0.030a

background

a: determined from results of U3O8 in the top 1 m of core samples from exploratory drilling onb: determined from the ratio of U3O8 of orebodies 1 and 3, multiplied by the dose rate measurc: determined from the measured γ-dose rate on orebody 3.d: determined using a 222Rn flux density per 226Ra activity concentration ratio: 1.85 Bq∙m−2∙s−

the surface in places and characterised on ground by three strong sepa-rate radiometric anomalies (Hegge et al., 1980). The Ranger orebodieshave been mined (Orebody 1: 1980–1994; Orebody 3: 1997–2012),the known uranium prospect at Anomaly 2 was not, due to its closeproximity to Mount Brockman and its aboriginal sacred sites in KakaduNational Park (RUEI, 1977).

We hypothesized that qualitative data from historic airbornegamma surveys can be used for quantitative radiological assessmentover a large area, if groundtruthed and calibrated retrospectively. Thishypothesis was tested by identifying a suitable undisturbed radiologicalanomaly, groundtruthing the airborne gamma survey data for thatanomaly through fieldmeasurements, extrapolating the groundtrutheddata for the anomaly to the greater Ranger region and comparing themodelled data with existing pre-mining measurements at Orebodies 1and 3. The method presented here is applicable to uranium mines orindustrial sites elsewhere, where pre-mining airborne gamma surveydata are available and an undisturbed radiological analogue exists.

2. Location and geological setting

The Ranger Project Area is a 78 km2 mineral lease that is located in,but not part of, the World Heritage listed Kakadu National Park in theNorthern Territory of Australia. Kakadu National Park forms part of thegreater Alligator Rivers Region (Fig. 1). The regional centre of Jabiru,with a population of approximately 1200 people, is located approxi-mately 8 km west of the mine. There are various outstations in thevicinity of Jabiru, which are largely inhabited by aboriginal people. Thetraditional lifestyle of many of the aboriginal people means that areason and around the mine were accessed before mining started for thehunting and gathering of traditional bush foods and for various culturalreasons. These areas may again be accessed once rehabilitation of themine area is complete.

The Ranger uranium mine is part of the Alligator Rivers Uraniumfield, within the Pine Creek Inlier. The geology is dominated byProterozoic volcanic, carbonate, and sedimentary sequences thatunconformably overlie Archaean granitic gneiss of the NanambuComplex. The following geological description is adapted from McKayand Miezitis (2001). Most of the ore in the mined out orebodies wasin the Upper Mine Sequence, which is mainly chloritised biotite–quartz–feldspar schist and microgneiss with thin carbonaceous lenses(Eupene et al., 1975). The orebodies extended into the Lower Mine Se-quence, which is mainly recrystallised magnesian or dolomitic marble,chloritic in the upper part. Both are broadly conformable with the hostrocks, strike north–south, with an overall dip to the east. Within theore zone there were several periods of brecciation with associatedchloritisation and remobilisation of uraniummineralisation. The prima-ry ore consists of uraninite with minor brannerite and amorphous mix-tures of uranium oxides (pitchblende) with titanium and phosphates.Gangue minerals associated with the pitchblende mineralisation arechlorite, quartz, titanium oxides, hematite, apatite, pyrite, chalcopyrite,and predominantly radiogenic galena.

densities determined for orebodies 1 and 3 and background sites, fromKvasnicka and Auty

Eγ[μGy∙hr−1]

226Ra[Bq∙kg−1]

222Rn[10–3 Bq∙m−2∙s−1]

0.96b 2230 ± 2300b 4.1 ± 3.4d

0.58 1350 ± 1090c 2.5 ± 2.0d

0.06–0.08 70–120 0.13–0.22d

the orebodies.ed on top of orebody 3.

1 per Bq∙g−1 measured on orebody 3.

Table 2Survey parameters for the 1976 Alligator River Geophysical Survey and the 1997 Rio TintoRanger uranium mine airborne gamma survey. In addition, statistics for the eU channelover the Anomaly 2 area (as outlined in Figs. A2 and A3 in the Supplementary material)are given.

Survey parameter

Survey date 1976 1997

Flight line direction (degrees) 030–210 090–270Line spacing 300 m 100 mFlying height unknown 50 m (approx)Total km flown 21,700 (approx) 500 (approx)Counts reported eU, eTh, K, TC eU, eTh, K, TCSpectrometer Unknown 256 channel PGAM-1000Volume Unknown 33.56 lResolution of raster data set 70 m × 70 m 25 m × 25 meU stats (min; max; mean; n) 144.3; 994.3; 384.0; 63 137.8, 673.3, 265.3, 479

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3. Methods

3.1. Airborne gamma surveys

Existing airborne gamma survey data covering the Alligator RiversRegion and available from the Northern Territory Geological Surveywere reviewed and compared with data in the Mineral OccurrenceDatabase. This database was developed to give spatially accurate pointdata of known mineral occurrences within the Northern Territory(NTGS, 2007). Pfitzner and Martin (2003) have investigated the data-basewith a focus on uranium (and thorium) commodities in the region.Fig. A1 in the Supplementary material shows the location of the poten-tial ore deposits found in their investigation overlaid on a mosaic of thespatial coverage of airborne gamma survey data available from theNorthern Territory Geological Survey. Airborne gamma survey datawas also acquired over Ranger mine, the rehabilitated Nabarlek mine(Martin et al., 2006), the historic Sleisbeck area (Bollhöfer et al., 2008),and the upper South Alligator River valley (Pfitzner et al., 2001), flownat flying heights of ≤50 m with a line spacing as tight as 25 m(Bollhöfer et al., 2008). Such a tight line spacing and low flyingheight are required for detailed radiological mine site and remediationassessment, where features can be small in size, sometimes less than0.1 ha.

