# Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface
## 1 Introduction  
Willow shrub cultivars are unequivocally among the most versatile environmental plant approach. These fast growing phreatophytic woody plants are frequently used in short rotation intensive culture (SRIC) for biomass production [1], often intended for bioenergy and biofuel processes [2]. Willows also show strong tolerance to several contaminants, such as nitrogen rich wastewater [3], trace elements (TEs) [4], various petroleum hydrocarbons compounds [5, 6], as well as pesticides [7], making them effective riparian buffer strips in agricultural systems [8]. More recently, their utilization has been extended to treatment wetlands [9] as well as vegetation filters designed to treat landfill leachate [10]. These plants can additionally be used in phytoremediation to extract or degrade contaminants [4, 11], or,as evapotranspiration (ET) covers, to contain them in the soil [12; 13]. Furthermore, the use of willow for environmental purposes can generally be considered low-cost compared to conventional approaches, and also benefits from strong social acceptability [14]. However, successful soil decontamination by phytoremediation is often challenging to demonstrate clearly, especially in field studies [15], characterized by many sources of variation that can influence the concentration and distribution of contaminants in soil [16].  
In past phytoremediation field experiments, our study group has observed stable, but also increased soil pollutant concentrations under a specific willow plantation (_Salix miyabenna_ 'SX61' and 'SX64'), even after almost a decade of cultivation [6; 17], contrary to initial hypotheses and objectives. The main findings concerning the establishment (first year) of this plantation can be found in [PERSON] et al. [6]. Although no scientific publication has reported the behavior of the soil contaminants on this site after the first four growing seasons, our research group was able to observe that no significant decrease in soil contaminant concentrations occurred for any of the compounds tested (several TEs, polychlorinated biphenyl (PCBs) and polycyclic aromatic hydrocarbons (PAHs)). These observations were hypothesized to be the result of several interacting factors, such as the heterogeneity of soil contamination, as well as the transport of contaminants from deeper to shallower soil depths, as a result of water uptake by the willows.  
At the beginning of the fifth growing season (corresponding to the T0 in [PERSON] et al. [17]), all analyzed contaminants on the site were found in significantly higher concentration under willows than under the control treatment. We hypothesized that the absence of convincing soil decontamination under the willow plantation could be attributable to the attraction of the dissolved contaminant fraction towards the root zones, facilitated by the high evapotranspiration rate of willow fields under SRIC management.  
Accordingly, removing the mature willow cover would theoretically limit the transfer of contaminants into the cut area. This study aimed to explore the effects of willow tree removal from a mature (seven-year-old) plantation on both organic and inorganic contaminant concentrations in surface soil over time (24 months) and was conducted inside the boundaries of a willow field established in 2010 on a former industrial site in southern Quebec, Canada [6].