Distribution of Se in the rocks, soil, water and crops in Enshi County, China

Previous studies on Se in Enshi County, which is known as the "world capital of selenium", mainly focused on typical excess-Se areas, such as Yutangba. In this paper, the distribution of Se in areas of Enshi County with different Se environmental effects were systematically studied using the geochemical method. The results showed that sedimentary rocks that contain Se (from high to low) are black shale, carbonate rock, fine clastic rock and coarse clastic rock. The geotectonic setting is the main factor controlling Se in sedimentary rocks, followed by org.C. The average total Se concentration in the topsoil was 1.12 +/- 2.18 mg/kg (n = 19734), and that in the excess, high and moderate-Se areas were 6.64 +/- 6.02 mg/kg (n = 1189), 0.84 +/- 0.49 mg/kg (n = 15633), and 0.32 +/- 0.06 mg/kg (n = 2774), respectively. The increase in Se by precipitation is lower than that leached from the rocks and soils by precipitation, and Se is generally lost from supergene soils and rocks under natural conditions in Enshi. The mean concentrations of Se in corn, rice, potatoes and Chinese cabbage were 2.365 +/- 2.614 mg/kg (n = 18), 1.122 +/- 1.118 mg/kg (n = 27), 0.117 +/- 0.182 mg/kg (n = 10) and 0.311 +/- 0.342 mg/kg (n = 11) in the excess-Se areas, respectively, and 0.151 +/- 0.402 mg/kg (n = 113), 0.148 +/- 0.335 mg/kg (n = 74), 0.015 +/- 0.040 mg/kg (n = 84) and 0.066 +/- 0.126 mg/kg (n = 50) in the high-Se area, respectively. The Se intake in excess-Se areas was approximately 512 mu g/d, indicating a potential risk of Se poisoning, and that in the high-Se area was 68-150 mu g/d. The crops planted in the high-Se areas of Enshi County are excellent Se-rich foods for people.