Understanding the variable impact of point sources on headwater stream water quality

In rural areas, point sources like wastewater treatment plants or combined sewer overflows are frequently overlooked or only simplistically considered in analyses of whole watersheds. The lack of available data and the difficulty of tracing hydrochemical signatures of (reactive) nutrients measured at the outlets of larger areas back to their origin are common reasons for that. The extent to which these sources influence nutrient dynamics in water bodies and how they interact with nutrients from diffuse sources has been examined in only a few studies.

As part of our study, we installed a comprehensive measurement setup in a selected rural watershed where a local wastewater treatment plant and combined sewer overflows discharge into a small river. To capture the dynamics of these point sources, pressure sensors, water quality probes, and automatic samplers were installed shortly after the treatment plant. The outlet was sampled weekly. Additional samples were taken upstream of the treatment plant and downstream of our monitoring station, to capture in-stream nutrient transformation.

In contrast to the common assumption that the influence of small treatment plants can be considered constant, the water quality of the treated wastewater undergoes significant fluctuations, especially during base flow. Comprehensive statistical analyses show that the treatment plant significantly influences the concentration-discharge relationship in the water body and is responsible for a large portion of nutrient loads. In the water body itself, ammonium constitutes half of the inorganic nitrogen It is detectable only downstream of the wastewater treatment plant outlet, where it undergoes rapid nitrification. During precipitation events, a complex interaction of the treatment plant, combined sewer overflows, and diffuse sources was observed. In particular, the increasing input of ammonium and ortho-phosphate leads to an increase in exported concentrations and loads of these nutrients. At the same time, the system is characterized by frequent activation of combined sewer overflows. Our investigations show that the effects of wastewater treatment plants in rural areas are more differentiated and extensive than commonly assumed. At the same time, there is still a high potential to reduce the discharge of nutrients from point sources and, thus, the discharge of nutrients from low order catchments.