Abstract: Radiometric investigation of radon in soil gas over Jurassic rocks of Northamptonshire, England G. Sharman Geology Department, University of Leicester, LE i 7RH, England
Radon is a health risk when it becomes concentrated in buildings. The British Government's radon Action Level for domestic properties is 200 Bq m-3. Elevated radon levels exist in homes on Jurassic sedimentary rocks in Northamptonshire. Soil gas measurements are a good indicator of radon arising from the underlying rock. A Pylon AB-5 emanometer has been used to measure radon in soil gas in Northamptonshire. The instrument and Lucas cells were calibrated at the British Geological Survey, Keyworth, and at the National Radiological Protection Board, Chilton. The survey method requires 750 mL of soil gas to be drawn through a 0.27 L Lucas cell, from a soil probe inserted to 500 mm depth. Three consecutive l-rain counts are taken, allowing differentiation of 222Rn (radon, half-life 3.8 days) and 22~ (thoron, half-life 55.6 seconds) activity, expressed as Becquerels per litre (Bq L-l). Typically, 1-2 % of soil gas may enter buildings (Scott, communication to NRPB, 1989), so soil radon activity greater than 10-20 Bq L-1 (10,000-20,000 Bq m-3) indicates a potential hazard. Intensive survey has revealed two major radioactive rocks in Northamptonshire; the Northampton Sand Formation (NSF), of Aalenian age, and the Marlstone Rock Bed (MRB) which is Upper Pliensbachian. Other lithologies have lower radon activity, (Table 1). (Specific activity of thoron is generally higher due to the shorter half-life). In the case of the NSF, which compromises ironstone in its lower part, and ferruginous and calcareous sandstones in its upper part, high radon levels are particularly associated with the base of the formation (Sutherland, this volume). Targeting of this horizon during sampling may account for a number of apparently high values on the Upper Lias Clay (ULC), which are probably
an artifact of two important factors in the local situation. Glacial disturbance (Hollingworth et al., 1944) causes mixing of the lower part of the NSF with the upper boundary of the ULC. Movement of radioisotopes in groundwaters (Andrews et aI., 1972; and others), which have moved downslope from the NSF could cause high radon values to be recorded over the ULC. Of the 222Rn values, 33 % on the MRB, and 31% on the NSF exceeded 20 Bq L-lo These figures compare well with indoor radon values (from passive detectors), where 27 % on the NSF exceed the 200 Bq m-3 Action Level (cf. Sutherland, this issue). This is consistent with 1% of soil gas typically being admitted. This correlation does not mean, however, that it is a simple matter to predict indoor values from soil gas readings. Local geological factors, differences ha construction and lifestyle all contribute to the indoor radon value, and variation of soil radon with meteorological variables is well known (Ball et al., 1983). Seasonal variations have been recorded on a small scale, on outcrop NSF. Eighteen sample locations on a 3 m grid have been sampled at monthly intervals around a property with an elevated indoor Rn value. Table 2 shows these results for the 8 months October 1990 to May 1991, illustrating seasonal variation. A trend has emerged, indicating radon activity in the soil pore spaces is less during winter months. This work also revealed 'hot spots' within the garden, which repeatedly produced elevated readings, suggesting favoured routes of radon emanation from the ground. This is more clearly shown by the 222Rn results than by 220Rn. Although not shown here, other points were repeatedly low during winter, but increased in spring. The locations of radon lows are closer to the house, suggesting depletion of radon in this zone during winter.
Table 1 Rn-222 (Rn) and Rn-220 (Tn) in soil gas on some Lower and Middle Jurassic rocks in Northants. Lithology
No. of Maximum readings Rn Tn
Rn
Tn
Rn
Tn
Blisworth Limestone (GOL) U. Esmarine Series (UES) Grantham Formation (LES) Northampton Sand (NSF) Upper Lias Clay (ULC) Marlstone Rock Bed (MRB)
