Analysis of recharge by paddy field irrigation using 222Rn concentration in groundwater as an indicator

Abstract

Groundwater recharge must be well understood in order that water resources can be effectively used and contamination of groundwater by surface water can be prevented. Evaluating recharge from groundwater levels alone does not show downward flow when outflow exceeds recharge. This article showed the variation of 222Rn concentrations in soil water according to degree of saturation in the aquifer. The theory used for analysis is based on gas solubility theory between two phases, liquid and gas. Field investigation confirmed that 222Rn concentration in the groundwater at the surface of a saturated zone decreases with downward flow in the unsaturated zone above. After that, we investigated the recharge by paddy fields with the method using 222Rn concentration in the groundwater at the surface of a saturated zone. The results showed that the state of recharge could be analyzed as expected, i.e., the occurrence of downward flow of soil water pushed out by irrigation water, unsaturated percolation during the irrigation period, and the redistribution of soil water after the release of ponding water. The degree of saturation of downward flow in the unsaturated zone was calculated to be about 50% from the ratio of 222Rn concentration in the irrigation period to that in the non-irrigation period. This shows that the downward flow in the underlying layer was very unsaturated. When the groundwater table is deep, and less pervious soil overlies more pervious soil, it is known that percolation from paddy fields is unsaturated in the underlying layer. Since the saturated hydraulic conductivity of the underlying layer was on the order of 10−1 cm/s and that of surface soil is 10−5 to 10−6 cm/s, it is likely that unsaturated downward flow occurred. The 222Rn concentration in the groundwater at the surface of a saturated zone is a useful indicator for detecting the existence of downward flow to the saturated zone through an unsaturated aquifer.