Influence of a stormwater sediment deposit on microbial and biogeochemical processes in infiltration porous media

In aquatic ecosystems, deposition of fine particles at the surface of bed sediments has a great influence on biogeochemical processes and organic matter mineralization. In urban area, the fine particle deposits collected in stormwater infiltration basins can strongly increase the concentrations of reduced solutes (as NH4+) in interstitial water infiltrating in the porous media. The purpose of this study was to test the relative influence of organic matter characteristics (quantity and quality) and the pollutant content of a stormwater particle deposit on mineralization processes, bacterial characteristics, and the release of nutrients in infiltration sediment systems. In microcosm experiments, two other natural particle deposits (one low and one rich particulate organic matter deposits) were studied to compare their effects with those of the stormwater deposit. The results showed that the biogeochemical processes (aerobic respiration, denitrification, fermentative processes) and the microbial metabolism (enzymatic activities) were stimulated in presence of the stormwater deposit and the natural particulate organic matter (POM)-rich deposit because of the quantity of the POM in these surface particle deposits. The high availability of the POM (indicated by its low C/N ratio) produced a higher stimulation of the microbial metabolism than the POM-rich deposit (with a high C/N ratio). In contrast, presence of pollutants (metals and hydrocarbons) in the stormwater deposit did not have significant influence on microbial processes in the porous media. Thus, stormwater sedimentary deposits strongly loaded in POM and pollutants have a great impact on functioning of water-sediment interfaces by stimulating biogeochemical and microbial processes: (1) interstitial water was deoxygenated, (2) several solutes (NH4+, PO43-, DOC) originating from POM were released to interstitial water and (3) these solutes, particularly DOC, that were transferred at depth can significantly affect the functional diversity of micro-organisms in the whole infiltration system.