Research On Prediction And Evaluation Of Ground Water Contamination In Changzhou Chemical Plant

Changzhou Chemical Plant has been an important manufacturer of pesticides andother organic chemicals in China, with a history of more than50years. Due to leakageof raw materials and products during production, the soil around the chemical plant, aswell the groundwater, has been severely contaminated.Based on historical hydrogeological data and site investigation, this studydetermined the hydrogeological conditions of the area under research. Within7m fromground surface is loam layer; between7and18m is slit layer. Unconfined aquifer isless than1m in depth, and the flow rate is relatively low. The flow direction in theunconfined aquifer varies with local topography, but generally goes southeast.Confined aquifer level is about-1m, also with a relatively low flow rate. The flowdirection in the confined aquifer is controlled by river boundary and groundwaterexploitation, and generally goes from the river to the central district.Through site investigation, this study also determined4main contaminants of thisarea, i.e. hexachlorocyclohexane, benzene, chlorobenzene, and dichloropropane, aswell as their distribution in the soil and groundwater. In soil, the contaminants diffusehorizontally from the workshops to the surrounding area, with a relatively highconcentration gradient. Vertically, the contaminants have penetrated the unconfinedaquifer, with maximum concentrations occurring in the middle and lower part of it.Horizontal distribution of the contaminants in unconfined aquifer is similar to that insoil, but a tendency of southward migration is observed. Maximum contaminantconcentrations in the confined layer superpose those in soil and unconfined aquifer, butexhibit a tendency to migrate northwards, as influenced by the groundwater flow. Thepresence of a thick layer of loam near the ground surface retards the contaminants andmitigates contamination of the confined aquifer.According to the site investigation results, a conceptual model is built up forpredicting solute transport in the groundwater of the studied area. The model isidentified and calibrated with monitored water level and contaminant concentrationdata, and the modeled results are in good agreement with monitored data,demonstrating the applicability of the model for prediction. Transport of main contaminants in the groundwater is predicted for the next5years using this model,with the assumption that the confined aquifer maintains a steady flow. Thecontaminants in the confined layer are predicted to keep moving northwards andincreasing in concentration, and get into wells for civil use in neighboring areas,affecting the life of people living there. If groundwater exploitation from these wellsincreases, the contaminants will get into the wells earlier and at higher concentrations.Several contaminant control measures are proposed, based on the predictionresults. Firstly, excavate contaminated soil in the unsaturated zone and unconfinedaquifer to use for cement production, thus removing contamination source and utilizingit as a resource. Secondly, set up a permeable reactive barrier (PRB) to the north of thecontaminated area in order to retard further migration of the contaminants. Thirdly, setup pumping stations to the north of the PRBs to control the flow field of thegroundwater and improve the dynamic conditions, so that the release of contaminantsin the confined layer is accelerated and the groundwater remediation efficiencyimproved. Finally, monitor the quality of pumped water on a regular basis, to ensurethe long-term effectiveness of groundwater remediation.