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Outdoor 222Rn-concentrations in Germany – part 2 – former mining areas
Journal of Environmental Radioactivity 132 (2014) 131e137
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Outdoor areas
222
Rn-concentrations in Germany e part 2 e former mining
M. Kümmel*, C. Dushe, S. Müller, K. Gehrcke Bundesamt für Strahlenschutz, 38226 Salzgitter, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2013 Received in revised form 17 January 2014 Accepted 19 January 2014 Available online 6 February 2014
In the German Federal States of Saxony, Saxony-Anhalt and Thuringia, centuries of mining and milling activities resulted in numerous residues with increased levels of natural radioactivity such as waste rock dumps and tailings ponds. These may have altered potential radiation exposures of the population significantly. Especially waste rock dumps from old mining activities as well as 20th century uranium mining may, due to their radon (222Rn) exhalation capacity, lead to significant radiation exposures. They often lie close to or within residential areas. In order to study the impact on the natural radon level, the Federal Office for Radiation Protection (BfS) has run networks of radon measurement points in 16 former mining areas, together with 2 networks in regions not influenced by mining for comparison purposes. Representative overviews of the long-term outdoor radon concentrations could be established including estimates of regional background concentrations. Former mining and milling activities did not result in large-area impacts on the outdoor radon level. However, significantly increased radon concentrations were observed in close vicinity of shafts and large waste rock dumps. They are partly located in residential areas and need to be considered under radiation protection aspects. Examples are given that illustrate the consequences of the Wismut Ltd. Company’s reclamation activities on the radon situation. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Radon Outdoor Mining Reclamation Detection threshold Detection limit
1. Introduction The results of a nation-wide study of outdoor radon (222Rn) concentrations in Germany are described in (Kümmel et al., 2013, hereinafter referred to as ‘Part 1’). In this three-year-measuring program radon concentrations were measured at 173 measuring points in an even grid with a grid length of approx. 50 km with solid-state track etch detectors. The explicit purpose of that study was to map natural radon levels. Hence, care had been taken to avoid setting up measurement points in the immediate vicinity of potential anthropogenic sources of radon in the atmosphere. Significant anthropogenic radon sources are, for example, tailings ponds and waste rock dumps from uranium and other ore mining and milling (e.g. Ettenhuber, 2002; Bollhöfer et al., 2006; Krizman et al., 2009), or heaps of phosphogypsum, a by-product from the phosphate industry (e.g. Rutherford et al., 1994). In Germany, facilities and residues from centuries of mining and milling activities in the three eastern Federal States of Saxony,
DOI of original article: http://dx.doi.org/10.1016/j.jenvrad.2014.01.012. * Corresponding author. Tel.: þ49 3018 3334244. E-mail address: [email protected] (M. Kümmel). 0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2014.01.011
Thuringia and Saxony-Anhalt, and specifically the 20th century uranium ore mining activities in Saxony and Thuringia are of special importance in this respect (Ettenhuber, 2002). Among others, large waste rock dumps play a significant role. With volumes of up to about 60 Mio m3, they may show average exhalation rates exceeding those known from the literature on tailings ponds (IAEA, 1992) (w1 Bq m2 s1/(Bq g1) of 226Ra) by an order of magnitude. Such high exhalation rates were found to be the result of convective transport processes which may prevail over diffusion in case of depositions of large volumes of coarse grained waste rock material (Dushe et al., 2003). In the fine grained materials of tailings ponds or phosphogypsum depositions diffusive transport dominates the exhalation. In contrast to many other countries, residues from uranium mining and milling in Eastern Germany are often situated close to, and partly directly within, residential areas. Under such conditions the inhalation of radon and its decay products may be a significant, sometimes even the dominating exposure pathway for the population (Ettenhuber, 2002). Complementary to the outdoor radon measurements of the natural background in Germany (Part 1), the study presented in this paper aimed at providing representative data on the height and range of the mining-related increase in outdoor radon
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Fig. 1. Overview of measurement areas with outdoor radon networks (area (17) is composed of 4 subareas).
concentrations in 16 former mining areas of the aforementioned three Federal States. A network of 485 radon measurement points was run over time periods partly exceeding 10 years in these mining areas and in two non-mining areas (for comparison). Thereby, we focused not only on the absolute height of concentrations but primarily on the mining-related contribution, seeing that only anthropogenic increments are of relevance from a regulatory point of view. Since there is no practical way to distinguish between anthropogenic and natural contributions to the measurement effect, retrospective background values needed to be established. Ideally, background levels should have been determined before the mining started. This was, however, neither possible nor even considered at those times. Retrospective estimations using “glass” detectors (e.g. Samuelsson, 2011) are also not possible because the necessary old
window glasses are not available. Therefore, a method was developed to estimate background values in the old mining areas. These values may be considered complementary to those given in Part 1. 2. Materials and methods 2.1. The radon measurement system Radon measurements in mining areas were carried out using the passive measurement system described in Part 1, based on nuclear track detectors made of MakrofolÒ, mounted in a diffusion chamber. The decision threshold and detection limit of the measurement system are 20 kBq h m3 and 40 kBq h m3, respectively. These figures correspond to concentrations of 5 Bq m3 and 10 Bq m3. The values are somewhat higher than those given in Part 1 (3 Bq m3 and
Table 1 Measurement areas with outdoor radon networks, the corresponding past mining activities and main rocks. No.
Measurement area
Mining activities
Main rock
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Annaberg-Buchholz Aue Crossen Dittrichshütte Freiberg Freital Gittersee Gottesberg Johanngeorgenstadt Königstein Lengenfeld Mansfeld Marienberg Mechelgrün Ronneburg Seelingstädt Görlitza Zwickaua
Mining (U; Ag, Co, Cu, Fe, Ni, Pb, Sn) Mining (U; Ag, Bi, Co, Cu, Fe, Ni, Sn) Ore (U) processing incl. storage of residues Mining (U; Fe) Mining (Ag, Pb, S, Zn) Mining (bituminous coal) Ore (U) mining & processing incl. storage of residues Mining (U; Ag, As, Bi, Co, Cu, Fe, Ni, Sn) Mining (U; Ag, Bi, Co, Cu, Fe, Sn) Mining (U) Ore (U) processing incl. storage of residues, mining (wolframite) Mining (Cu) & smelting Mining (U; Ag) Mining (U) Mining (U) Ore (U) processing incl. storage of residues
Slate, gneiss Granite, phyllite New red, sandstone Clay slate Gneiss Gneiss Gneiss Granite Granite, phyllite Sandstone Granite, phyllite, clay slate Sandstone Gneiss Granite (contact area) New red, sandstone, clay slate New red, sandstone, clay slate Granite Granite, phyllite, clay slate
a
Unaffected area (for comparison).
