Radioactive and radiogenic isotopes in sediments from Cooper Creek, Western Arnhem Land

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Journal of Environmental Radioactivity 99 (2008) 468e482 www.elsevier.com/locate/jenvrad

Radioactive and radiogenic isotopes in sediments from Cooper Creek, Western Arnhem Land A. Frostick a,b,*, A. Bollho¨fer b, D. Parry a, N. Munksgaard a, K. Evans b a

Charles Darwin University, Darwin, NT 0909, Australia b ERISS, GPO Box 461, Darwin, NT 0801, Australia

Received 15 October 2006; received in revised form 17 July 2007; accepted 9 August 2007 Available online 17 October 2007

Abstract Protection of the environment post-mining is a key objective of rehabilitation, especially where runoff and erosion from rehabilitated mine sites could potentially lead to contamination of the surrounding land and watercourses. As part of an overall assessment of the success of rehabilitation at the former Nabarlek uranium (U) mine, an appraisal of stable lead (Pb) isotopes, radionuclides and trace metals within sediments and soils was conducted to determine the off site impacts from a spatial and temporal perspective. The study found localised areas on and adjacent to the site where soils had elevated levels of trace metals and radionuclides. Lead isotope ratios are highly radiogenic in some samples, indicating the presence of U-rich material. There is some indication that erosion products with more radiogenic Pb isotope ratios have deposited in sediments downstream of the former ore body. However, there is no indication that the radiogenic erosion products found on the mine site at present have significantly contaminated sediments further downstream of Cooper Creek. Crown Copyright Ó 2007 Published by Elsevier Ltd. All rights reserved. Keywords: Stable lead isotopes; Mine site rehabilitation; Radionuclides; Trace metals

1. Introduction Lead (Pb) isotope ratios have been used as a source tracer for pollution for decades (e.g. Chow et al., 1975). They have been used as a source tracer for atmospheric studies in urban and developed areas (e.g. Bollho¨fer and Rosman, 2000, 2001; Chiaradia et al., 1997; Mukai et al., 1993) or in remote areas to study the sources and impacts of Pb pollution on the global atmosphere (e.g. Bollho¨fer et al., 2006; Rosman et al., 1994). Lake sediments have been investigated to decipher the history of Pb pollution in urban (e.g. Kober et al., 1999) or remote areas. This study provides a unique opportunity to develop the method as a monitoring tool for mining impacts in relatively remote areas. Remote areas are potentially more susceptible to impact signatures from industrial, mining related sources, as a result of the low background atmospheric Pb levels as compared to urban and developed areas. Protection of the environment post-mining is a key objective of rehabilitation, especially where runoff and erosion from rehabilitated mine sites can lead to contamination of the surrounding land and watercourses. This study is part of * Corresponding author. Charles Darwin University, Darwin, NT 0909, Australia. Tel.: þ618 8946 6923; fax: þ618 8946 7199. E-mail address: [email protected] (A. Frostick). 0265-931X/$ - see front matter Crown Copyright Ó 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2007.08.015

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an overall assessment of the success of the rehabilitation at the former Nabarlek uranium (U) mine (Bollho¨fer et al., 2006; Hancock et al., 2006; Martin et al., 2006). A combination of stable Pb isotope ratio, trace metal and radionuclide fingerprinting techniques has been used to determine the sediment deposition history and extent of erosion and pollution in the potentially mining influenced catchments of Cooper Creek, Western Arnhem Land. Lead has four stable isotopes three of which, 206Pb, 207Pb and 208Pb, are endmembers of the 238U (t½ ¼ 4.5  109 years), 235U (t½ ¼ 0.7  109 years) and 232Th (t½ ¼ 14  109 years) decay chains, respectively. Primordial Pb was formed at an isotope abundance of 2% (204Pb), 18.6% (206Pb), 20.6% (207Pb) and 58.9% (208Pb) (Tatsumoto et al., 1973). Radiogenic Pb continues to be formed by the radioactive decay of U and Th. Due to the U and Th content in the earth’s crust, the crustal Pb isotope ratios change over time (Cummings and Richards, 1975). The present day average crustal (PDAC) 206Pb/207Pb and 208Pb/207Pb ratios are approximately 1.20 and 2.47, respectively. In U- and Th-rich minerals, more radiogenic Pb has formed over time compared with the average. Hence, Pb isotope ratios associated with U and Th mineralisation are different from PDAC ratios. In minerals with high Th and U concentrations, respectively, Pb isotope ratios are much more radiogenic than PDAC. The 208Pb/207Pb and 206Pb/207Pb ratios as high as 142.8 and 16.7, respectively, have been reported by Bosch et al. (2002) in monazites with high Th/U ratios. Consequently, Pb isotope ratios in sediments with a relatively high proportion of heavy minerals and sands are usually more radiogenic. Munksgaard and Parry (2001) have reported Pb isotope ratios in sediments from the Gulf of Carpenteria for instance, which indicate the presence of detrital monazite from the Norman River catchment. In contrast, U ore bodies with low Th/U exhibit elevated 206Pb/207Pb ratios but are low in 208Pb/207Pb, as 208Pb is formed by the radioactive decay of Th. Gulson et al. (1992) for example measured 206Pb/207Pb (208Pb/207Pb) ratios in particulates from U tailings as high (low) as 9.690 (0.0494). 206Pb/207Pb and 208Pb/207Pb ratios of mixtures of natural material with PDAC Pb isotope ratios with U-rich sources usually fall along a trendline connecting the radiogenic endmember with the Pb isotope ratios along the growth curve (e.g. Bollho¨fer et al., 2006). Hancock et al. (2006) identified an area within the rehabilitated mine site where erosion of soil is enhanced and radionuclide activities are high (unit-7). Th/U is low, and the disequilibrium between 226Ra and 238U activities in the soil indicates that this erosional unit was in contact with radioactive tailing materials. Modelling demonstrated that the relatively small area of about 0.4 ha contributes approximately 75% of the total 238U flux to the Cooper Creek system from Nabarlek. Surface sediment from this area exhibits highly radiogenic 206Pb/207Pb (208Pb/207Pb) Pb isotope ratios of 13.14 (0.4480) (this study). 2. Study site The Nabarlek mine site is located 270 km east of Darwin near the western edge of East Arnhem Land to the east of Gunbalanya (Oenpelli), near Cooper Creek, a tributary of the East Alligator River (Fig. 1). The U ore body was discovered in 1970 by Queensland Mines Ltd (QML). QML operated the Nabarlek U mine from 1979 until 1988, and the mine was decommissioned in 1994/95. The ore body was small and concentrated, enabling the company to mine the ore over a 143-day operation during the 1979 dry season. The 600,000 tonnes of average 2% U3O8 ore were stockpiled on a specially prepared site while the mill was being built. It was then processed on site between 1980 and 1988 (UIC, 2001). The unique size, shape and position of the ore body enabled the return of all tailings from the processing plant directly to the mined-out pit. Although the Nabarlek U mine has been decommissioned, rehabilitation of the site has not reached a stage where the Northern Territory Government will agree to issue a Certificate of Closure to current site owners, Hanson Pty Ltd. The mine site is located in the tropical monsoonal climate of northern Australia, which has typical wet (NovembereApril) and dry (MayeOctober) seasons. Almost 96% of rainfall occurs during the wet season and almost 60% of that during JanuaryeMarch. As well as cyclonic rainfall events, which can last for days to weeks, the region is subject to frequent, short, and high intensity convective storms. Consequently fluvial erosion is the primary erosion process (Martin, 2003; Waggitt and Riley, 1992). The Nabarlek mine site covers approximately 173 ha and lies within three sub-catchments of the Cooper Creek system; namely Cooper Creek West, Buffalo Creek and Kadjirrikamarnda Creek (Fig. 1). These three sub-catchments drain into Cooper Creek, which in turn discharges into the mouth of the East Alligator River. Cooper Creek West and Buffalo Creek drain the majority of the mine site and might have been previously impacted by mining activities through runoff or erosion from the mine site. Other tributaries include Kadjirrikamarnda Creek, which has little

