Accumulation and persistence of sewage and fish cannery pollution in coastal sediments (northern Adriatic Sea)

Abstract

The study was carried out in sediments exposed to three different types of sewage pollution: primary treated sewage waters, urban/fish cannery untreated effluents, and discharges from boats (harbors) to evaluate the level and sources of pollution and the relationships between organic contaminants and chemical-physical factors that affect their accumulation and persistence in sediments. For this purpose, the occurrence and distribution of sterols, fecal indicator bacteria, hydrocarbons, granulometry, and redox conditions in sediments were analyzed. Sediment cores were collected by scuba divers and redox potential was determined in situ by vertical penetration of Pt electrode. For determining the granulometric composition, samples were wet sieved through ASTM sieves. The fraction <0.063 mm was analyzed using the Sedigraph method of particle sizing. Fecal coliforms (FC) and fecal streptococci (FS) were determined using the multiple test tube most probable number method. Sterols (foremost: coprostanol COP; cholesterol CHL) and hydrocarbons (n-alkanes and polycyclic aromatic PAH) were analyzed by GC/MSD. Specific ratios and plots of sterols were used as diagnostic tools for sources and levels of sewage contamination. The occurrence and diagnostic ratios of n-alkanes and PAHs have been used to trace the pollution and origin of hydrocarbons. At the sewage disposal site, COP, FC, and FS correlated and decreased with distance from the outfall, being predominantly associated with the very fine sand fraction. The primary treatment of sewage waters was ineffective and fecal pollution was diagnosed in a radius of 300 m around the outfall. The grain size composition of surface sediments was altered in comparison to the deeper sediments. In the sediments within the reach of the fish cannery effluents, CHL predominated or paralleled β-sitosterol, the principal sterol in harbors. COP was not correlated to FC or FS. PAHs within the extent of spread of the fish cannery effluents were comparable to those in the harbor sediments, regardless of considerably lower traffic intensity. In all sediments, PAHs were of pyrogenic origin. At the sewage disposal site, the level of pollution and the alterations in the granulometric composition of surface sediments followed a pattern consistent with dominant current direction. Accumulation of organic matter around the outfalls created highly anoxic conditions that favored persistence of fecal pollution, in situ formation of COP, enhanced accumulation of PAHs, and/or prolonged survival of fecal bacteria in the sediment. The industrial outfall, even if smaller than the urban, affected the sediment similarly.