Contribution of airflow to the control of seasonal variations in radon concentration in Ogof Ffynnon Ddu, Penwyllt, South Wales

A new winter and summer investigation of radon concentrations in the Ogof Ffynnon Ddu system at Penwyllt, South Wales, was carried out using 100 standard National Radiological Protection Board track etch detectors. Fifty detectors were installed in the system in December 1998 and August 1999 for a period of 28 days. The data obtained confirm that the system has moderately high radon concentrations with mean values of 2,318 Bq m^-3 in winter and 2,844 Bq m^-3 in summer. Traverse means showed that OFD I to Cwm Dwr has the highest mean concentration of the system in summer at 3,094 Bq m^-3. and has the lowest mean in winter, 1,946 Bq m^-3 The extremely high concentrations reported from the system in a previous study have not been reproduced. A significant find from the data was that the airflow directions at the entrances are not what might have been predicted. In the main, air appears to be emerging continuously from the lowest entrance but, counter to predictions, air enters at the higher entrances in winter and appears to come out or be variable in summer. Inside the cave there are sites that have very low radon concentrations in winter, and this can only be explained by ingress of fresh air from the surface. This ingress is a surprising result, given that warmer cave air might be expected to exit at these points. The low readings are not matched in the summer experiment results, indicating again that ingress of fresh air to some parts of the system is very variable. The complexity of airflow within a multientrance system is thus illustrated.