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Fig. 1. Location of the Nabarlek mineral lease and the sampling sites (modified from Hancock et al., 2006).

contact with the mine lease (Hancock et al., 2006). Cooper Creek upstream of the mine lease (core 6) is regarded as the control site (Fig. 1). Large scale bioturbation is common throughout much of the Alligator Rivers Region (ARR), caused by disturbance due to feral pigs and horses, but was previously controlled within the Nabarlek mine site by a high wire-mesh fence and cattle grids. Considerable damage was done to the fence by trees felled by Cyclone Monica in April 2006. In November 2005 and June 2006 much of the mineral lease was affected by a large bush fire. Hancock et al. (2006) have shown that impacts like these can have a dramatic impact on the runoff of sediments and consequently, radionuclides, from a rehabilitated mine site. Studies investigating these effects are currently underway. 3. Methods 3.1. Sample collection 3.1.1. Soil scrape samples Soil scrapes were collected from various cross sections along tributaries and drainage channels and from the main channel of Cooper Creek during the dry season in 2003 and 2005 (Table 1). Samples were collected using plastic polyethylene (PET) gloves and acid-cleaned plastic bags and approximately 3 cm of the top sediment was removed for analysis. Scrape samples were dried at 60  C and then ground in an automated agate mortar and pestle (Retsch RMO). A fraction of the sample was taken for heavy metal and Pb isotope analysis via inductively coupled plasma mass spectrometry (ICPMS). For direct gamma spectroscopy, approximately 150 g of the ground sediment was pressed into a standard geometry. The progeny of radon was allowed to reach secular equilibrium with its progenitor, 226Ra, in the sample. The sample was then counted after an ingrowth period of w20 days for a period of 1e2 days.

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Table 1 Description of the surface scrape and sediment core samples taken in the Cooper Creek catchment for gamma analysis, and ICPMS analysis for metals and Pb isotopes Site

Description

Sample Type

NS-01 BC NS-02 BC NS-03 BC NS-04 CCM NS-06 CCM NS-08 CCW NS-09 CCW NS-10 CCW NK04001 NK04002 NK04003 NK04004 NK04005 NK04006 NC2 NC4 NC6 NC9

Tributary Buffalo Creek adjacent to site fence Tributary Buffalo Creek w20 m d/s site fence Tributary Buffalo Creek ponded area Cooper Creek West u/s crossing Cooper Creek tributary u/s mine WRD settlement pond WRD settlement pond outflow channel Tributary Cooper Creek West d/s NS-09 CCW Buffalo Creek Buffalo Creek Buffalo Creek Buffalo Creek Buffalo Creek, overbank flow Buffalo Creek, overbank flow Kadjirrikamarnda Creek u/s Cooper Creek Cooper Creek d/s Kadjirrikamarnda Creek Cooper Creek u/s mine site Cooper Creek at Gauging Station d/s mine site

Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Core Core Core Core

scrape scrape scrape scrape scrape scrape scrape scrape scrape scrape scrape scrape scrape scrape

3.1.2. Core samples Core samples were collected from four locations within the Cooper Creek catchment. Core locations are shown in Fig. 1. Cores were taken from bank overflow areas of the creeks that seasonally overflow during the wet season. Samples were collected/extracted using a Dormer UWSS33 stainless steel sediment corer. Approximately 700 mm of sediment was recovered at each location. The core’s length was measured and then the core was cut into approximately 10-mm sections using an acid-cleaned stainless steel knife. At the lower parts of the core, the width of the sections generally increased. The core samples were dried at 60  C and ground in an automated agate mortar and pestle (Retsch RMO). A fraction of the sample was taken for heavy metal and Pb isotope analysis via ICPMS. For gamma spectroscopy, approximately 15 g of the sediment was pressed into a standard geometry and counted for a period of 1e2 days. 3.2. Gamma spectrometry Sediments were analysed using high-resolution high purity germanium (HPGe) gamma detectors at the Environmental Research Institute of the Supervising Scientist (ERISS). Procedures for sample collection, preparation and measurements of radionuclide activities via gamma spectrometry at ERISS are described in Murray et al. (1987) and Marten (1992). The counting system has been calibrated for the respective geometries, using certified U and Th standards. Detection limits for the geometry used were approximately 10 Bq kg1 for 210Pb and approximately 3.5 Bq kg1 for 226Ra, 228Ra and 228Th. Results of the gamma analyses contain the radionuclide activities of the long-lived progeny of the U and Th series, and miscellaneous other photopeaks, such as 40K and 137Cs. The stability of the detectors is checked fortnightly with a multielement standard containing elements of the U and Th decay chains. IAEA reference material 315 is measured monthly to check for the accuracy of the measurement system. During the course of this study, two detectors were used. Activity concentrations of 226Ra, 210Pb, 228Ra, 228 Th and 40K were determined in the reference material and the averages (uncertainty, k ¼ 2) amounted to 14.8  0.7, 26.0  2.2, 29.9  1.5, 30.5  1.2, and 285  8 Bq g1, respectively. These results are within the 95% confidence intervals of the certified values for U-series elements and 40K, but slightly higher for 228Ra and 228Th. Uncertainties given in Table 2 are based on counting statistics only, which is the dominating contribution to the uncertainty for low activity concentrations, and were typically less than 5%. Repeated measurements of high activity phosphogypsum material performed during the course of this study indicated a combined uncertainty for 226Ra and 210Pb of approximately 3%, and approximately 12% for 238U for the geometries used in this study. 3.3. Inductively coupled plasma mass spectrometry (ICPMS) Lead isotope ratios and trace metal concentrations of the sediments were analysed by ICPMS at Charles Darwin University (CDU). Samples were digested in a nitric/perchloric acid mixture and heated in a block digester at 180  C. An aliquot was

