Characterization of earthworms in an infiltration basin for maintaining water infiltration

Infiltration basins are among the most spread techniques for managing stormwater. Infiltration basins allow the infiltration of stormwater, which prevents their piping towards treatment systems. However, stormwater contains loads of pollutants and suspended solids that accumulate at the surface of the basin and form a sedimentary layer. That sedimentary lay may clog the infiltration basin partially, thus reducing its bulk infiltration capability. Fortunately, plants and fauna colonize spontaneously this sedimentary layer, thus preventing complete clogging and restoring soils' infiltration functions. The knowledge of the effect on restoring the infiltration function requires properly characterize fauna, notably earthworms, with the aim to predict their impact on infiltration. Besides, earthworms, considered as ecosystem engineers, are known to be good candidates for integrating soil chemical pollution.

If earthworms have been intensively studied in natural and agricultural soil, very few studies have focused on the characterization of earthworms' communities in urban soils and, in particular, in infiltration basins. This study presents the description of earthworms sampled at several places over one infiltration basins. This basin receives the stormwater collected over an industrial peri-urban catchment. The infiltration basin has been functioning for more than two decades, thus, plants and fauna have colonized the surface related to water ponding at surface and water infiltration. The sampled places were selected to follow three specific water pathways at the surface. High population variability was measured with densities ranging from 0 to 300 earthworms per square meter with the presence of adults but also juveniles. But, only endogenic and epigeic functional groups were found. The characterization of abundance, age, and species over the sampled places was correlated to water content and sediment thickness, in addition to pollutant loads.

The results show that earthworms require given edaphic conditions (including thick enough sedimentary layer) to settle. We then expect most earthworms to colonize those specific places, increasing water infiltration punctually at these places. Put all together, our findings participate in the understanding of colonization of basin infiltration by organisms and their contribution to their primary function: infiltrating water.