Combined Use of Caffeine and Turbidity to Evaluate the Impact of CSOs on River Water Quality

Several studies have demonstrated that the most effective way to control eutrophication is to reduce phosphorus input at the scale. Water quality monitoring programs need to separately evaluate the different polluting sources to provide a suitable estimate of their relative contributions and thus accurately prioritize possible restoration actions. Urban area cases, where a portion of untreated wastewater is often discharged directly into receiving surface waters by combined sewer overflows (CSOs) during rain events, remain unsolved. In this context, an urban watershed located in Northern Italy with 60 CSOs has been chosen as a case study, and four rainy events have been hourly monitored. The proposed monitoring program consists of the combined use of caffeine and turbidity to estimate the volume and phosphorus load spilled into the river from the CSOs, respectively. Caffeine proved to be a suitablemolecule to quantify the volume of wastewater discharged into water bodies, based on a per capita caffeine load of 10.8 mg inhab(-1) d(-1), estimated in the present work. This research showed that, on average, more than half of the total phosphorus loads transported by the river is due to the CSO discharges (56.5%). The knowledge of the prevailing responsibility of the CSO discharges for the Lambro River quality allows prioritizing effective restoration actions.