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Table 2 Radionuclide activity concentrations (Bq kg1 dry weight) for surface scrape samples in Cooper Creek catchment and sediment cores 6 and 9 Site Ref

Depth (cm)

238 U (Bq kg1)

226 Ra (Bq kg1)

210 Pb (Bq kg1)

137 Cs (Bq kg1)

228 Ra (Bq kg1)

228 Th (Bq kg1)

40 K (Bq kg1)

a

Scrapes NS-01 BC NS-02 BC NS-03 BC NS-04 CCM NS-06 CCM NS-08 CCW NS-09 CCW NS-10 CCW NK04001 NK04002 NK04003 NK04004 NK04005 NK04006

Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface

51  3 94  4 906  13 11  2 62 680  10 272  7 460  10 181  5 178  5 218  6 156  5 178  5 174  5

58  1 101  1 624  6 91 27  1 643  6 394  4 893  8 222  2 224  2 280  2 176  2 233  2 232  2

59  3 98  4 902  15 11  2 15  3 670  10 250  7 385  9 139  4 124  4 164  5 116  5 122  4 11  4

18  1 18  1 75  2 10  1 21  1 57  1 46  1 47  2 41  1 43  1 47  1 41  1 44  1 42  1

18  1 19  1 80  1 10  1 17  1 60  1 47  1 50  1 42  1 44  1 47  1 41  1 42  1 39  1

65  3 91  4 294  11 61 72 331  12 139  6 171  8 396  14 338  12 400  14 338  11 359  13 230  9

Core 6 NC6-01/02 NC6-03/04 NC6-05/06 NC6-07/08 NC6-09/10 NC6-11/12 NC6-13/14 NC6-15/16 NC6-17/18 NC6-19/20 NC6-21/22 NC6-23/24 NC6-25/26 NC6-27/28 NC6-29/30 NC6-31/32 NC6-33/34 NC6-35/36 NC6-37/38 NC6-39/40 NC6-41/42 NC6-43/44 NC6-45/46 NC6-47/48 NC6-49/50

2 4 6 8 10 12 14 16 18 22 22 24.2 26.6 29 31.4 33.7 35.9 37.9 39.9 42.1 44.3 46.3 49 53 58

17  10 47  10 15  7 97 15  5 13  6 29  7 24  7 96 28  7 17  5 20  16 35 86 15  6 10  6 11  6 10  6 21  8 20  7 14  17 16  18 43  17 29  16 40  18

97  3 133  5 102  3 86  2 27  1 51  2 101  3 85  2 56  2 77  2 96  2 49  2 21  1 24  1 46  2 33  1 34  2 87  2 148  4 76  2 55  2 77  2 37  2 27  1 18  1

176  12 220  10 189  8 90  7 34  5 44  6 113  8 110  7 42  6 61  7 71  5 41  6 32  6 39  6 42  6 35  6 46  6 61  7 110  8 73  7 44  6 66  7 37  6 37  6 11  6

a

58  4 62  4 49  3 28  2 11  2 15  2 32  3 24  2 15  2 19  2 24  2 11  2 92 12  2 15  2 10  2 82 18  2 27  3 25  2 16  2 19  2 14  2 11  2 11  2

69  2 72  2 51  1 26  1 14  1 17  1 38  1 30  1 14  1 22  1 25  1 13  1 10  1 10  1 14  1 81 14  1 19  1 30  1 22  1 18  1 20  1 16  1 14  1 13  1

44  13 77  13 47  10 30  9 17  8 15  8 36  10 31  9 28  8 32  9 20  7 18  8 99 19  8 18  9 38 23  4 16  9 45  10 15  9 22  8 35  9 88 28 69

Core 9 NC9-01/02 NC9-03/04 NC9-05/06 NC9-07/08 NC9-09/10 NC9-11/12 NC9-13/14 NC9-15/16 NC9-17/18 NC9-19/20 NC9-21/22 NC9-23/24 NC9-25/26

2.3 4.5 6.8 9.1 11.3 13.6 15.9 18.1 20.4 22.7 24.9 27.2 29.5

19  8 37  9 39  9 39  9 26  9 29  9 60  10 30  7 37  7 29  7 31  7 28  7 28  5

43  2 51  2 62  3 49  2 56  2 54  2 54  2 48  2 49  2 58  2 31  2 25  1 24  1

54  8 82  9 76  9 56  8 79  8 64  8 52  8 56  7 51  6 39  7 40  7 30  6 19  5

a

21  3 29  4 35  4 26  4 33  4 27  4 33  4 27  3 28  2 29  3 24  2 28  3 20  2

22  1 35  2 39  2 29  2 33  2 31  2 31  2 29  1 29  1 30  1 24  1 23  1 23  1

27  12 61  21 55  21 44  18 9  16 14  18 22  18 32  9 18  9 37  9 21  9 32  9 12  7

a

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Table 2 (continued ) Site Ref

Depth (cm)

