Urea monitoring in the Gulf of Trieste: a marker of anthropogenic impact due to the loads of treated urban wastewaters

Dissolved urea is still rarely included in the monitoring plans of the marine environments and scarcely considered in the oceanographic literature, although its importance as an emerging pollutant and as a relevant nitrogen-nutrient has already been recognised in some coastal zones. For this reason, the aim of this study is to analyse the dynamics of urea with respect to other physical-chemical parameters, in order to assess its role in the coastal waters of the Gulf of Trieste subjected to a strong anthropogenic pressure. This dataset was collected during monitoring and scientific activities carried out by the laboratories of Trieste of ARPA FVG and of CNR-ISMAR. It refers to the period 2002-2010 and it includes about 600 data of urea. The study was focused on three sites of the province of Trieste impacted by the release of treated urban wastewaters, which occurs through sewers located along the coast and through the terminals of offshore underwater pipelines. For comparison, background values of urea were obtained in zones not directly impacted by urban loads. The effect of the hydrological conditions on the concentration of urea and its linkage with the other nutrients have been investigated, paying a particular attention to the possible correlation with ammonium. The main anthropogenic origin of urea was proved by the large range of its concentration (up to 20 mol N l-1) and by its distribution near the sites of discharge. The analysis of the dataset led to consider the effectiveness of urea as a marker of the diffusion of the wastewaters, even more reliable than others traditionally used. Moreover, its behaviour in the coastal waters was proved to be an useful parameter for the assessment of the efficiency of biological abatement systems in wastewater treatment plants and of the diffusion systems in the pipelines.