Influence of sanitary infrastructure on nutrient transport mechanisms in a headwater catchment

Sanitary infrastructures draining smaller villages are often not taken specifically into account when discussing catchment transport process. One reason is the limited data availability, as they are usually not part of (high frequency) monitoring strategies, although it has been shown, that they can have a significant impact on water quantity and quality. Especially in first- or second-order streams, they can contribute a big share to the discharge volumes. At the same time, the dilution effect of small streams is limited. This is critical, as e.g. wastewater treatment plants (WWTPs) often have to meet lower requirements compared to their bigger counterparts treating water from cities, meaning, that especially nutrient concentrations can be still high in the effluent.

We measured discharge and different water quality data in a headwater stream (2.77 km²), which is influenced by agriculture, a small village and point sources: two combined sewer overflows (CSOs) and one WWTP. In comparison to other studies, we decided to implement our measurements shortly after the point sources, to measure the nutrient signal with little influence of in-stream processes.

The WWTP always contributed a high share of water, especially during dry periods. However, the discharge from the WWTP was much higher, than one would expect based on the number of inhabitants. Water quality data from the WWTP suggest, that groundwater is infiltrating into the sewer system and is additionally treated within the WWTP. This could also explain the high number of CSO events: infiltrating groundwater leads to the exceedance of the sewer system design capacities even at medium-sized rainfall events. As a consequence, not only CSO events occur more often, but also cleaning processes within the WWTP seem to be interrupted, explaining the increasing ammonium and ortho-phosphorus concentrations during events. Especially during long-lasting events with several peaks, the hysteresis analysis shows the activation of different nutrient sources, indicating a complex interaction between the sanitary infrastructure and the catchment itself.

Our data shows, that even small point sources from villages can have a significant influence on water quantity and quality. Similar to agricultural or natural catchments, their individual influence varies depending on season and pre-event conditions and are not constant throughout the year.