Examining changes in groundwater PFAS contamination from legacy landfills over a three-year period at Australia's largest urban renewal site.

Understanding groundwater contamination from legacy landfills, including fate and transport of Per- and polyfluoroalkyl substances (PFAS), is a critical challenge for sustainable urban renewal. We analysed groundwater within and surrounding legacy landfills at Fishermans Bend for PFAS and complementary hydrochemical indicators. Sampling in 2017 revealed extensive PFAS contamination from the landfills. We re-examined concentrations after a 3-year period, to assess natural source attenuation and evolution of the contaminant plumes. Total PFAS (∑PFAS) ranged from 88 to 973 ng/L, with relatively high concentrations (mean = 500 ng/L, n = 4) in samples directly within the waste mass of a large legacy municipal and industrial landfill (Port Melbourne Tip). Two samples on the boundary of a former construction and demolition waste landfill also had elevated PFAS concentrations (∑PFAS = 232 and 761.5 ng/L). Down-gradient of the landfills, groundwater showed reductions in total PFAS, though still maintained considerable loads (∑PFAS = 107.5-207.5 ng/L). Long-chained PFAS showed greatest reductions relative to chloride concentrations down-gradient of the landfills, consistent with sorption as the predominant removal mechanism. The dominant mass fractions detected were similar in 2017 and 2020 (median: PFOS > PFHxS > PFHxA > PFOA); comprising the widely known, persistent 'legacy' PFAS. Re-sampled bores returned similar concentrations of these PFAS in 2017 and 2020 (median %RPDs of 0.0, 9.3 and 15.4, for PFOS, PFOA, and PFHxS, respectively). However, there were marked increases in concentrations of certain PFAS in three bores - including a previously un-impacted background site. The results show limited attenuation of legacy landfill PFAS contamination in groundwater over a 3-year period.