Patrick Richon

Degassing the “Killer Lakes” Nyos and Monoun, Cameroon

better understood, thanks to the almost continuous scrutiny of the two lakes by scientists. Lakes Nyos and Monoun occupy the crater of a supposedly extinct volcano, in a region known by geologists for its numerous soda (not thermal) water springs, a common feature of old volcanic areas.The region belongs to the so-called volcanic chain of Cameroon, which culminates and ends 300 km further southwest at the still-active Mount Cameroon (4000 m asl.). Accumulated evidence strengthens the early hypothesis that the Monoun gas burst originated in the release of huge amounts of

Spatiotemporal variation of radon and carbon dioxide concentrations in an underground quarry: coupled processes of natural ventilation, barometric pumping and internal mixing

Radon-222 and carbon dioxide concentrations have been measured during several years at several points in the atmosphere of an underground limestone quarry located at a depth of 18 m in Vincennes, near Paris, France. Both concentrations showed a seasonal cycle. Radon concentration varied from 1200 to 2000 Bq m(-3) in summer to about 800-1400 Bq m(-3) in winter, indicating winter ventilation rates varying from 0.6 to 2.5 x 10(-6) s(-1). Carbon dioxide concentration varied from 0.9 to 1.0% in summer, to about 0.1-0.3% in winter. Radon concentration can be corrected for natural ventilation using temperature measurements. The obtained model also accounts for the measured seasonal variation of carbon dioxide. After correction, radon concentrations still exhibit significant temporal variation, mostly associated with the variation of atmospheric pressure, with coupling coefficients varying from -7 to -26 Bq m(-3) hPa(-1). This variation can be accounted for using a barometric pumping model, coupled with natural ventilation in winter, and including internal mixing as well. After correction, radon concentrations exhibit residual temporal variation, poorly correlated between different points, with standard deviations varying from 3 to 6%. This study shows that temporal variation of radon concentrations in underground cavities can be understood to a satisfactory level of detail using non-linear and time-dependent modelling. It is important to understand the temporal variation of radon concentrations and the limitations in their modelling to monitor the properties of natural or artificial underground settings, and to be able to assess the existence of new processes, for example associated with the preparatory phases of volcanic eruptions or earthquakes.

An experimental method for measuring the radon-222 emanation factor in rocks

Abstract An experimental method, based on a 21-day accumulation technique, is proposed for measuring the radon-222 emanation factor in undisturbed consolidated materials. The leakage rate is determined from the form of the radon growth curve in the measurement chamber. It was comparable to the radon decay. In order to obtain the “true” radon emanation factor, the thickness of the sample must be less than the radon diffusion length in the porous material. The method was used to measure the radon emanation factor in water-saturated claystones (argillites). The radon emanation factor, determined from experiments on a rock sample with a thickness of 5 mm , was 15%, a value typical for this kind of material.

Temporal variations of radon concentration in the saturated soil of Alpine grassland: the role of groundwater flow.

Radon concentration has been monitored from 1995 to 1999 in the soil of the Sur-Frêtes ridge (French Alps), covered with snow from November to April. Measurements were performed at 70 cm depth, with a sampling time of 1 h, at two points: the summit of the ridge, at an altitude of 1792 m, and the bottom of the ridge, at an altitude of 1590 m. On the summit, radon concentration shows a moderate seasonal variation, with a high value from October to April (winter), and a low value from May to September (summer). At the bottom of the ridge, a large and opposite seasonal variation is observed, with a low value in winter and a high value in summer. Fluctuations of the radon concentration seem to be associated with temperature variations, an effect which is largely delusory. Indeed, these variations are actually due to water infiltration. A simplified mixing model is used to show that, at the summit of the ridge, two effects compete in the radon response: a slow infiltration response, rich in radon, with a typical time scale of days, and a fast infiltration of radon-poor rainwater. At the bottom of the ridge, similarly, two groundwater contributions compete: one slow infiltration response, similar to the response seen at the summit, and an additional slower response, with a typical time scale of about a month. This second slower response can be interpreted as the aquifer discharge in response to snow melt. This study shows that, while caution is necessary to properly interpret the various effects, the temporal variations of the radon concentration in soil can be understood reasonably well, and appear to be a sensitive tool to study the subtle interplay of near surface transfer processes of groundwater with different transit times.

Evaluation of the effect of a cover layer on radon exhalation from uranium mill tailings: transient radon flux analysis.

An experimental study concerning the transport of 222Rn in uranium mill tailings (UMTs) and in the cover layer was launched in 1997 with the participation of the French uranium mining company (COGEMA). Evaluation of the cover layers effectiveness in reducing 222Rn flux emanating from UMTs was one of its objectives. In the first phase, the 222Rn flux densities were measured regularly on a UMT layer. In the second phase, the UMT was covered with a one-meter layer of compacted material consisting of crushed waste rock derived from mining activities. Radon-222 flux was then measured at the surface of this cover layer. Observations were compared with radon flux calculated using TRACI, a model for vertical water and gas flow and radon transport. The results show that the calculations bear a fair resemblance to the observations in both cases. They also show that the effectiveness of the cover layer calculated with TRACI, using the thickness and textural properties of the cover material, is very close to the measured effectiveness.

