Presence, distribution, and origin of radioisotopes in rock, soil, and groundwater in Central Mexico: implications of ionizing radiation for health

Several sources of environmental radioactivity in central Mexico were investigated for their potential association with health by assessing gamma dose rates, trace elements in rocks, gamma ray energy spectra, radon and thoron activity in groundwater, soil-air, and dwellings, and previously unpublished data on uranium, thorium, and enriched-tritium concentrations dissolved in groundwater. Results of the screening studies show that the predominant rocks are ignimbrites of rhyolitic composition contain ²³⁸U and ²³²Th. These rocks outcrop in hills and surrounding mountains and extend to depth where they constitute a fractured aquifer, which underlies a granular one, whose constituents and permeable soils also contain ²³⁸U and ²³²Th and their decay series. Gross Alfa Activity and ²²²Rn are present in groundwater for human consumption above the Maximum Contaminant Level suggested by the US Environmental Protection Agency, reaching the lower threshold for uranium ores and mining in northern Mexico. ²²²Rn and ²²⁰Rn were also measured in soil-air and in dwellings. Rhyolitic-ignimbrites have been used in industrial applications since the 1960s, and milling and tailing operations contain radionuclides that can be transported by air or leached into groundwater. The combined exposure to various sources of radioactivity and metals, primarily the use and ingestion of groundwater and dust may impact the human health in the study area, in addition to the inhalation of ²²²Rn-²²⁰Rn and dust. Additional and detailed research should be done for long-term measurements and risk assessment for radiological protection, also considering the evaluation and combined exposure to industrial and agricultural contaminants.