A case study of monitored natural attenuation of petroleum-hydrocarbon contaminated soil
Résumé
In general, natural attenuation processes refer to the use of natural processes, favorably without human intervention, in order to reduce the mass, toxicity, mobility, volume, or concentration of pollutants at contaminates sites. The natural processes include a variety of physical, chemical, or biological mechanisms such as biodegradation, dispersion, dilution, sorption, volatilization, and radioactive decay. Monitored natural attenuation is the use of natural attenuation processes to achieve site-specific remediation option for contaminated sites. In France, the use of natural attenuation like management measure includes two essential aspects: source control and long-term performance monitoring. The ATTENA program coordinated by the BRGM, consists in introducing a technical protocol for monitoring natural attenuation of organic compounds. A case study of monitored natural attenuation was carried out on a site who presents an organic pollution. This study focuses on contamination of soil and groundwater by benzene, toluene, ethylbenzene, xylene (BTEX) compounds, resulting from spills or leaks of petroleum hydrocarbons. A feasibility study of site management by monitored natural attenuation has consisted to compile historical data on the source of pollution and pollution plumes, to realize a vulnerability study of the environment and finally, to highlight the process of biodegradation of pollutants (biodegradation is considered to be the primary mechanism for attenuation of BTEX). The historical data reports two accidental oil spills. Remediation work resulted in the elimination of the majority of the source. Data on pollution plumes show pollution with BTEX, which extends about 80 m. The concentrations of BTEX in the plume are stable since the early follow-up. The concentrations of BTEX in the plume are stable since the early follow-up. Also, the vulnerability study allowed to identify compatibility between uses and the site condition. Hydrocarbons like BTEX can be degraded by microorganisms through electron transfer by various mechanisms. The availability of oxygen and other electron acceptors such as nitrate, sulphate, manganese (IV), iron (III) determine the rate of biodegradation. Chemicals indicators of microbiological activity in the groundwater chemistry such as consumption of oxygen, nitrate and sulphate and production of Fe (II), Mn (II), sulfure, nitrite, and ammonium are present on the site. Based on these results, management of site contamination by monitored natural attenuation is possible. Quantification of natural attenuation mechanisms, to achieve a predictive model determining the time of disappearance of pollution was conducted to define a monitoring plan for the site. Hydrocarbon contaminated site are characterized using traditional and innovative investigation methods. A careful interpretation of hydrogeological and hydrogeochemical data allowed for the conceptualization of the heterogeneous BTEX contaminant plume distribution in the subsurface. The model is used to simulate in particular, the combination of (abiotic) multicomponent chemical reactions, redox-dependent microbial activity, and subsurface transport occur simultaneously in the case considered here, contamination by benzene, toluene, ethylbenzene, xylene (BTEX) compounds. These contamination scenarios have a major impact on subsurface ecology; particularly, some microbial species are favoured due to contamination itself. Modeling of such systems, where justified, provides a sound quantitative basis for engineering predictions, for example the efficacy of natural attenuation as compared with active remediation. Subsequently, an in situ bioremediation will be conducted on this site. The model produced will be implemented by successive step in order to design and monitor the bioremediation system.