Non-aqueous phase liquids (NAPLs) are persistent sources of contamination in the ground, providing a long-term supply of dissolved phase contamination and taking significant periods to dissipate naturally. Light NAPLs (LNAPLs) take the form of a separate phase within the ground, often as individual ganglia in pore spaces within the capillary zone such that the contaminated region is diffuse and comprised of many unconnected small contaminant sources. Consequently, remedial action is challenging and success may be limited to ex-situ remediation techniques. The ability of plants to phytoremediate dissolved-phase contamination is well known, but the impact of LNAPLs on plant growth and subsequent contaminant behaviour is largely unknown. Experimental work with ryegrass (Lolium perenne) is presented, exploring the impact of the physical presence of an LNAPL (mineral oil) on plant growth, root distribution and oil removal. The presence of the oil was found to significantly impact root biomass and distribution, leading to zones of increased root growth alongside decreased shoot growth. Significant removal of the LNAPL was noted in both hydroponic conditions and planted soil.

The success of phytoremediation is dependent on the exposure of plants to contaminants, which is controlled by root distribution, physicochemical characteristics, and contaminant behaviour in the soil environment. Whilst phytoremediation has been successful in remediating hydrocarbons and other organic contaminants, there is little understanding of the impact of non-aqueous phase liquids (NAPLs) on plant behavior, root architecture and the resulting impact of this on phytoremediation. Light NAPLs (LNAPLs) may be present in pore spaces in the capillary zone as a continuous or semi-continuous phase, or as unconnected ganglia which act as individual contaminant sources. Experimental work with ryegrass (Lolium perenne) grown under hydroponic conditions in idealised pore scale models is presented, exploring how plant growth, root distribution and development, and oil removal are affected through direct physical contact with a model LNAPL (mineral oil). In the presence of low levels of LNAPL, a significant decrease in root length was observed, whilst at higher LNAPL levels root lengths increased due to root diversion and spreading, with evidence of root redistribution in the case of LNAPL contamination across multiple adjacent pores. Changes to root morphology were also observed in the presence of LNAPL with plant roots coarse and crooked compared to long, fine and smooth roots in uncontaminated columns. Root and shoot biomass also appear to be impacted by the LNAPL although the effects are complex, affected by both root diversion and thickening. Substantial levels of LNAPL removal were observed, with roots close to LNAPL sources able to remove dissolvedphase contamination, and root growth through LNAPL sources suggest that direct uptake/degradation is possible.