DOWNSTREAM CHANGE OF DISSOLVED URANIUM LOAD IN MOUNTAIN STREAMWATER; AN EXAMPLE FROM THE KANAMARU AREA, YAMAGATA, NORTHEASTERN JAPAN

The purpose of this study is to know U behavior in streamwater, which is important on fields such as radioactive wastes disposal and geochemical exploration for U. We studied U load change in streamwater downstreaming from a U-bearing pegmatite deposit, and revealed that U load decreases abruptly in particular sections such as that with a landslide preventing dam.

The studied stream Kaminosawa runs along the boundary between the Yamagata and Niigata Prefectures in northeastern Japan. Total distance of the studied section is 3.3 km, and the high U pegmatite deposit locates near the uppermost part of the section. The streamwater was sampled at seven and six points on the main stream and branches, respectively. The flow rate and water quality such as temperature, pH, EC and ORP were measured at the same points. The major and trace elements were analyzed with ion chromatography and ICP-MS. The stream survey was done in August 2004 under the low flow condition, which had about twice as base flow.

The flow rate of the mainstream increased downward due to inflow from branches. Degree of underflow looks to be very low, because of thiner floor sediments and rapid surface flow speed.

The U load of the uppermost point, where is the upstream side from the mineralized area, was lowest (0.3 mg / min). Then it suddenly increased (10.9 mg / min) after inflow from the mineralized area, which is the only section increasing U load. The U load decreased abruptly in two sections of downstream side from the mineralized area. One is the section immediately downstream of the mineralized section (reduced from 10.9 to 6.7 mg / min along 400 m), and the other is the section including a concrete dam built for preventing landslide (reduced from 4.7 to 2.9 mg / min along 300 m). In the other sections, the U load reduced slightly.

There is a possible interpretation for the reduction of the U load downstream of the dam.The dam was filled with sediments consisting mainly of sand and pebbles, and partly of muddy beds rich in organic materials. Measured lower ORP at the point downstream side of the dam compared to the other points suggests that the pore water of the fills in the dam was in reducued environment. A part of the dissolved U in streamwater was possibly absorbed by the organic matter and/or precipitated in the sediments in the dam.