Sylvain Charpentier

Specific stability of organic matter in a stormwater infiltration basin

PurposeIn stormwater infiltration basins, sediments accumulate at the soil surface and cause a gradual filling up of soil pores. These sediments are composed of a mixture of natural and anthropogenic (as oil products) organic matters (OMs). The degradation kinetics of these sediment OMs and their biological stability has been neglected. This study aimed to characterize sediments OMs to assess their evolution and their capacity to degrade.Materials and methodsTo characterize OMs from the sediment layer, we measured at several places in the infiltration basin, total OM and carbon (C) contents, C distribution and biochemical fractions of the OM in the different size fractions, the sediment’s C mineralization potential, soil microbial biomass, and organic pollutants (polycyclic aromatic hydrocarbons (PAHs)) in the sediment layer.Results and discussionOM contents were high and varied from 66 to 193xa0gu2009kg−1 from the inlet to the outlet of basin. Depending on rainfall intensity and volume, organic particles were deposited at varying distances in the basin by decantation; this was confirmed by analysis of sediment C distribution in the different size fractions. Despite high amounts of OM, organic C had a low biodegradability. Mineralization potentials were low compared to natural soil (i.e., from 0.3 to 1.1xa0g CO2–Cxa0kg−1 total organic carbon). Biochemical fractionation of the organic fractions indicated that they were mainly composed of a soluble fraction, which contributed to reducing OM biodegradability. The activity of the sediment microbial biomass was low. PAH contents seemed to be partly responsible for the high biostability of OMs.ConclusionsThere was limited capacity for biodegradation of sediment OMs probably due to inhibitory effects of soluble PAHs and consequently low microbial activity.

Bacterial and fungal communities vary with the type of organic substrate: implications for biocontrol of soilless crops

Biocontrol strategies using organic substrates such as wood fibers and biocontrol agents such as Trichoderma are currently developed to control soil pathogens such as Fusarium oxysporum. Nonetheless, such biocontrol methods give discording results, notably because microbial communities of organic substrates actually are not taken into account. Therefore, there is a lack of information concerning the variability of microbial composition related to the organic substrate type. Here we studied peat, wood and coir fibers, that are substrates known for their different biocontrol efficiency against Fusarium wilt of cucumber. We analyzed in microcosms the microbial composition of wood fibers, coir fibers and peat, incubated up to 60 days, by using an amplicon-sequencing approach based on 16S rRNA gene for bacteria and the internal transcribed spacer (ITS) for fungi. Diversity was assessed by sequencing the 16S rRNA for bacteria and ITS2 region for fungi. Results showed that bacterial richness was threefold higher for coir fiber and peat than for wood fiber. Fungal richness was three times higher for wood and coir fibers compared to peat. Bacterial and fungal patterns showed a dominance of α- and γ- Proteobacteria and Sordariomycetes for coir fiber; β- and γ-Proteobacteria and Eurotiomycetes for wood fibers; Flavobacteria, Leotiomycetes and Sordariomycetes for peat. In conclusion, results show that substrates have different microbial composition. Finally, for a proper use of a biocontrol strategy is important to take into account the type of substrate.

Clogging-up of a stormwater infiltration basin: a laboratory approach using image analysis

PurposeThe main function of stormwater infiltration basins is to favour stormwater drainage. However, the gradual clogging-up process caused by sediment accumulation inside these basins raises questions about their hydrodynamic functioning. Therefore, the objective of this work is to study the evolution of sediment pore distribution and its relationships with water retention and infiltration characteristics.Materials and methodsCheviré basin (Nantes, France) ageing was simulated in the laboratory, using PVC columns (10xa0cm diameter, 32xa0cm height). Seven columns were first filled in with 20xa0cm of thick sandy Loire river alluvia, i.e. the same material as in the basin over which sediment accumulates. The objective of the experiment was to simulate 36xa0months of basin ageing within 9xa0weeks in the laboratory. Every day, the columns were submitted to 2xa0cycles composed of 4xa0h of rain separated by 8xa0h without rain. During the experiment, (1) the water flow at the bottom of the column, (2) the sediment layer thickness, (3) sediment water retention and hydraulic conductivity at saturation (Ks), and (4) pore space distribution by image analysis were measured.Results and discussionAfter 36xa0months of experimental simulation, 3xa0cm of sediment had accumulated (i.e. 1xa0cmxa0year−1); this rate was representative of in situ observations. This progressive accumulation generated the formation of a water layer above the sediment, revealing early clogging-up by the sediment. Using HYDRUS 1D inverse resolution, a decreased Ks values from 25 to 6u2009×u200910−6xa0mxa0s−1 was observed after 6 and 36xa0months, respectively. The mean equivalent pore radius decreased 1.6-fold, from 606 to 380xa0μm after 6 and 36xa0months, respectively. These observations were confirmed by an image analysis study, whereby internal organisational changes were clearly evidenced in the sediment. Sediment particles, at first well individualised, progressively bound to one another, leaving hardly any voids.ConclusionsWhile the clogging-up process in stormwater infiltration basins has often been studied, very little has been done about sediment hydrodynamic properties. Pore space characterisation by image analysis is a major scientific progress and showed that the presence of high levels of organic matter did not favour sediment aggregation. On the contrary, sediment gradually constituted a barrier to water flow, leading to clogging-up.