Nitrate flux monitoring in small headwater catchments in the German low mountain range – Threshold exceedance during baseflow and snowmelt.

Nitrate is one of the key parameters for the assessment of water quality, since an excess of NO₃ in drinking water can lead to health risks, especially for young children. The purpose of our study was not only to detect occasionally exceedances of the water quality standards, but also to monitor the temporal highly resolved behavior of NO₃ concentrations in the stream water over a one year period. To reach this goal we picked the small headwater catchment of the Nesselbach, located at Grebenstein in the North-Hessian low-mountain-ranges as our study area. We installed a UV-ViS-probe (s::can spectro::lyser), capable of detecting small quantities of nitrate with a 5 minutes resolution and added an automatic sampler to collect samples for lab calibration. Additionally, we installed discharge measurement in the stream and collected event-based samples of stable water isotopes with the installed automated samplers. The stable water isotopes are used to perform hydrograph separation for differentiation between event-water and baseflow, and therefore to gain deeper insights into the hydrology of the Nesselbach catchment.  

The sampling results show that the E.U. environmental quality standard of 50 mg/l NO₃ is almost always exceeded during baseflow in the Nesselbach. Rain-event driven discharge dilutes the baseflow strong enough to reduce the concentrations below the threshold for a short time span, but snowmelt shows the opposite behavior, increasing the NO₃ concentrations in the discharge over a longer period of time. While headwater catchments are often considered to be of good water quality in comparison to the much bigger catchments downstream, our findings suggest, that in catchments with agricultural landuse even the headwaters lag a good water quality and exceed the E.U. environmental quality standards for NO₃. This first evaluation of the data of our study points to the relevance of monitoring NO₃ concentrations in the baseflow. Because headwater catchments are often used for the extraction of drinking water and cattle watering, it could be necessary to expand the monitoring of NO₃ to a higher number of headwater catchments.