The Influence of Sanitary Infrastructure on Event Nutrient Dynamics in a Headwater Catchment

Many low-order streams are recipients of effluents from wastewater treatment plants (WWTPs) and combined sewer overflows (CSOs). Not only do these facilities have to meet fewer requirements compared to their bigger counterparts in more densely populated areas, but they also discharge into smaller, more vulnerable streams, with low dilution potential. Although these local point sources can dictate the local water quality and quantity dynamics, they are barely monitored and often not included in catchment-wide analyses. In this case study we measured stream water quantity and quality in a first-order, point source-influenced stream. We have specifically addressed point sources in our monitoring programme, which included the installation of a low-budget probe at the CSO outlet to monitor overflow timing. By clustering hysteresis loops and using Principal Component Analysis, we were able to identify hydro-meteorological drivers and reveal seasonal patterns of discharge and nutrient export dynamics. Mobilisation of nitrate from agricultural sources clearly dominated event dynamics during periods with high soil moisture, while point source dynamics overlaid catchment responses during the rest of the time. Thus, the dilution potential of the stream was found to be a controlling factor for water quality dynamics. Groundwater infiltration into the sewer system probably increased the risk of CSO discharges, especially in winter and spring. In summer, CSO spillages occurred as a result of high-intensity rainfall. These events were related to an increase of turbidity and a mobilisation of particulate phosphorus. With our novel approach, including urban point sources in our monitoring setup, we were able to show the close relationship between sanitary infrastructure in rural areas and stream nutrient dynamics. Including point sources more closely into monitoring and analysis is essential to improve the process understanding.