NC9-27/28 NC9-29/30 NC9-31/32 NC9-33/34 NC9-35/36 NC9-37/38 NC9-39/40 NC9-41/42 NC9-43/44 NC9-45/46 NC9-47 NC9-48 NC9-49

31.7 34.0 36.3 38.5 40.8 43.1 45.3 47.6 49.9 56.7 62.3 69.1 76.5

238 U (Bq kg1)

226 Ra (Bq kg1)

210 Pb (Bq kg1)

14  6 14  6 11  6 66 20  6 12  6 18  6 66 96 47 11  8 85 25  8

19  1 18  1 18  1 20  1 17  1 19  1 20  1 16  1 17  1 15  1 13  2 13  1 14  2

18  6 15  5 14  5 13  6 13  5 14  6 25 15  5 21  6 36 24  7 13  5 20  6

137 Cs (Bq kg1)

228 Ra (Bq kg1)

228 Th (Bq kg1)

40 K (Bq kg1)

a

21  2 16  2 18  2 14  2 17  2 19  2 12  2 14  2 14  2 12  2 17  4 19  2 20  3

21  1 17  1 18  1 16  1 16  1 16  1 19  1 17  1 16  1 16  1 18  2 17  1 18  2

20  9 23  8 11  8 18 28 88 15  8 78

238 U was inferred from the results of the 234Th measurements. BC, Buffalo Creek; CCW, Cooper Creek West; CCM, Cooper Creek Main channel; NC6, core 6 (upstream); NC9, core 9 (downstream) (absolute error). Uncertainties given are based on counting statistics only. a
evaporated down and redissolved in 10% nitric acid before being injected into the plasma torch of a Perkin Elmer Elan 6000 ICPMS. Details of the operating system and the sample preparation can be found in Munksgaard et al. (1998) and Munksgaard and Parry (2000). Typical uncertainties for the Pb isotope ratios are 0.5e1% relative standard deviation. The measured Pb isotope ratios were normalized to the NIST SRM981 (207Pb/206Pb ¼ 0.91464; 208Pb/206Pb ¼ 2.1681). Isotope ratio correction factors applied were usually between 0.994 and 1.005. In addition, with every set of samples, IAEA Soil 7 certified standard reference material was measured. Levels reported were well above the detection limits. Whereas measurements of aluminium, iron and manganese in IAEA Soil 7 via ICPMS were generally lower than certified values, results for all other metals agree well within uncertainties. A similar pattern has been reported by Munksgaard et al. (2003) for rare earth elements (REE) and has been related to incomplete digestion of the soil/ sediment matrix. Munksgaard and Parry (unpublished) found analytical repeatability (including digestion and ICPMS analysis) to be 1.5e7.0% (1 sigma) with a median of 2.7%. When including sampling (triplicate sampling of assumed homogeneous sediment) the same figures were 2.6e9.9% and 3.6%.

4. Results 4.1. Gamma spectrometry Table 2 shows the results of radionuclide activity concentrations in Bq kg1 (dry weight) from scrape samples from the Cooper Creek catchment and core 6 and core 9, upstream and downstream, respectively, of the mine site. Reported uncertainties are one standard deviation from counting statistics only. Uranium activity concentrations shown are derived from the 234Th activity measurements, which is assumed to be in radioactive equilibrium with 238U in the soil. Regression analysis of 238U activity concentrations measured via gamma spectrometry and results inferred from ICPMS reveal that, on average, gamma spectrometry results are in accordance with the ICPMS measurements in the soil scrapes, although there is a discrepancy for lower activity samples. This discrepancy is also shown by comparison of 228Th (gamma spectrometry) and 232Th (ICPMS) activity concentrations, with the gamma spectrometry results being on average, 50% higher than ICPMS data. This is especially true for samples from the Cooper Creek main channel with a relatively high sand and a low silt and clay component. Differences are most likely due to incomplete digestion of some of the resistant sample matrix containing U and Th. This has previously been observed by Munksgaard et al. (2003) for REE. The comparison shows, however, that most of the U in the high activity surface scrapes is recovered by the digestion procedures. Some surface scrape samples, especially samples from the Cooper Creek West catchment, immediately downstream from the mine site, show much higher 226Ra activity concentrations as compared to 238U and 210Pb, whereas in sample NS-03 BC from the Buffalo Creek catchment 238U activity concentration is higher. In general, 228Ra and 228 Th are in radioactive equilibrium in all scrapes and 40K activity concentrations follow the trend of 238U activity concentrations within Buffalo Creek and Cooper Creek West channels.

