# Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface
## 4 Discussion  
### General Pattern  
Following the 24-month experiment, five PAHs showed significantly higher concentrations under the Salix treatment than in the plots without willows (Cut) (Table 2). The changes over time in the concentration of four of the five PAHs (acenaphthylene, chrysene, 1-methylnaphthalene and 2-methylnaphthalene) suggest that increases under the Salix treatment led to their differences between treatments. Although it is less obvious for the fifth PAH (acenaphthene), an increase under the Salix treatment may have once again led to the significant difference observed between treatments after two years, since the statistical difference was marginally significant (\(p\) = 0.0533). This level of significance is considered fairly, rather than extremely, reliable, given the many potential contributing factors involved in a dataset gathered on a full-size contaminated former industrial site [15]. The heterogeneity of soil contamination is a common feature of post-industrial zones and systematic soil sampling will most likely produce data with substantial variance, which can make it more challenging to show significant treatment effects statistically [15]. Accordingly, it has been proposed that a value of 10% (\(p\)\(\leq\) 0.1) instead of 5% (\(p\)\(\leq\) 0.05), could be an acceptable level of significance in such circumstances [15]. Following this recommendation, the \(p\)-values between 5% and 10% are presented in Table 2. They will be used to complement the interpretation of the results and will be referred to as tendencies.  
When considering these tendencies, five additional PAHs contribute to the previously mentioned pattern of higher concentrations of contaminants under the Salix treatment in the final samples (Tf). Again, increases in concentrations under the Salix treatment over time seem to have produced most of these differences, but some decreases in Cut might also have played a role. Phenanthrene also appeared to have increased over time in the Salix treatment, since it initially showed a tendency towards higher concentrations under the Cut treatment, but by the end of the experiment was similarly concentrated in both treatments. Anthracene was the only compound of importance in this study to show a tendency to increase under both treatments over time.  
Overall, these results clearly show that removal of the willow trees in the mature plantation limited the contaminants' increase in the soil surface, suggesting that such increases were mainly driven by the presence of willows. This finding strongly reinforces an earlier hypothesis that SRIC of willows may facilitate the migration of contaminants towards their roots, possibly by means of their high evapotranspiration rate, thus increasing their concentration in the surrounding soil [17].  
The finding that soil contaminant concentrations increase under a willow phytoremediation crop differs from the results of most phytoremediation studies that monitored changes in soil concentrations (i.e., the reduction of pollutant concentrations in soil (see [PERSON] et al. [25] for an example). Increases in soil pollutant concentrations under willows may appear surprising and incompatible with the objective of soil phytoremediation. Nevertheless, it is essential to consider possible dynamics at play that could generate such data, as well as the implications of these results for further phytoremediation field work.