Spatial patterns and controlling factors of radon concentration in Quaternary alluvial-lacustrine aquifer systems

Abstract

Radon (222Rn), naturally occurring in subsurface environments, is a significant environmental carcinogen when consumed via 222Rn-enriched groundwater and acts as a valuable geochemical tracer of the interaction between groundwater and surface water. Identifying the spatial patterns and influential factors of 222Rn in groundwater is therefore critical. However, there is a scarcity of specific studies on alluvial-lacustrine plains. In this study, 170 groundwater samples were collected to analyze the spatial patterns and controlling factors of 222Rn in the central Yangtze River plain, a typical alluvial-lacustrine plain. Through correlation and principal component analyses, as well as a random forest model (RF), it was determined that the groundwater 222Rn concentration was affected by several factors, including the hydrogeochemical environment, its associated water-rock interactions, the provenance of aquifer sediments, their mineral composition, and spatial distances from bedrock. Specifically, a thinner layer of Quaternary strata closer to the bedrock, combined with a less reducing hydrogeochemical setting and shorter-term water-rock interactions, along with a higher presence of silicate minerals in the aquifer sediments, promoted the accumulation of 222Rn in groundwater. An RF model was employed to predict the spatial distribution of the 222Rn concentration in the alluvial-lacustrine aquifer, revealing elevated groundwater 222Rn concentrations around local uplifts within the plain, at its edges, and near shallow-buried faults. The insights gained from this study can be applied to similar alluvial-lacustrine plains globally.