474

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Radionuclides in core 6 (upstream) and core 9 (downstream) exhibit comparatively low activity concentrations compared to the Buffalo Creek and Cooper Creek West scrape samples. Analysis of ICPMS and gamma results for 238 U and 232Th shows that, on average, the nitric/perchloric acid leach solubilised only w55e75% of total U and Th in these samples due to their high sand content. Generally, 226Ra activity concentrations in the sediment cores are higher than 238U, specifically in core 6, and 40K follows the general trends observed in the U and Th series radionuclides. Both sediment cores show relatively higher U-series radionuclide activity concentrations in their top sections, although variations in the bottom section of core 6 are quite large. The 226Ra/228Ra activity ratios are w1.8 in the top 12.5 cm of core 9, and approximately 1.1 in the bottom section (25e49 cm). Average 226Ra/228Ra activity ratios of core 6, upstream of any possible mine contaminants, are much higher than in core 9, being 2.5 in the top 12 cm. Thorium series radionuclides show less variation with depth and average 228Th concentration are 23 and 24 Bq kg1, for core 9 and core 6, respectively. The 210Pb activity concentrations are generally in equilibrium (to within two standard deviations) with 226Ra in the bottom section of the cores. However, both core 9 and core 6 exhibit higher 210Pb activity concentrations in the top section as compared to 226Ra (Fig. 2). From Fig. 2, it is apparent that there is 210Pb in excess of 226Ra down to w6 cm in core 6 and down to w13.5 cm in core 9. Due to the possibility of these sediments being bioturbated, we did not attempt a quantitative 210Pb excess dating of the sediments. However, due to the half-life of 210Pb of 22.3 years, it can be proposed that the respective core sections contain sediments, which are less than w100e150 years old. The 137Cs activity concentrations measured in the cores do support this proposition, with 137Cs only detectable in the top sections. Sediments from deeper sections of the core show no detectable 137Cs activity. 4.2. ICPMS 4.2.1. Trace metals Table 3 summarizes the results of the ICPMS measurements of trace metals (Al, Fe, Mn, Ni, Co, Cu, Pb, Th and U) and 206Pb/207Pb and 208Pb/207Pb isotope ratios in surface scrape samples. Fig. 3a and b shows the results for core sediments from the Cooper Creek catchment. Core samples from the catchment outside of the mine site display comparatively low concentrations of metals. Substantially higher metal concentrations are found in the Buffalo Creek and Cooper Creek West surface scrape samples, on or adjacent to the mine site. Scrape samples from the main channel upstream and downstream of the mine site exhibit relatively higher Th concentrations compared to U. Generally, metal concentrations in the sediment cores decrease with depth. Fig. 3 shows the metal concentrations with depth measured in cores 6, 9, 4 and 2 (see Fig. 1). Core 6 shows far greater variability throughout the core compared to the other cores, which can also be observed in the radionuclide profiles. Core 6 exhibited distinct bands of sandy and organic rich sediment at the time of sampling, and the variations in metal and radionuclide activity concentrations are due to changes in core lithology. Metal concentration in the top 10e20 cm are generally higher than in the bottom sections. The 238U activity concentrations and 226Ra/228Ra activity ratios in the top core sections are higher as well. This can mainly be attributed to the higher clay and organics and lower sand content of the cores in the top sections, as indicated by the dark colouring as opposed to the lighter colour of deeper core sections with larger amount of sands. Some cores show a distinct layering of dark humic sections alternating with sandier layers, and variations in metal concentrations throughout the cores. It has previously been shown (Alloway, 1995) that clays and humic soils usually exhibit higher metal concentrations. There is a strong positive correlation between Fe (and Al), and 238U (and the other trace metals) in the cores, suggesting that iron rich clays are a predominant carrier of radionuclides and metals. Pearson correlation coefficients are 0.923 (core 2), 0.964 (core 4), 0.864 (core 6) and 0.846 (core 9), respectively, and p-values are generally <0.001. Furthermore, regression analysis of 238U and Fe concentration in the cores reveals that in core 9 the U concentration normalized to the Fe content of the sediment is higher than in cores 6, 4 and 2, which indicates that material relatively higher in 238U (and 226Ra) was deposited downstream of the Nabarlek mine in the past. 4.2.2. Pb isotopes Fig. 4 shows the 206Pb/207Pb plotted versus the 208Pb/207Pb ratios measured in surface scrapes and sediment cores collected in 2004 and 2005, and a comparison with typical common Pb isotope ratios and ratios measured in aerosols worldwide (Bollho¨fer and Rosman, 2000, 2001), in Ngarradj sediments (a creek in the Jabiluka catchment of the

A. Frostick et al. / Journal of Environmental Radioactivity 99 (2008) 468e482 226Ra, 210Pb

0

475

[Bq kg-1]

100

200

0

~ 100 yrs core 6 10 ~ 100 yrs core 9

20

depth [cm]

30

40

50

core 9 Pb-210 60 core 9 Ra-226 core 6 Ra-226 core 6 Pb-210 70

Fig. 2. 210Pb and 226Ra versus the depth in core 6 (upstream of the mine site) and core 9 (downstream of the mine site). Uncertainties are one standard deviation due to counting statistics only.

Alligator Rivers Region) (Bollho¨fer and Martin, 2003) and aerosols measured next to the operating Ranger U mine with the two dashed lines indicating the range of values expected from mixing of PDAC with Ranger ore Pb isotopic ratios (Bollho¨fer et al., 2006). The arrow in Fig. 4 indicates the direction of the Pb isotopic composition measured in soils from unit-7 (206Pb/207Pb ¼ 13.14; 208Pb/207Pb ¼ 0.4480), which has previously been shown to be responsible for 75% of the flux of radionuclides from the Nabarlek mine site (Hancock et al., 2006). Pb isotope ratios in many of the surface scrape sediment samples exhibit highly radiogenic206Pb/207Pb isotope ratios as well, and no corresponding increase in 208Pb/207Pb ratios. The high 206Pb/207Pb ratios are due to contamination of surface soils with U-rich material, possibly originating from unit-7. Of the scrape samples the most radiogenic are NS-08 Cooper Creek West and NS-03 Buffalo Creek, which also recorded the highest 238U activities. These samples were collected next to a sediment settling pond on the rehabilitated mine and along a drainage line towards the Buffalo Creek channel. The 206Pb/207Pb and 208Pb/207Pb isotope ratios measured in the sediment core samples indicate that most radiogenic Pb are likely to originate from Th-rich and U-rich minerals in the local geology rather than U-rich minerals

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Table 3 ICPMS results for trace metal concentrations in mg kg1 (dry weight) and lead isotope ratios in surface scrapes collected at Nabarlek Sample

Al

Fe

Mn

Ni

Co

Cu

Pb

Th

U

208/207

NS-01 NS-02 NS-03 NS-04 NS-06 NS-08 NS-09 NS-10 NK04001 NK04002 NK04003 NK04004 NK04005 NK04006

14,100 13,700 88,900 1700 2140 74,300 72,100 47,900 93,000 96,200 86,700 90,500 85,000 79,200

10,700 11,300 41,700 2970 1470 51,100 81,700 68,800 86,300 86,700 107,000 81,000 94,100 130,000

18.2 35.7 193 35.8 4.70 163 1170 4970 694 531 1000 401 731 1300

4.42 4.62 24.8 0.788 0.199 16.9 48.5 154 N/Da N/Da N/Da N/Da N/Da N/Da

9.43 8.52 47.6 0.864 0.492 35.6 35.8 34.1 N/Da N/Da N/Da N/Da N/Da N/Da

11.9 14.6 75.5 1.6 0.6 53.1 47.1 44.8 60.5 68.9 58.0 63.6 63.5 74.3

2.39 2.76 20.5 1.35 1.27 11.5 16.4 24.1 10.21 8.88 13.43 8.05 10.21 13.87

3.30 3.14 11.6 2.01 1.49 9.20 8.40 6.93 6.90 7.40 8.63 7.00 7.37 6.88

2.98 6.50 69.3 0.299 0.158 53.1 23.2 38.9 13.52 13.68 18.75 11.89 13.37 14.55

2.702 2.570 2.322 3.143 3.100 2.320 2.456 2.460 2.432 2.447 2.452 2.441 2.453 2.470

a

Pb

206/207

Pb

2.238 2.802 2.853 1.669 1.491 3.108 1.678 1.618 2.391 2.078 2.292 2.155 2.187 1.809

N/D, data not available.

associated with U mining. Both isotope ratios are higher than typical PDAC ratios. Similar findings were reported for Ngarradj (Swift Creek) sediments (Bollho¨fer and Martin, 2003). Lead isotope ratios in the core sediment samples exhibit a grouping in their 206Pb/207Pb and 208Pb/207Pb isotope ratios. Whereas 208Pb/207Pb isotope ratios are slightly less radiogenic for core samples from upstream of the mine (core 6), downstream of the mine (core 9) and within Kadjirrikamarnda Creek (core 2), samples from the main channel of Cooper Creek, downstream of Kadjirrikamarnda Creek (core 4) are more radiogenic with 206Pb/207Pb and 208 Pb/207Pb ratios as high as 1.761 and 3.795, respectively.

