Geological basement control on 222Rn accumulation as an input function for hydrogeological systems on a loess aquifer, Argentina

Abstract

The activity of 222Rn of the groundwater was measured in different wells in the Pampeano aquifer in southeastern Buenos Aires to assess how the hydrogeological characteristics of the study area, which involves type and depth of the bedrock, aquifer mineralogy and residence time along flowpaths, influences on 222Rn contents. Water samples, as well as sediment samples, were collected in multilevel piezometers in different sites of the Buenos Aires province for the determination of stable isotopes, 222Rn concentrations and mineralogical analyses. The identification of uranium-bearing minerals in the loess sediments of the Pampeano aquifer was carried out with a scanning electron microscope allowing identifying zircon and monazite. The 222Rn was determined with a RAD7 detector, showing a range of activities from 3.1 to 14.9 Bq/L. The results showed a relation between the 222Rn activity, the depth of the wells, and the proximity of the Precambrian basement. The activity of 222Rn increases linearly with increasing depth at a rate of 0.09 Bq·L−1·m−1. However, at the same depth the concentration of 222Rn in wells near the Precambrian basement showed to be three times higher than the concentrations in the wells that have a Paleozoic bedrock, clearly showing that the proximity of granitoids rocks results in higher concentrations of radon.