# Willows Used for Phytoremediation Increased Organic Contaminant Concentrations in Soil Surface
## 4 Discussion
### Convective Transport of Dissolved Chemicals towards the Root Zone  
Plant transpiration is known to create water potential gradients from leaves to bulk soil, which can generate convective transport of dissolved chemicals from the adjacent bulk soil towards the roots [26; 27]. Rhizospheric accumulations of these chemicals can then be expected, especially if the quantity transferred by water mass flow surpasses plant requirements [28]. Soil scientists generally describe this phenomenon in a macronutrient acquisition context, but it could also concern micronutrients (i.e., Cu, Mn, Zn and Co) [29] and some TEs, as observed for Pb in [PERSON] et al. [30]. In their controlled laboratory study, the exclusionary mechanisms of metal resistance were suspected to promote the accumulation of Pb in the rooting zone of _Betula occidentalis_ after its mobilization in the rhizosphere. Moreover, the simultaneous mobilization of other chemicals like sulfate orphosphate in the rhizosphere may promote the precipitation of soluble elements, leading to their enrichment over time in a relatively less-mobile form [28; 30]. Interestingly, our results showed no effect of treatments on soil TE concentrations. Although TEs may have been subject to convective transport towards the roots, the ability of willows to bioaccumulate many of these TEs in their tissues [4] may have prevented their accumulation in the surrounding soil. The impacts of plants on the mobilization of hydrophobic organic compounds are less well documented. However, there is some evidence that plants can contribute to the accumulation of organic chemicals such as PAHs [31], which generally sorb to soil particles, decreasing their transport rate and increasing the time required for their remediation [32]. The release of organic acids with root exudates can increase their solubility and therefore their mobility [33]. Colloids, as mobile bacteria, may also enhance the transport of PAHs in the subsurface of soil [34]. Once in the rhizosphere, relatively hydrophobic compounds, such as high molecular weight (HMW) PAHs, can then adsorb and bind strongly to the roots [35]. Such adsorption to the roots can apparently increase with lipid content [36], as well as with plant age, due to a greater total root mass [36]. Although it is generally accepted that low molecular weight (LMW) PAHs are potentially more soluble and therefore more mobile than heavier compounds [37], our investigations did not reveal any statistical relationships between the behavior of the concentration (increase, stable or decrease) of individual PAHs and their molecular weight (data not shown). The mobilization of chemicals by mass flow, driven by plant transpiration, has been mostly documented on the individual plant scale (i.e., glass tubes [31]), and less is known about this phenomenon on a larger scale (i.e., field scale). It is now relatively well documented that the ET rate of most willow species used in environmental projects is high enough to affect the hydrology of the soil below [38]. Rapidly growing willow crops are indeed able to act as a \"biological pump\", thus influencing groundwater flow patterns, whereby the trees are able to reach the water table [39; 40]. Hydraulic control has been developed as a technique that uses trees as ET cover to remove contaminated soil water, in order to contain or control the migration of water-soluble contaminants in the subsurface [40]. We suspect that the high evapotranspiration rate of the willows in our experiment led to increases of pollutant concentrations under the Salix treatment over time through similar mechanisms. The ET of willows may have created water potential gradients from leaves to the peripheres of the plantation, or to the deeper soil layers, thus generating convective transport of dissolved chemicals from these zones towards the surface soil under the plantation, where samples were collected.