Analytical Study Of Groundwater-head Fluctuation In A Coastal Confined Aquifer System With A Sub-outcrop Covered By A Thin Silt Layer

This paper presents an analytical solution of groundwater head response to tidalfluctuation in a coastal multilayered aquifer system consisting of an unconfinedaquifer, a leaky confined aquifer and a semi-permeable layer between them. At itssubmarine outcrop, the confined aquifer is usually covered by a thin silt layer. Amathematical model is used to describe this system and the analytical solution of thismodel is derived. The silt layer reduces the amplitude of the hydraulic headfluctuation by a constant factor, and increases the phase by a positive shift (time lag),both of which depend on the leakance of the silt layer, the leakage and the storativityof the semi-permeable layer. The time lag does not exceed 1.5 hours and 3.0 hours forsemidiurnal and diurnal sea tides, respectively. When the leakance of the silt layer, theleakage and the storativity of the semi-permeable layer assumes certain special values,the solution becomes the existing solutions derived by several previous researchers.The amplitude of the hydraulic head fluctuation in the confined aquifer increasesstrictly with the leakance of the silt layer and decreases strictly with the leakage andthe storativity of the semi-permeable layer. The phase shift of the fluctuationdecreases strictly with the leakance of the silt layer and increases strictly with thestorativity of the semi-permeable layer. The impacts of the silt layer, the leakage andthe storativity of the semi-permeable layer on the tide-induced groundwater headfluctuation in the confined aquifer were analyzed and demonstrated to be significant.Hypothetical example shows that neglecting the silt layer may result in significantparameter estimation errors.