Abstract:In light of the complex management of chlorobenzene (CB) contaminated sites, at which a hydraulic barrier (HB) for plumes containment is emplaced, compound-specific stable isotope analysis (CSIA) has been applied for source apportionment, for investigating the relation between the upgradient and downgradient of the HB, and to target potential CB biodegradation processes. The isotope signature of all the components potentially involved in the degradation processes has been expressed using the concentration-weighted average δ 13 C of CBs + benzene (δ 13 C sum ). Upgradient of the HB, the average δ 13 C sum of −25.6‰ and −29.4‰ were measured for plumes within the eastern and western sectors, respectively. Similar values were observed for the potential sources, with δ 13 C sum values of −26.5‰ for contaminated soils and −29.8‰ for the processing water pipeline in the eastern and western sectors, respectively, allowing for apportioning of these potential sources to the respective contaminant plumes. For the downgradient of the HB, similar CB concentrations but enriched δ 13 C sum values between −24.5‰ and −25.9‰ were measured. Moreover, contaminated soils showed a similar δ 13 C sum signature of −24.5‰, thus suggesting that the plumes likely originate from past activities located in the downgradient of the HB. Within the industrial property, significant δ 13 C enrichments were measured for 1,2,4-trichlorobenzene (TCB), 1,2-dichlorobenzene (DCB), 1,3-DCB, and 1,4-DCBs, thus suggesting an important role for anaerobic biodegradation. Further degradation of monochlorobenzene (MCB) and benzene was also demonstrated. CSIA was confirmed to be an effective approach for site characterization, revealing the proper functioning of the HB and demonstrating the important role of natural attenuation processes in reducing the contamination upgradient of the HB.
Remediation actions at contaminated sites are based on multiple numerical model scenarios considering different parameter\ud distributions, source positions and contaminant transport paths. In some cases the excess of scenarios is due to uncertainties in the\ud conceptual model as a result of the spread of contamination through heterogeneities in the physical system. Reduction of project\ud hypotheses and conceptual model uncertainty is therefore needed. This result can be achieved by coupling hydrogeological\ud investigations with environmental forensic techniques, better localization of the source and understanding of contamination history.\ud In this respect, in the present study, compositional fingerprinting and groundwater flow modeling were applied to a former oil\ud storage facility where, even though a hydraulic barrier had been built to stop the hydrocarbon plume, the presence of some\ud hydrocarbons was still found in downgradient monitoring wells. The final aim was to evaluate the efficacy of the hydraulic barrier\ud and identify of the source of pollution. Fingerprinting results indicated pollution with a gasoline-diesel mixture much altered by\ud water washing and/or biodegradation. Comparison of seven groundwater samples collected in wells and monitoring wells was\ud performed by analyzing the volatile fraction (BTEX) and the total ion chromatogram (TIC), focusing attention on: n-alkanes\ud (m/z 85), alkylcyclohexanes (m/z 83), isoprenoids (m/z 113), C4-alkylbenzenes (m/z 134), C3-C6 alkylbenzenes and polycyclic\ud aromatic hydrocarbons (PAHs). The most probable scenario was then identified by combining the results of fingerprinting with\ud different contaminant paths obtained using the numerical model
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