Factors controlling the distribution of radon (222Rn) in groundwater of a tropical mountainous river basin in southwest India

Studies on occurrence of radon in the environment are receiving growing attention worldwide due to its adverse impact on human health. Despite that, the dissolved radon in water is found to be a useful tracer in many hydrogeological studies. Although, several studies focused on the occurrence of high Rn-222 in groundwater, the processes responsible for its variation is still not well understood. Hence, an attempt has been made in the present study to elucidate the underlying factors influencing the abundance Rn-222 in hard rock (gneissic) aquifers of Karamana River Basin, southwest India. Rn-222 in groundwater was analyzed in 71 dug wells during the pre-monsoon period of 2017. A large variability in Rn-222 activities (170-68,350 Bq/m(3)) was noticed in groundwater and high activities were mainly seen in the khondalite formation. No significant dependencies between Rn-222 activity and depth to water table, groundwater temperature and electrical conductivity were observed. However, majority of the presence of high Rn-222 activity in groundwater matches with the location of lineaments. Furthermore, radium content in the host rock, degree of weathering and fracturing and the emanation coefficient of the rock were found to have an important bearing on the occurrence of radon in groundwater. The underlying factors influencing the abundance of radon in hard rock aquifers were also conceptualized. Thus, the study highlights the usefulness of Rn-222 as a potential tool in delineating the macro-structural features like fractures/lineaments that are significant repositories of groundwater, which could be explored for groundwater development in hard rock terrains. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The study found a large amount of radon-222 in groundwater of hard rock aquifers, mainly concentrated in the khondalite formation, and high radon-222 activity matched with the location of lineaments. Although no significant correlations were observed between radon-222 activity and depth to water table, groundwater temperature, and electrical conductivity, the presence of high radon-222 activity in groundwater was associated with geological structural features.