URANIUM CONCENTRATIONS AT SHALLOW SEDIMENTARY SECTION IN NORTHEAST JAPAN : A CASE STUDY OF BEDROCK BUFFER AND RETARDATION FOR RADIONUCLIDES TRANSPORT

To help the understanding of the mass transport and retardation processes of solute, especially of radionuclides in a shallow environment, we have conducted a series of surface and drilling surveys on a Tertiary sedimentary sequence in a fixed geometrical extension, i.e., 0.25 square kilometers with a depth of 50m, in Yamagata district in Northeast Japan.

The Uranium concentrations up to 120 ppm sporadically occur in fluvio-brakish Neogene sediments in the Cretaceous granite basements. The sedimentary sequence conprises in arkosic nature where only few intercalations of silty, less permeable beds contain particularly organic materials and iron-sulfides. The U concentrations extends within one of these fine intercalations where the interstitial pore water contains an elevated value of dissolved U, up to 2.5 to 3.0 ppb.

An array of shallow explorating wells were drilled to study; geochemical extension of the anomaly, monitoring of the hydlauric pressure and sampling of pressured pore-water, in-situ hydraulic conductivity, hydraulic head distribution, and direct measurements of ground-water flow direction. The pore-water chemistry at these bore-holes have been monitored through the seasons, where we could recognize three principle aquifers in these depths.

Preliminary discusison leads to the model that the Uranium originating from the Cretaceous magmatic intrusives first transported in a form of solute in relatively reducing porewater in rapid fluvial sedimentations, to the redox boundary slightly below the sedimentation surface. The Present chemistry and mineralogy at the anomaly suggest the U attaching to a phosphate or amorphous mixture around the surface of pyrite and organic materials, are now extracting to the solution, some part in coloids, to the surface of the ground. This model suggests the elevated discharge of the retardation flux of radionuclides, at the shallow sedimentary horizons.