Long-Term Monitoring of a Lagooning Basin Used as Pretreatment Facility for a WTP: Effect on Water Quality and Description of Hydrological and Biological Cycles Using Chemometric Approaches

The drinking water treatment plant (WTP) of the city of Turin (NW Italy), with a treatment capacity of 40 × 10⁶ m³/year, has a basin that is employed as a lagooning pretreatment facility. This study aims to assess the effect of the basin on several environmental parameters (temperature, dissolved oxygen (DO), turbidity, pH, chloride, nitrite, and total chlorophyll) of the river water before entering the WTP and monitor the changes inside the basin caused by the seasonal hydrological and biological cycles. Sampling was carried out on 16 dates over 3 years at the inlet and outlet channel of the basin and in five locations along three depth values (1, 6, and 12 m, i.e., at the bottom). The results of the 3-year monitoring campaign demonstrated that the basin had an effect on pH (p = 6.6 × 10⁻⁹), DO (p = 0.000072), turbidity (p = 0.011), and chlorophyll (p = 0.033). No significant changes regarding nitrite (p = 0.11), chloride (p = 0.94), and temperature (p = 0.66) were detected. The results gathered from the sampling campaign inside the basin demonstrated that, during the year, the basin experienced the following: two states of complete mixing in early spring and fall, when the differences in temperature between the surface and the bottom of the basin were less than 1 °C; a condition of late spring/summer stratification with a temperature difference between the surface and the bottom of 4–5 °C and a difference in DO, pH, and total chlorophyll concentration that increased throughout the spring season; and one or more states of summer circulation due to the weak stability of the warm season stratification. During the states of circulation, the persistent algae photosynthetic activity tended to cause a quick change in the concentration of DO, total chlorophyll, and pH value in the most superficial layer of the basin. The results of the principal component analysis (PCA) showed a strong direct relationship between the weight of the first component and the hydrodynamic states of the basin (stratification/circulation) and an inverse relationship between the weight of the second component and the intensity of photosynthetic activity of algae species.