Field Study On Process And Mechanism Of Natural Attenuation For The Multi-solvents Co-contaminated Groundwater In The Pesticide Manufacturing Plant

Benzene,toluene,ethylbenzene,and xylene(collectively known as BTEX),as well as chlorinated aliphatic hydrocarbons(CAHs),are prevalent organic solvent pollutants in groundwater of pesticide manufacturing plants,and pose serious threats to the groundwater environments as well as to the public health and ecosystems.Monitored natural attenuation(MNA)is an eco-friendly remediation technology used for the remediation of pesticide manufacturing sites,in which biodegradation makes the major contribution.However,due to the distinct degradation pathways and environmental conditions requirements of BTEX and CAHs attenuation,there may be complex interrelationships between them and affect the natural attenuation potential of commingled plumes,which is less studied and poorly understood.In this paper,a former pesticide manufacturing plant co-contaminated with BTEX and CAHs in southern Jiangsu was selected for the 5-years monitoring of groundwater.The natural attenuation characteristics of BTEX/CAHs commingled plume in groundwater,especially the natural attenuation of the more refractory CAHs,were studied.The effects of different pollution status on the composition,succession,metabolism and symbiosis of microbial communities were analyzed,and the mechanism of contaminants and spatial/seasonal variation factors on the enrichment and activity of reductive dechlorination bacteria was discussed.With the help of cooccurrence network and metabolic pathway analysis,the co-attenuation model of BTEX and CAHs in the commingled plume was established.In addition,a method for predicting the natural attenuation rate of BTEX and CAHs based on the composition of groundwater microbial communities was established,and the predictive performance of two common machine learning models was evaluated and compared.The results of long-term groundwater monitoring showed that BTEX and CAHs in commingled plume reduced species richness and evenness,respectively,and jointly led to the decrease of species diversity.The groundwater microbial communities in different polluted zones originated from the same primitive microbial system.The pollution status led to microbial community succession,and commingled pollution intensified the separate influence of BTEX and CAHs respectively on community succession.Sulfate reduction and methanogenesis conditions dominated in BTEX contaminated zone,while aerobic and nitrate reduction dominated in the zone with CAHs alone.In addition,cell growth and energy metabolism in BTEX contaminated zones were significantly higher than those in other zones,indicating that BTEX degradation provides more abundant carbon sources and energy.BTEX significantly affected the key metabolic process of groundwater microbial community,and weakened the interspecific competition relationship,and was the key factor affecting the function assemblage of microbial community in the commingled plume,while CAHs played a negative role.The key species in the commingled plume and the zone with BTEX alone were mainly related to aromatic hydrocarbon degradation,fermentation and fatty acid degradation,and those in the zone with CAHs alone and the pollution-free group were mainly nitrate reducing bacteria and sulfur oxidizing bacteria.The shrinkage of the CAHs plume in the commingled zone was only about 20%-30% of that in the CAHs single pollution zone.The abundance of reductive dechlorinators was positively correlated with BTEX concentration,but not with CAHs concentration,and the abundance of dechlorinatiors in the commingled plume was about 7.6-fold of that in the zone contaminated with CAHs alone.In depth,reductive dechlorinators as anaerobic bacteria are more likely to be enriched(14.5% higher in average)in deeper groundwater with lower ORP.In season,the higher temperature of deep groundwater in rainy season was the key factor for the high abundance of reductive dechlorinators.However,the seasonal enrichment of dechlorinators only occurred in the deeper groundwater with the presence of BTEX,which further indicated that the degradation of BTEX played an important role in the enrichment of redutive dechlorinators.There was a synergistic symbiotic effect among BTEX degrading bacteria,fermentation bacteria and reductive dechlorinators in the commingled plume,and they jointly promoted the dechlorination reaction.In addition,BTEX also enhanced a variety of CAHs degradation pathways,including cometabolism and aerobic oxidation of low-chlorianted aliphatic hydrocarbons.In this study,a machine learning model based on microbial community composition was developed.The random forest classification model can accurately classify the natural attenuation feasibility of BTEX and CAHs.In the prediction of natural attenuation rate,the random forest regression model showed a relatively high accuracy for BTEX(the absolute percentage error was about 7%～20%),but showed a much high error for CAHs(the absolute percentage error was about 20%～160%).The artificial neural network model accurately predicted the natural attenuation rate of BTEX and CAHs(the absolute percentage error is 2% to 5% and 7% to 20%,respectively).To sum up,this paper systematically studies the characteristics of natural attenuation behaviors of pollution plumes in groundwater co-contaminated with BTEX and CAHs,and the effects of commingled pollution on microbial community composition and metabolism.The co-attenuation mechanism of commingled pollution as well as the impact of contaminants and spatial/seasonal variations emphatically discussed.With the help of machine learning models,a prediction method based on the composition of groundwater microbial communities for the prediction of natural attenuation rate of pollutants was established.These findings and results provided necessary theoretical and technical support for the application of monitored natural attenuation technology in the remediation of pesticide manufacturing plants with multisolvents co-contamination.