76 41 35 280 72 57
6 7 6 17 10 18
22 18 18 40 35 32
6 5 4 12 7 15
21 20 19 30 28 29
19 35 16 101 50 76
45 31 37 146 96 75
Mean
Median
147 Table 2 Seasonal variations of radon and thoron.
Average of 18 positions, by month Rn-222 Bq L-1 Rn-220 Bq L "1 Date Mean Median Mean Median Oct l0 Nov 5 Dec7 Jan4 Feb 4 Marl Apr5 May 8
11 18 16 8 9 13 10 25
4 22 6 4 8 10 5 19
35 32 21 22 20 18 26 27
35 32 20 21 19 19 27 27
Average of 8 months, by position Rn-222 Bq L -1 Rn-220 Bq L-1 Position Mean Median Mean Median 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
10 15 17 9 3 28 28 26 13 28 8 21 8 12 2 5 6 6
7 10 11 6 3 26 25 22 7 25 5 22 5 11 2 4 4 4
25 26 27 29 21 22 26 30 25 26 24 22 26 24 21 22 26 30
20 25 27 30 20 22 27 30 24 27 22 22 22 23 21 20 25 28
These results indicate that soil gas surveys can indicate potential areas of radon risk, and that these can be linked with geological horizons. They also illustrate the difficulty in utilising this type of data as a prediction for indoor values.
Acknowledgement This research, in conjunction with Diana Sutherland and Prof. John Hudson at The University of Leicester, and with the collaboration of Dr Keith Ball of the British Geological Survey, is partially funded by the Local Authorities in Northamptonshire.
References Andrews, J.N. and Wood, D.F. 1972. Mechanism of radon release m rock matrices and entry into groundwaters. Trans. Inst. Min. Metall., 81, B198--209. Ball, T.K., Nicholson, R.A. and Peachey, D. 1983. Effects of meteorological variables on certain soil gases used to detect buried ore deposits. Trans. Inst. Mia. Metall., 92, B183--190. HoUingworth, S.E., Taylor, J.H. and KeUaway, G.A. 1944~ Large scale superficial structures in the Northampton Ironstone Field. Quart. ]ourn. Geol. Soc., vol c, 1-44. National Radiological Protection Board, 1990. Human exposure to radon in homes. In: Documents of the NRPB, 1, no I. HMSO.
Radon is a health risk when it becomes concentrated in buildings. The British Government's radon Action Level for domestic properties is 200 Bq m-3. Elevated radon levels exist in homes on Jurassic sedimentary rocks in Northamptonshire. Soil gas measurements are a good indicator of radon arising from the underlying rock. A Pylon AB-5 emanometer has been used to measure radon in soil gas in Northamptonshire. The instrument and Lucas cells were calibrated at the British Geological Survey, Keyworth, and at the National Radiological Protection Board, Chilton. The survey method requires 750 mL of soil gas to be drawn through a 0.27 L Lucas cell, from a soil probe inserted to 500 mm depth. Three consecutive l-rain counts are taken, allowing differentiation of 222Rn (radon, half-life 3.8 days) and 22~ (thoron, half-life 55.6 seconds) activity, expressed as Becquerels per litre (Bq L-l). Typically, 1-2 % of soil gas may enter buildings (Scott, communication to NRPB, 1989), so soil radon activity greater than 10-20 Bq L-1 (10,000-20,000 Bq m-3) indicates a potential hazard. Intensive survey has revealed two major radioactive rocks in Northamptonshire; the Northampton Sand Formation (NSF), of Aalenian age, and the Marlstone Rock Bed (MRB) which is Upper Pliensbachian. Other lithologies have lower radon activity, (Table 1). (Specific activity of thoron is generally higher due to the shorter half-life). In the case of the NSF, which compromises ironstone in its lower part, and ferruginous and calcareous sandstones in its upper part, high radon levels are particularly associated with the base of the formation (Sutherland, this volume). Targeting of this horizon during sampling may account for a number of apparently high values on the Upper Lias Clay (ULC), which are probably