M. Kümmel et al. / Journal of Environmental Radioactivity 132 (2014) 131e137
5 Bq m3) due to the lower exposure time of 6 months (12 months in Part 1) scheduled in order to detect possible differences between summer and winter seasons. The relative measurement uncertainty at an exposure level of 100 kBq h m3, corresponding to 20 Bq m3 at a 6 months’ exposure time, is about 20%. Measurements were carried out at a height of 1.5 m above ground. 2.2. Measurement areas The measurement areas were defined based on the results of previous measurements and various types of documentation, including historical information on past mining activities. The studies were primarily aimed at determining radiologically relevant alterations of the radon levels caused by mining. Thus, we focused on residential areas. Consequently, the measurement points are not uniformly distributed in the measurement areas. The density of measurement points is normally highest close to mining residues or facilities. Altogether we defined radon networks in 16 mining areas and 2 networks in regions not influenced by mining for comparison purposes. The mining or measurement areas differ in terms of (i) the mining activities performed, (ii) geology and (iii) orography. Fig. 1 shows the position of the measurement areas in the respective Federal States. Table 1 provides information on the respective mining activities in the past and on the main geology. Up to 1990 uranium ore mining or milling were carried out in the measurement areas (2), (3), (6), (7), (10), (15) and (16), followed by extensive remediation activities by the Wismut GmbH in these areas. Areas (3), (7), (12), (15) and (16) are mainly characterized by flat terrain, in the other areas mountainous terrain prevails. In total, data were taken at 595 measurement points. Out of these 485 were installed in residential areas, the remaining 110 near mining facilities. 2.3. Reconstruction of background values To determine the local radon background concentrations we had to find a compromise between the somewhat contradicting criteria that measurement points should be located (i) close enough to the areas of interest to be comparable with respect to geology, orography and meteorology, but (ii) sufficiently distant from the major mining-related radon emitters so as to avoid significant contributions to the radon concentrations. In order to quantify the latter requirement we defined two criteria. First, only measurement points with an estimated maximum mining-related influence of less than 3 Bq m3 were considered. This figure corresponds to the measurement uncertainty at background levels (Part 1). For the estimates we used the 226 Ra inventories of the relevant mining residues gathered within the framework of a large federal project in the 1990-ies (Ettenhuber, 2002). Simplified models of both the radon exhalation rate and atmospheric transport (BfS, 2011) were then used for upper estimates of the radon concentrations. Second, a minimum distance of the measurement points from the nearest radon emitter of 500 m was defined to confine possible uncertainties of the calculated radon concentration, resulting from the simplifying assumption of a circular base area of all sources. Using this approach, between 3 and 18 measurement points were determined in the different mining areas to represent the respective natural background concentrations.
133
latter from the background concentrations (Gehrcke et al., 2008). For that purpose, the well known concept of decision thresholds and detection limits (see, e.g. Currie, 1999) was applied. 2.4.1. Decision thresholds The decision threshold allows a decision as to whether or not the physical effect quantified by the measurand is present. Here, the measurand of interest is the increment C of the radon concentration by anthropogenic sources. For the purpose of this study, C was defined as C ¼ Cgross Cbackground, the difference of the gross measurement effect of an individual measurement (Cgross) minus a predefined background concentration (Cbackground) assumed to be representative of the measurement area in question. The decision threshold (DT) is an upper percentile of the expected distribution of C values when actually no anthropogenic contribution is present. If C is less than the decision threshold DT, it is concluded that there is no anthropogenic contribution. The probability a of making a false conclusion about the null hypothesis H0: C DT was chosen to be 10%, which means that DT equals the 90th percentile of the distribution of C without mining influence. As the distribution of background values in the respective measurement areas turned out to be essentially log-normal (see Section 3.2), the question arose which distribution parameter is considered representative of the background. We decided for the median. This way, the true value of the anthropogenic radon concentration has equal chances to be over- and underestimated in individual measurements. Note that the decision threshold as used here is determined by three components, namely the spatial variability of radon concentrations within the respective mining areas, the variation of the concentration in time, and the statistical uncertainty of the measurement. 2.4.2. Detection limits The detection limit indicates which smallest true value of anthropogenic radon concentration can still be detected with the measurement procedure applied. It is a fictitious concentration that, if the measurement system is exposed to it, leads to a net measurement result greater than the detection limit with predefined probability b. b represents the probability of falsely concluding that there was only natural background concentration whilst there actually was an anthropogenic contribution. A value of 10% was chosen. As the detection limit refers to a single future measurement at an arbitrary but fixed point in the measurement area and at a fixed time interval, only the measurement uncertainty has to be taken into account. It is known from laboratory calibration experiments
2.4. Characteristic limits for anthropogenic contributions As the expected background values are in the same order of magnitude as at least part of the mining-caused radon concentrations, statistically based criteria are needed to discriminate the
Fig. 2. Illustration of the decision threshold, detection limit and error probabilities. C denotes the net radon concentration, Ca the anthropogenic contribution to the total radon concentration, DT the decision threshold and DL the detection limit.
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to be in the range of 4 Bq m3 in the relevant concentration range. The respective distribution is assumed to be Gaussian. Fig. 2 illustrates the described relations.
3.1. Radon concentrations in residential areas The 3904 annual (i.e. average of summer and winter values) radon concentrations in residential areas show a great variability with a minimum concentration of 5 Bq m3 and a maximum of 1700 Bq m3. Fig. 3 gives an overview of measured concentrations in residential areas, except for the reference areas (confined to concentration up to 300 Bq m3 for practical reasons). The frequency distribution reflects measurement results, which originate from different measurement areas with different measurement densities, fractions of mining-influenced measurement points and observation periods. It is positively skewed, but it is not well approximated by any of the respective standard probability density functions, such as the lognormal. The histogram shows a distinct maximum in the class 10e15 Bq m3 that corresponds to the average background concentrations (see Section 3.2). Many values above 50 Bq m3 represents a variety of measurement points that are significantly influenced by anthropogenic radon emitters. A value of approximately 50 Bq m3 may be considered an upper bound of outdoor radon concentrations in Germany at locations not influenced by radon from human activities (Part 1). A summarizing overview of the radon situation in the different measurement areas is given in Fig 4. From Fig. 4 it can be seen that values exceeding 50 Bq m3 were observed in six out of 18 measurement areas only. About 90% of all values are less than 50 Bq m3. Values above this level originate in all likelihood from locations that are influenced by anthropogenic radon emissions. Indeed, all measurement points with radon concentrations exceeding 50 Bq m3 were located in close vicinity to large mining-related radon emitters, the majority of them being large waste rock dumps. Others were located close to discharge air shafts (see examples in Section 3.4). The highest measured radon concentration (area number 9) of 1700 Bq m3 was caused by exhalation from a waste rock pile with dominating convective transport (Dushe et al., 2003). In most measurement areas the median values are in the range of 11 Bq m3e16 Bq m3, which is only slightly higher than the
-3
70 60
800
50
600
40 30
400
20
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10
0
-3
80
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> 50 Bq m
90
Absolute frequency for concentrations
100
1200
≤ 50 Bq m
Absolute frequency for concentrations
1400
0
5
20
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70 100 130 160 190 220 250 280
> 300
Radon concentration [Bq m-3]
Fig. 3. Histogram of 3904 annual mean radon concentrations measured between 1990 and 2005 at 469 stations in residential areas in mining regions of Saxony, Thuringia and Saxony-Anhalt. Note that the horizontal scale is subdivided into two ranges with class widths of 5 Bq m3 (up to 50 Bq m3) and 10 Bq m3 (>50 Bq m3).
