Effectivity of dissolved SF6 tracer for clarification of rainfall–runoff processes in a forested headwater catchment

Understanding rainfall-runoff processes is crucial for prevention and prediction of water-related natural disasters. Sulfur hexafluoride (SF6) is a potential tracer, but few researches have applied it for rainfall-runoff process studies. We observed multiple tracers including SF6 in spring water at 1- to 2-hr intervals during rainstorm events to investigate the effectivity of SF6 tracer in rainfall-runoff studies through the clarification of rainfall-runoff process. The target spring is a perennial spring in a forested headwater catchment with an area of 0.045 km(2) in Fukushima, Japan. The relationship between the SF6 concentration in spring water and the spring discharge volume was negative trend; the SF6 concentration in spring water becomes low as the spring discharge volume increases especially during rainstorms. The hydrograph separation using SF6 and chloride ion tracers was applied for determining the contribution of principal sources on rainfall-runoff water. It suggested more than 60% contribution of bedrock groundwater at the rainfall peak and high percentage contribution continued even in the hydrograph recession phase. Based on observed low SF6 concentration in groundwater after heavy rainfall, the replacement of groundwater near the spring with bedrock groundwater is indicated as a mechanism for water discharge with low SF6 concentration during rainfall events. Consequently, rainstorm events play an important role as triggers in discharging water stored in the deeper subsurface area. In addition, SF6 tracer is concluded as one of the strongest tracers for examining rainfall-runoff process studies. And, therefore, this study provided new insights into the dynamics of groundwater and its responses to rainfall in terms of SF6 concentration variance in water in headwater regions.