Applications of radon isotopes to mass exchange processes in marine and terrestrial environments

Abstract

The naturally occurring radioactive noble gas radon (222Rn) has been extensively applied to determine mass exchange processes in marine and terrestrial environments due to its chemically conservative behavior, enrichment in groundwater and interstitial air, and suitable half-life. The main goals of this study are the applications of Rn isotopes to determination of air exchange and prediction of earthquake in a cave system and to investigation of groundwater-seawater exchange and groundwater input in estuarine and subterranean estuarine systems. To investigate cave air ventilation mechanism, the activities of 222Rn and 220Rn were continuously measured in a limestone cave, Seongryu Cave, Korea, from May 2010 to June 2011. 222Rn and 220Rn activities showed seasonal variations. While the activity of 222Rn showed a significant positive correlation with the temperature difference between outside and inside cave, 220Rn activity showed a negative correlation. These results may be due to air-stagnant effect in the summer (denser inside air

higher 222Rn and lower 220Rn activities) and more effective air ventilation in the winter (denser outside air

lower 222Rn and higher 220Rn activities) as well as different half-lives of 222Rn and 220Rn. This mechanism can be applied to the diurnal variations in Rn isotope activities in the spring and fall. In particular, spike-like peaks of 222Rn 220Rn activities were observed in February 2011. Based on correlation analyses between weather parameters and radon isotopes, these anomalous peaks observed in February could be suggested as precursor signals of the Tohoku-Oki Earthquake (M = 9.0) which occurred on March 11, 2011 in Japan: (1) 220Rn peaks during the anomalous period were much higher than those during normal periods over the year

(2) a positive correlation was observed between 220Rn and 222Rn only during the anomalous days (negative correlation was observed in general)

(3) the hourly 220Rn and 222Rn activities showed no normal diurnal pattern. These results suggest that radon isotope pair, 222Rn-220Rn, is a reliable earthquake precursor, and meteorological parameters and potential carrier gases (CO2 and CH4) should also be monitored to filter out other environmental forcing factors and understand gas transport mechanism. To determine mechanism of seasonal changes in recharge and residence time of saline groundwater in a subterranean estuary, 222Rn activity in groundwater was continuously measured from September 2010 to July 2011 in a coastal zone, Korea. The activity of 222Rn showed a large seasonal variation, resulting from seasonal changes in sea level, water temperature, and tides. Sudden decreases of 222Rn activities coincided with episodic drops in groundwater temperature revealed that lower seawater temperature in winter may result in density-driven seawater intrusion. Furthermore, 222Rn activities were more clearly affected by semi-monthly and diurnal tidal pumping during the wet season. The estimated residence times of saline groundwater using 222Rn activity ranged from 0 to 26 days (average: 1.8 ± 1.3 days). These results imply that groundwater residence time, which may control biogeochemical reaction time in subterranean estuaries, should be significantly considered to estimate groundwater-driven terrestrial materials. 222Rn and dissolved inorganic nutrients in water at a fixed station of the Nakdong River estuary were continuously measured from October 2014 to May 2015 to investigate groundwater effect on nutrient levels in an estuary. The estimated benthic 222Rn flux (groundwater input) showed a significant correlation with groundwater level, suggesting that groundwater level can be used as a representative of the groundwater input. Based on correlation analyses, dissolved inorganic nitrogen (DIN) was found to be dependent primarily on river water input but dissolved inorganic phosphorus (DIP) and silicate (DSi) were predominantly controlled by groundwater input. These results suggest that groundwater input may be an important source of DIP which limits biological productivity in this estuary. This study demonstrates that measurements and modeling of Rn isotopes are important because these radionuclides have become primary tools for determining air and water mass exchange processes in environmental sciences.