Monitoring And Numerical Simulation Of Saltwater Intrusion In The Eastern Coast Of Laizhou Bay, China

Saltwater intrusion is arguably the most common and widespread contamination issue in the coastalaquifers around the world. The best means to understand, predict, and ameliorate saltwater intrusion,as well as to manage aquifers subject to saltwater intrusion, mainly involves monitoring andcomputer numerical simulation.This paper selected the east coast of Laizhou Bay of China as the key research area to study thistypical problem and draw some important conclusions.(l)The sandy coastal geomorphology and the Quaternary sediments are widely distributed in theeastern coast of Laizhou Bay. Due to two times transgression in the geology history, the aquiferformed by flood and alluvial deposits in the landward extends far away to the bottom of the sea.This kind of aquifer construction and coastal environment offers good conditions and veryconvenient path for saltwater intruding freshwater.(2) Water resources especially the groundwater in Laizhou city are very shortage compare to theaverage amount of Shandong province. By the analysis of water supply and demands, the presentwater resources can not meet the water requirements every year, even if the big project like WaterTransfer Project from South to North in China is completed in the future. Water resources lack andgroundwater over exploitation are the key induced factors of saltwater intrusion.(3)Groundwater table and quality monitoring work were done from 1979. For these many yearssaltwater intrusion passed through three periods of which includes primary development, fastincreasing and retreat. The recently retreat of the interface between the saltwater and the freshwaterfrom land to the coast is not due to the decrease of precipitation and the fresh groundwaterexploitation but caused by pumping of saltwater in the submarine aquifer.(4)The submarine groundwater along the eastern coast of Laizhou Bay is distributed in theQuaternary porous sediments below the shallow sea bed. This kind of submarine salt water storiedin the semi-confined aquifer has strong hydraulic connection with the landward groundwater storiedin the fresh water aquifer. Now large amounts of submarine groundwater discharge (SGD) are beingexploited which is mainly recharged by the inter-land groundwater. Studies show that the chemicalcharacteristic values such as PH, density, sodium absorption ratio (SAR) and rCl/rBr in thegroundwater follows the special rule from the land to the sea. In the mean time, the high value of Nitrates (NO3") in the land aquifer indicates the relative high value in the submarine groundwater. The hydrochemistry of groundwater from land to the sea is dominated by Cl-Ca.Na.Mg waters to Cl-Na.Mg waters and Cl-Na waters. In another hand, the ion strength (IS) and the mineral saturation index (SI) of the groundwater also follows the basic rule indicating the general model of hydrogeochemistry evolution in the coastal area. By using this method, saltwater can be delineated easily.(5) Except for the chemical methods, geophysical exploration techniques can also be used for saltwater intrusion monitoring which include the Vertical Electrical Sounding (VES), the Transient Electromagnetic Sounding (TES) and the Induced Polarization Sounding (IPS), etc. In the mean time, the geophysical logging including the Gamma ray and induced conductivity logging were used for typical profile detecting. With the interpretation of these detecting curves, a series of monitoring indexes are produced to determine the dynamic variation of salt-freshwater interface. Based on analysis of lots of measured field data, this paper gained two kinds of indexes to monitor saltwater intrusion. One is hydrogeochemical monitoring index mainly composed of characteristic chemical ion ratio, and the other one is geophysical exploration monitoring index mainly composed of resistivity and Gamma values. And then elaborated the relationship between the two kinds of indexes and introduced some problems must be paid attention to in the monitoring work. (6)The numerical simulation is the most important work in this paper. Pumping test were done in a small area to validate the hydrogeology parameters and it is found result is very good and lots of parameters obtained by this kind of pumping test in this research area can be used for the computer simulation.(7) FeFlow is a finite element based model with a wide selection of numerical solvers for performing complex 2D and 3D steady-state or transient groundwater flow and contaminant transport modeling. Saltwater intrusion in the confined aquifer (Henry problem) and unconfined aquifer (Huyakom problem) which are the classical issues were simulated by using the FeFlow software. It is found that the simulated results are similar with others in the publications. In the meantime, the migration of salt-fresh water interface drew by the groundwater funnel and the effect of subsurface barrier for saltwater prevention were also simulated. It is obviously seen that the software can solve this kind of density-dependent flow and mass transport problem.(8) 3D transient flow and mass transport model were set up to simulate the development ofsaltwater intrusion of Laizhou from 1998 to 2003. It is found that the computed results fit to the truedata well. Groundwater funnel are found in the submarine aquifer near the coastline. Thoughgroundwater table declined in the recent years, the salt-freshwater interface moved from the inlandto the coastline.(9)Saltwater intrusion has good relationship with the precipitation, groundwater and saltwaterdischarge. Using artificial neuron net model can analog and predict the area variation of saltwaterintrusion along with the three factors. If it encounters several continuous dry seasons in the futureand in the mean time keeping the small scale of groundwater exploitation just like the present year,the saltwater will not intrude to the inland because of the submarine saltwater pumping, but ifincreasing the amount of groundwater pumping along the coastal area, the saltwater intrusion wouldoccur from the beginning.(10)Much success of saltwater prevention is achieved in the area of coastal aquifer and phreaticaquifer. New types of prevention engineering have being constructed in the coastal area fromseveral years ago, of which including land-filling tidal prevention project, subsurface barrier projectin the estuary and submarine salty water exploration project for breed aquatics. Whether theseprojects do harm to the coastal environment needs more constant monitoring and forecastingresearch.Using the advanced international theory and technique of coastal and river ecosystem restoration,the ecological prevention theory should be the guide of the proper way to prevent saltwaterintrusion, but much work should be done to make the theory perfect and make progress with theengineering technology in the future.