Influence of inundation of ground surface on 222Rn concentrations in shallow groundwater

Abstract

Radon-222 (222Rn) is a good indicator for analyzing the speed at which surface water infiltrates the ground and reaches groundwater. Preferential flow, which is very fast, is expected to occur when the ground surface is inundated. Piston flow, which is very slow, is expected to occur when the ground surface is not in undated but is only sprinkled with water such as during a light rainfall. We hypothesized that preferential flow would reduce the 222Rn concentration in groundwater but that piston flow would not ; our study verified this hypothesis experimentally. We then measured 222Rn concentrations in groundwater for one year in low land and upland locations. 222Rn concentrations in groundwater decreased at the lowland site, where the ground surface was inundated by irrigation and heavy rainfall, while 222Rn concentrations in groundwater did not decrease at the upland site, where the ground surface was not inundated.