P2 Pollutant Turnover in the Groundwater - Stream Transition Zone of Contrasting Lower-Order Sub-Catchments

More than 50 % of a landscape are drained by first and second order streams. These small streams are typically well connected to groundwater and consequently are primary receptors for diffuse contaminant inputs from sub-catchments. Hence the initial chemical signature of stream water as well as the overall water quality in rivers is largely governed by processes in these lower order headwater streams. Here, the transition zone between groundwater and surface water is commonly rich in organic material and often characterized by steep redox gradients and high biogeochemical activity, providing potentially highly reactive conditions for the turnover of redox-sensitive species such as pesticides and nutrients. This project will evaluate the importance of this transition zone for pollutant transformation and subsequent effects on stream water quality. Specific goals are to

• identify integral indicators to evaluate and quantify reaction potentials in the transition zone
• assess locations in the transition zone showing high reactivity and mass flux for NH₄⁺, NO₃^–, and pesticides
• determine the variability of concentrations and fluxes of the lead substances over time and at integrating locations
• unravel key microbial populations involved in oxidative and reductive nitrogen cycling in groundwater-stream transition zones, elucidate the controls of their activity as impacted by seasonal dynamics and differential land-use
• derive effective turnover rates in the transition zone compared to turnover in other compartments of a sub-catchment based on process-based reactive transport modeling at local scales

Spatially highly resolved monitoring of water and solute fluxes using novel online field probes are coupled to a new method for sensitive ¹⁵N/¹⁴N isotope analysis and molecular biological tools to address the above mentioned research objectives. Field investigations are complemented by spatially explicit numerical models as depicted in the interdisciplinary toolbox above. This unique combination of approaches helps:

• To characterize the spatial and temporal patterns of groundwater infiltration and exfiltration in two contrasting first-order streams.
• To unravel spatio-temporal patterns of reactivity and nitrogen turnover for different land uses and hydraulic conditions.
• To evaluate the microbiology and transformation potential of the transition zone along highly instrumented stream transects.
• To evaluated the importance of the streambed and the near-stream aquifer for solute turnover.