Assessment Of Toluene Biodegradation And Fate In Groundwater By Compound Specific Isotope Analysis

A comprehensive understanding of the physical processes and biodegradation process during pollutant migration is required for assessment of site natural attenuation.Those processes can be recognized by Compound Specific Isotope Analysis(CSIA),provided that an accurate isotope enrichment factor is obtained.However,the enrichment factor tends to be overestimated by applying Rayleigh fractionation equation directly in field,and easily be underestimated by laboratory experiment.That will result in inaccurate biodegradation assessment and bring uncertainty to the monitoring natural attenuation applications,and the simulation and prediction of pollutant migration.In this paper,the in-situ degradation of toluene in groundwater was evaluated by CSIA and its migration trend was predicted in an oil spill contaminated site having dynamic data of toluene concentration and isotope ratio.Besides,mathematical statistical method,microbial community analysis,microcosmic experiment,theoretical model derivation and construction,and numerical simulation of toluene migration were also used in this paper,and the conclusions are as follows:(1)The conceptual model of hydrogeology and geochemistry of the site is constructed,and the redox zones of electron acceptors are divided,confirming the corresponding relationship between the microbial community and the redox zones which are both affected by groundwater flow.The response mechanism of carbon isotope fractionation in toluene degradation was analyzed based on the CSIA technology.The temporal and spatial variation of the 813C value is greater than+2‰,which confirmed the occurrence of toluene biodegradation in the field site.(2)The microcosm experiment simulated the biodegradation of toluene in sitegroundwater,identified the degradation stage and the dominant anaerobic denitrification bacteria Thauera,and obtained the indoor enrichment factor(average-0.943 ‰).(3)The Rayleigh fractionation model with hydrodynamic constraints was constructed:(?)for time dimension and(?)for spatial dimension.The enrichment factors of isotope fractionation with hydrodynamic constraints(average-0.861‰)were calculated by this method.(4)The enrichment factors obtained by the three methods are as follows:field enrichment factors(average-0.638‰)>enrichment factors corrected with hydrodynamic constraints(average-0.861‰)>laboratory enrichment factors(average-0.943‰).The field enrichment factors were tend to be overestimated and the indoor enrichment factors were easily to be underestimated.The corrected enrichment factors with hydrodynamic constraints between the two were closer to reality.(5)The corrected enrichment factors were used to evaluate the biodegradation degree of toluene in site,and the degradation rate was calculated(0.014d-1).The degradation ratewas introduced into the unsteady two-dimensional toluene transport model.After identified and verified by the groundwater level and concentration series,the model can reproduce and predict the migration process of toluene in the aquifer very well,and give the following monitoring suggestions.The results in this paper indicate that the corrected enrichment factors with hydrodynamic constraintsare more representative of the field biodegradation process,and they can be used to evaluate the pollutants degradation degree and calculate the biodegradation rateaccurately.This study has certain guiding significance for monitoring the natural attenuation of BTEX polluted aquifer and site management and control.