Radon concentration values in the field: Correlation with underlying geology

Radiation Measurements PERGAMON

Radiation Measurements 31 (1999) 271-276

RADON CONCENTRATION VALUES IN THE FIELD CORRELATION WITH UNDERLYING GEOLOGY

:

S.A. DURRANI School of Physics and Astronomy, University of Birmingham, Birmingham B 15 2TT, UK ABSTRACT In epidemiological and ecological studies of radon as a health hazard m a given area or region, it is becoming widely reeo~ized that it is necessary to establish the significance of correlations, if any, between the incidence of diseases (e.g. lung cancer or leukaemias) and local radon concentration values. Measurements of soil-radon concentration levels in the field, especially under geostatistieally controlled conditions, have underscored the highly erratic nature of radon emission on a scale often of only a few metres. It would appear that, while underlying geology does determine the longer scale of variation in soil-radon, many localized parameters, e.g. fissures, moisture content, thickness of overburden, etc., make it imperative that detailed in-situ measurements of radon emission are made before a reliable correlation can be established between, say, leukaemia clusters and radon concentration levels. A broad survey of measurement methods and reported results, examined in the above context, is presented and conclusions drawn in this paper.

KEYWORDS Geostatistics; radon concentration values; variance; unbalanced nested sampling; sampling interval; lithology; transect; correlation; radon affected areas; health hazards.

INTRODUCTION Much work has been done over the years on correlations of health hazards (especially, lung cancer) with Rn levels in mines and in dwellings. But more statistical studies need to be conducted in the field to test how closely correlations can be established between the levels of Rn prevailing at a specific location and the general levels expected in the area on geological grounds (Tanner, 1994). For instance, the question arises as to whether the description "Radon-affected area" (in the terminology, say, of the UK National Radiological Protection Board : NRPB) is sufficiently fine-tuned to indicate what Rn level may have been experienced by a person dwelling at a specific location in that area. This is a valid question, since the actual radon exhalation values depend on a number of factors such as soil composition and grain size, fissures, water-content of the soil, atmospheric conditions, underground carrier gas, etc. - in addition to rock types/lithologies and discontinuities/singularities within a few metres of the surface. To test such correlations, we have carded out a number of field studies over the last five or six years, m which statistically well-designed experiments have been performed in areas of known lithology both single and multiple. The data have been analysed by using geostatistical techniques. The investigations have been conducted by multidisciplinary teams comprising geologists, geostatisticians, epidemiologists and physicists. This paper summarizes our findings and highlights the conclusions drawn from these investigations. 1350-4487/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S 1350-4487(99)00105-5

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GEOSTATISTICAL PROCEDURES

The basic analytical tool used by us in our investigations is geostatistics (for a full description, see our earlier papers : Badr etal., 1993, 1996; Durrani and Badr, 1995; Durrani etal., 1997; and also Webster and Oliver, 1990). While, in geostatistical analysis, one can use any variable of relevance, e.g. soil composition or grain size, the pH value, etc., the variable chosen by us has been the 'lag' - or the separation distance, h, between points of observation : viz., the measurement of Rn levels. This process allows one to make a rough estimation of the spatial scale of variation of Rn concentrations in the soil. The variation is expressed in terms of the variance (i.e. the square of the standard deviation from the mean value), and plotted as a function of h. The fitted curve is termed the 'variogram'. Where there is a steep rise in the value of the variance, that underscores the critical value of h; i.e. if the separation distance between neighbouring measurement points exceeds that value of h, a significant change takes place in the measured value (viz., the Rn level in our case). In determining the measurement positions, we have primarily used the 'nested sampling' technique, in which the 'population' (i.e. the number of positions or samples) is progressively subdivided into increasingly finer 'subclasses' at each subsequent 'stage' - viz., for each decreasing separation distance h. For the purposes of economy of effort, without sacrificing precision, we have employed the 'unbalanced nested sampling' approach - which is illustrated in the section entitled Sample Configuration MEASUREMENT OF RADON IN SOIL GAS