Despite the coarser spatial resolution of surveys flown for mineralexploration,with a line spacing in the order of 500 mand a flying heightN100 m, data are still sufficient to map landscape scale radiologicalconditions. As the purpose of determining pre-mining radiologicalconditions across the Ranger lease is tomodel radiation doses to peopleaccessing the area for hunting and gathering of food stuffs beforemining started, such a spatial averaging was considered acceptable, inparticular with the lack of other, more detailed pre-mining informationavailable. Data and metadata for historic ground surveys andairborne gamma surveys flown formineral exploration are often impos-sible to retrieve as much of the information has been lost over thedecades.

To avoid correcting for survey differences in resolution,flyingheight,detector volume, and other parameters, only pre-mining data fromboththeRanger Project Area and a potential analogue site thatwere acquiredduring the same airborne gamma survey, using the same survey speci-fications and instrumentation, should be studied. Consequently, it wasdecided to study the Alligator River Geological Survey in more detail(‘Alligator’ in Fig. A1 in the Supplementary material) and investigatewhether this survey delineated radiation anomalies suitable as a pre-mining analogue for the Ranger 1 and 3 orebodies.

3.1.1. Alligator River Geophysical SurveyThe Alligator River Geophysical Survey was flown by Geometrics in

1976 by fixed wing aircraft (Table 2). The data were acquired from RioTinto by the Northern Territory Geological Survey and are available inthe public domain. Line data and 70 m × 70 m raster data (Fig. 2a)were supplied for this study by the Northern Territory Department ofMines and Energy. The processed data has had a height correctionapplied (but no height information is available) and has been strippedof the noise. An average background radon contribution was removedas a result of radon in air on the overall image, rather than strippingradon point by point. Detailed algorithms applied for data processingare unknown.

A comparison of signal intensity in the 1976 airborne gammasurvey with the location of known uranium occurrences in the MineralOccurrenceDatabase showed a strong airborne gamma signal associatedwith Anomaly 2 to the south of the mine area (Fig. 2b and c), similar tothe original 1969 data published in Eupene et al. (1975). As the greaterregion of Anomaly 2 was not disturbed significantly over the past threedecades, it provided a suitable area to groundtruth the 1976 airbornegamma survey data and an extensive fieldwork programme to do thiswas commissioned in 2007.

3.1.2. Rio Tinto airborne gamma surveyRio Tinto Mining Company commissioned an airborne gamma

survey in 1997, 20 years after the Alligator River Geophysical Survey,at a flying height of approximately 50 m (30 m–150 m) and a higherspatial resolution than the 1976 survey (Table 2). The survey wasflown byWorld Geoscience Corporation and covered the Ranger ProjectArea and Anomaly 2. Data reduction and gridding were carried out byWorld Geoscience Corporation. The data were processed using SpectraPlus, a spectral fitting approach which uses the spectra information inthe energy range of 400 keV to 3 MeV, developed by, and proprietaryto, World Geoscience Corporation. Stripping coefficients were appliedand data corrected for the contribution from aircraft and cosmic back-ground and a height correction was applied to determine uranium (eUin Bq kg−1) and total count (TC in s−1) values. At the above flyingheight, about 90% of the signal detected on board the plane originatesfrom within a strip with a width of 230 m (Pitkin and Duval, 1980;Duval, 1997).

Initially, thedatawas used to delineate the exact location of the threeoutcropping radiation anomalies associated with Anomaly 2 (Fig. A2 inthe Supplementary material), after some spatial discrepancies occurredwhen groundtruthing the1976 airborne gamma survey data. Itwas theninvestigatedwhether the better resolution 1997 airborne gamma surveycould be used as an intermediate step for groundtruthing of the lowerresolution 1976 survey.

3.2. Groundtruthing

Geographic Positioning System (GPS) readings were taken at eachmeasurement location of gamma dose rates, soil radionuclide activityconcentrations and 222Rn exhalation flux densities. Eastings andNorthings were recorded in the World Geodetic System 84 (WGS84)map datum, Universal Transverse Mercator (UTM) coordinate systemwithin Zone 53 south. The spatial accuracy of the GPS readings wasbetween 2 and 3 m.

3.2.1. External gamma dose rateGamma dose rates were measured using three environmental dose

rate meters (Mini-Instruments, Environmental Meter Type 6–80). Themeters have both an analogue display giving the measured dose ratein μGy h−1 and a digital display of counts in a pre-selected time, usually100 s. Gammadose rateswere calculated from themeasured count rate.One dose rate meter was calibrated throughout the duration of the pro-ject and results of the calibrations showed that it operated within itsspecifications. The remaining two instruments were cross calibratedagainst the first and respective correction factors were applied to themeasured count rates. A total of 1872 gamma dose rate measurementswere taken during five separate surveys over Anomaly 2 betweenAugust 2007 and July 2009. Measurements were taken to delineate

Fig. 2. (a) Counts per second in theuranium channel of the1976 70 m × 70 mpixel data over thewhole scene (N25 s−1) and (b) over theRanger and (c)Anomaly 2 areas only (N250 s−1),overlaid on an aerial photo mosaic of the region from 2007. The locations of Ranger orebodies 1 and 3, and Anomaly 2 are clearly visible in the uranium data. The black dots indicate theradon and soil sampling locations.

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the exact position and extent of the three radiation anomalies associatedwith Anomaly 2, and to get a representation of the local background. Theresolution of the surveys was between 10 and 20 m, covering an area ofapproximately 38 ha (Fig. A2 in the Supplementary material).

