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

Abstract

Abstract. 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.

Soil gas radon emanometry: A tool for delineation of fractures for groundwater in granitic terrains

“…High radon and helium anomaly location, as discussed later, represent vents for soil gas rich in radon and 4 He, which emanate from deep fracture system and connect through the network of smaller fractures to the top horizon where soil gas is sampled. It is argued that such emanation could be an indicator of ''fractures'' which are in turn a potential source of groundwater (Pointet, 1989;Gascoyne et al, 1993;Lachassagne et al, 2001). In order to validate this hypothesis, we drilled wells on the high radon anomaly as well as off anomaly sites.…”

“…The unit ( Fig. 3) was installed in an augered hole fitted with a $6 cm diameter PVC tube (monitoring unit diameter 5 cm, Lachassagne et al, 2001). The bottom portion of the tube was kept open and bottom soil was loosened for proper placement of the Rn unit as well as for free air circulation.…”

“…Nevertheless, the search for deeper fractures continues. It is therefore considered worthwhile to investigate whether soil gas radon emanometry can help in locating potentially productive fracture zones (Pointet, 1989;Gascoyne et al, 1993;Lachassagne et al, 2001). Here, we describe our attempt in this direction.…”

“…The wells were selected randomly and radon concentrations were measured in the soil gas samples collected near the wells at about 60 cm depth. Lachassagne et al (2001) reported that the radon emanometry could reveal fractured zones in hard rock since there may be an upward flux of deep origin gases (CO 2 , 4 He, etc.) from the fracture zone.…”

Optimization of a Groundwater Monitoring Network for a Sustainable Development of the Maheshwaram Catchment, India

“…Rocks have undergone a variable degree of weathering, usually with a thickness of 15 m to 20 m. An underlying fractured zone extends up to 50 m below ground level (mbgl) (Figure 2). Crystalline basement aquifers can be divided in several compartments that together constitute the aquifer but which are characterized by distinct hydrogeological properties [13,14]. In the area of study, these compartments can be described as: (i) the upper zone which consists of weathered and decayed rocks of clayey-sandy composition.…”