Solute transport characteristics and influencing factors in a coastal unconfined aquifer under tidal action identified by image monitoring in a laboratory experimental setup

Studies of solute transport in unconfined coastal aquifers typically neglect oceanic oscillations and assume a static seaward boundary condition defined by the mean annual sea level. In this paper, an image monitoring method is used to establish the quantitative relationship between the concentration of a colored tracer and the hue value of the image. The digital image was used to determine the tracer concentration distribution. Controlled laboratory experiments were conducted to quantitatively study the relationship between the tidal amplitude and the solute transport time and dispersion. Experiments show that the solute transport time is not associated with the tidal amplitude, but only with the average hydraulic gradient between the seaward boundary and the landward boundary. The dispersion range of the solute plume increases with the increase of the tidal amplitude. On large-scale beaches in nature, when the average hydraulic gradient of the aquifer between the sea side and the land side is small enough, the dispersion coefficient is proportional to the tidal amplitude in equal proportion. Therefore, when it comes to the transport time of pollutants, it is possible to set the seaward boundary as the mean annual sea level. In a study of the dispersion range of pollutants, it is inappropriate to set the seaward boundary as the mean annual sea level.