| Groundwater hydrodynamics and submarine groundwater discharge(SGD) in coastal aquifer systems subjected to various marine driving forces are an important research focus and topic of coastal hydrogeology as a consequence of the rapidly development of social and economic development. Researches on the groundwater hydrodynamics and SGD, an important part of global water cycle, can give sight into the groundwater flow and solute transport, the mechanism of land-sea interface flux exchange and its effect on coastal ecosystems and biodiversity, and provide a scientific basis and guidance for solving and preventing coastal ecological disasters.Based on field observations of groundwater head and salinity in a muddy tidal flat in Laizhou Bay, the groundwater hydrodynamics and SGD in a multi-layered aquifer system were investigated using two-dimensional numerical model MARUN. The factors influencing groundwater flow and solute transport were taken into account as more as possible, including seepage face, aquitards, tidal fluctuations, geomorphic configuration with slope break, and density difference between seawater and groundwater, etc. The SGD in the multi-layered aquifer system were quantitatively estimated considering the groundwater discharge from the seepage faces and the leakage flow through the aquitard.Both the field observations and numerical results provided evidence that extensive seepage faces develop in the tidal flat, with an almost fixed water-table exit point on the landward side of the slope break. The seepage face development changes periodically with tidal forcing and reaches its maximum length of ~580 m at spring low tide. Such a large-scale of seepage faces was rarely implemented in numerical studies but widely exists in gently sloping beaches. Two tide-induced circulations were revealed in the gently sloping tidal flat with the multi-layered aquifer structure: a seawater-groundwater circulation through the aquifer-ocean interface, and a leakage circulation through the aquitard. Solute transport associated with the two tide-induced circulations led to the formation of an upper saline plume and a freshwater discharge ―tube‖ corresponding to the maximum upward leakage and an increase of SGD. The freshwater discharge ―tube‖ differs from the ―tube‖ in many previous studies which is located near the low-tide mark and formed by the seawater-groundwater circulation.The total SGD was estimated to be 4.95 m3/d/m, and SGD from the seepage faces was 2.88 m3/d/m, accounting for ~58 % of the total SGD. The tide-induced leakage circulation was 1.05 m3/d/m. The freshwater recharge from the deep confined aquifer accounted for ~85 % of the total inland-originated SGD. It was indicated that the seepage face, leakage flow and slope break have great impacts on the groundwater discharge path and solute transport, which should be paid more attention in the studies of hydrodynamics and SGD in multi-layer coastal aquifers.Taking into account the effects of seepage face and density, a simple and efficient method for estimating seawater-groundwater exchange rate in gently sloping beaches was proposed based on the generalized Darcy’s law. With the observed groundwater head, temperature and salinity, the SGD, inflow and SGD from the seepage faces were estimated in the gently sloping tidal flat. The estimated results were in line with numerical results, with which the efficiency and reliability of this method were verified. The estimation results of SGD under the influence of waves indicated that the groundwater-seawater exchange is significantly strengthened especially in the subtidal zone.Finally, the SGD in offshore submarine aquifer driven by sea level fluctuation were studied using analytical method with the loading effect and elastic storage considered. Based on the analytical solution, a simple experiment was designed to identify the loading efficiency and hydraulic diffusivity. Then the method was applied in subtidal zones of a sandy beach at the east coast of Laizhou Bay, China. With the obtained loading efficiency, the wave-induced SGD in subtidal zones were estimated using typical values of hydrogeological parameters for sandy sediments and waves of Laizhou Bay. The results indicated that wave-induced SGD in subtidal zones can be as many as 104 m3/d/m, and comparable to the estimated SGD by radium and radon environmental isotope tracer methods, which is of the order of magnitude of 102–103 m3/d/m. The wave-induced SGD in subtidal offshore submarine aquifers should be emphasized and paid more attention in further studies. |
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