Radon concentration values were measured by using the well-established 'can technique' (Durrani et al., 1993). Fifly-cm deep holes (11 cm in diameter) were dug into the soil at the chosen sampling positions to locate 70 cm long PVC tubes, at the bottom of each of which a cylindrical can, 10 cm high and 7 cm in diameter, was placed, with the open mouth lowermost, but covered by a filter to deter the entry of 22°Rn. A CR-39 plastic detector 2 c m x 2 cm in size and 1000 ~tm thick, was attached at the inner surface of the closed top of each can. The detectors were left in situ concurrently for -3 weeks, to allow the radon concentration to reach a steady-state value (x~ of 222Rn being 3.825 d). The detectors were subsequently etched under standard conditions, either electrochemically or chemically, and the tracks counted under an optical microscope. The track density was converted (see Badr et al., 1993) to radon activity concentration by calibration against a known radon activity. SURVEY AREAS

Altogether four surveys were carried out. The first (Survey I) was over a 15 km x 15 km area in the West Midlands of England to the west of Worcester and the northeast of Hereford. It is underlain by a single geological formation - the Lower Old Red Sandstone. Survey II was in the County of Derbyshire - declared a 'radon affected area' by the NRPB - to the southeast of Buxton, covering an area 7.5 km x 7.5 kin. Here the geology comprises two principal rock types, viz., Bee Low Limestone and the Monsal Dale Limestone, each covering rougldy haft the surveyed region. Our sampling was done in the top-soil. Survey III was in an area around N o t t i n ~ . Here we sampled along a 2 km long transect, that started in the northwest of the City of Nottingham and continued northwestward, reaching close to the Nottinghamshire-Derbyshire border. The transect crossed several rock formations (some of them occurring repeatedly). There were three main lithologies, viz., the Mansfield Marine Band ('coal measure'), the Lower Magnesian Limestone, and the Middle Permian Marl, as well as some other less prominent formations. In this survey, detectors were spaced at 20 m intervals alon~ this 2 km long transect, giving a total of 101 sampling points. Survey IV covered an area of 4 km ~ near Biggin, which lies to the south of Buxton in the County of Derbyshire. The area chosen is underlain by only one formation, viz., the Monsal Dale Limestone, and the purpose of the survey was to study the variation of radon concentration within a simple lithology. An irregular twodimensional sampling scheme was used, and the variogram was computed more intensively at the short lag h - by distributing 93 detectors at sampling intervals varying from 50 m to 250 n~ SAMPLE CONFIGURATION We shall illustrate the scheme of sample distribution in the field by describing the procedures adopted for our Survey II, near Buxton in Derbyshire, England (Badr et aL, 1996). The area chosen was

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7.5 km x 7.5 km in extent, an 'unbalanced nested sampling' scheme comprising 7 stages (with replication at the first three stages only) being employed. The sampling intervals (or lag) h ranged from 3750 m at stage 1 down to 1 m apart at stage 7. The sampling points were located round each of 9 Mare Centres, i.e. nine symmetrically-placed 'nodes' of a square grid 7500 m x 7500 n t There was also an approximstely 4-fold (decreasing) geometric progression between stages to ensure independence of the components of variance. From each of the nine main centres the other sampling points were located on random vectors as follows. From each main centre, a second site was selected in a random direction at a distance of 950 m ( = ¼ x 3750 m) to provide stage 2. From each of these 9 + 9 points, another site was chosen at a distance of 240 m to form stage 3. At stage 4, sites were located at a distance of 60 m from just half the stage 3 sites, again on random vectors. Replicating just half of the sampling points was repeated for stages 5, 6, and 7 at distances of 15 m, 4 rn, and 1 rn, respectively. Care was taken to ensure that all sites associated with a particular main centre were located on the same rock formation (lithology), i.e., the Bee Low Limestone or the Monsal Dale Limestone. The number of samples at each stage is shown in Table 1, and the sample configuration in Fig. 1. Table 1. Stages, sampling intervals, and number of samples in the unbalanced nested survey of radon concentrations in Survey II near Buxton in Derbyshire Stage in hierarchy

Sampling distance h (metres)

Number of Samples

1

3750

9

2

950

9

3

240

18

4

60

18

5

15

18

6

4

18

7

1.

