Degradation potential of S-metolachlor in an artificial wetland ecosystem: microcosm assessment

Meeting of SWS/WETPOL and Biogeochemistry symposium, Prague, CZE, 03-/07/2011 - 08/07/2011International audienceA new method to estimate the biodegradation potential of S-metolachlor, in an artificial wetland ecosystem (AWE) in Bray, France, was developed. For that propose, batch experiments were performed with sediments collected from AWE. The half-life of S-metolachlor was determined under biotic and abiotic conditions (sterile microcosms), aerobic and anoxic conditions and in the presence of different carbon sources (crush plants or acetate). One assay was performed without any carbon source. The results suggest that the S-metolachlor degradation is a biotic phenomenon catalyzed by microorganisms. There is a significant decrease of the half-lives in the biotic microcosm (15.5 days in the case of the aerobic microcosm and 20.5 days in the anoxic) comparing to the sterile microcosms (60.2 days in the case of the aerobic and 45.5 days in the anoxic) (

Artificial Wetland and Forest Buffer Zone: Hydraulic and Tracer Characterization

As part of a European LIFE ArtWET project, two on-site buffer zones, an artificial wetland and a forest plot, are being evaluated for their capacity to mitigate pesticide pollution. As treatment efficiency is highly dependent on the systems' hydrology, the present work focuses on the watershed and both systems' hydrological functioning. The design strategy involved limited inlet flow rates to 70 L s-1: 99% of watershed outlet flow rates were lower than this limit. Approximately half of the flows of greatest concern passed through the artificial wetland, whereas the forest only received 2% of these flows. A tracer experiment was conducted under a low steady flow rate while little vegetation was present in the artificial wetland. A water dye tracer (sulforhodamine B, SB) and two molecules of contrasting properties, uranine (Ur, photodegrading) and isoproturon (mobile and only slightly sorptive, IPU) were injected. Dilution, sorption, and photodegradation were observed. The forest plot, which presented a high organic matter content, showed more sorption (IPU, SB) but lower photodecay (Ur) than did the artificial wetland. Total IPU losses in the forest buffer were high (79%). In the artificial wetland, 30% IPU losses were found, whereas a 66.5-h mean retention time was determined and good hydraulic efficiency (0.55) was calculated. Few dead zones and short-circuits were found, suggesting good hydrological functioning. Implementing buffer zones in subsurface pipe-drained watersheds actively participates in the reduction of pesticide transfer to natural water bodies

Polar organic chemical integrative sampler (POCIS) allows compound specific isotope analysis of substituted chlorobenzenes at trace levels

Compound specific isotope analysis (CSIA) is an established tool to demonstrate in situ degradation of traditional groundwater contaminants at heavily contaminated sites, usually at mg/L range aqueous concentrations. Currently, an efficient preconcentration method is lacking to expand CSIA to low aqueous concentration environmental samples. This work demonstrated the compatibility of polar organic chemical integrative sampler (POCIS) with CSIA of C, H, and N isotopes for four NH2- and NO¬2-substituted chlorobenzenes at low μg/L. Diffusion and sorption showed insignificant carbon isotope fractionation (<0.7‰) in laboratory experiment, except for a reproducible shift of 1.6‰ for 3,4-dichloronitrobenzene. A similar constant reproducible shift of 0.8-2‰ was evident for N-CSIA. Whereas, the compatibility of POCIS for H-CSIA seems to be analyte specific possibly reflecting the adsorption mechanism to POCIS by H-bonding. Performance of the POCIS-CSIA method was evaluated in a pilot constructed wetland where comparable C- and N-CSIA results were obtained from grab sampling and POCIS. This work opens the potential of CSIA application to the low concentration polar emerging contaminants in the environment, such as pesticides, pharmaceuticals, and flame-retardants

Diffusion Sampler for Compound Specific Carbon Isotope Analysis of Dissolved Hydrocarbon Contaminants

Compound Specific Isotope Analysis (CSIA) is widely utilized to study the fate of organic contaminants in groundwater. To date, however, no method is available to obtain CSIA samples at a fine (cm) spatial scale across the sediment–surface water interface (SWI), a key boundary for discharge of contaminated groundwater to surface water. Dissolved contaminants in such discharged zones undergo rapid temporal and spatial changes due to heterogeneity in redox conditions and microbial populations. The compatibility of a passive sediment pore water sampler (“peeper”) to collect 40 mL samples for CSIA of benzene, toluene, monochlorobenzene, and 1,2-dichlorobenzene at field-relevant concentrations (0.1–5 mg L^–1) was evaluated in laboratory experiments. Results demonstrate that physical diffusion across the polysulfone membrane does not alter the carbon isotope values (±0.5‰). Measured δ¹³C values also remain invariant despite significant adsorption of the compounds on the peeper material, an effect which increased with higher numbers of chlorine atoms and sorption coefficient (<i>K</i>_oc) values. In addition, isotope equilibrium between the peeper chamber and the sediment pore water occurred in less than a day, indicating the peeper method can be used to provide samples for CSIA analysis at fine spatial and temporal sampling resolutions in contaminated sediments

Sediment Monitored Natural Recovery Evidenced by Compound Specific Isotope Analysis and High-Resolution Pore Water Sampling

Monitoring natural recovery of contaminated sediments requires the use of techniques that can provide definitive evidence of in situ contaminant degradation. In this study, a passive diffusion sampler, called “peeper”, was combined with Compound Specific Isotope Analysis to determine benzene and monochlorobenzene (MCB) stable carbon isotope values at a fine vertical resolution (3 cm) across the sediment water interface at a contaminated site. Results indicated significant decrease in concentrations of MCB from the bottom to the top layers of the sediment over 25 cm, and a 3.5 ‰ enrichment in δ¹³C values of MCB over that distance. Benzene was always at lower concentrations than MCB, with consistently more depleted δ¹³C values than MCB. The redox conditions were dominated by iron reduction along most of the sediment profile. These results provide multiple lines of evidence for in situ reductive dechlorination of MCB to benzene. Stable isotope analysis of contaminants in pore water is a valuable method to demonstrate in situ natural recovery of contaminated sediments. This novel high-resolution approach is critical to deciphering the combined effects of parent contaminant (e.g., MCB) degradation and both production and simultaneous degradation of daughter products, especially benzene