Polycyclic aromatic hydrocarbons (PAH) derived from natural precursors have been analysed in superficial sediments and waters in the deltaic area of the Rhone river. This area is characterized by major water and solid inputs from the Rhone river into the Mediterranean Sea. Nine superficial sediment samples were collected in February 1985 and September 1986, and 15 water samples were collected in September 1986, January 1987 and June 1987, with further separation of particulate and dissolved phases. After extraction of lipids, isolation and fractionation of hydrocarbons by HPLC, PAH were analysed by GC and GC-MS. Retene and tetrahydrochrysenes (THCs) were the major naturally derived PAH detected in the Rhone delta. They derive from diterpenoid and triterpenoid precursors, which are common constituents of terrestrial plants. In sediments, intermediate products were absent or at trace levels, suggesting that formation of natural PAH occurred mainly before deposition. Concentrations of retene and THCs were in the range 12.92-52.86 ng g-1 and 16.56-162.68 ng g-1, respectively. Their distributions were different from those of anthropogenic PAH. Despite their common origin, retene and THCs did not show identical distributions and relations to parameters such as total organic carbon and terrestrial n-alkanes; these findings indicate that they follow different transport pathways, as a result of different particle associations. Estimations of retene and THC fluxes and the use of these markers for determining the percentage of 'terrestrial organic carbon' are presented. Analysis of water samples confirmed that naturally derived PAH were mainly formed before deposition, in soils of the drainage basin or during the early stages of riverine transport. The distributions of natural and anthropogenic PAH were different, which suggests that the carrier particles of natural PAH were coarser and settled more rapidly than those carrying anthropogenic PAH. However, retene and THCs did not appear to be associated with the same particle types, shown by distributional differences and by the presence of retene in the dissolved phase in contrast to THCs. Consequently, the distributional trends observed in sediments can be explained by the different aquatic transport of the various PAH classes as demonstrated by the analysis of water samples. Instantaneous fluxes were estimated in three seasons for both dissolved and particulate phases. They again showed decoupling between natural and anthropogenic PAH. Fluxes of natural PAH were strongly dependent on the river flow, whereas those of anthropogenic PAH were more dependent on emission rates. The dissolved phase was a significant factor in the transport of retene and of anthropogenic PAH. Naturally derived PAH, and more specifically THCs, appear to be promising tracers of inputs and dispersal of continental organic matter.
