A case study of sewage discharge in the shallow coastal area of the Northern Adriatic Sea (Gulf of Trieste)

A case study was carried out in 2000 in the shallow coastal area of the Northern Adriatic Sea (Gulf of Trieste) where untreated domestic sewage and industrial wastes are discharged at rate of 5500 m(3).day(-1). The sewage plume above the outfall was followed using faecal coliforms (FC) and overturning length scale (l(T)). The latter was rejected as a marker as the discharge conditions prohibit following the turbulence of sewage water. Intermittent sewage discharge is reflected in the minimal effect of eutrophication. Increase of phytoplankton biomass is thus only minor compared with the unpolluted area regardless of elevated concentrations of sewage-derived nutrients (confirmed by correlation coefficients between FC and NH4+, TP, PO43-: 0.78, 0.71 and 0.67, respectively). Deteriorated trophic status, determined by the TRIX index, was observed only in the surface layer (average TRIX: 5.67). High FC content well above the regulation limit (up to 2.6 x 10(5) FC.100 ml(-1)) represents, therefore, the major negative impact of the improperly treated waste for the risk to human health.