Abstract
Lima City is situated on alluvial fan deposits of rivers flowing through geological formations that contain different levels of uranium. In this paper, a study is made on the average spatial and temporal behavior of radon gas in soils of Lima City. Radon concentration was determined using the LR-115 type 2 track detector during 36 periods, of 14 days each, in twenty holes distributed in the fifteen districts of Lima City. Radon concentration in soil pores ranged from 0.1 to 64.3 kBq/m3 with an average value of 5.6 kBq/m3. The average radon concentration in soil gas was about two times lower in winter than in the other seasons. High radon values during October/November 2017 were related to the earthquakes perceived in Lima City in that period. The highest radon concentrations were found in areas of alluvial deposits whose parental material has been removed from the Quilmaná and Huarangal volcanics by the Chillón and Huaycoloro Rivers. Soil gas radon concentrations were even higher in areas closer to volcanic and less distant from rivers. During the period of maximum flooding of the Chillón, Rímac and Lurín rivers, due to the natural phenomenon “El Niño Costero”, anomalous high soil radon concentrations were observed in most of the measurement sites located near rivers. These high radon values were associated with ground vibrations caused by rock and debris avalanches in rivers and creeks.
Highlights
Radon gas is produced by the radioactive decay of 226Ra, which in turn is derived from the 238U series
The average radon concentration ranges from 1.5 kBq/m3 in winter to 11.9 kBq/m3 in spring
There is no history of Rn measurements in soil gases in Lima City
Summary
Radon gas is produced by the radioactive decay of 226Ra, which in turn is derived from the 238U series. Radon generated within the mineral grains of rocks and soils can escape from them by recoil into air or fluid filled cracks and into the contiguous pore space, respectively. This can be quantified by the emanation coefficient, which depends on the availability of 226Ra, porosity, moisture content and temperature (Nazaroff, 1992). Radon transport is influenced by local geology as rock types, but by other geophysical and geochemical parameters as joints, fractures, hydrology, porosity, permeability and presence of other gas carriers These characteristics are, in turn, determined by the character of the bedrock, glacial deposits, or transported sediments from which the soil was derived (Gundersen et al, 1992; Alonso et al, 2019). The most important meteorological factors appear to be barometric pressure, wind, relative humidity, rainfall, and temperature
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