Evaluations of the downward velocity of soil water movement in the unsaturated zone in a groundwater recharge area using δ18O tracer: the Kumamoto region, southern Japan

Water and substances from the surface infiltrate the unsaturated zone before reaching ground-water. Yet, little study has been done on the unsaturated zone due to the difficulty of sampling. A lot of studies have been carried out on the top soil down to a depth of one metre and on shallow aquifers because they are easily accessible for sampling. The unsaturated zone of the Kumamoto region recharge areas is important due to concerns about groundwater pollution from agriculture. The aim of this study was to estimate the downward velocity of soil water movement through the unsaturated zone and the recharge rate using delta 18O as a tracer. Five sampling sites were selected and a core was taken from each site. The cores were cut into 0.1 m pieces and soil water was extracted from each to analyze for delta D and the delta 18O content. Average delta D and delta 18O compositions of soil water were similar to the isotopic compositions of summer precipitation. Annual average recharge rate and the downward velocity of soil water in each site were estimated by fitting a vertical delta 18O profile pattern to a precipitation delta 18O time series as a theoretical water displacement flow model for recharge. An estimated annual average recharge rate in the recharge area ranged from 745 to 1058 mm/yr with the annual average downward velocity of 1.37 to 2.34 m/yr. Based on the estimated downward velocity, the infiltration time for soil water to reach the aquifer was determined as ranging from 9 to 24 years, which corresponds with previous groundwater age estimations presented in an earlier published study on the same area. It was assumed that contaminants will reach aquifers in 9 to 25 years if the effects of diffusion and microbiological reaction are not taken into account.