Sources of anthropogenic pollution were categorised by combining elemental and isotopic techniques (C/N ratios and δ¹³C, δ¹⁵N) on samples of surficial sediments in the harbour of Naples (Southern Tyrrhenian Sea, Italy). The study area, due to the intense and diversified industrial and commercial activities, is an appropriate natural laboratory to verify reliability and relevance of geochemical methodologies applied to exploration of sources, pathways and fate of contaminants in highly polluted coastal marine systems. Application of cluster analysis to the whole dataset and resolution of a δ¹³C and δ¹⁵N Moore–Penrose system equation provided information to reliably discriminate and identify anthropogenic point sources in the studied sediments. In particular, effects of oil spilling and wastewater discharge within the area of the harbour of Naples were clearly discriminated thus suggesting high-potential of the two numerical techniques, applied to the C and N elemental and isotopic values, to explore effects of anthropogenic impact in marginal and confined coastal basins. The δ¹⁵N values showed high sensitivity to discriminate sewage discharges (treated or untreated organic matter), clearly indicating the point of emissions. The δ¹³C shows indirect capability of discriminating crude oil and petroleum products.