Air exchange and 222Rn concentrations in the Carlsbad Caverns

Radon 222 concentrations in the air in Carlsbad Caverns have been measured at different times of year in order to define certain features of the natural circulation of the cave atmosphere and to estimate the internal radiation exposure to visitors and Park Service personnel. Concentrations average 48 pCi/l. in the summer when the temperature lapse rate provides a stable cave atmosphere. In the winter months air from outside having a 222Rn concentration of only about 0.2 pCi/l. mixes with the cave air reducing radon levels to about 15 pCi/l. A simple model obtained by equating the input of 222Rn atoms from the rocks and soils of the cave's interior surfaces to the sum of terms for radioactive decay and dilution of the cave atmosphere by outdoor air enables one to predict, for steady-state conditions, the approximate levels of activity to be expected for all months of the year. If it is assumed that the short-lived daughter products of 222Rn are present in equilibrium amounts, Park Service personnel spending most of their time underground approach the 4 WM/yr (working level months per year) exposure limit established for the uranium industry. A visitor on a 3-hr trip through the Caverns would receive only a few per cent of the exposure guides for the general population. (author)