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Differences in electrical properties of media are the basis for determining the type and extent of contamination using geophysical methods. However, differences in heavy metals and organic matter complicate the electrical properties of compound-contaminated media, and existing geophysical methods cannot independently identify compound contamination. Therefore, this study proposes a geophysical detection system that combines electrical resistance tomography (ERT) and induced polarization methods and establishes a solid theory as the basis for the system application through laboratory experiments, model analysis, and site applications. The study reveals that as the organics volume proportion increases, the resistivity and normalized chargeability of contaminated media increased slowly, followed by a rapid increase, and finally reached a stable state. The specific type of compound significantly influences the electrical properties, while the resistivity of different kinds of compound-contaminated media reaches the same maximum value as the organics volume proportion increases. The medium type determines the contaminated media's lower resistivity limit and upper normalized chargeability limit. Additionally, the interplay between heavy metal type, content, and medium complicates the electrical properties of the media, with the compound type exerting a significant impact on resistivity. Archie's law and random forest modeling reveal that the inflection point for resistivity change occurs at 40 % and 80 % organics volume proportions, while the inflection point for normalized chargeability change occurs at 30 % and 70 % organics volume proportions in compound-contaminated media. These inflection points depend on the types of compounds, compositions, proportions, and media, and their importance for the electrical properties of the media changes with the increasing organics volume proportion. Based on the changing patterns of resistivity and normalized chargeability in heavy metal-organic compound contaminated media, the modified geophysical detection system can effectively identify the pollution type and intensity, which provides accurate pollution information to develop effective treatment strategies.

RESUMEN

In situ chemical oxidation (ISCO) is widely used as an efficient remediation technology for groundwater pollution. However, quantitative studies of its reactive remediation process under coupled thermal desorption technology are scarce. Based on laboratory experiments and site remediation, the chemical oxidation remediation reaction process was quantified, and the apparent reaction equation of the ISCO process was constructed. And then, a numerical model coupled with Hydraulic-Thermal-Chemical (HTC) fields was built to quantitatively describe the remediation process of an actual contaminated site. The simulation results fit well with the site monitoring data, and the results indicated that thermal desorption strengthens the ISCO remediation effect. In addition, the HTC model is expanded to build a conceptual and numerical model of a coupled remediation system, including heating and remediation wells. The results showed that high-temperature conditions enhance the activity of remediation chemicals and increase the rate of remediation reaction to obtain a better remediation effect. The heating wells increase the regional temperature, accelerating the diffusion of pollutants and remediation chemicals, and promoting adequate contact and reaction. Based on this crucial mechanism, thermal desorption coupled with ISCO technology can significantly improve remediation efficiency, shorten the remediation cycle, and precisely control agent delivery with the help of numerical simulation to avoid secondary contamination.

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Seawater intrusion is a global coastal environmental issue of great concern and significantly impacts the regional biogeochemical environment and material cycles, including nitrogen cycling. To reveal the mechanism of seawater intrusion altering nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions, the Bayesian mixing model (δ15N-NO3- and δ18O-NO3-) and 16S rDNA gene amplicon sequencing are used to establish nitrogen cycling pathways and microbial functional network. The results show that the nitrate in the coastal groundwater is from manure and septic waste (M&S, over 44 %), soil organic nitrogen (SON, over 20 %), and nitrogen fertilizer (FN, over 16 %). The hydrological interaction has promoted the coupling between material cycling and microbial community in the coastal groundwater systems. Among them, precipitation infiltration has caused the gradual decrease of specific microbes along the flow direction, such as Lactobacillus, Acinetobacter, Bifidobacterium, etc. And seawater intrusion has caused the mutations of specific microbes (Planktomarina, Clade_Ia, Wenyingzhuangia, Glaciecola, etc.) and convergence of microbial community at the salt-freshwater interface in the aquifer. In the coastal intruded aquifer systems, the nitrogen cycling pattern can be divided into oxidation and reduction processes. The oxidation process involves the enhancement of nitrification while the weakening of denitrification and anammox with the increase of aquifer depth. The reduction process consists of the enhancement of denitrification and anammox while the erosion of nitrification and ammonification with increased seawater intrusion. In addition, seawater intrusion can mitigate nitrate contamination by promoting denitrification and anammox in coastal areas.

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Seawater intrusion (SWI) seriously affects the economic development of coastal areas in southern China, and understanding its mechanisms is the basis for effective control of SWI. Hydrogeochemical methods and slack-based measurement data envelopment analysis (SBM-DEA) are used to study the characteristics and potential risk of SWI in coastal cities of southern China. Types and distribution of SWI, coastal groundwater evolution, geological-geographic and economic threatens of SWI, potential SWI risk, and environmental management recommendations are explored. The results show that the intrusion areas of Zhejiang and Guangdong account for 94.1 % of the total intrusion area of southern China, and the intrusion degree in Zhejiang is the highest, followed by Guangdong and Fujian. SWI is prone to occur on the sandy and silty coasts of the plain area of southern China; it accelerates the groundwater evolution speed and shortens the evolution path. SBM-DEA can be well applied to evaluate the potential risk of SWI events, and the results indicate a noticeable difference in the environmental performance level of coastal cities in southern China. The low environmental performance level (<0.3) and severe SWI of Taizhou and Zhanjiang indicate that SWI gradually worsens with economic development. In contrast, the high environmental performance level (>0.7) and low SWI of Wenzhou, Fuzhou, Quanzhou, Shantou, and Beihai indicate that the potential risk of SWI is gradually decreasing. Moreover, this study confirms that the environmental Kuznets curve (EKC) phenomenon exists in SWI events for southern China, and SWI-EKC indicates that the urban development of south China is approaching maturity. The specific case of SWI and EPL in coastal cities of south China jointly indicates that optimizing industrial structure, implementing a resources management policy, and improving citizens' environmental awareness are fundamental measures to resolve the contradiction between economic development and environmental problems.

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The seawater intrusion in the Dagu River Basin, China, has attracted intensive attention from the government and scholars. Increasing data have become available with the development of data acquisition technology. This situation brings unprecedented opportunities and challenges to the hydrochemical evolution analysis and improvement of seawater intrusion. The hydrochemical evolution process of groundwater is studied on the basis of our collected data in the Dagu River Basin by using mathematical statistics, end-element mixing, Durov, and Gibbs. The negative influencing factors of the groundwater environment are determined. Results show that the groundwater on the north side of the cutoff wall is mainly affected by residual saltwater, sulfuric acid leakage, and NO- 3 pollution. The groundwater on the south side of the cutoff wall is seriously affected by seawater invasion, followed by NO- 3 pollution. Meanwhile, the groundwater on the west bank of the Dagu River Basin is mainly affected by NO- 3 pollution. The groundwater on the north side of the cutoff wall must be discharged on a large scale for its improvement. Meanwhile, unpolluted water is recharged to renew the groundwater resources. Groundwater exploitation should be reduced, and the exploitation area should be dispersed to mitigate seawater intrusion and increase the recharge of the groundwater resources. NO- 3 pollution mainly comes from the sowing of chemical fertilizers and pesticides and domestic waste. Therefore, we should reduce the use of pesticides and fertilizers to control the generation, storage, and treatment of domestic pollutants strictly.