Radon concentration [Bq m-3 ]
3. Results and discussions
10000
1000
100
10
1 1
2
3
4
5
6 7 8 9 10 11 12 13 14 15 16 17 18 Measurement area number
Fig. 4. Box plot of outdoor radon concentrations in all measurement areas (comp. Table 1). The ends of the whiskers represent the minimum and maximum values. The rectangle denotes the interquartile range and the horizontal line in the rectangle the median.
observed median background values for Saxony, Thuringia and Saxony-Anhalt of about 10 Bq m3 (Part 1). In the areas without mining activities (17, 18) the median values are relatively high compared to most measurement areas with mining activities (20 Bq m3 and 17 Bq m3), see Fig. 4. Since the areas (17) and (18) are mainly characterized by granite bedrock (comp. Table 1), which is known to show increased 226Ra activity concentrations, this result is not surprising. Median values of around 20 Bq m3 were also observed in most of mining areas with granite in the geological set-up. As expected, the range of measured concentrations in the areas (17) and (18) is smaller than in most mining areas, due to the absence of mining influence. 3.2. Background values & characteristic limits 3.2.1. Background values Fig. 5 shows the frequency distribution of annual background radon concentrations (as defined in Section 2.3) from all measurement areas. The values from the areas (17) and (18), being background values per se, are also included. This distribution and those for all individual measurement areas, fit to three-parameter log-normal distributions (KolmogoroveSmirnov-test, p values > 0.15). The distribution parameters were estimated according to part 1, eqns. (3)e(6). The median of 14 Bq m3 is comparable with background values established for these regions according to Part 1, but is slightly higher than the median values for Saxony and Thuringia as a whole (10 Bq m3). Table 2, column 2, shows the results of the estimated median background concentrations for the individual measurement areas. For the majority (13 out of 16) of areas, the concentrations lie in a relatively narrow range of 12 Bq m3 up to 15 Bq m3. Higher means were observed for both unaffected areas (18 Bq m3 and 19 Bq m3), as well as for areas 11a and 8, the latter with an exceptionally high value of 29 Bq m3. Areas 11a and 8 are characterized by prevailing granite bedrocks with increased 226Ra content. We did not find significant differences of the means and standard deviations between measurement areas with flat terrain (3, 12) and mountainous terrain (1, 2, 13) having similar 226Ra contents in bedrocks. It would appear plausible to assume lesser mixing with low e 222Rn air masses from the sea in mountainous areas,
M. Kümmel et al. / Journal of Environmental Radioactivity 132 (2014) 131e137 100%
250
90% mean = 15 Bq m
Absolute frequency
200
80%
standard deviation = 7.5 Bq m median = 14 Bq m geometric standard deviation = 1.8
documents that mining activities and residues did not have large area impacts on the outdoor radon level. 3.4. Effects of reclamation
70%
shift parameter = 3.3 Bq m
150
135
60% 50%
100
Since 1991 reclamations of uranium mining sites are in progress. Many mining relics were, among others, cleaned up because of increased radon releases.
40% 30%
50
20% 10%
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0% 0
4
8
12
16
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24
28
32
36
40
44
48
52
56
60
Radon concentration [Bq m ]
Fig. 5. Histogram of background radon concentration of all measurement areas (the solid line indicates the cumulative percentage distribution).
especially in valleys. However, our measurements did not support this assumption. 3.2.2. Characteristic limits Table 2, columns 3 and 4 show the decision thresholds and detection limits for anthropogenic contributions to the measurement effect. The 95%-confidence interval of the decision threshold was determined by a bootstrap procedure. Detection limits between 7.4 Bq m3 and up to 19 Bq m3 were determined. This implies that, depending on the measurement area, anthropogenic contributions to atmospheric radon concentrations of up to almost 20 Bq m3 cannot be detected with sufficient statistical certainty using the methodology described here. According to ICRP 65 recommendations (ICRP, 1993) such radon concentrations may result in radiation exposures of up to 0.4 mSv effective dose per year, assuming an exposure time of 7000 h a1 (indoors) and 1760 h a1 (outdoors) and an equilibrium factor of 0.4 (indoors) and 0.6 (outdoors). After (ICRP, 2010), the newly introduced dose coefficient for radon inhalation will increase the corresponding dose to about 0.8 mSv a1, a value that cannot be neglected from the radiation protection point of view. This result underlines the deficiencies of the retrospective approach to the background determination, which was, however, necessary here. 3.3. Spatial extent of mining-related increases in radon concentrations We have studied the spatial extent of mining-related increases in outdoor radon levels by comparing the percentage of measurement points with mean radon net concentrations greater than the decision threshold (chosen from the data in Table 2) for different distances to the nearest mining residues. Data from all distances (<500 m up to >2000 m) were available for seven measurement areas only, resulting in a total of about 200 measurement points taken into account in the analysis. About 13 percent of all measurement points showed mean radon concentrations exceeding the decision threshold. Fig. 6 shows the results as a function of distance d, classified in d 500 m, 500 m < d 1000 m, 1000 m < d 1500 m, 1500 m < d 2000 m and d > 2000 m. The diagram shows the decrease of the mining influence on the outdoor radon concentration with increasing distance to the mining residues. It
3.4.1. Reclamation of a waste rock dump The following example illustrates the influence of a waste rock dump on the adjacent outdoor radon situation and the effect of the reclamation on outdoor radon concentrations. The measurements in the summer and winter months (AprileSeptember and Octobere March, respectively) indicate the seasonal influence on the radon exhalation rate caused by convective processes. The studied waste rock dump “Zentralschachthalde” is located in the city of Johanngeorgenstadt, a town in the mountainous region Erzgebirge (measurement area no. 9, see Table 1 and Fig. 1). It is one of many dumps in this area raised as a result of rigorous uranium mining from the end of the 1940s throughout the end of the 1950s. The volume of the deposited material amounts to about 8.25 105 m3, the whole surface is about 1.5 105 m2 and the average specific 226Ra activity of the material is 1.7 Bq g1. The radon release from the dump is characterized by diurnal as well as annual variations due to convective processes triggered by the temperature difference between the interior and the exterior of the dump (Dushe et al., 2003). As a result of these processes enlarged radon exhalation may occur in the summertime at the toe of the waste rock dump. This phenomenon can induce high radon exposure of people vicinity of the dump. In order to reduce the radon release, a new remediation concept of partial covering at the toe of the dump of about 2 104 m2 was applied. Fig. 7 shows the outdoor radon levels before and after the remediation (2004e2005) at three different distances to the toe of the dump. Close by the dump, at a distance of 20 m, expectedly the
Table 2 Measurement area-specific median of natural background, decision threshold with 95% confidence interval (CI) and detection limit. Measurement area
Background radon concentration (median) [Bq m3]
Decision threshold (95% CI in brackets) [Bq m3]
Detection limit [Bq m3]
1 2 3 4 5 6 8 9 10 11aa 11ba 12 13 14 15 þ 16b 17 18 All areas
13 12 13 13 12 13 29 13 12 21 14 12 13 13 15 19 18 14
7.3 (5e9) 4.9 (3e6) 5.8 (4e7) 12 (7e16) 6.1 (4e7) 6.4 (3e9) 12 (7e15) 7.6 (5e9) 8.2 (6e10) 16 (13e18) 9.6 (6e12) 6.7 (4e8) 7.7 (5e9) 9.4 (5e13) 7.1 (5e9) 6.2 (2e9) 9.5 (6e12) 10 (9e11)
9.9 7.4 8.3 15 8.7 9.0 14 10 11 19 12 9.3 10 12 9.6 8.8 12 13
a The measurement area 11 is characterized by two main geological formations with different 226Ra content (granite, phyllite) and was therefore divided into two subregions 11a (granite) and 11b (phyllite). b Data of the measurement areas 15 and 16 (with similar geological formation) were pooled because there were too few background measurement points in either area to allow for a statistical analysis.
