PREDICTION OF DISPERSION IN COASTAL AQUIFER CONCIDERING ADSOPTION AND RETENTION OF SALINITY

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The prediction of sea water intrusion into the coastal aquifers is the most important to control quality of ground water. In the sand box model experiments of salt water transport, the observed results require the defferent values of dispersion coefficient in the intruding process and the excluding process, respectively. In order to investigate this didcrepancy, some fundamental works are carried out. The first one is a column test to measure the change of salinity concentration with time in a series of the intruding process and the excluding process. The change is measured by means of the embedded probes of Electric Conductivity Meter in the column. The gradual sand, natural river sand and parasitic bead are selected for the test materials. In the repeated measurements with constant flux, the transport velocities gradually decrease with the number of repeat. Using the curve fitting method woth observed consentration and theoretical one , the dispersivity, decay constant and the retardation factor are identified. The second work is a sand box test to simulate seawater intrusion into the aquifer. The movement of fresh-salt water interface and the change of salinity concentration are measures. The shapes of fresh-salt water interface are different in the intruding process and excluding process. The third work is the measurement of vertical distribution of gronudwater electrical conductivity as an actual salinity distribution in the aquifer. From the measured results, it is confirmed that the dimensions of the dispersion zone in the intruding process are quite narrow, however, ones in the excluding process are wide in the excluding process due to the retention of salinity. From these works, it is clarified that adsorption of salinity is sufficiently feeble to the sand grains but the retardation effect is not negligible to simulate the salinity transport in the sand and to determine the dispersion coefficient.