3.2.2. Soil 226Ra activity concentration

3.2.2.1. In-situ soil activity concentration measurements. Soil activityconcentrations were measured in-situ during a survey in August 2007using a 512 channel portable sodium iodide (NaI) field gamma detector(Geofyzika, now SatisGeo, model GS-512). A total of 160measurementswere taken for 600 s each at a height of 1 m above ground. At eachlocation gamma dose rates were also measured. The GS-512 displaysand stores the concentrations of potassium [%], uranium and thorium[mg kg−1]. Stripping coefficients (which define the ratios of countrates, caused by a single element in a particular energy window, to thecount rate in its principle energy window (IAEA, 2003)) and calibrationconstants determined by the manufacturer were used to convert countrates to nominal activity concentrations for the three elements. Theuranium concentrations are reported as equivalent uranium (eU) asthe instrument is using the gamma rays detected at 1.73–1.76 MeVfrom thedecay of 214Bi as a proxy for 226Ra, and assumes secular equilib-rium between 226Ra and 238U. Consequently, as secular equilibrium isnot always a given (Martin et al., 2006), eU is a measure of 226Ra ratherthan uranium concentration. The stability of the instrument was testedregularly by measuring the signal on a test spot at the Darwin laborato-ries of the Environmental Research Institute to ensure that the calibrationhad not changed.

3.2.2.2. Laboratory soil activity concentration measurements. Soil sampleswere collected in July 2009 from 25 locations (Fig. 2c) where 222Rnexhalation flux densities were also determined. At each site, the top2 cm of soil was scraped at 5 random locations within 1 m of the char-coal canisters deployed for 222Rn measurements. The 5 samples werethen combined in a plastic bag. Location 13 was on top of Anomaly 2Aand this site was dominated by coarse gravel of a few centimetres

thickness overlying rock strata. Similar soil morphology was encoun-tered at Sites 14 and 15 where a relatively thin layer of gravel andsand was overlying rocky terrain. Location 24 was on top of a rockheap associated with a small exploration trench. The remainder of thesites were located in areas of sandy loam or gravel of various sizes.

Samples were transported to the Darwin laboratories for radionu-clide analyses. Radionuclide activities were determined using HighPurity Germanium (HPGe) gamma spectrometry. Details of the gammaspectrometry methods are described in Murray et al. (1987). TheVisualGamma spectrum analysis programme developed at theEnvironmental Research Institute of the Supervising Scientist wasused for analyses of radionuclide activity concentrations (Esparon andPfitzner, 2010). The stability and background of the HPGe detectorswas checked weekly with a multi isotope standard containing radionu-clides of the uranium and thorium decay chains and a blank container,respectively. Detection limits for soil samples using gamma spectrome-try are dependent on sample size, detector efficiency and backgroundcount rates of the given nuclide, and were typically b3 Bq kg−1 for226Ra for a one day count.

3.2.3. 222Rn exhalation flux densityRadon-222 exhalation flux density was measured by trapping the

222Rn that is emitted from the ground surface in canisters filled withactivated charcoal (Countess, 1976). The canisters used in this studywere a brass cylindrical design with an internal diameter of 0.061 m.If the “open face” of the canister is sealed against a surface, then allthe 222Rn coming from the surface area underneath the canister willdiffuse into the canister andbe adsorbed onto the charcoal. The 222Rn ex-halation flux density over the period of exposure can then be estimatedusing the methodology described in Spehr and Johnston (1983) andBollhöfer et al. (2006).

Radon-222 exhalation flux density was measured over the extent ofAnomaly 2 at 25 locations and 75 individual charcoal-filled canisterswere deployed. To prevent leakage of 222Rn around the edge of thecanisters they were embedded in the earth to a depth of about 1 cmor, if the surface was too hard or irregular, the rim was sealed with a

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“putty”. There was no rainfall during the deployment period. Threecanisters were deployed at each location and collected after a periodof three days. Five canisters were knocked over by wildlife during theperiod of deployment and were excluded from further analysis. Conse-quently, 70 individual 222Rn exhalation flux density measurementswere made at the 25 soil sampling locations. After collection thecanisters were transported to the Darwin laboratories for radioactivitycounting.

A NaI gamma spectrometer, housed in a lead castle to reduce back-ground, was used to determine the activity of 222Rn decay productsadsorbed on the charcoal. Each canister was counted for 600 s. Fourregions of interest representing the photopeaks of 214Pb at 242, 295and 353 keV and of 214Bi at 609 keVwere used to determine the activityof 222Rn progeny adsorbed onto the charcoal. The procedure for radioac-tivity counting included a delay of at least 3 h between the collection ofthe canisters and the start of the count period to allow the progeny214Pb (t1/2 = 26.8 min) and 214Bi (t1/2 = 19.9 min) to in-grow towardsa secular equilibrium with their progenitor, 222Rn (t1/2 = 3.82 days).The average counting efficiency was 10.6% ± 0.2% determined usingthe methodology in Bollhöfer et al. (2006).

3.3. Approach tomodel the 1976 airborne gamma survey data to radiologicaldata on ground

Differences in survey parameters, height of the detector aboveground, detector type and calibration do not allow direct comparisonof spatial data from the two airborne gamma surveys with theground-based gamma dose rate measurements. Up-scaling of theground-based survey is required to make the footprints of the surveyscomparable. Spatial referencing and data processing (which is largelyunknown for the 1976 survey) can complicate the process of up-scaling the data. Due to the better spatial resolution of the 1997 airbornegamma survey data it was decided that up-scaling of the gamma doserates measured in the field to the uranium count data measured duringthe 1976 airborne gamma survey is performed via an intermediate step,using the data from the 1997 survey. The underlying assumption is thatbetween 1976 and the time of the groundtruthing, no changes thatwould significantly alter the radiological conditions at Anomaly 2have occurred. This assumption is reasonable, as Anomaly 2 has beenexcluded from mining due to its close proximity to Mount Brockmanand aboriginal sacred sites.