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natural background. Fig. 7 also shows the decreasing concentrations with increasing distance from the dump.
Fig. 6. Percentage of measurement points with mean radon net concentration exceeding the decision threshold (DT) for five classes of distance d between the measurement point and the closest mining residue.
20 m 50 m 150 m
4. Summary
-3
Radon concentration [Bq m ]
300
3.4.2. Decommission of a discharge shaft Fig. 8 shows the time series of radon discharges from a shaft (AW 389) in measurement area 15, decommissioned February 1997, and the outdoor radon concentrations in the lee of the main wind direction at different distances. It is evident that the outdoor radon concentrations decrease with the decreasing radon effluents as well as increasing distances from the shaft. At 600 m distance, the concentrations decrease to background level. However, at 190 m distance the concentrations remain slightly increased even after the complete decommission of the shaft. An influence of other radon emitters cannot be ruled out, but is less than the decision threshold for that measurement area of 7 Bq m3. Thus, the concentrations observed at 190 m distance after the closure of shaft AW 389 are considered part of the natural variation of background concentrations.
250 200 150 100 50
summer 2009
winter 2009/10
summer 2008
winter 2008/09
summer 2007
winter 2007/08
summer 2006
winter 2006/07
summer 2005
winter 2005/06
summer 2004
winter 2004/05
summer 2003
winter 2003/04
winter 2002/03
summer 2002
winter 2001/02
0
Exposure time
Fig. 7. Outdoor radon concentrations at 1.5 m above-ground at different distances (in the same direction) to the toe of the waste rock dump “Zentralschachthalde” in Johanngeorgenstadt (Saxony, Germany). The summer period includes the months from April to September and the winter period from October to March.
highest values were measured, ranging up to more than 200 Bq m3, with the typical seasonal variations caused by convective processes. After reclamation, the radon concentration at this distance was reduced down to values in the range of the 600 m
background
70
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40
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30
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-3
190 m
Radon concentration [Bq m ]
Discharge of radon [TBq]
discharge
0 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Year
Fig. 8. Time series of radon discharge of an air shaft in measurement area 15 and the outdoor radon concentration at two distances, i.e. 190 m and 600 m in the lee of the main wind direction and the natural background. The air shaft was decommissioned finally in February 1997.
To investigate the impact of mining and milling activities on the natural radon level, networks of radon measurement points with nuclear track detectors were run in 16 former mining areas, together with 2 networks in regions not influenced by mining for comparison purposes. Altogether, data has been taken at 485 measurement points. The annual radon concentrations show a great variability, with a minimum measured concentration of 5 Bq m3 and a maximum of 1700 Bq m3. The latter value was measured close to a waste rock pile with dominating convective transport. However, the former mining and milling activities did not result in large-area impacts on the outdoor radon level. At distances of more than 1000 m, a mining-related contribution was very rarely detected, depending, of course, on the strength of the radon emitter. The radon-reducing effect of different reclamation measures could clearly be demonstrated. A procedure to establish background radon levels for the former mining areas resulted in a median of 13 Bq m3. This value is compatible with results of outdoor radon measurements of natural background in this part of Germany (see part 1). References BfS, 2011. (Bundesamt für Strahlenschutz); Federal Office for Radiation Protection. In: Calculation Guide for the Determination of Radiation Exposure Due to Environmental Radioactivity Resulting from Mining. http://nbn-resolving.de/ urn:nbn:de:0221-201109056212. Bollhöfer, A., Storm, J., Martin, P., Tims, S., 2006. Geographic variability in radon exhalation at a rehabilitated uranium mine in the Northern Territory, Australia. Environ. Monit. Assess. 114 (1e3), 313e330. Currie, L.A., 1999. International recommendations offered on analytical detection and quantification concepts and nomenclature. Anal. Chim. Acta 391, 103e134. Dushe, C., Kümmel, M., Schulz, H., May 2003. Investigations of enhanced outdoor radon concentration in Johanngeorgenstadt (Saxony). Health Phys. 84 (5), 655e 663. Ettenhuber, E., 2002. Investigations and radiological assesment of mining residues in Germany. In: Proceedings of the 5th International Conference on High Levels of Natural Radiation and Radon Areas. September 4e7, 2000. International Congress Series No. 1225. ELSEVIER SCIENCE B.V, Munich, Germany. Gehrcke, K., Kümmel, M., Dushe, C., 15e20 June 2008. Background radon levels in Germany and how to take them into account in radiological evaluations. In: Proc. International Conference on Radioecology & Environmental Protection, pp. 92e95. Bergen, Norway, Part 1. IAEA (International Atomic Energy Agency), 1992. Measurement and Calculation of Radon Releases from Uranium Mill Tailings. Technical Reports Series No. 333. ICRP, 1993. International Commission on radiological Protection: Protection against Radon-222 at Home and at Work, ICRP Publication 65. Ann. ICRP 23 (2). ICRP, 18.7 .2010. International Commission on Radiological Protection, Statement on Radon. ICRP Ref 00/902/09. www.icrp.org/docs/ICRP_Statement_on_Radon% 28November_2009%29.pdf.
M. Kümmel et al. / Journal of Environmental Radioactivity 132 (2014) 131e137 Krizman, M.J., Peter, J.E., Rojc, J., 2009. Study on radon dispersion modes from the U mine disposal sites at Zirovski Vrh (Slovenia). Radioprotection 44 (5), 469e474. Kümmel, M., Dushe, C., Müller, S., Gehrcke, K., 2014. Outdoor 222Rn e concentrations in Germany e Part 1-Natural background. J. Environ. Radioact. submitted for publication to.
