OSUR is pleased to announce that Christopher Green and Gunnar Lischeid hold a seminar on Monday, the 3rd of July in the OSUR conference room (building 14B, Campus Beaulieu, Université Rennes1).
14:00 - 15:00 - Christopher Green (USGS)
Reactive transport of nitrate in groundwater in the US
15:00 - 16:00 - Gunnar Lischeid (Leibniz Centre for Agricultural Landscape Research - ZALF)
Complex coupling between groundwater and surface water in lowlands
>>> These two seminars will take place before Tamara Kolbe's thesis defence
Reactive transport of nitrate in groundwater in the US (Christopher Green)
Nitrate flux in the subsurface is an issue of global importance, yet little is known about the extents and rates of reactions (e.g. denitrification) affecting concentrations in groundwater. The US Geological Survey has conducted studies to better understand the fluxes and reactions of nitrate in groundwater in a range of settings in the US. Monitoring wells were sampled and sediment cores collected in flow-transects between agricultural fields and streams. Groundwater samples were analyzed for atmospheric tracers of groundwater age, stable isotopes of N and O in nitrate, and dissolved gases to characterize reaction progress. Sediments were analyzed for solid phase electron donors that promote denitrification. Modeling methods included detailed numerical models for understanding processes in heterogeneous aquifers, and analytical solutions to allow systematic comparisons among many sites across the US. Among the key findings of these studies are that mixing in heterogeneous aquifers reduces the apparent rates of reactions and isotope fractionations, denitrification depends on solid phase electron donors, denitrification rates are often too low to substantially mitigate contamination before reaching receptors, and most shallow aquifers are at or near equilibrium with respect to the depth of nitrate. These findings indicate, in general, that extensive nitrate contamination will persist under current agricultural practices and may worsen if labile, solid-phase electron donors are depleted in aquifers.
Complex coupling between groundwater and surface water in lowlands< /strong> (Gunnar Lischeid)
Lowland regions often exhibit a high density streams, lakes, ponds and wetlands. This holds, e.g., for Northeast Europe. As a consequence of heterogeneous sedimentation during and after glaciation, and temporarily dead ice blocks left behind in the late stage of the last glaciation, the landscape is now spotted by small depressions that are often filled with water, called kettle holes.
Tight coupling between groundwater and surface water with non-stationary gradients and flow directions is often considered rather complex and surely is a challenge for hydrological models. However, on the other hand, close links imply that the one system bears information about the other and vice versa. For example, for the 170 km2 Quillow catchment in Northeast Germany a detailed groundwater map was generated from airborne remote sensing data exclusively, that is, from laser scan data of water level elevation in streams and kettle holes. On average, about 12 points of support per km2 were used. Such a high density can hardly be achieved at that scale based on groundwater wells.
The resulting map was checked for consistency and plausibility. The groundwater map allowed delineating the catchments of the single kettle holes in order to assess the impact of land use of water quality. These catchments differed substantially from those determined based on topography. In general, the former usually exhibited a clearly elongated shape of up to some km length, resembling more single groundwater flowpaths rather than exhibiting the typical shape of stream catchments. Detailed modelling at a smaller scale, however, revealed non-steady and complex interactions between lake and adjacent groundwater with inverting groundwater flow direction during dry periods.
Analysis of water quality monitoring data revealed that kettle hole water reflected the concentration of alkaline (earth) metals of the shallow groundwater. However, this did not hold for nutrients and redox sensitive solutes due to rapid biogeochemical turnover in the eutrophic to hypertrophic small lentic systems. In addition, there is strong evidence that the history of periodically felling dry imposes major effects on observed biogeochemical processes. Taking together these effects make any direct proof of land use effects on the aquatic systems a tedious task.