HYDROLOGY OF URANIUM-BEARING GROUNDWATER IN FOREST CATCHMENTS IN THE HUMID TEMPERATE CLIMATE; THE KANAMARU AREA, YAMAGATA, JAPAN

The purpose of this study is to make clear the principal features of hydrogeochemistry and hydrogeology of uranium-bearing groundwater in a small area composed of two first-order stream catchments in the humid temperate climate.

The study area of the Kanamaru is located in the northeastern part of the Honshu island, Japan. The area is in a foot part of a backborn mountain region in the Honshu island, with altitude of 150 to 310 m ASL, annual precipitation of about 3000 mm and average year-round temperature of 12 C. Cretaceous granitic and overlying Miocene sedimentary rocks are distributed around the study area. In total 12 boreholes were drilled and geological structure, lithology, rock and water geochemistry and hydrogeologic features were investigated.

Groundwater in the sedimentary rocks principally flows southward from the northern end of the small ridge to the southern end of the Kuzuresawa stream, concordant with the surface topography. Stepwise decrease of the hydraulic head with increasing depth is one of the most important hydrogeological features. The depth zone of the largest drop of the hydraulic head agrees to the weatherd topmost zone of the Cretaceous granite where many hair cracks were developed but completely filled with secondary hematite. Chemical change in the groundwater with increasing depth from the water table (GL -2 to 5 m) to the 50 m deep in the area is summarized as follows; increase of pH (5 to 7) and EC (5 to 30 mS/m), and decrease of DO (10 to 0 mg/l) and Eh (500 to 100 mV), and increase of Ca++ and HCO3-, and decrease of SO4-- in solute composition. The groundwater in the shallower part of the sedimentary rocks has the major ion and isotopic composition of D and 18O similar to those of the stream water. This suggests an important contribution of the groundwater in the shallower part of the sedimentary rocks to the source of the stream water. Although drilling-derived distubance is still recognized in minor elements in the groundwater in deeper levels, dissolved uranium concentrations look to be the highest (around 0.5 ppb) in the depth of the rock uranium anormaly (up to 150 ppm).