Phytoremediation of an acid tar pit at Cinderhill, Derbyshire, UK: Lessons learned and future challenges

Acid tar pits represent some of the most heavily contaminated land and are widespread in the UK, and globally. They pose significant challenges to remediate, with options typically consisting of financially and logistically expensive mixing and/or moving of contamination and disruption to surrounding areas. Phytoremediation offers a great opportunity to remediate hydrocarbon-related pollution, but many studies take place at small spatial and temporal scales and in controlled conditions. Here we present the monitoring results of a large-scale field trial (30 m × 16 m area) of willow (Salix) phytoremediation of an acid tar pit at Cinderhill, UK. Results show that tars had not degraded since deposition in the 1970s before the trial but, after planting with willow spilling, evidence of degradation was clear, with a reduction in n-alkane and PAH concentrations. After 2 years, tar breakdown was only recorded in close association with willow root systems, but after 3 years was documented in tars not targeted by root growth. Willow also stabilised the tar pit surface, altered its topography and enabled the establishment of a diverse understory flora and, along with initial liming of the area, contributed to reduced surface pH from highly acidic to neutral. eDNA of microbial communities in tars near willow roots were highly distinct from tars away from roots, and surrounding soils. The community around willow roots included species known to process PAHs and hydrocarbons, and also to include species that aid vegetation growing in stressful environments. This holistic assessment of the phytoremediation of an acid tar pit indicates its significant potential as a remediation option. Challenges and unforeseen issues during the trial, future work, and knowledge gaps are discussed.