Simulation Study On The Formation And Evolution Of Groundwater In An Artificial Coral Reef Island In The South China Sea

Groundwater is a very important resource within the coral reef island,which is of great significance for the establishment of the island’s animal and plant ecosystem and the subsequent development and construction of the island.This part of fresh water resources is stored in the interior of coral reef island,floating on the sea water due to rainfall infiltration displacement and density difference.Its shape is thick in the center,thin in the edge,and similar to the lens,so it is called “fresh water lens”.The formation of underground fresh water in the coral reef island is affected by many factors,such as geological structure,stratum lithology,atmospheric precipitation,etc.In order to explore the mechanism of formation and evolution of underground fresh water in the coral reef island,based on the background of an artificially dredged coral reef island in the South China Sea,this paper studies the stratum structure,rainfall The influence of permeability and other factors on the formation of fresh groundwater.The main research contents and conclusions are as follows:(1)The coral reef island in the study area is artificially dredged from the original coral reef flat,which is formed by the accumulation and cementation of coral reef fragments,broken branches and some biological debris.Its main component is calcium carbonate.In order to obtain more detailed stratum data,several boreholes were arranged on the island for drilling and coring,and then the stratum characteristics of the island were further determined through particle analysis tests.The deep stratum of the coral reef island is reef limestone,while the shallow stratum of the island has fine particles,mainly including fine sand,medium sand and gravelly sand.With the increase of the stratum depth,the particle size of the particles gradually increases and large particles such as breccia appear,which has poor stratification.Through the permeability test analysis,it can be seen that the permeability of the stratum in the middle area of the coral reef island is poor,while the permeability of the stratum in the surrounding area is strong;The permeability of the shallow strata of the coral reef island is poor,and the closer to the deep reef limestone,the stronger the permeability is,which is consistent with the characteristics of the stratum structure.(2)Coral reef islands in this study area have experienced multiple sedimentary cycles during their development and evolution.In combination with drilling test data,physical model tests of different stratigraphic structure characteristics were carried out respectively.The model test results show that the thickness of groundwater fresh water that can develop is in direct proportion to the rainfall infiltration.With the increase of rainfall frequency,the rainfall infiltration increases,and the freshwater lens and the brackish water transition zone can further develop to the deep stratum as a whole.Different stratigraphic structures will lead to different development rates of underground fresh water.A single stratigraphic structure model can more easily retain fresh water.The more complex the stratigraphic structure is,the less able it is to retain fresh water.The formation of underground fresh water is affected by the formation permeability.In areas with strong formation permeability,the thickness of the freshwater lens decreases correspondingly,and the thickness of the brackish water transition zone increases correspondingly.In addition,the deep high permeability stratum controls the maximum depth that the freshwater lens can develop.(3)Taking the artificial coral reef island in the study area as an example,this paper uses Visual MODFLOW software to conduct numerical simulation on the formation process and influencing factors of underground fresh water in the island.The simulation test results show that the formation permeability,seepage path,time and other factors will affect the development process of freshwater lens in the coral reef island.The stronger the stratum permeability in the coral reef island is,the thinner the freshwater lens is,and the thicker the transition zone is;The poorer the permeability of the formation,the thicker the freshwater lens and the thinner the transition zone.The path of seepage from the middle area of coral reef island to the surrounding sea water is long,and fresh water is not easy to lose,so more fresh water can be retained here;The seepage path from the area around the coral reef island to the sea water is short,and the fresh water is easy to lose,so it becomes more difficult to retain fresh water.In the early desalination stage of forming freshwater lens,the stronger the permeability of the formation,the faster the advancing speed of the freshwater line;In the middle and late stages of the desalination stage,the stronger the permeability of the formation,the easier it is for the freshwater line to reach a stable position,while the weaker the permeability of the formation,the easier it is for it to be progressive.The simulation results show that after 10 years of development,the freshwater lens of the coral reef island has roughly formed,and then can continue to expand slightly,reaching a basically stable state in the 20 th year.This paper includes 46 pictures,4 table and 93 references.