Résultat de la recherche
Use of dual carbon-chlorine isotope analysis to assess the degradation pathways of 1,1,1-trichloroethane in groundwater
Compound-specific isotope analysis (CSIA) is a powerful tool to track contaminant fate in groundwater.However, the application of CSIA to chlorinated ethanes has received little attention so far. Thesecompounds are toxic and prevalent groundwater contaminants of environmental concern. The highsusceptibility of chlorinated ethanes like 1,1,1-trichloroethane (1,1,1-TCA) to be transformed via differentcompeting pathways (biotic and abiotic) complicates the assessment of their fate in the subsurface. Inthis study, the use of a dual CeCl isotope approach to identify the active degradation pathways of 1,1,1-TCA is evaluated for thefirst time in an aerobic aquifer impacted by 1,1,1-TCA and trichloroethylene (TCE)with concentrations of up to 20 mg/L and 3.4 mg/L, respectively. The reaction-specific dual carbonechlorine (CeCl) isotope trends determined in a recent laboratory study illustrated the potential of adual isotope approach to identify contaminant degradation pathways of 1,1,1-TCA. Compared to the dualisotope slopes (Dd13C/Dd37Cl) previously determined in the laboratory for dehydrohalogenation/hydro-lysis (DH/HY, 0.33±0.04) and oxidation by persulfate (∞), the slope determined fromfield samples(0.6±0.2, r2¼0.75) is closer to the one observed for DH/HY, pointing to DH/HY as the predominantdegradation pathway of 1,1,1-TCA in the aquifer. The observed deviation could be explained by a minorcontribution of additional degradation processes. This result, along with the little degradation of TCEdetermined from isotope measurements, confirmed that 1,1,1-TCA is the main source of the 1,1-dichlorethylene (1,1-DCE) detected in the aquifer with concentrations of up to 10 mg/L. This studydemonstrates that a dual CeCl isotope approach can strongly improve the qualitative and quantitativeassessment of 1,1,1-TCA degradation processes in the field.
Benzene dispersion and natural attenuation in an alluvial aquifer with strong interactions with surface water
Field and laboratory investigations have been conducted at a former coke plant, in order to assess pollutant attenuation in a contaminated alluvial aquifer, discharging to an adjacent river. Various organic (BTEX, PAHs, mineral oils) and inorganic (As, Zn, Cd) compounds were found in the aquifer in concentrations exceeding regulatory values. Due to redox conditions of the aquifer, heavy metals were almost immobile, thus not posing a major risk of dispersion off-site the brownfield. Field and laboratory investigations demonstrated that benzene, among organic pollutants, presented the major worry for off-site dispersion, mainly due to its mobility and high concentration, i.e. up to 750 mg L-1 in the source zone. However, benzene could never be detected near the river which is about 160 m downgradient the main source. Redox conditions together with benzene concentrations determined in the aquifer have suggested that degradation mainly occurred within 100 m distance from the contaminant source under anoxic conditions, and most probably with sulphate as main oxidant. A numerical groundwater flow and transport model, calibrated under transient conditions, was used to simulate benzene attenuation in the alluvial aquifer towards the Meuse River. The mean benzene degradation rate used in the model was quantified in situ along the groundwater flow path using compound-specific carbon isotope analysis (CSIA). The results of the solute transport simulations confirmed that benzene concentrations decreased almost five orders of magnitude 70 m downgradient the source. Simulated concentrations have been found to be below the detection limit in the zone adjacent to the river and consistent with the absence of benzene in downgradient piezometers located close to the river reported in groundwater sampling campaigns. In a transient model scenario including groundwater–surface water dynamics, benzene concentrations were observed to be inversely correlated to the river water levels, leading to the hypothesis that benzene dispersion is mainly controlled by natural attenuation.