Radon, an environmental hazard: A geological case study of Northamptonshire.

Systematic soil-gas surveys were carried out in Northamptonshire to investigate the distribution of radon and its controlling factors. This followed reports in 1988 of indoor radon levels in the county exceeding the Action Level of 200 Bq m-3. Over 3300 observations were made using a soil-gas emanometer in traverses crossing all geological formations. Over each formation results were found to be log-normally distributed, falling within 95% Confidence Limits. The results are compared with 1026 indoor measurements provided by local authorities. Direct comparisons cannot be made due to different controlling factors; the concentration of radon in soil-gas is determined by the permeability of the source rock and overlying soil and by climate and weather, particularly temperature and wind run. Indoor radon is also affected by the nature of the house construction, particularly the floor, and heating and ventilation. Both sets of observations show that radon occurrence is primarily controlled by the underlying geology; the relationship is presented as a Radon Hazard Map for Northamptonshire. The highest soil-gas radon levels occur over the Northampton Sand Formation where indoor levels reach 1600 Bq m-3, clearly presenting an elevated health hazard. The Marlstone Rock Bed, Upper Lincolnshire Limestone and Glacial gravels are also an elevated hazard. Hazard groups are defined as elevated on permeable strata where soil-gas Rn-222 is 11 Bq L-1 or over; moderate where soil-gas Rn-222 is 6-8 Bq L-1 on permeable strata, or 11 Bq L-1 or over on impermeable strata; slight where soil- gas Rn-222 is 5 Bq L-1 or less on permeable strata, or 6-8 Bq L-1 on impermeable strata; and low on impermeable strata where soil-gas Rn-222 is 5 Bq L-1 or less. Indoor results were also used in defining the hazard groups. Of the formations designated as elevated, all extend into adjacent counties, where soil-gas radon surveys are strongly recommended.