Application Research Of 1,2-dichloroethane Contaminated Groundwater Remediation Through In Situ Chemical Oxidation Enhanced By Hydraulic Regulation

1,2-dichloroethane（1,2-DCA）is a volatile chlorinated organic compound which characterized as refractory contaminant with low solubility,high density and low mobility.When1,2-DCA enters the groundwater,its high density makes it easy to be trapped in the porous medium or trapped in the bottom of the aquifer,and turn out to be a persistent pollution source.The 1,2-DCA received more attention in groundwater remediation due to its human health and environmental hazards.In Situ Chemical Oxidation（ISCO）technology is a traditional effective strategy for groundwater remediation in chemical contaminated sites.However,slow and range-limited agent transportation limits the chemical oxidation of contaminant,and makes the application of ISCO technology facing serious problem in remediation efficiency,operational cost and remediation effectiveness.This study aims to establish a high performance ISCO technology based on the idea of"hydrodynamic regulation and enhancement of oxidant migration",trying to enhance the mass transfer efficiency of Sodium Persulfate Activated by Sodium Hydroxide（SPASH）.The feasibility and mechanism of 1,2-DCA removal from groundwater were studied.A novel ISCO-P remediation mode retrofitted with pumping was proposed based on the hydrogeological conditions and pollutant transferring.Oxidative degradation mechanism of1,2-DCA in SPASH system was studied.The 1,2-DCA distribution and method of chemical agent consumption calculation has been revealed in san box analogy models.Pilot tests had been conducted to verify the feasibility and to collect design parameters that were critical for full scale application of ISCO-P.Finally,numerical simulation of groundwater flow assisted the design of ISCO-P system based on these parameters obtained from pilot tests,and realized the hydrodynamic regulation to strengthen in situ chemical oxidation technology.The main research contents and results are stated as follows:（1）Investigation of hydrogeology and pollutant migration background and proposal of remediation strategy.The stratigraphic texture is made up of plain fill,silty clay,boulder and mud stone from top to bottom.The groundwater in this water-enriched region was categorized into quaternary pore water of loose rock that mainly contained in gravel layer,in which permeability coefficient ranges from 2.98 to 21.02 m/d and pumpage from each well equals to 1.001-1.154 L/s·m.The maximum concentration of 1,2-DCA exceeded the Class-IV quality requirement of groundwater quality standard（GB/T 14848--2017）by 125.9 times,which generated unacceptable public health threaten to surrounding resident.The results of risk assessment suggested that the targeted 1,2-DCA concentration for contaminated site remediation shall be 147μg/L.Pollutant migration and distribution in groundwater generally powered by groundwater convection,following by significant adsorption retardant and biological attenuation in the water-bearing media.In view of this background,an optimizing remediation mode of hydrodynamic regulation to enhance in situ chemical oxidation（ISCO-P）was proposed,aiming to break the bottlenecks of hydrodynamic conditions in the contaminated sites.（2）Optimization of SPASH parameters and clarification of 1,2-DCA degradation mechanism and path.SPASH parameters for 1,2-DCA removal had been optimized by response surface methodology and given as followed:Na₂S₂O₈ injection dosage was 5 g/L;then,groundwater alkalization by Na OH injection with a Na₂S₂O₈:Na OH ratio of 3 by weight.An average 1,2-DCA removal of 96.24%was achieved under the optimized condition when influent 1,2-DCA concentration was 3 mg/L.Free radicals reaction played the dominant role in 1,2-DCA degradation.Free radicals presented in SPASH system were O₂^-·、O·^-、OH·、SO₄^-·and O₂^-·was relatively stable and played the major role in the oxidation.The intermediate products of 1,2-DCA degradation were vinyl chloride,dichloromethane,trichloromethane and carbon tetrachloride.vinyl chloride was the key intermediate in the degradation path of 1,2-DCA.（3）Simulation of groundwater contamination and remediation were assessed through sand chamber experiments,to clarify the pollutant distribution and theoretical oxidant dosage demand for remediation.The surface microstructure varied between size-graded gravels and presented mesopore structure in command.The occurrences of1,2-DCA was not adsorption state in the gravel layers,because only 17.3%-33.3%of the adsorbed 1,2-DCA released into water during desorption experiment.Therefore,the estimation method for calculating the theoretical oxidant dosage was developed according to the mode of 1,2-DCA occurrence.The experiments showed that molecular diffusion coefficient,reaction duration and 1,2-DCA reduction ratio were dramatically enhance by 1.78,7.5-9.8 and 3.5 folds with the assistance of hydrodynamic regulation.Pumping accelerated groundwater flow and enhanced migration and diffusion of oxidation agents in groundwater.In this way,the scope and duration for thoroughly groundwater remediation were extended and shorten,beneficial to guarantee effective and efficient in situ remediation.（4）The design scheme of ISCO-P technology was established and proved general solution for full-scale contaminated sites remediation,based on the pilot scale experiments.The pilot tests conducted in a point source contaminated plume showed that ISCO-P technology could reduce local 1,2-DCA concentration below the limiting value or even detection line within a short period.Further,the oxidation agent provided a long-term pollutant control in aquifer with an effective duration of 28 days.The oxidation ratio constant and half-life time of 1,2-DCA degradation was 0.022 d^-1 and 29 days.SPASH process accompanied with sulfate generation and p H recovery（from acidic to neutral）.During the restoration process,the concentration of sulfate and p H increased first and then decreased,and recovered to the original concentration level within 140 days.The secondary pollution of the test site was controllable,providing an reference to further application of SPASH system in groundwater remediation.Combined with pilot test parameters and through numerical simulation,a design scheme for ISCO-P remediation of groundwater is proposed,which has been verified in full-scale implementation:The concentration of 1,2-DCA restored to below the limiting value within merely seven months,and continuously kept up to standard level for the 24 months monitoring period without rebound.It was also founded that the potential secondary pollution risk was controllable,which completely solved the problem of incomplete repair by traditional technology.In comparison with traditional ISCO technology,ISCO-P technology showed significant advantage in technology and economic feasibility.