18

GrandTotalofsamples

= 108

The number of samples shown at each stage in the totalfor all (9) main ccntrcs. The grand totalof samples for n (=9) ccntresand m stagesgiven by n [2(m-l)],m being 7 in our case. Had we used the 'balancednested sampling' scheme the grand totalwould have been 576 insteadof I08. 12(3) e , ~

l'o another Main Centre

~

~

"

3750m away

Main Centre I "~1)

\ Fig. I.

The spatial configuration of the sampling x points for one of the main eentres(CentreI) for the nested survey near Buxton. The number at each sampling point refers to the sample number, and the stage in the hierarchy is shown in parentheses m,, ~

\gs)m

~

s

)

7 1

4(4)

•

I#°m

4xn17(51

6~41~""~ N6)

~9(6) 11(7)~ lm

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RESULTS AND DISCUSSION

Taking Survey II (at Buxton, Derbyshire), once again, for the purposes of illustration: the radon values were measured at each of the 108 sampling points described in the preceding section. These values were analysed using a hierarchical analysis of variance (see detailed description in Badr et al., 1993) where, essentially, the components of variance for each stage are a~ 2 , or22. . . . <~m2 , leading to a total variance of a 2 = a~ l + a22 + ... + Crm2 . Since the standard deviations from the mean are expressed as kBq m "3, the variances have the units (kBq m'3) 2. Furthermore, an extra stage (8~) was included in this analysis to represent the different lithologies. Table 2 records the components of variance as well as the percentage variance contributed by each stage to the total (or accumulated) variance for Survey H. In interpreting the table in terms of the variogram, any negative contributions from a given stage to the cumulative variance are ignored, i.e. treated as if they were o, and one skips over the corresponding stage or stages - considering only the upward lift of the variogram curve. For instance, the cumulative variance at stage 6 is 39.6; at stage 5 it is 24.2 - hence that value is ignored; the cumulative value of variance is 43.4 at stage 4. Hence the % variance contributed between stage 6 (where h is 4 In) and stage 4 (where h is 60 m) is 100 x (43.4 39.6)/173.8 = + 2.2%, where 173.8 is the total accumulated variance over all the (8) stages. Table 2. Components of variance and percentage variance contributed by each stage from the unbalanced nested survey of radon activity concentration values near Buxton, Derbyshire (Survey U). Stage in hierarchy

Sampling distance h (metres)

Component of variance (kBq m-3):

Cumulative variance (kB qm3) 2

% of total variance accounted for

Geology (m=8) *

-

97.3

173.8

46.3

1

3750

-12.8

76.5

0.0

2

950

-4.1

89.3

0.0

3

240

50.0

93.4

28.8

4

60

19.2

43.4

2.2

5

15

-15.4

24.2

0.0

6

4

35.5

39.6

20.4

Residual (m=7)

1

4.1

4.1

2.3

*Stage 8 represents lithological differences. There are two main geological formations in the survey area : Bee Low Limestone and Monsal Dale Limestone, each occupying about one-half of the surveyed area. Some minor formations have been subsumed into the above two lithologies if the radon values were not too dissimilar(see Badr et al., 1996, for details). Figure 2 shows the variogram based on the measurements corresponding to Table 2. On analysing Table 2 and Figure 2, it is clear that:

(i)

(ii)

by far the greatest influence on the Rn concentrations in the soil comes from lithology or other characteristics that are included in this factor - soil moisture, soil type, carrier gas, etc. (since stage 8 for geology contributes = 46% of the total variance), especially at distances greater than 3750 m, viz. at the first stage in the hierarchy; large changes in the Rn levels occur over distances of between 60 m and 240 m (contribution to the variance between stages 4 and 3 being ~ 29%), and also over lag values (distance

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180 160 140

thology 120 100

40 20

~

6b

2io

g~o

aT~o

Log distance (m)

Fig. 2..Reconnaisance variogram for Buxton, Derbyshire (Survey I1), based on Table 2. The closest distance between sampling points was 1 m. An extra (8th and final) stage was added to take into account the two lithologies. Steepest % contributions to the accumulated components of variance have been marked; the largest contribution (46%) comes from lithological differences.