3.3.1. Modelling the 1997 survey line data to field gamma dose ratesThe airborne gamma survey line data were projected into the

WGS84 map datum, UTM Zone 53S. A shapefile was then created inArcGIS, defined by the boundary of the field data obtained for theAnomaly 2 area (Figs. A2 and A3 in the Supplementary material).Airborne gamma survey points within the boundary of the shapefileand with integer time records were then extracted. The basic assump-tion here is that all non-integer second data is interpolated based onthe integer second records. The integer records were then given aunique ID and the uranium (eU) (in Bq kg−1) and total count (TC)data (in s−1) were used for modelling purposes. The average distancethe plane travels between individual measurements is 60 m. Lineswere then created which represent the flight path for each of therecords and each line adopted the TC and eU values from the integertime stamp of the 1997 survey line data file.

A series of buffers of the shape shown in Fig. A3 in the Supplementarymaterial was created in ArcGIS and the buffer radius was varied. Ratherthan using a circular shape, the buffer shape takes into account that theplane moves at a speed of approximately 60 m s−1. The mean gammadose rates (μGy h−1) measured on ground within the chosen buffersfor each line were then calculated. The buffer radius was then varied in5 m steps to obtain the radius giving the best correlation betweenaveraged gamma dose rates and uranium data measured on board theplane.

3.3.2. Modelling the 1976 to the 1997 survey raster dataBoth the 1976 (70 m × 70 m) and 1997 (25 m × 25 m) raster

datasets were reprojected toWGS84 UTM53S and a subset of the rasterdata was created. This subset included the full extent of the 1997 rasterimagery including Anomaly 2 at the southern edge of the dataset, butexcluded the mine footprint. The assumption was that, apart fromareas at the mine, radiation levels have not changed between 1976and 1997.

Both the 1997 and 1976 data were then clipped to this subset. Avector grid of the 1976 data was created using the Hawths Tool inArcGIS and each grid cell was populated with the uranium counts ofthe 1976 raster,with the centre of each grid cell taking on the respectiveuraniumcount value. The 1997 raster eU datawas then overlaid and theaverageswere calculated of those 1997 raster data thatwere completelycontained within the respective vector grid cell of the 1976 survey data,using the Spatial Analyst Tool in ArcGIS.

4. Results and discussion

4.1. Airborne gamma survey data and field gamma dose rates

In this study, line and raster data from two airborne gamma surveyswere available over thegreater Anomaly 2 area. Statistics in theuraniumchannel of the raster data over the Anomaly 2 region are given inTable 2. Fig. 2a, b, and c show the raster data of the uranium channel(s−1) of the 1976 airborne gamma survey overlaid on an aerial photomosaic of Kakadu National Park from 2007. The locations of RangerOrebodies 1 and 3, and of Anomaly 2, are clearly visible. Due to thelow spatial resolution of the 1976 airborne gamma survey only twoseparate anomalies can be distinguished across Anomaly 2. In contrast,the 1997 airborne gamma survey data, and the 1872 gamma doserates measured on ground at Anomaly 2 (shown in Fig. A2 in theSupplementary material), confirmed that there are 3 separate anoma-lies associated with Anomaly 2, in agreement with Hegge et al. (1980).The strongest of those was named Anomaly 2A. The anomalies to thenortheast and southwest of Anomaly 2A were named Anomaly 2B and2C, respectively.

Analysis of the average field gamma dose rate data within buffersaround the 1997 survey flight lines indicated that the best coefficientof determination between the average gamma dose rates and eUmeasured on board of the plane was gained for a radius of 90 m(r2 = 0.73; p b 0.005). According to Pitkin and Duval (1980) andDuval (1997) at 50 m flying height, about 85% of the signal detectedon board the plane originates from within a strip with a width of180 m. The mean gamma dose rates within the 54 buffers were thenplotted against the eU data of the respective flight lines and the slopeand coefficient of determination calculated. To ensure that results ofthe correlations were not affected by variations in ground samplespacing 29 buffers with a consistent distribution of ground points with-in them were used for further analysis (Fig. A3 in the Supplementarymaterial). The correlation (r2 = 0.76; p b 0.005) between averaged ter-restrial gamma dose rates in those buffers and 1997 airborne gammasurvey eU data above Anomaly 2 (Fig. 3a) was only slightly betterthan using all 54 buffers.

The uranium raster data sets were used to model the 1997 data tothe 1976 data, as described in Section 3.3.2. Using this method a totalof 6916 recordswere obtained. Results of the correlation and the coeffi-cient of determination are shown in Fig. 3b.

4.2. Soil activity concentrations

Soil 226Ra activity concentrations varied by almost four orders ofmagnitude and ranged from 14 Bq kg−1 to 111,000 Bq kg−1. Soil226Ra activity concentrations are highest on top of Anomaly 2A, at loca-tions 13, 14 and 15,where 210Pb activity concentrationwas significantlylower. Radon-228 and 228Th are in radioactive equilibrium and vary by

eU19

97 [

Bq

kg

-1]

0 200 400 600 800 1000

µG

y h

r-1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

mean uGy hr-1 vs U Regr95% conf

µGy hr-1 = (0.00126±0.00028)*eU-(0.023±0.086) r ² = 0.76; p < 0.005

a

eU1976 [s-1]

1000100

1010

100

1000

mean 1997 vs 1976 eU Regr95% conf

eU1997 = (0.58±0.01)*eU1976+(14±1) r ² = 0.65; p < 0.005

b

eU1997 [Bq kg-1]

Fig. 3. (a) Average measured terrestrial gamma dose rates plotted against the eUdata (Bq∙kg−1) of the 1997 airborne gamma survey for buffers with evenly distributedground survey points. R = 90 m, n = 29; and (b) average eU data (Bq∙kg−1) from the1997 AGS within a 1976 AGS vector grid cell plotted versus the 1976 data (s−1) (notethe logarithmic scales).

