Reference Sensitivity in Coastal Sediment Pollution Indices: Spatial Priorities Are More Robust than Classifications and Dominant-Element Attribution

Sediment pollution indices convert metal concentrations into contamination classes, hotspot rankings, and dominant-element assignments (the metal with the largest single-element risk factor, Er). For seven metals at 49 stations in three northern Beibu Gulf bays, we examined sensitivity across nine geochemical backgrounds, three potential ecological risk index (RI) cut-point conventions, and a threshold effect level (TEL) toxicity-normalised denominator. First, for within-source envelope variation (varying one regional background across its published lower, central, and upper bounds), 62.9% of 294 station–metal geoaccumulation (Igeo) classes and 38.8% of station RI classes were unchanged; moving from lower to upper bounds reduced stations with pollution load index (PLI) > 1 from 31 to 12, reversing the pooled majority verdict without changing reference sources. Second, across four regional backgrounds at central values, agreement was 59.2% for Igeo class, 79.6% for PLI > 1, and 87.8% for RI class at fixed cut-points (42.9% across the 12 reference-by-cut-point combinations). Station rankings were robust across geochemical backgrounds (Spearman ρ = 0.895–1.000), whereas switching to the TEL-normalised construct changed the dominant element at 42 of 49 stations. Assessments should report benchmark sources, denominator meanings, cut-point conventions, and sensitivity across defensible choices.