Influence of urban infrastructure on headwater streams – first insights into water quantity and quality measurements in two rural areas

Water quantity and quality of headwater catchments can react very sensitive to human impacts. While many studies focus on the influence of bigger cities on urban streams, the influence of rural villages and their associated infrastructure onto stream discharge and water quality dynamics is not often part of research.

We installed discharge measurements, UV-Vis probes Nitrate (NO₃^-) monitoring and conductivity probes on two neighboured headwater catchments, the Kelze (2.64 km²) and the Nesselbach (3.23 km²) catchment. All probes sample with a high temporal resolution of five minutes. We additionally equipped the sites with automatic samplers for also monitoring Nitrite (NO₂^-), Ammonium (NH₄⁺), ortho-Phosphate (PO₄^3- ) and total Phosphate (_totP). All over the catchments are characterized by agriculture and forests, while the Kelze catchment is also influenced by a village. A part of the village is drained by a stormwater sewer while most of the area is drained by mixed sewer. A small wastewater treatment plant (WWTP), which is very common in rural areas, treats solely the wastewater of this village. The WWTP consists of four ponds in a series. Water flows solely driven by gravity and it is not possible to manually control the discharge of the WWTP.

First measurements show that during low flow conditions Nitrate concentrations are generally higher in the Nesselbach, which is more influenced by agricultural areas. While the outflow of the WWTP dilutes the NO₃^- concentration in the Kelze, it causes increased levels of NO₂^-, NH₄⁺ and PO₄^3- concentrations. Even though the village is comparatively small, the sealed area, which is connected to the sewer system, as well as private drainages lead to a fast runoff during rainfall events. The rainwater is directly transported to the WWTP. Due to the limited storage capacity of such WWTP high discharge peaks can be observed shortly after the event. Depending on the water storage in the WWTP, even small events can produce a discharge wave, leading to short time rise of water levels in the Kelze stream, while the Nesselbach catchment shows smaller peak flows and thus bigger storage effects for the same events.

Overall, the first measurements show, that understanding the interplay between agricultural and urban areas is crucial to understand the coupling of different hydrologic and biogeochemical processes and could lead to a better understanding of catchment processes.