Fig. 3. Trace metal concentrations (mg kg1 dry wt) measured in cores 6, 9, 2 and 4. Cores 9 and 2 are represented with open symbols and dashed lines, whereas cores 6 and 4 are represented with solid symbols and solid lines.

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477

3.00 global aerosol Ngarradj sediments

Fit: Unit 7 - PDAC

Leaves KNP Nabarlek Scrapes 2003 Nabarlek Scrapes 2005 Nabarlek Core 9 at Gauging Station Nabarlek Core 6 u/s Mine Nabarlek Core 4 d/s Gadji Ck Nabarlek Core 2 Gadji Ck

206Pb/207Pb

2.00

Fit: Ranger Ore- PDAC

1.50

1.00 1.40

1.30 2.65

0.00 1.50

2.00

2.50

3.00

2.85

3.50

3.05

4.00

4.50

208Pb/207Pb

Fig. 4. 3-isotope plot of soil scrape and core sediment samples (cores 2, 4, 6 and 9) taken at the rehabilitated Nabarlek mine. The inset shows a close up of the box. The regression lines indicate mixing of PDAC Pb and ore from the Ranger uranium mine (95% confidence intervals indicated by the dashed lines) and a linear fit to the data of surface scrape samples of this study and samples with a 206Pb/207Pb (208Pb/207Pb) ratio of 13.14 (0.45) as measured in soils from unit-7.

The in-between as well as the within core variations of the 208Pb/207Pb ratio can be attributed to the varying relative contributions of sand and clay in the samples. Bollho¨fer and Martin (2003) have associated less radiogenic 206Pb/207Pb and 208Pb/207Pb ratios in sandy sediments from Ngarradj with a relatively larger amount of clays in some samples, whereas Munksgaard and Parry (2000) have shown that the more radiogenic Pb isotope ratios in sediments from the Norman River catchment, in the south-east corner of the Gulf of Carpentaria, Australia, are most likely due to a higher contribution of heavy minerals such as monazites or ilmenites in the sand fraction of the sediments. This is demonstrated in our cores also, with lower 206Pb/207Pb associated with higher iron contents in the sediments. 5. Discussion 5.1. Characterization of Cooper Creek sediments There have been several studies focussing on radionuclide activity concentrations within sediments in the Alligator Rivers Region. For example, Clark et al. (1992) investigated sediments of the Magela floodplain in order to constrain sediment transport and deposition. They report U activity concentrations of 70e90 Bq kg1 across the floodplain, and 232 Th and 40K activity concentrations of 73e104 Bq kg1 and 156e221 Bq kg1, respectively. Winde (2002) reports 50e100 Bq kg1 of 238U in sediments of Magela Creek, downstream of the Ranger mine, similar to sediments from the Magela floodplain. Magela Creek sands show low activity concentrations of approximately 15, 12 and 5 Bq kg1 for 238U, 228Th and 40K, respectively (Supervising Scientist, 1985), whereas Bollho¨fer and Martin (2003) report activity concentrations in sandy sediments from Ngarradj (Swift Creek) of 10, 12 and 9 Bq kg1. On average, Cooper Creek samples from core 9 show 238U activity concentrations of 23 Bq kg1, an average of 23 Bq kg1 for 228Th and 20 Bq kg1 for 40K. Core 6 exhibits average 238U, 228Th and 40K activity concentrations of 16, 18 and 25 Bq kg1, respectively. In addition to results from earlier studies, Table 4 shows average concentrations of radionuclides (Bq kg1), metals (mg kg1), the inventory of 210Pb excess and 137Cs (Bq cm2) at the time of sample analysis, and average Pb isotope ratios for the different core sections. Using the 210Pb excess profile, the cores have been divided into sediments which

478

Depth

238 U (Bq kg1)

228 Th (Bq kg1)

210

Pbxs (Bq cm2)

137 Cs (Bq cm2)

Ni (mg kg1)

Co (mg kg1)

Cu (mg kg1)

Pb (mg kg1)

206

208

Scrapes Buffalo Ck Cooper Ck west

Surface Surface

49e1039 274e690

18e75 46e57

N/Da N/Da

N/Da N/Da

4e25 17e154

9e48 34e36

12e75 45e53

2e21 12  24

2.238e2.853 1.618e3.108

2.322e2.702 2.320e2.460

Core 6 Top <100 years Top >100 years Bottom

0e6 6e10 10e58

26  18 12  4 15  8

64  11 20  8 18  8

4.1  0.6 0.1  0.2 N/Da

0.024  0.010 0.000  0.008 0.010  0.010

2.9  0.6 1.0  0.6 0.9  0.5

11  2.5 3.3  1.9 3.2  1.7

10  2.2 1.7  1.1 1.8  1.2

8.25  1.60 3.17  1.90 3.02  1.35

1.329  0.01 1.363  0.06 1.403  0.06

2.690  0.02 2.755  0.10 2.918  0.18

Core 9 Top <100 years Top >100 years Bottom

0e13.5 13.5e27 27e76.5

32  8 36  12 13  7

32  6 28  3 18  2

7.5  2.1 1.8  1.8 N/Da

0.027  0.010 0.000  0.012 0.00  0.00

3.3  0.8 3.2  0.8 0.8  0.5

6.2  1.5 6.3  1.7 1.6  0.9

4.6  0.8 4.5  1.0 1.8  0.4

3.87  0.62 3.36  0.64 1.97  0.30

1.440  0.013 1.453  0.012 1.484  0.026

2.751  0.033 2.745  0.024 2.897  0.096

Cooper Creek main Magela floodplain Magela Ck S.Alligator R. Ngarradj

Surface Surface Surface Banks Surface

8.5  3.5 70e90 15 20e40 7e29

13.5  5.0 73e104 12 N/Da 8e26

N/Da N/Da N/Da N/Da N/Da

N/Da N/Da N/Da N/Da N/Da

0.5  0.4 N/Da N/Da 4.5e11.1 N/Da

0.7  0.3 N/Da N/Da 3.2e6.6 N/Da

1.1  0.7 N/Da N/Da 3.3e9.4 0.2e2.55

1.31  0.06 N/Da N/Da 4.3e9.5 0.5e2.4

1.580  0.126 N/Da N/Da N/Da 1.445e1.717

3.122  0.030 N/Da N/Da N/Da 2.858e3.208

In addition, 210Pb excess and 137Cs inventories (Bq cm2) are shown. a N/D, data not available.

