Radon (~(222)Rn) in groundwater studies in two volcanic zones of central Mexico

摘要

The distribution of radon (~(222)Rn) concentrations in groundwater from two basins of volcanic origin is presented. Regions have different physiographic characteristics with fractured mafic/intermediate and felsic rocks. Samples were taken from deep wells and springs. Concentrations were field measured by two methods: i) scintillator, coupled to a photomultiplier, and ii) passive method, using Nuclear Track Detectors. Qualitatively, results of ~(222)Rn measured with both techniques are comparable only when concentrations have values less than 1 Bq/l. For the Basin of Mexico City the data shows an average difference of 0.13 Bq/l. Results of ~(222)Rn concentrations in 46 groundwater samples indicate that the data are below 11.1 Bq/l, with both methodologies. Low concentrations of ~(222)Rn in the Basin of Mexico City are related to the mafic intermediate composition rocks such as basalt. The anomalies with high values are correlated with the transition zone between volcanic units and clays from ancient lakes. In San Luis Potosí 10 samples show an average of 4.2 Bq/l. These concentrations compared with those of the Basin of Mexico City are related to the composition of the felsic (rhyolite) volcanic rocks.