Transport of Solutes With Different Release Mode Under Tidal Action in Coastal Unconfined Aquifers

Abstract

Previous studies on solute transport in coastal aquifers have mostly focused on the transport characteristics of instantaneous point sources. In actual situations, due to various causes of pollution, pollutants usually do not enter the aquifer instantaneously but continue to discharge into the groundwater at a certain speed. Due to different pollution conditions, pollutants usually enter the aquifer in various forms such as point sources and line sources. It is of great significance to study solute transport characteristics with different release modes in coastal aquifers for environmental impact assessment of coastal groundwater. This paper explored a laboratory-controlled experiment to research the transport characteristics of solute with different release modes in coastal unconfined aquifers under tidal action. This paper presented that the solute plumes of each release mode affected by tidal fluctuations all follow the transport characteristics of a large step to the sea and a small step to the land. The rising tide suppressed the original change trend of the concentration at each point. The spreading range of the solute plume in each release mode had periodic fluctuation. With the increase of the hydraulic gradient during the falling tide, the spreading range of the solute plume increased, whereas with the difference of the transport velocity of the leading edge and trailing edge of the solute plume during the rising tide, the spreading range decreased slightly. The variation characteristics of the spreading range of the continuous source mainly depend on the continuously released rate of the tracer. When the continuous release rate of the tracer and the discharge to the sea reach a dynamic equilibrium, the solute transport velocity of the continuous source will eventually oscillate around a relatively stable velocity value, and the solute concentration value of each point in the aquifer will eventually oscillate around a relatively stable concentration value, especially the concentration of each point in the continuous line source will be approximately equal to the initial concentration.