Research On In-situ Monitoring Of The Seawater Intrusion Process Using A High-density Resistivity Probe

The seawater intrusion problem has been researched for a long time, and thereare lots of topics at home and abroad. The research methods can be summarized asmodel study, field investigation study and laboratory study. However, previous modelstudies are most based on the abrupt interface model and transition model. Fieldstudies are also just focused on the regional survey. It lacks method studies on thein-situ and real-time monitoring of seawater intrusion problem.In this dissertation, the status of seawater intrusion studies and the main factorsaffecting the seawater intrusion are briefly described. The existing monitoringmethods and monitoring indicators for seawater intrusion problem are introduced, thejudgment methods of salt-fresh interface position are also discussed. In this article, anin-situ real-time monitoring system for seawater intrusion is designed. The seawaterintrusion process is simulated through a sand tank in laboratory. The process iscontinuous monitored in real time by the high-density resistivity probe. Acomprehensive and systematic research of the seawater intrusion process dynamicchanges is finished. The data processing and analysis methods of the high-densityresistivity method are summarized. The effectiveness of the monitoring method isverified. The core part of this paper is the experimental simulation study. In thelaboratory study, the dynamic effect of water level changes to seawater intrusionwedge was studied through single-phase medium and multilayer medium. The resultsprovide a reliable experimental basis for the prevention and control of seawaterintrusion. The research achievement includes the following sections:1. The resistivity of sand saturated by pore-water with different salt contention ismeasured by high-density resistivity probe. The exponential relationship between porewater salinity or ion concentration and sand resistivity is well.2. The simulation test results in homogeneous medium show that the size ofseawater intrusion area and intrusion velocity is mainly affected by the salt-freshwater head difference. When the salt and fresh water contact each other smoothly, adynamic balance of salt-fresh interface will be formed. A drastic change of thesalt-fresh water head will cause the destruction of the original salt-fresh interface inthe medium, and cause short-time turbulence of pore water and expansion of saltwater area. While if the head difference remains the same for a long time, thatphenomenon eventually disappeared, a new balance between salt and fresh water isformed. If the head difference change slowly or small, the original interface isgenerally not destroyed. Increase the salt water level or reduce the freshwater levelwill raise the position of the interface between salt and freshwater. Otherwise theopposite phenomenon will occur. When the head difference is large, the contributionof infiltration is dominant and abrupt interface shows between salt and freshwater.While the head difference is small, the contribution of diffusion plays a dominant role, and a transition zone is formed between salt and freshwater.3. The test results in the horizontal layered medium show that the permeabilitydifferences of the media in the vertical direction caused the complexity and diversityof the intrusion path. At the intrusion stage of seawater toward freshwater, thelow-permeability layer will hinder the invasion of sea water, blocked the migration ofsaltwater to the bottom of the silt layer. At the recession stage of the seawater, thepresence of low-permeability layers impedes the seepage of fresh water, causing thedifficulty of the contamination control for the area below the low permeability layer.In addition, once the low-permeability layers are polluted by the saltwater, it is moredifficult to governance.4. Through the simulation experiment, the real-time monitoring of pore waterquality using high-density resistivity probe is achieved. In the process of resistivitydata analysis, vertical resistivity distribution map and contour map in the monitoringprofile both can reflect the changes of the medium pore water quality. In the dataanalysis, the contrast between the two maps can grasp the seawater intrusion situationbatter.In this essay, the flume experiments was setup to simulate seawater intrusion, thetest conditions are closer to the actual situation; by analyzing the real-time monitoringresult of transition zone and its dynamic characteristics under various conditions, thefour evolutionary processes of the transition zone were found, that are: the formation,development, change and stability of the transition zone; the dynamic characteristicsof the evolutionary stage of the transition zone was educed and explained theoretically;the relationship between transition zone and hydraulic conditions or mediacharacteristics was obtained, such as Water level and layered media determine thelocation of the transition zone, while permeability affects the transition zonedevelopment.Through the experiment study and data analysis, seawater intrusion monitoringresearch step forwards the practical application, and the results obtained has a largersignificance and practical value to the seawater intrusion monitor and early warning,planning of water resources management and prevention of seawater intrusion.