Integrative assessment of in situ combined bioremediation strategies applied to remediate soils spilled with sewage sludges

Landfills and waste disposal sites in the Basque Country are summarized in the inventory of soils that either currently support or have supported potentially polluting activities or facilities (Law 4/2015). Notably, “Landfill 17,” located in Gernika-Lumo, has been receiving, for decades, sewage sludges from the local wastewater treatment plant (WWTP) as agricultural amendment. In order to decontaminate and recover soil functionality, a combination of bioremediation (which involved bioagumentation and phyto- and vermitechnologies) and complementary bioremediation strategy (i.e., promotion and maintenance of the native vegetation) was implemented in situ . Physicochemical and ecotoxicological characterization were achieved. Furthermore, an ecotoxicological assessment of the soils upon flora and fauna was carried out through the application of different bioassays and biomarkers. Additionally, an integrative biomarker response (IBR/n) index was calculated to provide a holistic view of the soil general status. Critical pollutants [Cd, Cr, Ni, Pb, benzo(a)pyrene, and dieldrin] were observed in most of the treated sites. Microbial parameters did not present remarkable differences among sites. However, plant indicators pointed the non-treated site (MN8) as the unhealthiest. This was also observed in earthworms’ immune system, where cytotoxicity appears when exposed to non-treated soils. In conclusion, this field study showed that the combination of bioaugmentation, phytoremediation with native species, and vermiremediation is highly useful in eliminating mixed contamination, improving soil health, and ultimately restoring ecosystem functionality and biodiversity.