Vertical fluxes and organic composition of settling material from the sewage impacted Buenos Aires coastal area, Argentina

Settling particulates collected upstream and downstream of Buenos Aires main sewer in the Río de la Plata were analyzed for total organic carbon (TOC), nitrogen (TN), resolved aliphatic hydrocarbons (ALI), unresolved complex mixture (UCM), terpane biomarkers, linear alkylbenzenes (LABs) and PCBs to evaluate the composition, sources and variability of anthropogenic fluxes. Sediment traps collected a substantial amount of material (19.9 ± 13.5 g cm−2 yr−1) with relatively high organic contents (TOC: 7.7 ± 5.5%; TN: 0.65 ± 0.34%; C/N: 13 ± 3.2) and contaminant loads (UCM: 949 ± 737, ALI: 115 ± 111, LABs: 18 ± 12; PCBs: 0.07 ± 0.04 μg g−1). The resulting vertical fluxes are very large (TOC: 1.9 ± 2.5 g cm−2 yr−1; total hydrocarbons: 21 ± 23 mg cm−2 yr−1; LABs: 0.42 ± 0.62 mg cm−2 yr−1; PCBs: 1.6 ± 1.8 μg cm−2 yr−1). The settling material showed a clear organic enrichment downstream the sewer and during rainy, warm months. A variance component analysis revealed that spatial differences were less significant (13–27%) than the seasonal trend (30–63% total variability) which is inversely related to compound stabilities. The organic composition indicated slightly altered inputs of petrogenic hydrocarbons, detergents and Aroclor 1242:1254:1260 residues, with limited contribution of terrestrial plant waxes and negligible from plankton. The seasonal variation of individual compounds revealed a summer enrichment of labile components inversely correlated to molecular weight, with terpane biomarkers showing an opposite pattern. Rainy, warm months showed higher total mass fluxes and C/N ratios with signatures enriched in C30 hopanes suggesting a more efficient washout of fresher organic residues. Conversely, cold months showed more degraded signatures depleted of low molecular weight components and enriched in UCM, internal LABs and tricyclic terpanes.