Table 3222Rn flux densities, soil radionuclide activity concentrations and terrestrial gamma dose rates

loc Easting Northing 222Rn[Bq m−2 s−1] ± stdev (geomean)

226Ra[Bq kg−1]

1 272,968 8,593,810 0.25 ± 0.01 (0.25) 224 ± 22 273,084 8,593,837 0.69 ± 0.41 (0.58) 1330 ± 103 273,106 8,593,792 1.05 ± 0.15 (1.04) 1230 ± 104 273,141 8,593,890 0.76 ± 0.01 (0.76) 766 ± 65 273,273 8,593,846 0.87 ± 0.10 (0.86) 429 ± 36 273,263 8,593,696 0.29 ± 0.05 (0.29) 91 ± 17 273,200 8,593,597 0.92 ± 0.05 (0.92) 130 ± 18 272,988 8,593,601 0.222 ± 0.013 (0.22) 212 ± 29 272,793 8,593,599 0.305 ± 0.027 (0.31) 168 ± 110 272,606 8,593,588 0.140 ± 0.014 (0.14) 171 ± 211 272,511 8,593,428 0.61 ± 0.03 (0.61) 291 ± 212 272,712 8,593,449 6.63 ± 0.46 (6.6) 1330 ± 1013 272,894 8,593,454 10.0 ± 3.9 (9.5) 10,580 ± 7014 272,894 8,593,453 5.2 ± 5.4 (3.4) 110,700 ± 80015 272,856 8,593,432 5.0 ± 2.2 (4.6) 41,100 ± 30016 272,867 8,593,385 5.7 ± 0.8 (5.7) 7800 ± 6017 273,092 8,593,445 0.104 ± 15 (0.103) 38 ± 118 272,798 8,593,299 0.12 ± 0.03 (0.12) 79 ± 119 272,993 8,593,295 0.23 ± 0.02 (0.23) 39 ± 120 272,495 8,593,306 0.93 ± 0.15 (0.92) 213 ± 221 272,416 8,593,047 0.006 ± 0.002 (0.006) 14 ± 122 272,524 8,593,172 0.56 ± 0.03 (0.56) 502 ± 423 272,598 8,593,113 3.5 ± 1.3 (3.4) 654 ± 524 272,643 8,593,165 8.7 ± 6.7 (6.6) 9950 ± 7025 272,731 8,593,124 0.23 ± 0.07 (0.23) 30 ± 1

N/A: not analysed.

770 A. Bollhöfer et al. / Science of the Total Environment 468–469 (2014) 764–773

less than one order of magnitude across the sites. The range of 40Kactivity concentrations is from 10 Bq kg−1 to 600 Bq kg−1. Dose ratesmeasured at these sites ranged from 0.1 μGy h−1 to 21 μGy h−1.Results of the HPGe measurements are shown in Table 3. A value of0.066 μGy h−1 was subtracted as the contribution from cosmic radia-tion to themeasured gammadose rates (Marten, 1992) to derive terres-trial gamma dose rates.

Terrestrial gamma dose rates were also calculated from the activityconcentrations of 226Ra, 228Th and 40K shown in Table 3 using equationsgiven by Saito and Jacob (1995) and assuming secular equilibrium in the238U and 232Th decay series. Most of the gamma dose rate (and thesignal measured by an in-situ portable NaI gamma detector) in air at aheight of 1 m above the ground originates from radionuclides locatedin the top 0.5 m of the soil (ICRU, 1994; Saito and Jacob, 1995). Hence,gamma dose rates calculated from radionuclide activity concentrationsin surface scrapes may over (or under) estimate the terrestrial gammadose rate if soil activity concentrations are higher (or lower) at the soilsurface. At locations 14 and 15, which were located on top of Anomaly2A, and had the highest 226Ra soil activity concentrations measured inthis study, it appears that activity concentrations of the surface soilscrapes were significantly higher than the average activity concentra-tions throughout the top about 0.5 m of soil. Site 24, which was locatedon top of a rock pile, exhibited relatively smallermeasured gammadoserates due to the geometry of the surface where the measurement wasconducted (the measurement was done on the pile crest).

Fig. 4 shows the results of both laboratoryHPGe and in-situNaI 226Raactivity concentrations plotted against the measured terrestrial gammadose rates. Only the results for gamma dose rates from 0 to 2 μGy h−1

are shown, as the calibration of the environmental dose rate meterswas strictly valid for dose rates in that range only. A conversion factorof 2330 ± 50 Bq kg−1 per μGy h−1 was determined. This conversionfactor is similar to the theoretical value of 2165 Bq kg−1 per μGy h−1

for 238U in equilibrium with all its decay products (Saito and Jacob,1995). The x-intercept is 0.07 ± 0.01 μGy h−1, which is only slightlyhigher than the average terrestrial gamma dose rate calculated fromaverage measured soil 232Th and 40K activity concentrations of 0.034 ±0.030 μGy h−1.

(measured and calculated from soil activity concentrations) at Anomaly 2, locations 1–25.

210Pb[Bq kg−1]

228Ra[Bq kg−1]

228Th[Bq kg−1]

40 K[Bq kg−1]

Eγ,meas Eγ,calc[μGy∙hr−1]

193 ± 5 27 ± 1 34 ± 1 36 ± 3 0.14 ± 0.01 0.12803 ± 14 24 ± 1 24 ± 1 10 ± 4 0.40 ± 0.01 0.63930 ± 20 52 ± 2 58 ± 1 b10 0.65 ± 0.02 0.60560 ± 10 28 ± 1 33 ± 1 38 ± 5 0.40 ± 0.01 0.37414 ± 8 24 ± 1 28 ± 1 26 ± 4 0.23 ± 0.01 0.21141 ± 4 24 ± 1 25 ± 1 15 ± 2 0.070 ± 0.004 0.06197 ± 4 21 ± 1 25 ± 1 16 ± 2 0.09 ± 0.01 0.07168 ± 5 51 ± 1 56 ± 1 17 ± 2 0.15 ± 0.01 0.13159 ± 5 32 ± 1 36 ± 1 35 ± 3 0.10 ± 0.01 0.10227 ± 6 56 ± 1 62 ± 2 69 ± 4 0.072 ± 0.006 0.12379 ± 7 25 ± 1 26 ± 1 35 ± 3 0.14 ± 0.01 0.15

