Monitoring Salt Water Intrusion Using High Density Resistivity Technology In Dagu River Zone

Water is an essential resource for human.As an important part of water resource, groundwater is one of the major source of irrigation and domestic water in many city.Dagu River is the most important groundwater reservoir of Qingdao.Seawater intrusion occurred in downstream of Dagu River by reason of the overexploitation of groundwater. In 1998, in order to solve this problem fundamentally, Mawanzhuang seepage-prevention wall was constructed in downstream of Dagu River.Seawater intrusion had been effectively prevented, however, some salt water remained in the town of Ligezhuang at the same time. Therefore, effective monitoring means are urgently needed, to find out the extent and scope of saltwater intrusion, forecast the development trend of saltwater intrusion, and provide support for the rational management.Based on the analysis of existing water quality data of monitoring wells,scientific and feasible salt water intrusion monitoring methods were studied.The conclusions are as follows:For saltwater intrusion in Dagu River zone, mineralization can reflect the change of saturated sand layer resistivity,and the characteristic value of resistivity 23Ω· m which was obtained in the experiment can be used as saltwater intrusion criterion for research area. There is no obvious effect of salinization degree and clay layer resistivity on the measurement of salt and fresh water interface. Measuring electrode spacing mainly affects the resolution of inversion profile, the resolution of resistivity profile increases with the decrease of electrode spacing. Resistivity profiling method can accurately reflect the location and morphology of saltwater intrusion interface,-as well as the extent and trend of saltwater intrusion.The main work in the paper includes the following five parts:(1) A salt-fresh water displacement experiment was performed to establish the characteristic relationships between of ion concentration, mineralization and resistivity of Dagu saltwater intrusion zone. It can provide the relevant scientific evidence for accurately delineating the scope of saltwater intrusion by using resistivity profiling.(2)Use large scale sink to undertake physical model tests of saltwater intrusion, study the dynamic changes of intrusion interface by coloring salt water, as well as the correspondencing relationship between resistivity profile and the scope of saltwater intrusion.This result further verified the feasibility of determining saltwater intrusion situation by using the resulting the resistivity threshold.(3) The finite difference time domain was used to establish two different geoelectric model of salt water intrusion, the position and shape of the invasion interface were obtained by 2D resistivity forward and inversion. And as a case study of mutation model,the effect of salinization degree,clay layer resistivity and measuring electrode spacing on the measurement results of resistivity profiling were analyzed by numerical simulation method,which can provide a theoretical basis for the foregoing experimental results.(4) Based on the groundwater monitoring data of the research area, resistivity method was carried out on site along the vertical direction of salinity contour. And analyze the measurement data by combining with relevant hydrogeologicai data and existing research results,which aimed at verifying the accuracy of monitoring saltwater intrusion in the study area by using resistivity method.(5) The laboratory experiments, numerical simulation and field measurement were combined to preliminary explore a generally applicable resistivity technology system for monitoring saltwater intrusion, which has high practical significance for the identification of other salty (sea) water intrusion.