Groundwater response to tidal fluctuation in a two-zone aquifer

A coastal aquifer may consist of two zones of distinctly different hydraulic properties, with the coastal zone of limited width and the inland zone of infinite width. The analytical solutions for groundwater level response to tidal fluctuation in such a two-zone aquifer are derived and the underlying physical meanings are discussed. A dimensionless zone-contrast parameter r is introduced, which is controlled by ratio between the product of the transmissivity and storativity of the inland zone and that of the coastal zone. In the coastal zone, the amplitude of groundwater head fluctuation attenuates more quickly when r becomes greater. The amplitude declines exponentially with distance from the coastline, and the phase lag increases linearly only when r = 1. In the inland zone, however, the head fluctuation always exhibits an exponential attenuation of the amplitude and a linear increase of the phase lag for any value of r. Thus, the diffusivity of the inland zone can be readily assessed with measurements of fluctuation amplitudes or lags, as are frequently used when estimating aquifer parameters in homogeneous aquifers. Compared with Jacob (1950) solution for a semi-infinite homogeneous aquifer, the existence of the coastal zone reduces the amplitude in the inland zone by a spatially constant coefficient and increases the phase lag by a spatially constant shift. Inverse estimation of aquifer parameters are discussed based on the analytical solutions. The analytical solutions are used to estimate the aquifer parameters in a coastal two-zone aquifer located in the west part of Dongzhai Harbor, Hainan Province, China.