1300 ± 20 29 ± 2 31 ± 1 75 ± 6 0.74 ± 0.02 0.646510 ± 90 38 ± 4 45 ± 2 182 ± 16 5.2 ± 0.1 4.92

64,800 ± 830 N/A N/A N/A 21.0 ± 0.1 51.129,010 ± 370 78 ± 7 113 ± 5 290 ± 25 11.3 ± 0.1 19.16910 ± 90 42 ± 3 43 ± 2 80 ± 11 2.00 ± 0.03 3.6

77 ± 3 18 ± 1 20 ± 1 21 ± 2 0.045 ± 0.003 0.03107 ± 4 26 ± 1 30 ± 1 37 ± 3 0.075 ± 0.005 0.06109 ± 3 18 ± 1 20 ± 1 27 ± 2 0.060 ± 0.004 0.03267 ± 6 21 ± 1 25 ± 1 24 ± 3 0.16 ± 0.01 0.1145 ± 3 15 ± 1 16 ± 1 44 ± 3 0.033 ± 0.002 0.02

420 ± 8 20 ± 1 23 ± 1 22 ± 4 0.31 ± 0.01 0.25729 ± 12 16 ± 1 16 ± 1 20 ± 4 0.63 ± 0.02 0.31

7660 ± 100 77 ± 4 98 ± 3 616 ± 22 1.94 ± 0.03 4.780 ± 3 15 ± 1 16 ± 1 45 ± 3 0.055 ± 0.004 0.03

Eterr [µGy hr-1]

0.0 0.5 1.0 1.5 2.0

226 R

a [B

q k

g-1

]

0

1000

2000

3000

4000

5000

226Ra = 2330 * Eterr - 150

R2=0.98p<<0.001

Fig. 4. In-situ (NaI) (o) and laboratory (HPGe) (•) measured soil 226Ra activity concentra-tions plotted against measured terrestrial gamma dose rates (b2 μGy∙hr−1).

Eterr [uGy hr-1]0.01 0.1 1 10

rad

on

[B

q m

-2 s-

1 ] 14

1513

2416

Rn = (3.4+-0.7)*Eterr

R2 = 0.83p < 0.0050.01

0.1

1

10

Fig. 5. 222Rn flux densities plotted against themeasured terrestrial gamma dose rates. Theline of best fit excludes measurements from directly on top of the Anomaly (circled) andsample 24. Note the logarithmic scales for both axes.

771A. Bollhöfer et al. / Science of the Total Environment 468–469 (2014) 764–773

4.3. 222Rn flux densities

The minimum and maximum flux densities measured in individualcharcoal canisters were 0.004 and 15.7 Bq m−2 s−1, respectively. Thearithmetic and geometric means of all measurements were 2.2 and0.7 Bq m−2 s−1, respectively. Arithmetic and geometric mean 222Rnexhalation flux densities at the 25 locations are shown in Table 3. The222Rn exhalation flux densities on top of Anomaly 2A were highest atlocation 13 (10 Bq m−2 s−1), 14 and 15 (about 5 Bq m−2 s−1),which exhibited a similar soil morphology with a relatively thin layerof gravel and sand overlying rocky terrain.

Exhalation of 222Rn from the soil surface depends on a number ofvariables including soil 226Ra activity concentrations, soil porosity andpermeability, soil thickness and soil moisture. Deeper layers of the soilmake a decreasing contribution to surface exhalation but 222Rn canreach the surface from soil depths of several metres, a typical value is1.5 m (Porstendörfer, 1994). Exhalation of 222Rn from the soil surfacealso depends on the emanation power of 222Rn from the soil grain,which has been shown to typically range from 0.1 to 0.4 in weatheredsoils (Porstendörfer, 1994; Sakoda et al., 2010) but can be as low as0.006 for rock minerals (Sakoda et al., 2010). If shallow layers ofweathered soils overly areas of mineralised rock strata with a low222Rn emanation power, then 222Rn exhalation flux densities measuredat the soil surface are lower than expected compared to weathered soilsthroughout the entire soil profile.

Fig. 5 shows the 222Rn exhalation flux densities at locations 1–25plotted against the terrestrial gamma dose rates. The line shows a linearmodel fit to the data points but excludes Sites 13, 14 and 15 on top ofAnomaly 2A and Site 24 for the reasonsmentioned above. A conversionfactor of 3.4 ± 0.7 Bq m−2 s−1 per μGy h−1 was determined. Thevariability in terrestrial gamma dose rates accounts for approximately80% of the variability observed in 222Rn exhalation flux densities usingthis linear model. The coefficient of determination for this fit is signifi-cantly better than that for 222Rn exhalationfluxdensities plotted against226Ra activity concentration in the surface scrapes (R2 = 0.56). Where-as samples taken for 226Ra analysis via HPGe gamma spectrometryweretaken from the top 2 cm only, most of the in-situ gamma signal at aheight of 1 m above the ground originates from radionuclides locatedin the top 0.5 m of the soil (ICRU, 1994; Saito and Jacob, 1995). This issimilar to the typical radon diffusion length of 1.5 m (Porstendörfer,1994). In a uranium mineralised area the gamma signal is dominatedby 214Bi (Saito and Jacob, 1995), a radioactive decay product of 226Ra.Hence, the gamma signal at a height of 1 m is a better proxy of the226Ra activity throughout the top 0.5 m of the soil, which improvesthe coefficient of determination compared to plotting the 222Rn exhala-tion flux densities against 226Ra in surface scrapes.

