Natural radioelement solution by circulating ground waters in the Stripa granite

Natural radioelement concentrations in ground waters from the fracture flow regime in the Stripa granite were monitored during the period 1977-1986. Uranium concentrations from depths of 80 m or less and in mine waters from above the hydraulic sink produced by the deepest mining level (410 m), range from 10 to 90 ppb. {sup 234}U/{sup 238}U activity ratios are characteristic of chemical leaching processes at the shallower depths whilst at about 350 m ground waters occur which have an additional component of {sup 234}U by alpha-recoil processes. At greater depths, U concentrations are indicative of the changed redox conditions and are subject to activity ratio enhancement by the alpha-recoil process. Groundwater residence times may be estimated from the rate of such recoil solution of {sup 234}U in zones where chemical leaching of U is not active. The {sup 222}Rn contents of the mine waters are extremely high and can be related to the {sup 222}Rn flux from fracture surfaces and the fracture width. The fracture apertures range from 10 mm for the shallow groundwater zone ({sup 222}Rn contents < 400 Bq kg{sup {minus}1}) to 20 {mu}m for water from the deepest borehole section ({sup 222}Rn content 100 kBq kg{sup {minus}1}). The activity ratios {sup 234}Th/{sup 238}U, {sup 210}Pb/{sup 222}Rn and {sup 210}Po/{sup 222}Rn have been used to estimate residence times for Th, Pb and Po isotopes. Solution of Ra is controlled by chemical rather than alpha-recoil processes. The {sup 226}Ra content of the groundwater increases with salinity and is well correlated with groundwater Ca content. The {sup 228}Ra/{sup 226}Ra activity ratios in the ground waters suggest that water/rock interaction occurs predominantly with the average granitic matrix rather than with highly uraniferous fracture mineralization.