Pb/207Pb

Pb/207Pb

A. Frostick et al. / Journal of Environmental Radioactivity 99 (2008) 468e482

Table 4 Averages (or ranges) of 238U and 228Th activity concentrations (Bq kg1) and heavy metal concentrations (mg kg1) in sediments of the Alligator Rivers Region (Clark et al., 1992; Supervising Scientist, 1985; Winde, 2002; Bollho¨fer and Martin, 2003), and averages for different core sections of Cooper Creek cores 6 (upstream) and 9 (downstream)

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are most likely younger than approximately 100 years, upper sediments most likely older than 100 years and with a substantial amount of clays and black organic material present, and bottom sediments which are predominantly sandy. Core 6 shows some variation with alternating sandy and darker, organic rich layers throughout the core. The 137Cs inventory data support the assumption that the top 6 cm (core 6) and 13.5 cm (core 9), respectively, are younger than w100 years. 137Cs was injected into the atmosphere of the southern hemisphere from 1950s to the 1970s (Ritchie and McHenry, 1990) and the presence of 137Cs in our samples indicates the presence of relatively young sediments in the top sections. Quite possibly, core 9 is bioturbated to a greater extent than core 6. The surface scrape sample activity concentrations for the Cooper Creek main channel (CCM) are 8.5 Bq kg1 238U, 13.5 Bq kg1 228Th and 6.5 Bq kg1 40K. Surface scrape samples from Buffalo Creek within the mine site are on average 273  256 Bq kg1 238U, 41  18 Bq kg1 228Th and 279  125 Bq kg1 40K and Cooper Creek West scrape samples are on average 471  204 Bq kg1 238U, 52  7 Bq kg1 228Th and 214  103 Bq kg1 40K. Hancock et al. (2006) have reported a wide range of radionuclide activity concentrations of 48e2530 (64e6780) Bq kg1 for 238U and 67e2380 (92e15400) Bq kg1 for 226Ra, respectively, in the Buffalo Creek (Cooper Creek West) catchments and our measurements are within the ranges reported earlier for the respective catchments. Uranium activity concentrations in the majority of the Cooper Creek surface scrape samples and sediments are low and comparable with concentrations at other creeks within the Alligator Rivers region. The top of the cores exhibit higher concentrations of radionuclides and heavy metals, most likely related to the higher clay and organics content. There is no significant variability in the top sections of core 9 for sediments older and younger (0e13.5 cm) than 100 years, respectively. In contrast, top sediments (0e6 cm) less than 100 years old in core 6, upstream of the Nabarlek ore body, exhibit metal concentrations which are on average 2e5 times higher than sediments older than 100 years. Only Buffalo Creek and Cooper Creek West adjacent to the mine site have sediments that appear to contain material with higher radionuclide activity concentrations. The influence of these sediments off site appear to be limited at this stage and no erosion of U-rich sediments can be concluded from the concentration of radionuclides in the cores or surface scrape samples in Cooper Creek. 5.2. Radiogenic and radioactive isotopes There is limited data available on Pb isotope ratios of sediments within the Alligator Rivers region. Gulson and Mizon (1980) and Gulson (1986) have demonstrated the advantages of using Pb isotopes for mineral exploration and report highly radiogenic Pb isotope data for the Jabiluka and Koongarra ore bodies, whereas Dean and Gulson (1987) have used Pb isotopes in soils and plants from the Koongarra deposit for biogeochemical prospecting. Gulson et al. (1992) investigated particulates in tailings dam water of the ERA Ranger mine site exhibiting very radiogenic 206 Pb/207Pb ratios. Bollho¨fer and Martin (2003) examined sediments from Ngarradj (Swift Creek), a catchment potentially influenced by the Jabiluka mineral lease, whereas Munksgaard et al. (1998) and Munksgaard and Parry (2000, 2001) have investigated sediments in coastal and estuarine zones in northern Australia. General trends observed in sediments in Cooper Creek, both in the core and main channel scrape samples, are similar to those seen in Ngarradj (Bollho¨fer and Martin, 2003). There it was assumed that relatively radiogenic sands including heavy minerals, possibly monazites or ilmenites with comparatively low Pb concentrations and high 208 Pb/207Pb ratios, are mixed with natural dust, clays and silts, that contribute some of the Pb within the sediment and have a significantly different Pb isotopic composition closer to PDAC ratios. The contribution of the heavy minerals to total Pb within the samples may be small but due to their different isotope ratios this small contribution will cause a drastic shift towards higher 208Pb/207Pb isotope ratios. Uranium-rich materials associated with U mining are characterized by very high 206Pb/207Pb but low 208Pb/207Pb ratios as seen in the sediments from Unit-7 at Nabarlek. As shown in Table 3 and Fig. 4, erosion products from the Nabarlek mineral lease are contaminating the sediments of Buffalo Creek and Cooper Creek West adjacent to the site. Radium isotopes also indicate that the relative contribution of U to the sediments appears to be significantly higher for Buffalo Creek and Cooper Creek West, which supports the conclusion that present day variations in radiogenic and radioactive isotopes and trace metals within the sediments of these tributaries are due to mine-derived erosion products. As previously discussed, however, radioactive isotopes in the Cooper Creek sediment cores do not exhibit a clear influence of U-rich materials. Examining the Pb isotope ratios more closely, sediment samples downstream of the Nabarlek ore body, in particular in core 9, clearly exhibit more radiogenic 206Pb/207Pb ratios as compared to the upstream core 6. This is most