The terrestrial gamma dose rate to 222Rn flux density (Fig. 5) and tosoil 226Ra activity concentration (Fig. 4) conversions provide a reliableapproach to convert 226Ra activity concentrations to average 222Rnexhalation flux densities in the region. A conversion factor of (1.4 ±0.3) · 10−3 Bq m−2 s−1 per Bq kg−1 was calculated. This value iswithin the range of values determined previously in the region whichrange from (0.4–4.9) · 10−3 Bq m−2 s−1 per Bq kg−1 for environmen-tal sites (Lawrence et al., 2009).

4.4. Model output and validation

The linear models shown in Figs. 3–5 are summarised by Eqs. (1),(2), and (3). Uncertainties (95% confidence) were propagated fromthe empirical fits, and are dominated by uncertainties in (a) the conver-sion of 1997 eU data to terrestrial gamma dose rates measured onground (Fig. 3a) and (b) the conversion of gamma dose rates to radonexhalation fluxes (Fig. 5).

EterrnGyh

� �¼ 0:73� 0:16ð Þ nGy � s

h

� �� eU1976 s−1

h i− 5� 86ð Þ nGy

h

� �ð1Þ

222RnmBqm2 � s

� �¼ 2:48� 0:76ð Þ mBq

m2

� �� eU1976 s−1

h i− 20� 290ð Þ

� mBqm2 � s

� �ð2Þ

226RaBqkg

� �¼ 1:70� 0:38ð Þ Bq � s

kg

� �� eU1976 s−1

h i− 160� 200ð Þ Bq

kg

� �ð3Þ

Despite the relatively large uncertainties, these equations providea framework for calculating pre-mining gamma dose rates, 222Rnexhalation flux densities, and 226Ra soil activity concentrations withan associated uncertainty from the uranium counts per second of the1976 raster data set.

Shapefiles were created in ArcGIS and overlaid on the 1976 uraniumraster data (s−1). For each of those shapefiles the sum of the counts persecond in the uranium channel of the 1976 survey was determinedusing the Zonal Statistics tool in ArcGIS. This number was used inEqs. (1)–(3), and the result divided by the number of pixels within theshapefile, to determine mean pre-mining gamma dose rates, 222Rnexhalation flux densities, and soil 226Ra activity concentrations in thegreater Ranger region for various areas. Fig. A4 in the Supplementaryinformation shows the shapefiles, including the area of the townshipof Jabiru, overlaid on an aerial photo from 2007 of the region and theuranium count rates of the 1976 raster data set over the same region.Table 4 shows the calculated means of the pre-mining gamma dose

Table 4Pre-mining mean external gamma dose rates E (including cosmic component), 226Ra soilactivity concentrations and 222Rnfluxdensities estimated for features in thegreater Rangerregion. Uncertainties were calculated from Eqs. (1), (2) and (3).

Infrastructure Area[ha]

Eave[μGy h−1]

226Raave[Bq kg−1]

Rnave[Bq m−2 s−1]

1—Pit 1 40 0.87 ± 0.18 1880 ± 430 2.7 ± 0.82—Pit 3 77 0.44 ± 0.09 880 ± 200 1.3 ± 0.43—Anomaly 2 38 0.33 ± 0.06 630 ± 140 0.9 ± 0.34—Djalkmarra LAA 9 0.20 ± 0.03 310 ± 70 0.46 ± 0.145—Corridor Ck LAAs 139 0.14 ± 0.02 170 ± 40 0.25 ± 0.086—Tailings Dam 110 0.11 ± 0.01 110 ± 30 0.16 ± 0.057—Magela LAAs 56 0.12 ± 0.01 110 ± 30 0.17 ± 0.058—RP1 17 0.11 ± 0.01 90 ± 20 0.14 ± 0.049—RP1 LAAs 38 0.11 ± 0.01 90 ± 20 0.13 ± 0.0410—Djalkmarra ex LAA 9 0.10 ± 0.01 70 ± 20 0.12 ± 0.0411—Jabiru East LAA 50 0.10 ± 0.01 90 ± 20 0.13 ± 0.0412—Jabiru 306 0.11 ± 0.01 90 ± 20 0.14 ± 0.04Ranger Project Area 7881 0.11 ± 0.01 110 ± 20 0.15 ± 0.05

772 A. Bollhöfer et al. / Science of the Total Environment 468–469 (2014) 764–773

rates, 222Rn exhalation flux densities and soil 226Ra activity concentra-tions. Orebody 1, Orebody 3 and Anomaly 2 exhibit the highest externalgamma dose rates, 222Rn exhalation flux densities and soil 226Ra activityconcentrations, averages have been determined in ArcGIS for the foot-print of the two pits in 2007 rather than the orebodies.

Kvasnicka and Auty (1994) determined terrestrial gamma doserates for the two orebodies (Table 1). Whereas the dose rate on topof Orebody 3 was measured directly before mining started, doserates for Orebody 1 were estimated from radionuclide activity concen-trations in drill core taken above Orebody 1. Differences in averageterrestrial gamma dose rates between our study (Orebody 1: 0.81 ±0.18 μGy h−1; Orebody 3: 0.38 ± 0.09 μGy h−1), and Kvasnicka andAuty (1994), arise from the different areal extent of the anomaliesinvestigated. The surface area of Orebody 1 investigated by Kvasnickaand Auty (1994) was 44 ha, which is similar to the 40 ha used for aver-aging in our study, and average external gamma dose rates and 226Rasoil activity concentrations agree well. In contrast, measurements ofthe gamma dose rates on top of Orebody 3 were only taken over anarea of 66 ha compared to an average calculated above 77 ha in ourstudy. Lower values at the edges of the orebody have been included inour calculation of the average external gamma dose rate, leading tothe difference observed between the two studies. The average externalgamma dose rates determined above Anomaly 2 using our geographicinformation system is 0.33 μGy h−1 (Table 4) the average of all fieldmeasurements (shown in Fig. A2 in the Supplementary material) is0.40 μGy h−1. This translates to an average soil 226Ra activity concentra-tion of about 600 Bq kg−1.

