| The coastal aquifer is at the center of the sea-land boundary zone,and it is the key zone for the conversion of substance and energy between terrestrial groundwater and seawater.The conversion relationship between substance and energy is not only controlled by the flow field of terrestrial groundwater,but also affected by the marine hydrodynamic conditions,especially the seawater tide.Different groundwater system structure,flow field characteristics and ocean hydrodynamic conditions have very different transformation relationship between matter and energy.For example,due to the small elastic water yield and large pressure transmission coefficient of confined aquifer,the confined aquifer in coastal area is sensitive to tidal energy(water pressure),and the amplitude and influence range of groundwater fluctuation caused by seawater fluctuation are large.Because of the large gravity water supply and the small pressure transmission coefficient of the phreatic aquifer,the phreatic aquifer in the coastal area is not sensitive to tidal energy(water pressure),and the amplitude and influence range of groundwater fluctuation caused by seawater fluctuation are small.It is precisely because of this reason that there are many studies on the tidal effect of groundwater in confined aquifer,and the tidal energy effect in phreatic aquifer is usually ignored.But what changes will happen to the velocity and direction of material transport due to the change of groundwater velocity and direction under tidal action? There are relatively few studies on which factors control it.Compared with confined water,the problem of phreatic water pollution is more prominent,and the mechanism and control factors of solute transport in phreatic aquifer under tidal influence are not clear enough.Therefore,it is urgent to study the solute transport mechanism of coastal phreatic aquifer.This study is not only a basic theoretical problem to be solved urgently in coastal pollution hydrogeology,but also has important significance to deepen the understanding of the solute transport rule of coastal phreatic aquifer.At the same time,it has great practical significance for groundwater environmental protection,rational development and utilization,prediction of groundwater pollutants under sudden pollution accidents,control of pollutants and remediation of contaminated sites in coastal areas.Therefore,based on the theories of hydrogeology,groundwater dynamics and solute transport in groundwater,and on the basis of collecting and summarizing the previous research data and achievements,this paper explores the characteristics,mechanism and influencing factors of solute transport in coastal phreatic aquifer under tide by indoor controlled experiments.The main key problems involved are:(1)The mechanism of solute transport in coastal phreatic aquifer under tide;(2)Quantitative relationship between tidal amplitude and solute transport trajectory and dispersion range;(3)Effects of saline-freshwater interface on transport of tracers with different densities in coastal phreatic aquifers;(4)The solute transport characteristics of tracers with different release forms(simulated pollution mode)in coastal phreatic aquifer;(5)The solute transport characteristics of tracers in different locations(simulated pollution leakage location)in coastal phreatic aquifer.Based on the research objectives,the main contents and methods of this paper include the following parts.(1)Based on the indoor control experiments of different boundary types,the influence of tide on the groundwater flow field and solute transport in the phreatic aquifer is quantitatively analyzed.Through the solute transport experiment that ignores the tidal fluctuation and the solute transport experiment with periodic tidal fluctuation,the tidal effect and the characteristic mechanism of solute transport in the groundwater of coastal phreatic aquifer under the action of tide are analyzed in combination with the water level data of each monitoring hole and the statistical data of solute plume trajectory shape.(2)Based on indoor controlled solute transport experiments under different tidal amplitudes,the influence of tidal amplitude on solute transport in coastal phreatic aquifer is quantitatively studied by analytical method,and the quantitative relationship between tidal amplitude and solute transport trajectory and solute plume dispersion range is established.(3)Based on the indoor controlled solute transport experiments of tracers with different densities under different seawater densities,the indoor controlled solute transport experiments of multiple groups of tracers with different densities were carried out by controlling the seawater density,and the influence of tracer density on solute transport and the influence of salt-fresh water interface on tracer transport with different densities were analyzed.(4)Based on the indoor control experiments of tracers with different release forms,the transport characteristics of tracers with different release forms in coastal phreatic aquifer affected by tides were analyzed by setting tracers into four different release forms(instantaneous point source,continuous point source,instantaneous line source and continuous line source)(simulating different ways of pollutant-contaminated aquifers respectively).(5)Based on the indoor control experiments of different placement positions of tracers,five groups of different placement areas(simulating the different initial positions of pollutant-contaminated aquifers respectively)were set up to analyze the solute transport characteristics when solute leakage occurred in each area of coastal aquifer by setting the placement position of tracers as the saturated zone in the recharge area,the vicinity of the phreatic surface in the recharge area,the capillary saturated zone in the recharge area,the saturated zone in the discharge area and the capillary saturated zone in the discharge area.Through the above research,the following research results are obtained.