Contrasting petroleum hydrocarbon contamination across active, retired and reference wetlands in the Liao River estuary

Introduction Assessing residual contamination in coastal wetland soils after oil-well retirement requires comparisons with both active oil-well sites and natural reference wetlands. Methods We analyzed a September 2022 soil survey from the Liao River estuary wetlands in northeastern China, comprising five active oil-well plots, five retired oil-well plots, and three reference wetland plots. Three separately collected surface soil samples were averaged per plot, giving 13 independent analytical units. Soil properties, total petroleum hydrocarbons (TPHs), and five trace metals were compared among well-status groups. Results Mean TPH concentrations were 35.71 g/kg in active plots, 12.95 g/kg in retired plots, and 1.20 g/kg in reference plots (exact Kruskal–Wallis P < 0.001; false-discovery-rate-adjusted q = 0.0015). Total organic carbon (TOC) followed the same ordering (43.15, 29.76, and 18.99 g/kg, respectively; P < 0.001; q < 0.001) and was positively associated with TPHs (Spearman rho = 0.76, P = 0.003). No overall group differences were detected for As, Cd, Cr, Cu, or Pb after multiple-testing correction, although one active plot showed locally elevated As, Cd, and Cr benchmark ratios. Discussion Retired plots had lower mean TPH concentrations than active plots, but all retired-plot values exceeded the reference range. These findings support monitoring residual petroleum contamination after oil-well retirement and targeted assessment of local metal enrichment.