(iii)

between sampling points) of I m to 4 m (the contribution to variance between stages 7 and 6 being = 20%); and there is virtually no residual variance (stage 7 accounts for only z 2% to the change in the value of the variogram) : i.e. samples separated by 1 m are very similar to each other; it also suggests that little of the variance is attributable to measurement errors.

The above conclusions differ from those reported by us in the early stages of our investigations (cf. Badr et al., 1993), which had indicated that the largest variations in Rn values occurred over distances of less than 10 m between sampling points even over a simple lithology, i.e. that Rn concentration varied very erratically over short distances without any reliable correlation with geological conditions. That correlation of Rn levels with lithology does indeed exist (as we now conclude) was also clearly demonstrated in our Survey III near the City of Nottingham, where we conducted a linear sampling at 20 m intervals along a 2 km long transect, which crossed several lithologies (see Badr et al., 1996). The transect crossed six main formations; our variogram analysis picked out the positions of seven distinct boundaries, which in five cases coincided with geological boundaries. CONCLUSION The use of geostatistical methods for the design of field experiments to measure the values of Rn activity concentration in the soil, and the analysis of the results based on geostatistical parameters (in particular, the variogram), in several large-scale surveys conducted in and around the English Midlands, leM us to conclude that : (i) geology has a strong influence on the general Rn levels in the soil, especially on the larger, viz. Ion, scale (see Badr et al., 1996); (ii) over intermediate inter-sample distances, appreciable differences in Rn levels can occur independently of lithology (possibly caused, for example, by factors such as carrier gases - e.g. CO2 - in the soil, moisture content, soil composition, fissures, etc.); and (iii) only in situ measurements can establish what Rn values may be expected to prevail locally (over distances of the order of 1 m). From these observations (especially (iii) above) it is clear that any correlations of radon levels with phenomena such as the incidence of disease (lung cancer, leukaemias, etc.) must only be based on in situ and highly localized measurements inside dwellings, etc.

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Acknowledgement--The author wishes to thank his former colleagues on the investigating team for their cooperation,namely Ishmail Badr, Ali Khayrat, Mm'garetOliver and Graham Hendry.

REFERENCES

Back I., Oliver M.A., Hen&y G.L. and Durrani S.A. (1993) Determining the spatial scale of variation in soil radon values using a nested survey and analysis. Radiat. Prot. Dosim. 49, 433-442. Back I., Oliver M.A. and Dutrani S.A. (1996) Statistical evidence of the geological control over radon soil gas concentrations and its implications for mapping radon potential. Radiat. Prot. Dosim. 63, 281-291. Durrani S.A. and Back I. (1995) Geostatistically controlled field study of radon levels and the analysis of theft spatial variation. Radiat. Meas. 25, 565-572. Durrani S.A., Khayrat A.H., Oliver M.A. and Back I. (1997) Estimating soil radon concentration by kriging in the Biggin area of Derbyshire (UK). Radiat. Meas. 28, 633-639. Durrani S.A., Karamdoust N.A., Griffiths C.J.M. and AI-Najjar S.A.R. (1993) Radon measurements at the site of a former coal-burning power station. In: Proc. Int. Conf. on High Levels of Natural Radiation; Ramsar, Iran, Nov. 1990 (eds: M Sohrabi et al.), IAEA, Vienna, pp. 207-220. Tanner A.B. (1994) Measurement and determination of radon source potential : A literature review. National Institute of Standards and Technology Report NISTIR 5399. NIST, Gaithersburg, Md 20899, USA. Webster R. and Oliver M.A. (1990) Statistical Methods for Soil and Land Resource Surveys. Oxford University Press, Oxford.