Tides And Waves Induced Groundwater Dynamics In Coastal Aquifers

Research of groundwater dynamics at coastal aquifer is of great significance, it contributes to the grasp of groundwater flow pattern, understanding of the aquifer structure, the evaluation of aquifer hydrogeological parameters; it contributes to solve the increasingly concerned environmental geological problems(such as transport of pollutant in the coastal zone, seawater intrusion, submarine groundwater discharge and nutrient flux into the sea, etc.); it contributes to the protection of fresh groundwater in coastal zone, the protection of tidal flats ecosystem, the development of coastal zones and so on.This thesis contains the following three studies. The first is analytical studies for groundwater flows in common coastal aquifers incorporating the effects of tides and a single-well pumping. The second includes field observations and numerical simulations of groundwater flow and solute transport in a coastal sandy beach aquifer system in the Laizhou Bay subjected to tides and waves. The third one provides field observations and analysis of groundwater-seawater exchange induced by waves in the subtidal zones. The details of the three studies are described as follows.Groundwater table or head in the coastal aquifers usually fluctuates due to tidal effects. There have been a lot of analytical solutions describing tidal groundwater table or head fluctuations in various coastal aquifers. Tidal effects have been widely used as an economical and effective method to estimate aquifer parameters. With the development of social and economy in coastal zone, especially the increasing demand for aquaculture, beach dewatering, marine retaining structures and seawater intrusion protection, water pumping from or injecting to coastal aquifers is very common. However, there are few analytical solutions describing the groundwater flow subjected to pumping as well as tides. Here we derived analytical solutions for groundwater table or head variations during a constant rate pumping from a single, fully-penetrating well in tidally-influenced coastal aquifer systems using the image method and superposition principle. A series of relevant mathematical models and their analytical solutions were given with different types of boundary conditions and/or aquifer structures. Solution analyses indicate that the shape of drawdown curve strongly depends on the initial tidal level at the initial time of pumping, which should be paid great attention in applications.Marine forces such as tides and waves affect the groundwater dynamics, solute transport and pollutants migration in coastal aquifer systems. Due to complexity and nonlinearity of the density-dependent groundwater flow, it is almost impossible to predict the groundwater salinity distributions subjected to tides and waves using analytical solutions. At present, even numerical simulations of groundwater dynamics under the joint effects of tides and waves are very limited. In fact, there are no reports in literature on numerical simulations to field data subjected to both tides and waves. Here we used field observation data in a sandy beach in Laizhou Bay, which was subjected to both tides and waves, to simulate the density-dependent groundwater flow by means of the MARUN code coupling groundwater flow and solute transport. In the model, the superposition of the observed tidal data and the time-varying phase averaged wave data are used to is set the seaward boundary. Simulated results reproduced the observed groundwater heads and salinities in 8 observation wells in the intertidal zone. Water and salt exchange across the aquifer-ocean interface of the intertidal zone shows that waves enhance water and salt exchange rate across the interface, making the intertidal zone a more active area.Submarine groundwater discharge(SGD) includes fresh groundwater discharge and recirculated seawater. Researchers commonly recognized that recirculated seawater accounts for most of SGD and it is composed of three major components: density-driven flow, tidally driven flow, and wave-induced flow. Many existing studies showed that the tide or wave induced seawater-groundwater recirculation in the intertidal zone is at least one order of magnitude smaller than that estimated by the radium isotopes method. The difference to some extent is due to the neglect of groundwater discharge in the subtidal zone of the shallow sea, where wave pumping may be a main driving force of seawater-groundwater exchange. Based on this idea, field monitoring work was done at the subtidal zone of two sites in Laizhou Bay. High frequency groundwater data were obtained to quantify the effects of waves on the seawater-groundwater exchange. Water exchange across the aquifer-ocean interface is calculated according to the generalized Darcy’s law. Generalizing the method to the whole Laizhou Bay combined with the data of water depth and sediments, the total exchange between seawater and groundwater in the subtidal zone of shallow water caused by wave pumping was estimated to be 1.35×108 m3/d, which could accounts for 22%-32% of the total SGD estimated by radium isotopes.