【作者】 郭敏；

【导师】 万军伟；

【作者基本信息】 中国地质大学 ， 水文地质学， 2022， 博士

【摘要】 滨海含水层处于海-陆交界带的中心,是陆地地下水与海水物质和能量转换的关键带,其物质和能量的转化关系既受陆地地下水流场的控制,同时也受到海洋水动力条件,尤其是海水潮汐的影响。不同的地下水系统结构、流场特征以及海洋水动力条件,其物质和能量的转化关系极不相同。例如,承压含水层由于其弹性给水度很小,压力传导系数很大,滨海地区承压含水层表现出对潮汐能量(水压力)的响应较为敏感,海水波动引起地下水波动的幅度和影响范围较大;而潜水含水层由于其重力给水度较大,压力传导系数很小,滨海地区潜水含水层表现出对潮汐能量(水压力)的响应不敏感,海水波动引起地下水波动的幅度和影响范围较小,正是由于这一原因,目前承压含水层地下水潮汐效应的研究较多,而潜水含水层中的潮汐能量效应则通常被忽略。但潜水含水层在潮汐作用下由于地下水流速、流向的变化会导致物质运移的速度和方向发生怎样的变化?受哪些影响因素控制的研究相对较少。相比承压水,潜水污染问题更为突出,潮汐影响下潜水含水层溶质运移机理及控制因素尚不够清晰。因此,亟待开展滨海潜水含水层溶质运移机理研究,该研究不仅是海岸带污染水文地质学的一个亟待解决的基础理论问题,对深化认识滨海潜水含水层溶质运移规律有重要意义,同时对滨海地区地下水环境保护、合理开发利用以及突发污染事故情况下地下水污染物的预测、污染物的控制以及污染场地修复等都具有重大的实际意义。为此,本文以水文地质学、地下水动力学、地下水溶质运移等理论为支撑,在搜集总结前人研究数据和成果的基础上,采用室内控制性实验,探索潮汐作用下滨海潜水含水层中溶质运移的特征、机理及影响因素,这其中涉及的主要关键问题有:(1)潮汐影响下滨海潜水含水层地下水潮汐效应及溶质运移机理;(2)潮汐振幅与溶质运移轨迹和弥散范围的定量关系;(3)咸淡水界面对滨海潜水含水层不同密度的示踪剂运移的影响;(4)滨海潜水含水层中示踪剂的不同投放形式(模拟污染方式)溶质运移特征;(5)滨海潜水含水层中示踪剂的不同投放位置(模拟污染泄露位置)溶质运移特征。基于研究目标,本文的主要研究内容和方法包括以下几部分内容:(1)基于不同边界类型的室内控制性实验,定量分析潮汐对潜水含水层中地下水流场和溶质运移的影响,通过开展忽略潮汐波动的溶质运移实验和存在潮汐呈周期性波动的溶质运移实验,结合各监测点的水位数据和溶质晕轨迹形态的统计学数据,分析潮汐作用下滨海潜水含水层地下水的潮汐效应和溶质运移的特征机理。(2)基于不同潮汐振幅下的室内控制性溶质运移实验,结合解析法定量研究潮汐振幅大小对滨海潜水含水层溶质运移的影响,建立潮汐振幅与溶质运移轨迹和溶质晕弥散范围的定量关系。(3)基于不同海水密度下不同密度的示踪剂室内控制性溶质运移实验,通过控制海水密度开展多组不同密度示踪剂的室内控制性溶质运移实验,分析示踪剂密度对溶质运移的影响,以及咸淡水界面对不同密度的示踪剂运移的影响。(4)基于示踪剂不同投放形式的室内控制性实验,通过将示踪剂设置为瞬时点源、连续点源、瞬时线源、连续线源的四种不同投放形式(分别模拟污染物污染含水层的不同方式),分析不同投放形态的示踪剂在受潮汐影响的滨海潜水含水层中运移特征。(5)基于示踪剂不同投放位置的室内控制性实验,通过将示踪剂的投放位置设置为补给区饱水带、补给区潜水面附近、补给区毛细饱和带、排泄区饱水带、排泄区毛细饱和带五组不同投放区域(分别模拟污染物污染含水层初始位置的不同),分析滨海含水层各区域发生溶质泄露时的溶质运移特征。