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likely due to more radiogenic runoff from the catchment, which includes the ore body (pre-mining) and the mine and rehabilitated site (post-mining). The inset in Fig. 4 shows the between core variations in the Nabarlek catchment. Cores 4 and 9, downstream of the Nabarlek mine and U ore body, generally exhibit more radiogenic 206Pb/207Pb isotope ratios compared to the upstream core 6, and core 2 from the Kadjirrikamarnda catchment. The Kadjirrikamarnda catchment at present only delivers 3.3% of the total 238U flux from the Nabarlek mine site (Hancock et al., 2006) and thus core 2 is relatively little influenced by mine runoff. Core 9 is closer to the mine and as such is potentially influenced to a higher degree by runoff from the mine site and former ore body than core 4, which is further downstream. There is no indication of significant differences between top sediments in core 9 older and younger than 100 years, respectively (Table 4), and the variations within the core are related to changing core lithology rather than variable contribution of mine origin material with high 206Pb/207Pb ratios. In an inverse concentration plot, the Pb isotope ratios are plotted versus the inverse of the stable Pb concentration. The Pb isotope ratio of the contaminating endmember is then displayed as the intercept with the y-axis assuming that the background concentration and isotopic composition remains constant. Table 5 shows the results of inverse concentration plots for both 206Pb/207Pb and 208Pb/207Pb isotope ratios. The lowest Pb concentrations were measured in the sandy bottom sediments of core 4 and the ranges in Pb isotope ratios of those samples are labelled background. Samples from cores 2, 4, 6, and 9 show endmember Pb isotopic composition with 206Pb/207Pb and 208Pb/207Pb ratios between 1.29e1.37 and 2.54e2.61, similar to Pb isotope ratios measured in sediments and clays elsewhere (Doe, 1970), although the 206Pb/207Pb endmember ratio in core 9 is slightly higher than in the other cores. Surface scrapes collected in 2006 from Buffalo Creek and Cooper Creek Main exhibit a much more radiogenic endmember 206 Pb/207Pb isotope ratio of 3.13. Assuming two component mixing of material with 206Pb/207Pb of 1.33 (top section of core 6, Table 4) with a contaminating endmember exhibiting a 206Pb/207Pb ratio of 3.13 (Table 5) is leading to a ratio of 1.44 (top section of core 9, Table 4) reveals that approximately 6% of the Pb deposited in the top sections of core 9 originates from U-rich material eroding off site. An approximately 1% contribution is required assuming, as one possible scenario, that the bulk of the radiogenic material originated from erosion unit-7 (Hancock et al., 2006) with an endmember 206 Pb/207Pb ratio of 13.14. This 1% contribution translates into approximately 0.039 mg of Pb per kg sediment. At erosion unit-7, a Pb concentration of 261 mg kg1 has been measured. Consequently, a 0.015% contribution of sediment is needed from this erosion unit in core 9 to shift the 206Pb/207Pb ratio from 1.33 to 1.44. The 226Ra and 238U activity concentrations in unit-7 soils have been determined and amount to 15,400 and 6780 Bq kg1, respectively (Hancock et al., 2006). Thus, approximately 2 Bq kg1 226Ra and 1 Bq kg1 238U in the top section of core 9 may be due to highly radiogenic erosion products from areas on the mine site. Such a contribution is generally masked by the natural variation throughout the core and goes unnoticed using the measurement of radioactive isotopes only. However, this study highlights the potential of the much more sensitive stable Pb isotope fingerprint to isolate a small contribution of such kind. As the mineralisation at Nabarlek extended to the surface (Anthony, 1975), erosion of radiogenic material could potentially have influenced the Pb isotope ratios in sediments downstream of Nabarlek before mining started. The pattern of generally more radiogenic Pb isotope ratios in core 9 as compared to cores 6 and 2 is consistent throughout the core (see Fig. 4) and does not seem to be a feature that happened at any particular point in time. Despite bioturbation observed in core 9 in the top section, activity concentrations of 210Pb and 137Cs indicate that deposition of more radiogenic material downstream of Nabarlek was an ongoing process before mining started at Nabarlek.

Table 5 Results of inverse concentration plots, and 206Pb/207Pb and 208Pb/207Pb intercepts, for cores 2, 4, 6 and 9 Pb/207Pb intercept

Surface scrapes Core 9 Core 4 Core 2 Core 6 Background a

N/A, not applicable.

206

R2

p

208

Pb/207Pb intercept

R2

p

3.13 1.37 1.34 1.33 1.29 1.55e1.75

0.88 0.53 0.92 0.61 0.49 N/Aa

<0.001 <0.001 <0.001 <0.001 <0.001 N/Aa

2.23 2.55 2.54 2.67 2.61 3.35e3.80

0.98 0.60 0.96 0.73 0.40 N/Aa

<0.001 <0.001 <0.001 <0.001 <0.001 N/Aa

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6. Conclusions This study provides the first quantitative post-rehabilitation assessment of stable Pb isotopes, radionuclides and trace metals in erosion products at the former Nabarlek U mine site. The objectives were to map the spatial and temporal distributions of U-rich erosion products leaving the Nabarlek site, and potentially contaminating sediments within the Cooper Creek catchment. Most of the variations in radiogenic and radioactive isotopes and trace metals between and within the cores are attributed to varying core lithology. Uranium, Th and metal concentrations in the majority of the Cooper Creek surface scrape samples and sediment cores are low and comparable with concentrations at other creeks within the Alligator Rivers Region. However, sediments from Buffalo Creek and Cooper Creek West, in close proximity to the mine site, contain U-rich mine-derived material. The spatial effects on the sediments off site appear to be limited at this stage. Using radionuclide activity concentration measurements alone, there is no indication of deposition of U-rich material in Cooper Creek sediments downstream of the former ore body. The measurement of Pb isotope ratios in the sediments, however, provides a much more powerful tool to determine whether erosion products from U-rich sources are being transported downstream. Using the 206 Pb/207Pb ratio in the sediments, our study reveals that approximately 6% of the Pb deposited in a sediment core from Cooper Creek, immediately downstream of Nabarlek, originates from a U-rich source with an endmember 206 Pb/207Pb ratio of 3.13. This contribution is likely to have existed pre-mining, due to erosion of the outcropping ore body, as indicated by the presence of radiogenic material throughout deeper pre-mining sections of the cores downstream of Nabarlek.

Acknowledgements The authors acknowledge the Australian Research Council for funding this study (Linkage Project LP0455703) and thank Gary Fox, Bruce Ryan, Therese Fox and Anthony Sullivan for field assistance and David Richard Jones for comments on the manuscript.

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