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Rutherford, P.M., Dudas, M.J., Samek, R.A., 1994. Environmental impacts of phosphogypsum. Sci. Total Environ. 149, 1e38. Samuelsson, C., 2011. Exerpts from the history of alpha recoil. J. Environ. Radioact. 102, 531e533.
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Outdoor areas
222
Rn-concentrations in Germany e part 2 e former mining
M. Kümmel*, C. Dushe, S. Müller, K. Gehrcke Bundesamt für Strahlenschutz, 38226 Salzgitter, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2013 Received in revised form 17 January 2014 Accepted 19 January 2014 Available online 6 February 2014
In the German Federal States of Saxony, Saxony-Anhalt and Thuringia, centuries of mining and milling activities resulted in numerous residues with increased levels of natural radioactivity such as waste rock dumps and tailings ponds. These may have altered potential radiation exposures of the population significantly. Especially waste rock dumps from old mining activities as well as 20th century uranium mining may, due to their radon (222Rn) exhalation capacity, lead to significant radiation exposures. They often lie close to or within residential areas. In order to study the impact on the natural radon level, the Federal Office for Radiation Protection (BfS) has run networks of radon measurement points in 16 former mining areas, together with 2 networks in regions not influenced by mining for comparison purposes. Representative overviews of the long-term outdoor radon concentrations could be established including estimates of regional background concentrations. Former mining and milling activities did not result in large-area impacts on the outdoor radon level. However, significantly increased radon concentrations were observed in close vicinity of shafts and large waste rock dumps. They are partly located in residential areas and need to be considered under radiation protection aspects. Examples are given that illustrate the consequences of the Wismut Ltd. Company’s reclamation activities on the radon situation. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Radon Outdoor Mining Reclamation Detection threshold Detection limit
1. Introduction The results of a nation-wide study of outdoor radon (222Rn) concentrations in Germany are described in (Kümmel et al., 2013, hereinafter referred to as ‘Part 1’). In this three-year-measuring program radon concentrations were measured at 173 measuring points in an even grid with a grid length of approx. 50 km with solid-state track etch detectors. The explicit purpose of that study was to map natural radon levels. Hence, care had been taken to avoid setting up measurement points in the immediate vicinity of potential anthropogenic sources of radon in the atmosphere. Significant anthropogenic radon sources are, for example, tailings ponds and waste rock dumps from uranium and other ore mining and milling (e.g. Ettenhuber, 2002; Bollhöfer et al., 2006; Krizman et al., 2009), or heaps of phosphogypsum, a by-product from the phosphate industry (e.g. Rutherford et al., 1994). In Germany, facilities and residues from centuries of mining and milling activities in the three eastern Federal States of Saxony,
DOI of original article: http://dx.doi.org/10.1016/j.jenvrad.2014.01.012. * Corresponding author. Tel.: þ49 3018 3334244. E-mail address: [email protected] (M. Kümmel). 0265-931X/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2014.01.011
Thuringia and Saxony-Anhalt, and specifically the 20th century uranium ore mining activities in Saxony and Thuringia are of special importance in this respect (Ettenhuber, 2002). Among others, large waste rock dumps play a significant role. With volumes of up to about 60 Mio m3, they may show average exhalation rates exceeding those known from the literature on tailings ponds (IAEA, 1992) (w1 Bq m2 s1/(Bq g1) of 226Ra) by an order of magnitude. Such high exhalation rates were found to be the result of convective transport processes which may prevail over diffusion in case of depositions of large volumes of coarse grained waste rock material (Dushe et al., 2003). In the fine grained materials of tailings ponds or phosphogypsum depositions diffusive transport dominates the exhalation. In contrast to many other countries, residues from uranium mining and milling in Eastern Germany are often situated close to, and partly directly within, residential areas. Under such conditions the inhalation of radon and its decay products may be a significant, sometimes even the dominating exposure pathway for the population (Ettenhuber, 2002). Complementary to the outdoor radon measurements of the natural background in Germany (Part 1), the study presented in this paper aimed at providing representative data on the height and range of the mining-related increase in outdoor radon
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Fig. 1. Overview of measurement areas with outdoor radon networks (area (17) is composed of 4 subareas).
concentrations in 16 former mining areas of the aforementioned three Federal States. A network of 485 radon measurement points was run over time periods partly exceeding 10 years in these mining areas and in two non-mining areas (for comparison). Thereby, we focused not only on the absolute height of concentrations but primarily on the mining-related contribution, seeing that only anthropogenic increments are of relevance from a regulatory point of view. Since there is no practical way to distinguish between anthropogenic and natural contributions to the measurement effect, retrospective background values needed to be established. Ideally, background levels should have been determined before the mining started. This was, however, neither possible nor even considered at those times. Retrospective estimations using “glass” detectors (e.g. Samuelsson, 2011) are also not possible because the necessary old
window glasses are not available. Therefore, a method was developed to estimate background values in the old mining areas. These values may be considered complementary to those given in Part 1. 2. Materials and methods 2.1. The radon measurement system Radon measurements in mining areas were carried out using the passive measurement system described in Part 1, based on nuclear track detectors made of MakrofolÒ, mounted in a diffusion chamber. The decision threshold and detection limit of the measurement system are 20 kBq h m3 and 40 kBq h m3, respectively. These figures correspond to concentrations of 5 Bq m3 and 10 Bq m3. The values are somewhat higher than those given in Part 1 (3 Bq m3 and
Table 1 Measurement areas with outdoor radon networks, the corresponding past mining activities and main rocks. No.
Measurement area
Mining activities
Main rock
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Annaberg-Buchholz Aue Crossen Dittrichshütte Freiberg Freital Gittersee Gottesberg Johanngeorgenstadt Königstein Lengenfeld Mansfeld Marienberg Mechelgrün Ronneburg Seelingstädt Görlitza Zwickaua
Mining (U; Ag, Co, Cu, Fe, Ni, Pb, Sn) Mining (U; Ag, Bi, Co, Cu, Fe, Ni, Sn) Ore (U) processing incl. storage of residues Mining (U; Fe) Mining (Ag, Pb, S, Zn) Mining (bituminous coal) Ore (U) mining & processing incl. storage of residues Mining (U; Ag, As, Bi, Co, Cu, Fe, Ni, Sn) Mining (U; Ag, Bi, Co, Cu, Fe, Sn) Mining (U) Ore (U) processing incl. storage of residues, mining (wolframite) Mining (Cu) & smelting Mining (U; Ag) Mining (U) Mining (U) Ore (U) processing incl. storage of residues
Slate, gneiss Granite, phyllite New red, sandstone Clay slate Gneiss Gneiss Gneiss Granite Granite, phyllite Sandstone Granite, phyllite, clay slate Sandstone Gneiss Granite (contact area) New red, sandstone, clay slate New red, sandstone, clay slate Granite Granite, phyllite, clay slate
a
Unaffected area (for comparison).
