Role of rhizoremediation in a complex PCB soil contamination gradient

In the last two decades there has been a growing interest in bioremediation technologies which use plants and microorganisms to degrade organic chemicals such as Polychlorinated Biphenyls (PCBs) in contaminated sites. These techniques represent a good alternative to traditional remediation technologies, being cheaper, not disruptive and more suitable for large contaminated areas. Different studies have been conducted to investigate the potential of plant-microbe interactions in the remediation of organic chemical contaminated soils with respect to natural attenuation, providing useful data such as chemical degradation rates (K D ) or half lives (HL). Such a type of data can be used to predict soil concentration temporal trend, as well as the time needed to achieve legal limit when using plants and their associated rhizosphere microbe to remediate contaminated sites. In the present work, rhizoremediation experiment derived KD or HL for PCBs will be used as input parameters in an existing dynamic air-plant-litter-soil model (SoilPlusVeg) to estimate PCB concentration temporal trend in the soil of a National Relevance Site (SIN) for remediation located in Northern Italy (SINBrescia Caffaro). Analyses of sample collected in this area have shown the presence of a high spatial variability of PCB concentrations with values varying up to 3-4 orders of magnitude. Therefore it could be interesting to understand if rhizoremediation might be a suitable technique for the entire site. A number of long term simulations were run with SoilPlusVeg model for some PCBs and results were used to 1) compare the influence of natural attenuation vs. plant/microbe interactions on soil concentrations, 2) evaluate the effectiveness of rhizoremediation when a complex contamination gradient is present. Simulations results showed that although the importance of natural attenuation vs plant/microbe interactions depends on PCB physical chemical properties, when considering rhizoremediation experiment derived degradation rates the time requested to achieve legal limits decrease. However, rhizoremediation must be accurately implemented (in terms of species to be selected, their density, etc) to account for effective remediation, especially when complex patterns of contaminant are present.