Radon exhalation from uranium mill tailings: experimental validation of a 1-D model

TRACI, a model based on the physical mechanisms governing the migration of radon in unsaturated soils, has been developed to evaluate the radon flux density at the surface of uranium mill tailings. To check the validity of the TRACI model and the effectiveness of cover layers, an in situ study was launched in 1997 with the French uranium mining company, COGEMA. The study consisted of continuous measurements of moisture content, suction, radon concentration at various depths inside a UMT cover, and flux density at its surface. An initial analysis has shown that radon concentration and flux density, as calculated with a steady-state diffusion model using monthly averaged moisture contents, are in good agreement with measured monthly averaged concentrations and flux densities.

Fractured porous media under influence: The Roselend Experiment

The deformation of the crust and fluid rock interactions are both important problems in the Earth sciences. Observing and measuring these coupled phenomena will improve our understanding of geologic processes acting in fractured porous media and the transfer properties of fluids. These observations can be made via laboratory experiments, but the sample scale is not always satisfying for understanding crustal-scale phenomena. On the contrary, monitoring a natural site at the field-scale provides clues that are directly interpretable in terms of geologic phenomena. The Roselend Experiment is one such endeavor that provides a setting for a multidisciplinary research project. Its goal is to determine transfer processes and transient phenomena at different scales in space and time within a highly dynamic system that is submitted to mechanical, hydrologic, meteorological, and thermal forces.

Persistence of radon-222 flux during monsoon at a geothermal zone in Nepal

The Syabru-Bensi hydrothermal zone, Langtang region (Nepal), is characterized by high radon-222 and CO(2) discharge. Seasonal variations of gas fluxes were studied on a reference transect in a newly discovered gas discharge zone. Radon-222 and CO(2) fluxes were measured with the accumulation chamber technique, coupled with the scintillation flask method for radon. In the reference transect, fluxes reach exceptional mean values, as high as 8700+/-1500 gm(-2)d(-1) for CO(2) and 3400+/-100 x 10(-3) Bq m(-2)s(-1) for radon. Gases fluxes were measured in September 2007 during the monsoon and during the dry winter season, in December 2007 to January 2008 and in December 2008 to January 2009. Contrary to expectations, radon and its carrier gas fluxes were similar during both seasons. The integrated flux along this transect was approximately the same for radon, with a small increase of 11+/-4% during the wet season, whereas it was reduced by 38+/-5% during the monsoon for CO(2). In order to account for the persistence of the high gas emissions during monsoon, watering experiments have been performed at selected radon measurement points. After watering, radon flux decreased within 5 min by a factor of 2-7 depending on the point. Subsequently, it returned to its original value, firstly, by an initial partial recovery within 3-4h, followed by a slow relaxation, lasting around 10h and possibly superimposed by diurnal variations. Monsoon, in this part of the Himalayas, proceeds generally by brutal rainfall events separated by two- or three-day lapses. Thus, the recovery ability shown in the watering experiments accounts for the observed long-term persistence of gas discharge. This persistence is an important asset for long-term monitoring, for example to study possible temporal variations associated with stress accumulation and release.

Rehabilitation of a uranium-ore processing residues storage site at Le Bouchet, Paris Region, France

Abstract From 1949 to 1971, the French Atomic Energy Commission operated a uranium-ore processing plant in the Paris region. After the plant was dismantled in the 70s, the plot of land that had been used to store the processing residues, and the area covered by the settling basin, were locked up. A study of the site in 1990 showed that the level of radon emissions and radon daughters from these premises was 10 to 100 times greater than the mean level above the ground. In 1992, the local authorities decided to rehabilitate the site. To limit the radon emissions, the land was buried under various materials, including a bed of clay several tens of centimetres thick. The measurements made on the site and in the surroundings of it, after the land was covered, showed clearly that the radon and the radon decay product concentrations were lowered to the same order of magnitude as the representative regional background noise in the Paris basin, typical of a sedimentary rock environment.

Subslab ventilation system: Installation and follow-up in a high-radon house in Brittany, France

Abstract A subslab ventilation system was designed and installed in a high radon house in Brittany, France. Radon concentration in the basement of this house was reduced from an average 10 000 Bq/m 3 to less than 200 Bq/m 3 , thanks to a subslab depressurization system with a double barrier and a 32 W fan. Sealing the concrete slab cracks greatly improved the system performance. Finally, the feasibility of using a passive double barrier ventilation system is shown.