Application Investigation Of Monitored Natural Attenuation Technology On Chlorinated Organic Compounds Contaminated Sites

Due to the high biotoxicity, environmental accumulation, and recalcitrance for degradation, and the known carcinogenic/teratogenic/mutagenic impacts on human beings, chlorinated organic compounds have been listed as a group of priority controlled pollutants by the United States Environmental Protection Agency (EPA). During the remediation of the chlorinated pollutants contaminated sites, traditional physical, chemical and biological methods generally impose relatively high costs and will seriously impact the surrounding environment. The monitored natural attenuation (MNA) is an effective technology for site remediation and management and has been worldwidely used. It has attracted more and more attentions in China recently. It relies on the pollutants'intrinsic potential for natural degradation meet the remediation requirements, and thereby would reduce the treatment costs and lower the on-site project risks. In this study, based on the microcosm experiments, the natural attenuation of chlorinated organic compounds ? biodegradation characteristics and microbial community dynamics in contaminated soils and groundwater sampled in situ; In addition,1,2-dichloroethane was added to the soil sample on a small pilot experiment, to further explore the natural attenuation of chlorinated organics. Results of this study can provide theoretic and technical supports for the restoration of MNA remediation of contaminated sites.During the natural attenuation study of chlorinated organic compounds in the soil sampled in situ from the depth of 3 m,6m, and 9 m, the samples were anaerobically incubated for 45 days and no significant biodegradation process were noticed. During the experiment, there were no significant changes in some of the earth's biological and chemical indicators of microcosm environment, such as catalase and dehydrogenase activity, dechlorination product (Cl-), electron acceptor (NO3-, SO42-), pH value, ORP. However, the 16S rDNA gene sequencing combined with the DGGE(denatured gradient gel electrophoresis) fingerprints analysis indicated the presence of the functional bacteria involved in the degradation of chlorinated organic compounds in all studied soil samples collected from the diffent depth of the contaminated site, such as Dehalococcoides?uncultured Dehalobacter sp?uncultured betaproteobacterium.Natural attenuation characteristics study of chlorinated organic compounds in aquifer soil and groundwater demonstratedd that natural attenuation and biodegradation of chlorinated organic compounds had occurred during the 120 days of the microcosm experiment. The increasing on concentration of Cl- and the decreasing concentration of NO3- in the experimental group further verified the occurrence of biodegradation. Catalase and dehydrogenase activity, pH value, ORP value and other indicators has no significant change. The microbial community structure analysis results indiacted that some bacteria which can participate in the degradation of chlorinated organic compounds exist in quifer soil and groundwater, such as uncultured deltaproteobacterium? Spirochaeta sp?Geobacter sp?and Clostridiales.Significant natural attenuation and biodegradation of 1,2-dichloroethane has occurred through 140 day microcosm experiments in artificially contaminated soil natural attenuation study. The concentration of Cl- in different concentration gradient of the experimental group rised significantly, while concentration of electron acceptors (NO3-, SO42-) has no significant changes.It can be inferred that 1,2-dichloroethane was degraded with other ions as the electron acceptor(such as Fe3+) by biological degradation or directly by microbial utilization rather than co metabolism. The catalase activity in different concentration gradient experiment group did not change obviously. The dehydrogenase activity showed a clear downward trend, and enzyme activity of experimental group of high concentration was lower than that of the medium concentration. The reason may be that 1,2-dichloroethane affected the microbial activity. In addition, changes of pH value and ORP value are relatively small, indicating anaerobic environment is relatively stable. Microbial community structure analysis showed that the functional bacteria, such as Pseudomonas? Geobacter, and uncultured Candidatus Saccharibacteria bacterium, which involved in the degradation of chlorinated organic compounds was found in different concentration gradient soils.