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5 Bq m3) due to the lower exposure time of 6 months (12 months in Part 1) scheduled in order to detect possible differences between summer and winter seasons. The relative measurement uncertainty at an exposure level of 100 kBq h m3, corresponding to 20 Bq m3 at a 6 months’ exposure time, is about 20%. Measurements were carried out at a height of 1.5 m above ground. 2.2. Measurement areas The measurement areas were defined based on the results of previous measurements and various types of documentation, including historical information on past mining activities. The studies were primarily aimed at determining radiologically relevant alterations of the radon levels caused by mining. Thus, we focused on residential areas. Consequently, the measurement points are not uniformly distributed in the measurement areas. The density of measurement points is normally highest close to mining residues or facilities. Altogether we defined radon networks in 16 mining areas and 2 networks in regions not influenced by mining for comparison purposes. The mining or measurement areas differ in terms of (i) the mining activities performed, (ii) geology and (iii) orography. Fig. 1 shows the position of the measurement areas in the respective Federal States. Table 1 provides information on the respective mining activities in the past and on the main geology. Up to 1990 uranium ore mining or milling were carried out in the measurement areas (2), (3), (6), (7), (10), (15) and (16), followed by extensive remediation activities by the Wismut GmbH in these areas. Areas (3), (7), (12), (15) and (16) are mainly characterized by flat terrain, in the other areas mountainous terrain prevails. In total, data were taken at 595 measurement points. Out of these 485 were installed in residential areas, the remaining 110 near mining facilities. 2.3. Reconstruction of background values To determine the local radon background concentrations we had to find a compromise between the somewhat contradicting criteria that measurement points should be located (i) close enough to the areas of interest to be comparable with respect to geology, orography and meteorology, but (ii) sufficiently distant from the major mining-related radon emitters so as to avoid significant contributions to the radon concentrations. In order to quantify the latter requirement we defined two criteria. First, only measurement points with an estimated maximum mining-related influence of less than 3 Bq m3 were considered. This figure corresponds to the measurement uncertainty at background levels (Part 1). For the estimates we used the 226 Ra inventories of the relevant mining residues gathered within the framework of a large federal project in the 1990-ies (Ettenhuber, 2002). Simplified models of both the radon exhalation rate and atmospheric transport (BfS, 2011) were then used for upper estimates of the radon concentrations. Second, a minimum distance of the measurement points from the nearest radon emitter of 500 m was defined to confine possible uncertainties of the calculated radon concentration, resulting from the simplifying assumption of a circular base area of all sources. Using this approach, between 3 and 18 measurement points were determined in the different mining areas to represent the respective natural background concentrations.
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latter from the background concentrations (Gehrcke et al., 2008). For that purpose, the well known concept of decision thresholds and detection limits (see, e.g. Currie, 1999) was applied. 2.4.1. Decision thresholds The decision threshold allows a decision as to whether or not the physical effect quantified by the measurand is present. Here, the measurand of interest is the increment C of the radon concentration by anthropogenic sources. For the purpose of this study, C was defined as C ¼ Cgross Cbackground, the difference of the gross measurement effect of an individual measurement (Cgross) minus a predefined background concentration (Cbackground) assumed to be representative of the measurement area in question. The decision threshold (DT) is an upper percentile of the expected distribution of C values when actually no anthropogenic contribution is present. If C is less than the decision threshold DT, it is concluded that there is no anthropogenic contribution. The probability a of making a false conclusion about the null hypothesis H0: C DT was chosen to be 10%, which means that DT equals the 90th percentile of the distribution of C without mining influence. As the distribution of background values in the respective measurement areas turned out to be essentially log-normal (see Section 3.2), the question arose which distribution parameter is considered representative of the background. We decided for the median. This way, the true value of the anthropogenic radon concentration has equal chances to be over- and underestimated in individual measurements. Note that the decision threshold as used here is determined by three components, namely the spatial variability of radon concentrations within the respective mining areas, the variation of the concentration in time, and the statistical uncertainty of the measurement. 2.4.2. Detection limits The detection limit indicates which smallest true value of anthropogenic radon concentration can still be detected with the measurement procedure applied. It is a fictitious concentration that, if the measurement system is exposed to it, leads to a net measurement result greater than the detection limit with predefined probability b. b represents the probability of falsely concluding that there was only natural background concentration whilst there actually was an anthropogenic contribution. A value of 10% was chosen. As the detection limit refers to a single future measurement at an arbitrary but fixed point in the measurement area and at a fixed time interval, only the measurement uncertainty has to be taken into account. It is known from laboratory calibration experiments
2.4. Characteristic limits for anthropogenic contributions As the expected background values are in the same order of magnitude as at least part of the mining-caused radon concentrations, statistically based criteria are needed to discriminate the
Fig. 2. Illustration of the decision threshold, detection limit and error probabilities. C denotes the net radon concentration, Ca the anthropogenic contribution to the total radon concentration, DT the decision threshold and DL the detection limit.
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to be in the range of 4 Bq m3 in the relevant concentration range. The respective distribution is assumed to be Gaussian. Fig. 2 illustrates the described relations.
3.1. Radon concentrations in residential areas The 3904 annual (i.e. average of summer and winter values) radon concentrations in residential areas show a great variability with a minimum concentration of 5 Bq m3 and a maximum of 1700 Bq m3. Fig. 3 gives an overview of measured concentrations in residential areas, except for the reference areas (confined to concentration up to 300 Bq m3 for practical reasons). The frequency distribution reflects measurement results, which originate from different measurement areas with different measurement densities, fractions of mining-influenced measurement points and observation periods. It is positively skewed, but it is not well approximated by any of the respective standard probability density functions, such as the lognormal. The histogram shows a distinct maximum in the class 10e15 Bq m3 that corresponds to the average background concentrations (see Section 3.2). Many values above 50 Bq m3 represents a variety of measurement points that are significantly influenced by anthropogenic radon emitters. A value of approximately 50 Bq m3 may be considered an upper bound of outdoor radon concentrations in Germany at locations not influenced by radon from human activities (Part 1). A summarizing overview of the radon situation in the different measurement areas is given in Fig 4. From Fig. 4 it can be seen that values exceeding 50 Bq m3 were observed in six out of 18 measurement areas only. About 90% of all values are less than 50 Bq m3. Values above this level originate in all likelihood from locations that are influenced by anthropogenic radon emissions. Indeed, all measurement points with radon concentrations exceeding 50 Bq m3 were located in close vicinity to large mining-related radon emitters, the majority of them being large waste rock dumps. Others were located close to discharge air shafts (see examples in Section 3.4). The highest measured radon concentration (area number 9) of 1700 Bq m3 was caused by exhalation from a waste rock pile with dominating convective transport (Dushe et al., 2003). In most measurement areas the median values are in the range of 11 Bq m3e16 Bq m3, which is only slightly higher than the
-3
70 60
800
50
600
40 30
400
20
200
10
0
-3
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> 50 Bq m
90
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100
1200
≤ 50 Bq m
Absolute frequency for concentrations
1400
0
5
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70 100 130 160 190 220 250 280
> 300
Radon concentration [Bq m-3]
Fig. 3. Histogram of 3904 annual mean radon concentrations measured between 1990 and 2005 at 469 stations in residential areas in mining regions of Saxony, Thuringia and Saxony-Anhalt. Note that the horizontal scale is subdivided into two ranges with class widths of 5 Bq m3 (up to 50 Bq m3) and 10 Bq m3 (>50 Bq m3).
