17:15 〜 18:30
[AGE28-P06] Tidal Method for the Estimation of Coastal Aquifer Properties Based on Theoretical Solutions: Advantages and Limitations
キーワード：coastal aquifer、tidal method、theoretical model、Overview
Determination of the hydraulic properties of coastal aquifer systems has practical implications that are related to the many important issues such as seawater intrusion, submarine groundwater discharge, migration of contaminants, assessment of water resources, and geotechnical engineering. Compared with conventional well techniques such as the pumping test performed in a localized area, the hydraulic properties obtained by the tidal method are more representative of the large-scale hydrogeological characteristics. This is because the results are derived from the signals of hydraulic heads transmitted from the coastline where ocean tides are imposed across a long distance to the observation well at which the hydraulic head fluctuations are measured. In addition, the tidal method is more cost effective as it requires fewer wells compared to the pumping test method, and uses natural waves as input signals, thus eliminating the need for producing hydraulic signals such as water flow or changes in hydraulic pressure for the measurements.
Over recent decades, many analytical solutions that consider different types of coastal aquifer systems or models have been developed. Major models include single-layer perfectly confined coastal aquifer; single-layer unconfined coastal aquifer; two-layer island aquifer; three-layer with different boundary conditions; L-shaped boundary single-layer; L-shaped boundary three-layer; and U-shaped boundary non-leaky aquifer. Theoretical solutions to these models can be used as a theoretical basis for the tidal method that characterizes the hydraulic properties of a coastal aquifer system using hydraulic response measurements in an observation well or wells induced by tidal waves in the ocean.
The hydraulic properties of an aquifer can in principle be estimated through fitting a series of time-dependent changes of hydraulic head detected in an observation well to those calculated based on theoretical solutions. For simplicity, however, most theoretical solutions only consider one tidal component and idealized boundary conditions, although in reality multiple tidal components exist simultaneously and boundary conditions can be more complicated. For practical applications of the tidal method and to increase its reliability, multiple tidal components should be considered and models considering more complicated boundary conditions should be developed. In addition, methods that can determine both the hydraulic conductivity and storage coefficient, rather than only the hydraulic diffusivity, i.e., the ratio of hydraulic conductivity to storage coefficient of coastal aquifer, should be developed. Cautions should be taken into account when using the tidal method because earth tides and changes in local atmospheric pressure may induce similar tidal fluctuations in the hydraulic head within inland observation wells.
In this presentation, we overview the advantages of the tidal method, currently available theoretical models, limitations and future perspective associated with the tidal method.
Over recent decades, many analytical solutions that consider different types of coastal aquifer systems or models have been developed. Major models include single-layer perfectly confined coastal aquifer; single-layer unconfined coastal aquifer; two-layer island aquifer; three-layer with different boundary conditions; L-shaped boundary single-layer; L-shaped boundary three-layer; and U-shaped boundary non-leaky aquifer. Theoretical solutions to these models can be used as a theoretical basis for the tidal method that characterizes the hydraulic properties of a coastal aquifer system using hydraulic response measurements in an observation well or wells induced by tidal waves in the ocean.
The hydraulic properties of an aquifer can in principle be estimated through fitting a series of time-dependent changes of hydraulic head detected in an observation well to those calculated based on theoretical solutions. For simplicity, however, most theoretical solutions only consider one tidal component and idealized boundary conditions, although in reality multiple tidal components exist simultaneously and boundary conditions can be more complicated. For practical applications of the tidal method and to increase its reliability, multiple tidal components should be considered and models considering more complicated boundary conditions should be developed. In addition, methods that can determine both the hydraulic conductivity and storage coefficient, rather than only the hydraulic diffusivity, i.e., the ratio of hydraulic conductivity to storage coefficient of coastal aquifer, should be developed. Cautions should be taken into account when using the tidal method because earth tides and changes in local atmospheric pressure may induce similar tidal fluctuations in the hydraulic head within inland observation wells.
In this presentation, we overview the advantages of the tidal method, currently available theoretical models, limitations and future perspective associated with the tidal method.