Effects of multi-constituent tides on a subterranean estuary

Abstract

Subterranean estuary, as a saltwater-freshwater mixing zone in a coastal aquifer, is significantly affected by tides. While many studies have been conducted to examine the effects of tides on groundwater dynamics in subterranean estuaries in relation to submarine groundwater discharge and seawater intrusion, the focus of the investigation, particularly for those based on numerical modelling, has been on tides with few frequencies, e.g., monochromatic tide and spring-neap tides (bichromatic). In this study, we examine numerically effects of measured multi-constituent tides on a coastal unconfined aquifer. With the antecedent tidal conditions accounted for using the Gamma distribution function, we conducted regression analysis and quantified the effects of measured multi-constituent tides on coastal unconfined aquifers in a prolonged and cumulative fashion. The results showed that the response of submarine groundwater discharge to the tidal fluctuation is mostly instantaneous. The coastal aquifer behaved like a low-pass filter that smooths out high-frequency tidal signals and leaves behind low-frequency signals to affect upper saline plume and lower saltwater wedge. The upper saline plume and lower saltwater wedge were significantly affected by antecedent tidal conditions at monthly and yearly time scales, respectively. Sensitivity analysis showed that the low-pass filter capacity can be altered by soil hydraulic conductivity and beach slope. This study revealed the importance of antecedent tidal conditions for seawater intrusion and will help to improve our understanding of hydrological processes in natural coastal unconfined aquifers.