ABSTRACT
In the Long Valley caldera, an area of recently (∼ 550 y) active volcanism and current seismic activity, 222Rn concentrations in hot, warm, and cold spring waters have been measured since 1982. Rn contents of the waters correlate inversely with temperature and specific conductance, with high concentrations (1500 to 2500 pCi/l) occurring in dilute cold springs on the margins of the caldera, and low concentrations (12 to 25 pCi/l) in hot to boiling springs. Rn correlates only slightly with the uranium contents of the rocks which host the hydrological system feeding the springs, which encompass a wide range of rock types.
Anomalous changes in groundwater Rn contents may accompany or precede earthquake activity, and a continuous Rn monitoring system was installed in 1983 to monitor short-term variations. A gamma detector is submerged in a natural pond fed by ~11ºC spring waters with ∼ 700 pCi/l Rn, and measured gamma activity is due almost entirely to 222Rn in the water. The gamma record, which is integrated hourly, shows a consistent, pronounced diurnal variation (∼ 30% of mean count rate), and weaker higher frequency variations. This pattern correlates well with small variations (< 1°C) in water temperature at the Rn monitoring point, and is strongly influenced by precipitation and by patterns of water flow in the pond. It does not adhere closely to a tidal pattern.
These environmental effects on the radon record may mask responses to small or distant seismic events. To date, anomalous changes in waterborne Rn have been observed in connection with at least one earthquake, which occurred close to the monitoring site. This continuing study points out that an understanding of the geological setting, its associated hydrological system, and environmental influences is necessary to properly evaluate concentrations and changes in groundwater radioactivity.
