Evaluation of Po river water quality in Torino (Italy): effect of diffuse origin and local point loads

The aim of this paper is to examine the Po river quality in a small stretch between two stations in Piedmont in the north of Italy, as a consequence of the introduced loads and the natural capacity of the river to receive these loads without creation of critical conditions for the different use of the water. In this stretch a large pollution load, deriving from the discharge of a very large wastewater treatment plant of the metropolitan area of Torino, is introduced and diluted in the fluent water of the river. The load has been quantified and about it we performed mass balances and modeling in order to understand the entity, the origin and the final fate of the immitted pollutants. The objective of the work was to highlight the impact produced by the Metropolitan Wastewater Treatment Plant of the area of Torino (more than 2 millions of inhabitants) on the quality of the water, in account of different hydrological conditions of the flow. The considered stretch is comprised between Torino and Chivasso, in Piedmont (north Italy). In order to properly evaluate the real meaning of the impact of the wastewater treatment plant on the river, it is important to consider that the studied stretch is interested by three principals types of loads that determine the Po river quality in the Torino area and in the stations downstream. Two of these three loads are of diffuse origin (one, upstream, is depending from agricultural discharges coming from country areas, and the other is relative to meteoric waters of the metropolitan areas); to these loads it is added an important point origin load, arising from the discharge of the metropolitan Torino treatment plant. The objective of the work was to define, by means of a qualitative and analytical study of the quality parameters of the river water, the entity of the loads and to analyze their fate, by taking also in account a perspective of load comparison. In particular the most important result was the possibility to evaluate, after a critical qualitative definition of the Po river water quality that was observed in the city stretch, the effects of the large wastewater treatment plant, in comparison with effects of the diffuse origin loads. In order to perform the illustrated evaluation we utilized the important tool of the material balance. This methodology has been utilized both for the determination of the pollutant loads in each examined station and also for the determination of the hydraulic flow rates in some stations where the data about this aspect supplied by regional water authority were not available. About this aspect, it is very important to underline that the conservative balance for chlorides was a very effective tool useful to supply good results for this aspect. The obtained results show that the immission of the wastewater treatment plant present very important consequences in order to determine the environmental state of the Po river in the stations downstream of the same treatment plant; but it is also important to take into account that, in comparison with the diffused loads that were originally present in the river upstream from the discharge, the percent contribution of the localized discharge can be quite different in consideration of the hydrological regime of the river and the capacity of pollutant transport of the flowing water.