An Experimental Simulation Of The Fresh Water-Salt Water Interface In Coastal Zones Under Constant Head Difference And Its Regime

Due to the increase in population and the rapid economic development in the coastal areas,the demand for water supply has increased.How to scientifically evaluate and rationally pump of groundwater resources in coastal aquifers have become a concern and research issue in human activities.Since the middle of the last century,the phenomenon of irrational exploitation of groundwater resources in coastal aquifers has repeatedly occurred.The decline in the groundwater levels has caused a dynamic imbalance and the fresh water – salt water interface moving inland to cause seawater intrusion.Seawater intrusion has also occurred in coastal areas such as Laizhou and Dalian in China,and it has become increasingly serious.The location,shape,migration,and prediction of the fresh water – salt water interface in coastal aquifers are the core contents of the study of seawater intrusion.In this paper,an experimental unconfined aquifer is constructed in a lab to simulate the coastal aquifer system.Based on the design and preparation of the experimental device,the device simulation experiment is used to model the fresh water – salt water interface in coastal aquifers,and the experimental results are numerically simulated.The study of the migration and change of the fresh water – salt water interface during seawater intrusion in coastal aquifers is carried out,and the influence of salt water density on the shape and position of salt water interface with the constant fresh water head and salt water head conditions is examined.The results shows that the head above and below the fresh water – salt water interface is basically stable in the same vertical line.There are obvious differences above and below the fresh water – salt water interface.The salt water level below the interface is significantly lower than the freshwater level above the interface.The greater the difference in density,the greater the distance from the salt water interface to the inland extension.The greater the difference in density,the greater the longest distance of the fresh water – salt water interface inland.When the density of salt water changes from large to small,the interface gradient shape gradually becomes steeper.FEFLOW is used to construct a finite element model and the experimental process is numerically simulated.The results of the modelling show that the greater the difference in density between fresh water – salt water,the farther distance of the interface extending inland,the faster the interface frontier rate is,and thus the longer the time required to achieve equilibrium stability is,and the longer the time until the interface reaches a steady state increases from about 0.042 day to about 0.07 day.The conclusion is consistent with that suggested by Song Chao's(2015).With the salt water density decreases,the farthest distance of the interface extending inland reduces from 89 cm to 47 cm,which is in agreement with the experimental results.The overall shape of the fresh water – salt water interface is parabolic in the process of retreat of the interface,and the water level is gradually reduced from the fresh water end to the salt water end.When there is no fresh water – salt water interface,the water-level fluctuation is very small,almost are at the same water level in the same vertical observation line.When there is a fresh water – salt water interface,the fresh water head is higher than that of the salt water head in the same vertical observation line.Besides,in the middle transition zone,the fresh water head on the interface and the salt water head under the interface change slightly with the increasing depth.The heads of all the measured points are in the range of 0-6 cm,which are in agreement with the actual situation of the experiment.The latest calculation formula proposed by Zhou et al.(2016)is used to calculate the longest extension distance of the fresh water – salt water interface inland.The calculated results are slightly larger than the actual measured values.The calculated depth values of the fresh water – salt water interface at 23 cm,46 cm and 69 cm are less than the actual measured values.Comparison of the values of the fresh water – salt water interface under three different densities at the same location show that the depth of the interface will increase with the decreasing density,which are in coincident with the actual observed values.