Iodine and Selenium Distribution in Soils and Groundwater in Rural Areas of Oryol Oblast as a Risk Factor for Thyroid Cancer Incidence

The territory of Russia, including Oryol oblast, is characterized by a deficiency of iodine and selenium in the environment, which causes an increased risk of thyroid cancer (TC) among the local population. The purpose of our study is to identify the features of spatial heterogeneity of iodine and selenium content in environmental components (soils and natural waters) in Oryol oblast in their relationship with the incidence morbidity of thyroid cancer among the local population. Samples of pasture soils (20 cm core (n = 54)) and groundwater using for drinking purposes (n = 104) have been collected in 52 rural settlements. A high heterogeneity in the content of the elements is shown in both soils and natural waters. The concentration of I and Se has proven to be lower in Greyic Phaeozems Albic (the median concentration of I is 1.68 mg/kg; Se = 0.155 mg/kg) than in Voronic Chernozems Pachic (I = 2.53 mg/kg; Se = 0.158 mg/kg). The iodine concentration was below the optimal threshold (5 mg/kg) in all soil samples; in 55% of samples, the selenium content was below the optimum (0.2–0.7 mg/kg). The distribution of the elements in groundwater is generally related to aquifers. Higher iodine concentrations are characteristic of the Frasnian carbonate-terrigenous complex (I = 9.89 μg/L; Se = 0.09 μg/L), while lower iodine concentrations are related to the Famennian carbonate complex (I = 3.83 μg/L; Se = 0.2 μg/L). The iodine concentration has proven to be below the optimum (10 μg/L) in 67% of groundwater samples and in 99% for selenium (2.5 μg/L). Significant differences in iodine and selenium concentrations have been observed in areas with low and high thyroid cancer risk. The results make it possible to assess the effect of the elemental status of environmental components on the distribution of thyroid cancer incidence among the local population.