J.L. Seidel

Radon in soil-air and in groundwater related to major geophysical events: A survey

Abstract The various technical aspects of radon concentration measurements in the ground or in the groundwater are reviewed. The emphasis is put on the most convenient techniques particularly suited for in situ monitoring in the field. Some attention is paid to the thoron problem. Historical and most recent findings in the field of earthquake related radon anomalies are presented and briefly discussed. A similar approach is made to rarer volcanic cases that have been documented up to now. A short discussion regarding the various models that intend to provide satisfactory explanations is presented, supported, whenever it is possible, by experimental laboratory results. Finally a glimpse on the possible and future application of the method to hydrogeology is given.

Radon in soil anomaly observed at Los Azufres geothermal field, Michoacan: a possible precursor of the 1985 Mexico earthquake (Ms = 8.1)

A radon anomaly in a nuclear track detector placed on a fault was detected prior to the destructive (Ms = 8.1) Mexican earthquake of 19 September 1985. The fault is a structural feature of the geothermal field ‘Los Azufres’, 260 km NE of the epicentral area. Since no other phenomenon can be found as a possible cause for the radon anomaly, it is proposed that preseismic regional stress perturbations may produce changes in the fluid transport patterns at the fault, suggesting that radon measurements in similar conditions could provide a good precursor in high seismic risk areas.

Short and long term radon in soil monitoring for geophysical purposes

Abstract The subduction of the Cocos Plate under the North America Plate defines a high risk seismic zone along the Pacific coast of Mexico. A seismic gap has been identified at the Guerrero State as a place where a large earthquake, up to MS=8.2, can occur within the next years. In this area an in-soil radon network, consisting of fixed SSNTD monitoring stations, has been operated during several years on a monthly exposure basis. A continuous radon probe that registers data on a short term basis is also operating since recently. The results of the two radon detection systems are compared and discussed as a function of the local seismic activity observed at the Guerrero coastal zone.

Variations of radon in soils induced by external factors

Experiments were designed to evaluate, in a controlled way, some of the variations that can be expected in radon concentration in soil due mainly to meteorological factors and to close spacing monitoring in the field. Electronic continuous radon monitoring and the track-etch method were used to record the radon α-decay. The radon emanation in daily measurements and in long term surveys showed a general effect modulated by temperature and pressure variations. Radon monitoring in soil for nearby stations showed that a 10% range of variation can be expected with the track-etch method and as a result of non-homogeneous emanation pattern of the soil.

Radon-in-soil variations related to volcanic activity

Two active volcanoes, the Popocatepetl in Mexico, and the Piton de la Fournaise on the island of La Reunion, France, have been monitored for soil radon for several years. The Popocatepetl, located 60 km from Mexico City, started an eruptive period in December, 1994. Two radon-in-soil monitoring stations are operated there, since 1993. The behaviour of soil radon showed an increase as a function of the eruptive phase. The Piton de la Fournaise volcano has also been under survey for several years. A radon network based on 20 Clipperton II automatic probes was set on July, 1994. Since then, the volcano has entered a quiescence phase. Thus it has been possible to draw the pattern of the radon behavior under the influence of atmospheric conditions.

Volcanic monitoring for radon and chemical species in the soil and in spring water samples

Abstract Soil radon has been monitored at two fixed stations in the northern flank of Popocatepetl Volcano, a high risk volcano located 60 km SE from Mexico City. Water samples from three springs were also studied for radon as well as major and trace elements. Radon in the soil was recorded using track detectors. Radon in the water samples was evaluated using the liquid scintillation method and an Alphaguard. The major elements were determined through conventional chemical methods and trace elements using an ICP-MS equipment. Soil radon levels were low, indicating a moderate diffuse degassing through the flanks of the volcano. Groundwater radon had almost no relation with the eruptive stages. Water chemistry was stable in the reported time (2000–2002).

Physical models related to radon emission in connection with dynamic manifestations in the upper terrestrial crust: A review

Abstract Radon emissions that are enhanced by forthcoming geophysical events as earthquakes or volcanic activity have been observed all over the World. Many of these observations have been carried out with equipments using Solid State Nuclear Tracks Detectors as sensors. However, such radon “anomalies” are not one-to-one events and the technique has not yet reached the necessary level of confidence. In the mean time, the data collected by scientists are casting some light on the involved phenomena. Radon emissions are now recognized as a rather complicated set of successive steps involving many aspects of geophysics and chemistry. Several models have been proposed as an explanation of the experimental field data. They are reviewed in this paper; ranging from radon outgassing related to fumaroles to radon emission related to the particular behaviour of aquifers in volcanic areas.

Radon signals related to seismic activity in Ecuador, March 1987

A radon concentration measurement survey was carried out in Ecuador using the SSNTD technique. The data analysis shows a definite radon anomaly prior to March 6, 1987;M=6.9 earthquake. Anomalies found before October 18 and October 28th mild earthquakes in 1986 could be correlated with these events. Other anomalies that we cannot correlate with any geophysical event were also found.

Radon variations in active volcanoes and in regions with high seismicity: internal and external factors

Abstract The results of 4 years of observations of radon concentrations in soils of active volcanoes of Costa Rica and a highly seismic region in Mexico are discussed. A distinction is made on external (mostly meteorological) and internal (magmatic or tectonic) factors of variation in radon levels. The geological meaning of radon data can be thus enhanced if the external factors are excluded.

Fluctuations of groundwater radon and chemical species in basaltic aquifers

Surveys of radon and trace elements concentration in groundwaters have been carried out. Two different series of aquifers were monitored. One is located along the Pacific coast of Mexico, characterised by an intense seismic activity. The second one is located on the coast of the Gulf of Mexico, where, on the contrary, low seismic activity is observed. Radon determinations are performed with track detectors LR 115 type II and with automatic radon probes. Concentration levels of trace elements in water samples were measured with an Inductively Coupled Plasma Mass Spectrometer. Differences of the geochemical response of each aquifer depend on its specific geological context and recharge pattern.