Radon 222 emanometry: A relevant methodology for water well siting in hard rock aquifers

To improve the understanding of soil gas emanations with the aim of groundwater prospecting in fractured substrates, radon anomalies were mapped in a tropical region (French Guyana) where the structural features of a potential gas source are masked owing to the thick regolith cover. Two anomalies were located. Two probes were then installed, one (B1) on the border of the north anomaly and the second (B2) at the center of the south anomaly, as well as a meteorological station, and monitoring was carried out for 6 months extending from the dry to the rainy season. Signal processing of radon signal-atmospheric pressure time series allowed us to determine the physical processes causing migration of the gases in the soil and the subsoil. The radon anomalies are very probably associated with the presence, in the hard rock substratum, of a permeable structure that permits the transit of deep origin gas to the surface. This transport by advection disturbs the radon diffusion profile from the first meters of soil into the atmosphere. The radon thus behaves as a marker of deeper gases percolating through the regolith cover. Indeed, in such a context with low-permeability weathered formations, because of its short half-life (3.8 days) the radon measured in the first decimeters of the soil displays an exclusively superficial origin. Hence the measurement of the radon content in soil, at an identical depth from one sampling site to another, helps locate the zones affected by advective gas transfers which explains why the radon anomalies in soil gases, identified at the scale of a hydrogeological prospecting site located in a hard rock context, usually coincide vertically in hydraulically active fracture zones. This fully justifies the use of this method to site water wells, provided some precautions are observed, particularly pertaining to the spatial and temporal variability of the signal associated with different types of soils and changes in the meteorological parameters and soil moisture contents. The methodology is simple since only robust equipment is required. The estimation of gas transport parameters also helped identify the extent of advection areas. During the dry season they are characterized by low gas velocity. These zones shrink with the early rains, and the advection area is confined vertically up the fault where a channeling effect occurs: Soil gas preferably follows the less constricted pores, and the increased pressure gradient offsets the decrease in permeability.