PAH–aliphatic hydrocarbon source disentanglement and ecological risk mapping in Ghanaian river sediments: National scale source apportionment across tropical basins

Abstract

Hydrocarbon contamination in tropical river sediments is often assessed using polycyclic aromatic hydrocarbons (PAHs), although PAH only approaches can compress petroleum weathering, plant wax dilution and mixed anthropogenic source histories into broad pyrogenic classes. This study presents a national PAH–aliphatic hydrocarbon source disentanglement and Hydrocarbon Risk Surveillance Index (HRSI) framework for Ghanaian river sediments, based on 168 surface sediment records from 21 rivers across six operational basin systems, four reach positions and two hydrological seasons. For the first time at national river basin scale in Ghana, EPA-16 PAHs are integrated with C10–C35 n-alkanes, pristane/phytane, carbon preference index, terrigenous/aquatic ratio, unresolved complex mixture fractions, exploratory PCA/HCA, constrained non-negative latent source classification and HRSI screening to separate petroleum weathering signals from combustion source signals. Basin scale hydrocarbon burden followed the surveillance priority order Ankobra > Pra > Coastal > Volta > Tano > Bia. PAH diagnostic ratios classified most samples as pyrogenic dominant, whereas aliphatic biomarkers and latent source fractions identified additional weathered-petroleum, mining-urban, traffic-combustion, biomass-combustion and terrestrial-biogenic signatures. Risk was spatially compressed: 114 samples were low risk, 40 were moderate, 12 were high and 2 were severe, with severe dry season records at the Densu estuary and Ankobra midstream. The results identify sediment capture, especially in depositional or low gradient reaches, as a critical process by which petroleum weathering and combustion signals are converted into local high risk hotspots. Mining pressure, urban traffic pressure and depositional reach position were positively associated with hydrocarbon burden and HRSI class, but the observational design and collinearity among urban land use, road density and population density require interpretation as associations rather than causal effects. The study provides a testable basis for basin prioritised hydrocarbon surveillance in Ghana.