Radon concentration [Bq m-3 ]
3. Results and discussions
10000
1000
100
10
1 1
2
3
4
5
6 7 8 9 10 11 12 13 14 15 16 17 18 Measurement area number
Fig. 4. Box plot of outdoor radon concentrations in all measurement areas (comp. Table 1). The ends of the whiskers represent the minimum and maximum values. The rectangle denotes the interquartile range and the horizontal line in the rectangle the median.
observed median background values for Saxony, Thuringia and Saxony-Anhalt of about 10 Bq m3 (Part 1). In the areas without mining activities (17, 18) the median values are relatively high compared to most measurement areas with mining activities (20 Bq m3 and 17 Bq m3), see Fig. 4. Since the areas (17) and (18) are mainly characterized by granite bedrock (comp. Table 1), which is known to show increased 226Ra activity concentrations, this result is not surprising. Median values of around 20 Bq m3 were also observed in most of mining areas with granite in the geological set-up. As expected, the range of measured concentrations in the areas (17) and (18) is smaller than in most mining areas, due to the absence of mining influence. 3.2. Background values & characteristic limits 3.2.1. Background values Fig. 5 shows the frequency distribution of annual background radon concentrations (as defined in Section 2.3) from all measurement areas. The values from the areas (17) and (18), being background values per se, are also included. This distribution and those for all individual measurement areas, fit to three-parameter log-normal distributions (KolmogoroveSmirnov-test, p values > 0.15). The distribution parameters were estimated according to part 1, eqns. (3)e(6). The median of 14 Bq m3 is comparable with background values established for these regions according to Part 1, but is slightly higher than the median values for Saxony and Thuringia as a whole (10 Bq m3). Table 2, column 2, shows the results of the estimated median background concentrations for the individual measurement areas. For the majority (13 out of 16) of areas, the concentrations lie in a relatively narrow range of 12 Bq m3 up to 15 Bq m3. Higher means were observed for both unaffected areas (18 Bq m3 and 19 Bq m3), as well as for areas 11a and 8, the latter with an exceptionally high value of 29 Bq m3. Areas 11a and 8 are characterized by prevailing granite bedrocks with increased 226Ra content. We did not find significant differences of the means and standard deviations between measurement areas with flat terrain (3, 12) and mountainous terrain (1, 2, 13) having similar 226Ra contents in bedrocks. It would appear plausible to assume lesser mixing with low e 222Rn air masses from the sea in mountainous areas,
M. Kümmel et al. / Journal of Environmental Radioactivity 132 (2014) 131e137 100%
250
90% mean = 15 Bq m
Absolute frequency
200
80%
standard deviation = 7.5 Bq m median = 14 Bq m geometric standard deviation = 1.8
documents that mining activities and residues did not have large area impacts on the outdoor radon level. 3.4. Effects of reclamation
70%
shift parameter = 3.3 Bq m
150
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100
Since 1991 reclamations of uranium mining sites are in progress. Many mining relics were, among others, cleaned up because of increased radon releases.
40% 30%
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36
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52
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60
Radon concentration [Bq m ]
Fig. 5. Histogram of background radon concentration of all measurement areas (the solid line indicates the cumulative percentage distribution).
especially in valleys. However, our measurements did not support this assumption. 3.2.2. Characteristic limits Table 2, columns 3 and 4 show the decision thresholds and detection limits for anthropogenic contributions to the measurement effect. The 95%-confidence interval of the decision threshold was determined by a bootstrap procedure. Detection limits between 7.4 Bq m3 and up to 19 Bq m3 were determined. This implies that, depending on the measurement area, anthropogenic contributions to atmospheric radon concentrations of up to almost 20 Bq m3 cannot be detected with sufficient statistical certainty using the methodology described here. According to ICRP 65 recommendations (ICRP, 1993) such radon concentrations may result in radiation exposures of up to 0.4 mSv effective dose per year, assuming an exposure time of 7000 h a1 (indoors) and 1760 h a1 (outdoors) and an equilibrium factor of 0.4 (indoors) and 0.6 (outdoors). After (ICRP, 2010), the newly introduced dose coefficient for radon inhalation will increase the corresponding dose to about 0.8 mSv a1, a value that cannot be neglected from the radiation protection point of view. This result underlines the deficiencies of the retrospective approach to the background determination, which was, however, necessary here. 3.3. Spatial extent of mining-related increases in radon concentrations We have studied the spatial extent of mining-related increases in outdoor radon levels by comparing the percentage of measurement points with mean radon net concentrations greater than the decision threshold (chosen from the data in Table 2) for different distances to the nearest mining residues. Data from all distances (<500 m up to >2000 m) were available for seven measurement areas only, resulting in a total of about 200 measurement points taken into account in the analysis. About 13 percent of all measurement points showed mean radon concentrations exceeding the decision threshold. Fig. 6 shows the results as a function of distance d, classified in d 500 m, 500 m < d 1000 m, 1000 m < d 1500 m, 1500 m < d 2000 m and d > 2000 m. The diagram shows the decrease of the mining influence on the outdoor radon concentration with increasing distance to the mining residues. It
3.4.1. Reclamation of a waste rock dump The following example illustrates the influence of a waste rock dump on the adjacent outdoor radon situation and the effect of the reclamation on outdoor radon concentrations. The measurements in the summer and winter months (AprileSeptember and Octobere March, respectively) indicate the seasonal influence on the radon exhalation rate caused by convective processes. The studied waste rock dump “Zentralschachthalde” is located in the city of Johanngeorgenstadt, a town in the mountainous region Erzgebirge (measurement area no. 9, see Table 1 and Fig. 1). It is one of many dumps in this area raised as a result of rigorous uranium mining from the end of the 1940s throughout the end of the 1950s. The volume of the deposited material amounts to about 8.25 105 m3, the whole surface is about 1.5 105 m2 and the average specific 226Ra activity of the material is 1.7 Bq g1. The radon release from the dump is characterized by diurnal as well as annual variations due to convective processes triggered by the temperature difference between the interior and the exterior of the dump (Dushe et al., 2003). As a result of these processes enlarged radon exhalation may occur in the summertime at the toe of the waste rock dump. This phenomenon can induce high radon exposure of people vicinity of the dump. In order to reduce the radon release, a new remediation concept of partial covering at the toe of the dump of about 2 104 m2 was applied. Fig. 7 shows the outdoor radon levels before and after the remediation (2004e2005) at three different distances to the toe of the dump. Close by the dump, at a distance of 20 m, expectedly the
Table 2 Measurement area-specific median of natural background, decision threshold with 95% confidence interval (CI) and detection limit. Measurement area
Background radon concentration (median) [Bq m3]
Decision threshold (95% CI in brackets) [Bq m3]
Detection limit [Bq m3]
1 2 3 4 5 6 8 9 10 11aa 11ba 12 13 14 15 þ 16b 17 18 All areas
13 12 13 13 12 13 29 13 12 21 14 12 13 13 15 19 18 14
7.3 (5e9) 4.9 (3e6) 5.8 (4e7) 12 (7e16) 6.1 (4e7) 6.4 (3e9) 12 (7e15) 7.6 (5e9) 8.2 (6e10) 16 (13e18) 9.6 (6e12) 6.7 (4e8) 7.7 (5e9) 9.4 (5e13) 7.1 (5e9) 6.2 (2e9) 9.5 (6e12) 10 (9e11)
9.9 7.4 8.3 15 8.7 9.0 14 10 11 19 12 9.3 10 12 9.6 8.8 12 13
a The measurement area 11 is characterized by two main geological formations with different 226Ra content (granite, phyllite) and was therefore divided into two subregions 11a (granite) and 11b (phyllite). b Data of the measurement areas 15 and 16 (with similar geological formation) were pooled because there were too few background measurement points in either area to allow for a statistical analysis.