通过以上研究,取得了以下成果:(1)相较于无潮汐作用条件,潮汐作用下,由于海水和地下水的补排关系交替改变,溶质晕运移过程中多次在海陆之间往复移动,溶质晕的整体弥散范围也出现周期性的波动;涨潮控制含水层中各点示踪剂浓度的增减。(2)潮汐幅度对地下水潮汐效应的相位的滞后没有影响,与地下水位波幅呈等比正比;在自然界大尺度海滩的溶质运移问题中,当平均海水水位与陆地水位水头差足够小时,潮汐振幅与地下水位波幅、流速、弥散系数呈等比正比。(3)低密度的示踪剂在含水层上部运移,而由于高密度示踪剂的不稳定性,高密度的溶质晕在垂向向海底下沉,密度越大,溶质晕在垂向上向下的运移更远;海水的密度对低密度示踪剂的运移和排泄影响并不大,而由于高密度示踪剂的不稳定性,若忽略海水的密度,由于溶质晕密度大于淡水密度,溶质晕将在含水层底部即靠近海床处排泄入海。(4)不同投放形态的溶质运移规律依次为:(1)瞬时点源:由于海水和地下水的补排关系交替改变,溶质晕运移过程中多次在海陆之间往复移动,含水层中任意一点浓度处于上升期,涨潮抑制其增大,其处于下降期,涨潮抑制其减小。(2)连续点源:由于示踪剂的持续释放量和溶质晕的向海排泄量达到了一个动态平衡,溶质晕形态会达到一个动态平衡状态,涨潮抑制含水层中各点浓度的增大。(3)瞬时线源:瞬时线源的运移和弥散规律均与瞬时点源的运移和弥散规律一致,仅仅由于在垂向上含水层底部流速相较于上部更大,而导致线源整体倾斜,线源下部相较上部向海运移更快。(4)连续线源:连续线源的运移和弥散规律均与连续点源的运移和弥散规律一致,仅仅由于在垂向上含水层底部流速相较于上部更大,而导致线源整体倾斜,线源下部相较上部向海运移更快,此外由于溶质晕的持续释放和向海排泄量最终会达到平衡状态,溶质晕各点浓度值最终均在1000mg/L附近波动。(5)含水层不同区域发生溶质泄露的运移规律依次分别为:(1)补给区饱水带:由于投放点距隔水底板较近,垂向分流速较小,因此溶质晕垂向位移较小;由于示踪剂的投放点位于远离排泄边界的补给区,且位于水平向分流速远大于垂向分流速的饱水带底部,因此水平向弥散范围远大于垂向。(2)补给区包气带:在垂向上由于毛细饱和带补给区和排泄区垂向水势规律相反,因此不同时期溶质晕垂向运移规律相反;弥散范围整体依旧为上述规律,但由于径流区较小的垂向水力梯度而在此期间弥散范围几乎不变。(3)补给区潜水面处:垂向运移规律同时受控于退潮时毛细胞带的等水势线和涨潮时饱水带中的等水位线,由于补给区中两者受潮汐影响呈现相反的变化规律,在排泄区中两者则呈现相同的变化规律,因此垂向运移在各时期存在较大差异。(4)排泄区饱水带:由于示踪剂投放点靠近排泄边界,因此溶质晕快速达到“稳态”,溶质晕轨迹和弥散范围受回灌水侵入和排泄的影响显著;由于示踪剂投放点位于含水层的排泄区,排泄区垂向分流速相对其他区域较大,因此垂向运移速度不再远小于水平向运移速度。(5)排泄区包气带:“稳态”前,随着潮汐涨落毛细饱水带垂向水势梯度方向的变化而在垂向上下移动;“稳态”后,溶质晕位置受海水上部“0”浓度的海水回灌和下部前期排泄物的回灌同时控制而小幅波动,溶弥散范围同时也受其影响波动。本文的主要创新点包括:(1)揭示了潮汐周期波动下,滨海潜水含水层地下水溶质的振荡式运移特征以及其主要的影响因素;(2)采用比色法构建了食用亮蓝溶液浓度与色调的定量关系,为采用影像技术定量记录地下水溶质运移过程提供了技术支撑,既克服了对含水介质结构和流场的干扰问题,同时也可以获得整个流场中不同空间点溶质浓度随时间和空间的变化规律,为地下水溶质运移室内物理模拟研究提供了新方法。更多还原

【Abstract】 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.更多还原