an artifact of two important factors in the local situation. Glacial disturbance (Hollingworth et al., 1944) causes mixing of the lower part of the NSF with the upper boundary of the ULC. Movement of radioisotopes in groundwaters (Andrews et aI., 1972; and others), which have moved downslope from the NSF could cause high radon values to be recorded over the ULC. Of the 222Rn values, 33 % on the MRB, and 31% on the NSF exceeded 20 Bq L-lo These figures compare well with indoor radon values (from passive detectors), where 27 % on the NSF exceed the 200 Bq m-3 Action Level (cf. Sutherland, this issue). This is consistent with 1% of soil gas typically being admitted. This correlation does not mean, however, that it is a simple matter to predict indoor values from soil gas readings. Local geological factors, differences ha construction and lifestyle all contribute to the indoor radon value, and variation of soil radon with meteorological variables is well known (Ball et al., 1983). Seasonal variations have been recorded on a small scale, on outcrop NSF. Eighteen sample locations on a 3 m grid have been sampled at monthly intervals around a property with an elevated indoor Rn value. Table 2 shows these results for the 8 months October 1990 to May 1991, illustrating seasonal variation. A trend has emerged, indicating radon activity in the soil pore spaces is less during winter months. This work also revealed 'hot spots' within the garden, which repeatedly produced elevated readings, suggesting favoured routes of radon emanation from the ground. This is more clearly shown by the 222Rn results than by 220Rn. Although not shown here, other points were repeatedly low during winter, but increased in spring. The locations of radon lows are closer to the house, suggesting depletion of radon in this zone during winter.
Table 1 Rn-222 (Rn) and Rn-220 (Tn) in soil gas on some Lower and Middle Jurassic rocks in Northants. Lithology
No. of Maximum readings Rn Tn
Rn
Tn
Rn
Tn
Blisworth Limestone (GOL) U. Esmarine Series (UES) Grantham Formation (LES) Northampton Sand (NSF) Upper Lias Clay (ULC) Marlstone Rock Bed (MRB)
76 41 35 280 72 57
6 7 6 17 10 18
22 18 18 40 35 32
6 5 4 12 7 15
21 20 19 30 28 29
19 35 16 101 50 76
45 31 37 146 96 75
Mean
Median
147 Table 2 Seasonal variations of radon and thoron.
Average of 18 positions, by month Rn-222 Bq L-1 Rn-220 Bq L "1 Date Mean Median Mean Median Oct l0 Nov 5 Dec7 Jan4 Feb 4 Marl Apr5 May 8
11 18 16 8 9 13 10 25
4 22 6 4 8 10 5 19
35 32 21 22 20 18 26 27
35 32 20 21 19 19 27 27
Average of 8 months, by position Rn-222 Bq L -1 Rn-220 Bq L-1 Position Mean Median Mean Median 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
10 15 17 9 3 28 28 26 13 28 8 21 8 12 2 5 6 6
7 10 11 6 3 26 25 22 7 25 5 22 5 11 2 4 4 4
25 26 27 29 21 22 26 30 25 26 24 22 26 24 21 22 26 30
20 25 27 30 20 22 27 30 24 27 22 22 22 23 21 20 25 28
These results indicate that soil gas surveys can indicate potential areas of radon risk, and that these can be linked with geological horizons. They also illustrate the difficulty in utilising this type of data as a prediction for indoor values.
Acknowledgement This research, in conjunction with Diana Sutherland and Prof. John Hudson at The University of Leicester, and with the collaboration of Dr Keith Ball of the British Geological Survey, is partially funded by the Local Authorities in Northamptonshire.
References Andrews, J.N. and Wood, D.F. 1972. Mechanism of radon release m rock matrices and entry into groundwaters. Trans. Inst. Min. Metall., 81, B198--209. Ball, T.K., Nicholson, R.A. and Peachey, D. 1983. Effects of meteorological variables on certain soil gases used to detect buried ore deposits. Trans. Inst. Mia. Metall., 92, B183--190. HoUingworth, S.E., Taylor, J.H. and KeUaway, G.A. 1944~ Large scale superficial structures in the Northampton Ironstone Field. Quart. ]ourn. Geol. Soc., vol c, 1-44. National Radiological Protection Board, 1990. Human exposure to radon in homes. In: Documents of the NRPB, 1, no I. HMSO.