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natural background. Fig. 7 also shows the decreasing concentrations with increasing distance from the dump.
Fig. 6. Percentage of measurement points with mean radon net concentration exceeding the decision threshold (DT) for five classes of distance d between the measurement point and the closest mining residue.
20 m 50 m 150 m
4. Summary
-3
Radon concentration [Bq m ]
300
3.4.2. Decommission of a discharge shaft Fig. 8 shows the time series of radon discharges from a shaft (AW 389) in measurement area 15, decommissioned February 1997, and the outdoor radon concentrations in the lee of the main wind direction at different distances. It is evident that the outdoor radon concentrations decrease with the decreasing radon effluents as well as increasing distances from the shaft. At 600 m distance, the concentrations decrease to background level. However, at 190 m distance the concentrations remain slightly increased even after the complete decommission of the shaft. An influence of other radon emitters cannot be ruled out, but is less than the decision threshold for that measurement area of 7 Bq m3. Thus, the concentrations observed at 190 m distance after the closure of shaft AW 389 are considered part of the natural variation of background concentrations.
250 200 150 100 50
summer 2009
winter 2009/10
summer 2008
winter 2008/09
summer 2007
winter 2007/08
summer 2006
winter 2006/07
summer 2005
winter 2005/06
summer 2004
winter 2004/05
summer 2003
winter 2003/04
winter 2002/03
summer 2002
winter 2001/02
0
Exposure time
Fig. 7. Outdoor radon concentrations at 1.5 m above-ground at different distances (in the same direction) to the toe of the waste rock dump “Zentralschachthalde” in Johanngeorgenstadt (Saxony, Germany). The summer period includes the months from April to September and the winter period from October to March.
highest values were measured, ranging up to more than 200 Bq m3, with the typical seasonal variations caused by convective processes. After reclamation, the radon concentration at this distance was reduced down to values in the range of the 600 m
background
70
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40
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-3
190 m
Radon concentration [Bq m ]
Discharge of radon [TBq]
discharge
0 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 Year
Fig. 8. Time series of radon discharge of an air shaft in measurement area 15 and the outdoor radon concentration at two distances, i.e. 190 m and 600 m in the lee of the main wind direction and the natural background. The air shaft was decommissioned finally in February 1997.
To investigate the impact of mining and milling activities on the natural radon level, networks of radon measurement points with nuclear track detectors were run in 16 former mining areas, together with 2 networks in regions not influenced by mining for comparison purposes. Altogether, data has been taken at 485 measurement points. The annual radon concentrations show a great variability, with a minimum measured concentration of 5 Bq m3 and a maximum of 1700 Bq m3. The latter value was measured close to a waste rock pile with dominating convective transport. However, the former mining and milling activities did not result in large-area impacts on the outdoor radon level. At distances of more than 1000 m, a mining-related contribution was very rarely detected, depending, of course, on the strength of the radon emitter. The radon-reducing effect of different reclamation measures could clearly be demonstrated. A procedure to establish background radon levels for the former mining areas resulted in a median of 13 Bq m3. This value is compatible with results of outdoor radon measurements of natural background in this part of Germany (see part 1). References BfS, 2011. (Bundesamt für Strahlenschutz); Federal Office for Radiation Protection. In: Calculation Guide for the Determination of Radiation Exposure Due to Environmental Radioactivity Resulting from Mining. http://nbn-resolving.de/ urn:nbn:de:0221-201109056212. Bollhöfer, A., Storm, J., Martin, P., Tims, S., 2006. Geographic variability in radon exhalation at a rehabilitated uranium mine in the Northern Territory, Australia. Environ. Monit. Assess. 114 (1e3), 313e330. Currie, L.A., 1999. International recommendations offered on analytical detection and quantification concepts and nomenclature. Anal. Chim. Acta 391, 103e134. Dushe, C., Kümmel, M., Schulz, H., May 2003. Investigations of enhanced outdoor radon concentration in Johanngeorgenstadt (Saxony). Health Phys. 84 (5), 655e 663. Ettenhuber, E., 2002. Investigations and radiological assesment of mining residues in Germany. In: Proceedings of the 5th International Conference on High Levels of Natural Radiation and Radon Areas. September 4e7, 2000. International Congress Series No. 1225. ELSEVIER SCIENCE B.V, Munich, Germany. Gehrcke, K., Kümmel, M., Dushe, C., 15e20 June 2008. Background radon levels in Germany and how to take them into account in radiological evaluations. In: Proc. International Conference on Radioecology & Environmental Protection, pp. 92e95. Bergen, Norway, Part 1. IAEA (International Atomic Energy Agency), 1992. Measurement and Calculation of Radon Releases from Uranium Mill Tailings. Technical Reports Series No. 333. ICRP, 1993. International Commission on radiological Protection: Protection against Radon-222 at Home and at Work, ICRP Publication 65. Ann. ICRP 23 (2). ICRP, 18.7 .2010. International Commission on Radiological Protection, Statement on Radon. ICRP Ref 00/902/09. www.icrp.org/docs/ICRP_Statement_on_Radon% 28November_2009%29.pdf.
M. Kümmel et al. / Journal of Environmental Radioactivity 132 (2014) 131e137 Krizman, M.J., Peter, J.E., Rojc, J., 2009. Study on radon dispersion modes from the U mine disposal sites at Zirovski Vrh (Slovenia). Radioprotection 44 (5), 469e474. Kümmel, M., Dushe, C., Müller, S., Gehrcke, K., 2014. Outdoor 222Rn e concentrations in Germany e Part 1-Natural background. J. Environ. Radioact. submitted for publication to.
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