Vertical fluxes and accumulation of PCBs in coastal sediments of the Río de la Plata estuary, Argentina

Settling particles and underlying sediments collected at 1, 2.5 and 4 km along offshore transects in the urbanized sector of the Río de la Plata were analyzed to evaluate the sources and accumulation of PCBs. Total PCB concentrations range from <0.1 to 100 ng g−1 and include variability associated to North-South and offshore gradients reflecting the impact of coastal discharges. Highest concentrations were recorded in the industrialized Central area close to Buenos Aires (61 ± 37 ng g−1 at 1 km) relative to cleaner northern stations (3.6 ± 2.2 ng g−1) and southward sites (37 ± 2.8 ng g−1), affected by transport of particulate PCBs by coastal currents. Sediment traps deployed in the Central area revealed large depositional fluxes of total matter (361 ± 124 g m−2 day−1) and PCBs (26 ± 19 μg m−2 day−1) and high sedimentation rates (5.0 ± 1.7 cm yr−1). Uniform PCB concentrations (66-89 ng g−1) down to 20 cm in sediment cores suggest continued PCB discharges during the last 4 years. PCB composition was dominated by hexa (43 ± 6.4%), hepta (23 ± 5.1%) and pentachlorobiphenyls (21 ± 5.5%) with lower proportions tri-tetra (7.4 ± 5.4%) and higher chlorinated congeners (5.1 ± 3.3%). A consistent weathering pattern with loss of 3-5 chlorobiphenyls and enrichment in higher chlorinated PCBs corresponding to a shift from a 1:1 to a 1:3 1254:1260 Aroclor mixture, was observed offshore. A principal component analysis performed with the relative contribution of PCB congener classes confirmed the offshore weathering pattern indicating that transformer oils containing Aroclor 1254-1260 are the most probable sources. Sediment inventories, sediment trap fluxes and Fugacity II calculations indicate an accumulation ∼500-800 kg PCB in superficial sediments of this coastal environment.