Abstract
Water and substances from the surface infiltrate the unsaturated zone before reaching groundwater. 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 Kumamotoregion 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 δ18O 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 δD and the δ18O content. Average δD and δ18O 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 δ18O profile pattern to a precipitation δ18O 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 soilwater 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.
Highlights
The excessive application of chemical fertilizers and manures to agricultural lands has increased the influx of substances into the ground, which eventually reach and contaminate groundwater (SPALDING et al, 1982; UMEZAWA et al, 2008; ZHANG et al, 2014)
The aim of this study was to estimate the downward velocity of soil water movement through the unsaturated zone and the recharge rate using δ18O as a tracer
Annual average recharge rate and the downward velocity of soil water in each site were estimated by fitting a vertical δ18O profile pattern to a precipitation δ18O time series as a theoretical water displacement flow model for recharge
Summary
The excessive application of chemical fertilizers and manures to agricultural lands has increased the influx of substances into the ground, which eventually reach and contaminate groundwater (SPALDING et al, 1982; UMEZAWA et al, 2008; ZHANG et al, 2014). A direct link is created between the characteristics of the groundwater and ions, isotopes and metals on the surface without taking the unsaturated zone into consideration even though substances pass through this zone before reaching groundwater. DELIN et al (2017) applied rhodamine and Br tracers to a crude oil contaminated site to detect the influence of oil on the water infiltration rate in the unsaturated zone. These studies show that the plurality of flow processes such as prefe rential flow and matrix flow complicates water infiltration and contaminant behaviour through unsaturated media. Understanding the behaviour of water in the unsaturated zone is important for making decisions about groundwater usage and formulating mitigation measures against groundwater contamination
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