Présentations Colloques

Oral Presentation
8.02
Session 8.02: Groundwater Development and Protection in Coastal and Volcanic Environments with Complex Geological Structures
Levanon Elad
The mechanism of groundwater fluctuations induced by sea tides in unconfined aquifers
In this study we examine effects of tides on fluctuations of groundwater level (GWL) and of fresh-saline water interface (FSI) in unconfined coastal aquifers using a two-dimensional numerical model. The time-lags of hydraulic heads and salinities compared to sea level fluctuations are analyzed using cross-correlation analysis. The results show that both the GWL and the FSI are affected harmonically by sea tide fluctuations. However, significantly different time-lags are obtained between hydraulic head in the deeper and upper parts of the aquifer, and between head and salinity in the FSI. The hydraulic head in the deeper part of the aquifer reacts much faster to sea level fluctuations than in the upper part. Surprisingly, a similar difference was detected between the time-lag of the hydraulic head in the FSI and the time-lag of the salinity at the same location. Furthermore, the time-lag of the salinity in the FSI is similar to the time-lag of the GWL fluctuations. We suggest a mechanism in which two main processes are acting simultaneously - 1) tidal forcing at the sea floor boundary, and 2) attenuation at the GWL boundary. Sea tides cause a pressure head wave which propagates into the saturated zone of the aquifer, governed by the diffusivity of the aquifer (K Ss). This pressure head wave is attenuated at the water table due to the unsaturated flow within the capillary fringe, which occurs during GWL oscillations. Since the tidal forcing and attenuation by the GWL act on different boundaries, two dimensional distributions of time-lag and hydraulic head amplitude are created. The unsaturated flow within the capillary fringe plays a key role in the pace of water flow and solute transport, since it controls the actual movement of the fresh water body, which results in simultaneous fluctuations of the GWL and the FSI.
Israel