Complex sentence: Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released in significant quantities: the Th 4 + ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can reach 40 ppm.
Split into simpler sentences: Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released in significant quantities. However, the Th 4 + ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can reach 40 ppm.