There are many data published on environmental backgroundgamma dose rates in the Alligator Rivers Region (Bollhöfer et al.,2008; Martin et al., 2006) and close to Ranger uranium mine (Toddand Akber, 1998; Lawrence, 2005). Values range from 0.09 μGy h−1

to 0.13 μGy h−1 at Jabiru East next to the mine site to around0.14 μGy h−1 at Jabiru. The values determined in the ArcGIS modelare in good agreement with the measured values for environmentalbackground sites.

The average pre-mining 222Rn exhalation flux density calculatedfrom our radiological geographic information system for Orebodies 1and 3, and for the Anomaly 2 area are 2.7, 1.3 and 0.9 Bq m−2 s−1,respectively. The values above Orebodies 1 and 3 are lower than thosereported by Kvasnicka and Auty (1994), partly explained by the largerareal extent of Orebody 3 used in our study. In addition, the conversionfactor of 1.85 · 10−3 Bq m−2∙s−1 per Bq kg−1 to determine the 222Rnexhalation flux density for known soil 226Ra activity concentrationsacross the two orebodies used by Kvasnicka and Auty (1994) is larger.Using (1.4 ± 0.3) · 10−3 Bq m−2 s−1 per Bq kg−1 determined in ourstudy, 222Rn flux densities for Orebodies 1 and 3 from Kvasnicka and

Auty (1994) would amount to 3.2 Bq m−2 s−1 and 1.9 Bq m−2 s−1,respectively, similar to the averages determined in our study. Thegeometric mean (or typical) 222Rn exhalation flux density from mea-surement in the field above Anomaly 2 is 0.7 Bq m−2 s−1, similar tothe average determined with the ArcGIS model of 0.9 Bq m−2 s−1.

Radon-222 exhalation flux densities for typical environmental back-ground sites have also been calculated using Eq. (2) and 35 counts persecond in the 1976 uranium channel (typical for environmental back-ground) up to 55 counts per second (determined for the Jabiru area).Environmental 222Rn exhalation flux densities for environmental back-ground areas are between 0.07 ± 0.02 Bq m−2 s−1 and 0.12 ±0.04 Bq m−2 s−1, in good agreement with dry season values between0.04 and 0.11 Bq m−2 s−1 reported previously for various sites in thearea (Lawrence, 2005; Lawrence et al., 2009; Todd and Akber, 1998).

With the average 222Rn exhalation flux densities and soil 226Raactivity concentrations shown in Table 4, and a simple diffusion equa-tion given in Porstendörfer (1994) (assuming an infinite layer of soilwith a homogeneous 226Ra activity concentration and radon emanationpower of 0.2) a mean radon diffusion coefficient D for the area has beenestimated. Values for D are at the upper end (10−5 m2 s−1) of typicalsoil values. This could be due to heterogeneous distribution of 226Ra insoil profiles and the highly weathered soils in the region, leading to ahigher radon emanation power. Detailed studies are currently under-way to determine radon diffusion length in soils of the region and ofwaste rockmaterial, whichwill be used as a covermaterial for the reha-bilitation of Ranger, to determine the optimum thickness required toderive at 222Rn fluxes similar to pre-mining fluxes in the area.

5. Conclusion

Pre-mining radiological conditions have been determined fromhistoric airborne gamma survey data by groundtruthing an undisturbedradiological anomaly and extrapolating to the whole extent of theairborne gamma survey including the unmined Ranger Project Area.The minimum footprint area that can be assessed was set by the opti-mum buffer radius determined when up-scaling the external gammadose rates measured on the ground to the airborne gamma surveydata. For the current case this is approximately 4 ha. The correlationmodels developed allow estimates to be made of the pre-mining base-line gamma dose rates, 226Ra soil activity concentrations, and 222Rnfluxes for any selected area larger than 4 ha covered by the 1976 surveyover the greater Ranger area and large parts of the Alligator RiversRegion. Pre-mining levels determined for themine site using the ArcGISmodel provide a radiological benchmark for rehabilitation of the Rangermine. Our approach for determining pre-mining radiological conditionscan be used at any mine or industrial site where historical airbornegamma survey data are available and an undisturbed analogue existsto groundtruth the historic data. Areal extent and thus resources forthe ground survey will ultimately depend on the spatial resolution ofthe historic airborne gamma survey.

Comparison with published data on external gamma dose rates,226Ra soil activity concentrations and 222Rn exhalation flux densities inthe Ranger region shows that our model estimates are in good agree-ment with radiation levels estimated previously via direct measure-ment on top of Orebody 3, and from previous environmental radiationsurveys. Our ArcGIS model will allow an estimate of pre-mining uptakeof uranium series radionuclides into biota over the footprint of theRanger mine, assuming secular equilibrium of the radionuclides insoils and using uptake factors determined for traditional foods in theregion (e.g. Martin et al., 1998; Ryan et al., 2005). This will ultimatelyfacilitate the estimation of pre-mining ingestion doses to humansfrom the consumption of foods harvested on site, which has beenidentified as one of the potential exposure pathways of the public, inaddition to an estimate of the internal and external radiation doses towildlife.

773A. Bollhöfer et al. / Science of the Total Environment 468–469 (2014) 764–773

Acknowledgements

The Northern Territory Geological Survey, Department of Mines andEnergy, Rio Tinto Exploration and Energy Resources of Australia,Darwin, are gratefully acknowledged for their support and making theairborne gamma survey data available. Many thanks go to two anony-mous reviewers and the Associate Editor for their valuable commentsthat greatly improved the manuscript. The Mirrar and the GundjeihmiAboriginal Corporation are thanked for their support for this projectand granting access to the Anomaly 2 area.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.scitotenv.2013.09.001.

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