(1)Compared with the condition without tidal action,under tidal action,due to the alternate change of the replenishment and discharge relationship between seawater and groundwater,the solute plume moves repeatedly between the sea and land during the transport process,and the overall dispersion range of the solute plume also fluctuates periodically.The rising tide controls the increase and decrease of tracer concentration at each point in the aquifer.(2)The tidal amplitude has no effect on the phase lag of groundwater tidal effect,and is proportional to the amplitude of groundwater level.In the solute transport problem of large-scale beaches in nature,when the difference between the average seawater level and the land water level is small enough,the tidal amplitude is proportional to the amplitude,velocity and dispersion coefficient of groundwater level.(3)The low-density tracer migrates in the upper aquifer,and the high-density solute plume sinks in the vertical seabed due to the instability of the high-density tracer.The higher the density,the farther the solute plume migrates in the vertical downward.The density of seawater has little effect on the transport and excretion of low-density tracers.However,due to the instability of high-density tracers,if the density of seawater is ignored,because the density of solute plume is greater than that of fresh water,the solute plume will be discharged into the sea near the seabed at the bottom of the aquifer.(4)The solute transport rules of different release forms are as follows:(1)Instantaneous point source: Due to the alternate change of the recharge and discharge relationship between seawater and groundwater,the solute plume moves repeatedly between land and sea in the process of transport.The concentration of any point in the aquifer is in the rising period,and the tide inhibits its increase.It is in the declining period,and the tide inhibits its decrease.(2)Continuous point source: Since the continuous release amount of tracer and the seaward discharge amount of solute plume reach a dynamic equilibrium,the shape of solute plume will reach a dynamic equilibrium state,and the rising tide inhibits the increase of concentration at each point in the aquifer.(3)Instantaneous line source: The transport and dispersion rules of the instantaneous line source are consistent with those of the instantaneous point source.Only because the flow velocity at the bottom of the vertical aquifer is larger than that at the top,the line source is inclined as a whole,and the lower part of the line source moves faster towards the sea than the upper part.(4)Continuous line source: The transport and dispersion of the continuous line source are consistent with the transport and dispersion of the continuous point source.Only because the flow velocity at the bottom of the vertical aquifer is larger than that at the upper part,the line source is inclined as a whole,and the lower part of the line source migrates faster than the upper part.In addition,due to the continuous release of the solute plume and the discharge to the sea,the concentration of the solute plume at each point will eventually fluctuate around 1000 mg/L.(5)The transport rules of solute leakage in different areas of aquifer are as follows:(1)Saturated zone in the recharge area: Since the delivery point is close to the bottom plate of the riser and the vertical velocity is small,the vertical displacement of the solute plume is small.Since the release point of tracer is located in the recharge area far from the discharge boundary and at the bottom of the saturated zone where the horizontal partial flow rate is much larger than the vertical partial flow rate,the horizontal dispersion range is much larger than the vertical.(2)Unsaturated zone in the recharge area: In the vertical direction,due to the opposite vertical water potential rule in the recharge area and discharge area of capillary saturated zone,the vertical transport rule of solute plume in different periods is opposite.The dispersion range remains the same as above,but the dispersion range is almost constant during this period due to the small vertical hydraulic gradient in the runoff area.(3)Near the phreatic surface in the recharge area: The vertical transport rule is also controlled by the water potential line in the hair cell zone at the ebb tide and the water level line in the saturated zone at the ebb tide.Since the two in the recharge area show the opposite change rule under the influence of tide,and the two in the discharge area show the same change rule,the vertical transport varies greatly in different periods.(4)Saturated zone in the discharge area : Because the tracer delivery point is close to the discharge boundary,the solute plume quickly reaches the steady state,and the solute plume trajectory and diffusion range are affected by the intrusion and excretion of recharge.Since the tracer release point is located in the discharge area of aquifer,the vertical flow velocity in the discharge area is larger than that in other areas,so the vertical transport velocity is no longer much smaller than the horizontal transport velocity.(5)Unsaturated zone in the discharge area: Before steady state,it moves up and down vertically with the change of vertical water potential gradient direction of tidal fluctuation capillary saturated zone;after steady state,the position of the solute plume is controlled by the ’ 0 ’ concentration of the upper seawater and the early excreta of the lower seawater.The main innovations of this paper are as follows.(1)The oscillatory transport characteristics and influencing factors of groundwater solute in coastal phreatic aquifer under tidal cycle fluctuation are revealed.(2)The image monitoring method was used to construct the quantitative relationship between the concentration of edible bright blue solution and hue,which provided technical support for quantitatively recording the transport process of groundwater solute by using image technology.It not only overcame the interference of water-bearing medium structure and flow field,but also obtained the variation rule of solute concentration at different spatial points in the whole flow field with time and space,providing a new method for the indoor physical simulation of groundwater solute transport. |
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