Sources, vertical fluxes, and accumulation of aliphatic hydrocarbons in coastal sediments of the Río de la Plata Estuary, Argentina

Abstract

Settling particles and underlying sediments collected at 1, 2.5 and 4 km off the metropolitan Buenos Aires coast were analyzed to evaluate the sources and accumulation of resolved (RES), unresolved (UCM), and biomarker aliphatic hydrocarbons. Sedimentary aliphatic concentrations (RES 0.11-14 microg x g(-1); UCM 0.1-800 microg x g(-1)) included variability associated with north-south gradients and an exponential offshore reduction. Highest concentrations were registered close to Buenos Aires port and sewer, compared to cleaner northern stations and southward sites affected by a seaward residue transport. Sediment traps deployed in the sewer area revealed large hydrocarbon (38 and 319 mg x m(-2) x day(-1), RES and UCM) and total organic carbon fluxes (29 +/- 26 g x m(-2) x day(-1). The composition of RES and hopanes evaluated by principal component analysis indicated a consistent offshore gradient defined bythe relative contribution of lower vs higher molecular weight components. Distant sites showed decreasing proportions of petrogenic n-C(17-26) alkanes, isoprenoids, and C(20-27) terpanes and relative enrichment of n-C(27,29,31,33) terrestrial plant alkanes and C(31-33) homohopanes. Sediment hydrocarbon profiles showed an average 2-fold reduction down to 20 cm depth with preferential removal of lower molecular weight components and enrichment of n-C(23-35) alkanes and hopanes. Sediment inventories and trap depositional fluxes indicate the accumulation of 5800-9700 tons of aliphatic hydrocarbons in the top 0-5 cm sediments with a strong interfacial alteration and selective preservation of refractory components: n-C(13-22) (1.0%) < isoprenoids (3.2%) < n-C(23-35) (6.1%) < hopanes (47%) approximately UCM (50%), compared to intermediate stability of organic carbon (12%) and quantitative preservation of polychlorinated biphenyls (PCBs) (91%).