Investigation on Shallow Groundwater in a Small Basin Using Natural Radioisotopes.

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The authors conducted an investigation on shallow groundwater using natural radioisotopes as indicators in the small basin of the Hinuma River, Kasama City, Ibaraki Prefecture, Japan. 3H concentrations in the groundwater showed that it originated from precipitation in the 1960's. Since 222Rn concentrations decreased as groundwater flowed downstream, they were influenced by infiltration of surface water. Especially, during the irrigation period, the decrease of 222Rn concentrations was remarkable in the lowland. From the distribution of 222Rn concentrations in surface water, the sections where groundwater seeped into a river were found, and a quantitative analysis of groundwater seepage in the two sections was conducted on the basis of 222Rn concentrations in groundwater and in surface water. The ratios of groundwater seepage to the flow at the upstream station for the two sections were about 5% and 10%, respectively. The water movement within the basin, i.e., the actual manner in which surface water infiltrated underground and groundwater seeped into a river, was clarified by analyzing the variations of natural radioisotope concentrations in water and the water balance of the basin.
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