Spatially variable organic-matter-driven clogging in a stormwater infiltration pond: Isotopic, microbiological and hydrogeological evidence

Stormwater infiltration ponds (SIPs) are nature-based solutions which tend to decrease their infiltration capacity over time due to pore clogging. Organic matter (OM) is a well-known clogging driver, but how OM affects the physical and biochemical processes in a SIP remains largely unknown. An analysis encompassing soil organic carbon (SOC) stable isotopes, extracellular polymeric substances (EPS) of biofilms, DNA-based identification of microbiological communities and hydrogeological tests was carried out to elucidate the main clogging mechanisms in a large SIP in Italy. Open pits revealed a stratified soil composed of different textures and compositions, associated with artificial recharge sequences and on-site maintenance practices. A very different isotopic and microbiological signature of soil samples collected at different depths within the first meter of the soil surface was observed. Such diversity was linked to the spatially variable permeability of OM-enriched sediments limiting the infiltration. The isotopic signature beneath the more permeable (i.e., less clogged) OM-enriched layers was similar to that of the isotopic value of the biological surficial crust (δ13C → −27 ‰). Below the less permeable (i.e., more clogged) OM-enriched layers, isotopic values were more consistent with advanced degradation of organic matter (δ13C → −23 ‰). The selective hydraulic isolation of the analyzed trenches could lead to the formation of microbial microenvironments, with direct consequences on local composition of EPS and biofilm production. Based on this multidisciplinary approach, a new conceptual model could be proposed to the site managers and authorities dealing with the SIP's maintenance.