Description

[Objectives]

   It is essential to quantify groundwater effluent and reservoir water influent in ponds in order to use water resources effectively and to predict water quality. However, conventional methods measure only the amount of surface water inflow and outflow, and can only reveal differences between groundwater effluent and reservoir water influent. It is not possible to quantify both parameters simultaneously. In this study, we concentrated on the radon-222 (²²²Rn) content in the environment and developed an analytical method using ²²²Rn- and waterbalance equations.

[Results]

   Fig. 1 shows ²²²Rn- and water-balance in a pond. Although groundwater influent and reservoir water influent are unknown quantities, it is possible to quantify them both by constructing ²²²Rn- and water-balance equations. The dispersion of ²²²Rn into the atmosphere, an important factor contributing to ²²²Rn loss, is calculated assuming that there is a stagnant film between water and air. The thickness of this stagnant film was empirically estimated to be 830μm.
   The developed method was applied to a pond near a landslide-prone area in Japan, where it is considered that reservoir water influent is a major cause of landslides. Table 1 shows the results of the field investigation. Using these data, we constructed ²²²Rn- and water-balance equations, and groundwater effluent and reservoir water influent were calculated to be 0.67 L/s and 0.41 L/s, respectively. As stated above, conventional methods could not quantify groundwater effluent and reservoir water influent simultaneously, but our new method has made this possible.
   The on-site application of the method is expected to facilitate effective water use, predict water quality, and prevent landslides.

Figure, table

1.  

   

   Fig. 1. ²²²Rn- and water balance in a pond (+ : supply, - : loss.).
   Water balance was maintained (water level remained constant) when the amount of surface water inflow and groundwater effluent was equal to the amount of reservoir water outflow, evaporation, and reservoir water influent. ²²²Rn balance was maintained (concentration remained constant) when the amount of surface water inflow and groundwater effluent equaled the amount of radioactive decay, dispersion, reservoir water outflow, and reservoir water influent.

2. Table 1. Results of field investigation.