Characteristics of Oxygen Isotopic Ratios of Soil Water and Groundwater at Paddy, Upland and Grove Fields in Tsukuba, Ibaraki Prefecture, Central Japan

Abstract

Groundwater, soil water, precipitation and flooded water were collected periodically at paddy, upland and forest fields in the National Institute for Agro-Environmental Sciences (NIAES) from April to December, 1999 to study whether the oxygen and hydrogen isotopic ratios of soil water and groundwater can be useful as tracer to estimate the soil water movement. To consider how the flooded water have effects on the river water, river, flooded and irrigation water samples were also taken in the Kinu River basin at four times. δ¹⁸O values of them were analyzed for all samples, and δD were also analyzed for precipitation water samples taken in NIAES, and river, flooded and irrigation water samples in the Kinu River basin.

δ¹⁸O values of soil water at paddy field showed that the flooded water percolated to the upper part of the Joso clay layer. δ¹⁸O values of soil and groundwater at upland and grove fields are different from those of paddy field. It is suggested that the estimation of the soil water movement could be possible by using the oxygen and hydrogen isotopic ratios of soil water.

The altitude effect was found for the Kinu and Ta rivers. Each rivers, in addition, have their own isotopic compositions in some places, thus isotopic compositions of river water can be used for estimation of the groundwater flow and presumption of the recharge area of groundwater.