RESIDENCE TIME OF GROUNDWATER IN A SAND DUNE

Abstract

A severe storm as shown in Figure 3 attacked Japan from January 31 to Feburary 1 in 1970 and caused one of the highest storm surge along the Japan Sea coast. At this time a great amount of sea water invaded a small fishing village named Ashizaki located on a sand dune at the periphery of the Kurobe Alluvial Fan. The topography and geology of the sand dune are shown in Figures 1 and 2. The invaded sea water infiltrated into the sand dune and about three-hundred wells which have been used for drinking were polluted by salt water. The decrease in Cl^- concentration had been investigated from April to November at the interval of one or two-month in order to estimate the residence time of groundwater in the sand dune. The residence time defined here refers to the mean residence time spent by individual water molecules in a reservoir, i. e. a sand dune aquifer in this case. An exponential decay of Cl^- concentration as can be read from Figures 5 and 7 is found by observation and the decay constant is calculated to be about 0.02 day^-1 by the method of least squares. Theoretical solution of Cl^- concentration in the sand dune may be expressed by the following equation provided the circulation is in steady state and the reservoir is well mixed;
S=S₀e^-αt
where S denotes the Cl^- concentration of groundwater, So refers to the initial value of S at the time of sea water invasion, t is the time, and 1/α means the residence time of groundwater. From the theory and observations it is concluded that the residence time of groundwater in this sand dune is about 50 days. The value is very short compared with the residence time of other groundwaters. This may be caused by the ample groundwater supply from the upstream side of the alluvial fan which have a relatively steep surface slope.