Matrix and bypass-flow in quaternary and tertiary sediments of agricultural areas in south Germany

In hilly areas of south Germany with tertiary and quaternary sediments, percolation has been studied applying an isotope tracer of the water molecule in small-scale field experiments to better understand the hierarchies and interconnections of quick and slow seepage. On a catchment-related scale, these results have been connected with traditional and environmental isotope analysis of discharge to better quantify quick and slow seepage components, and to better assess the export of agrochemicals into ground- and surface waters. Consequently, the development of improved application techniques of agrochemicals and better strategies for ground and surface water protection is possible. The unsaturated zone of the study area was traced with Deuterium on areas of about 50 m2. Results show that infiltration splits into bypass and matrix-flow. Bypass-flow exceeds flow velocities of 0.5 m day−1, which is close to flow velocities of overland-flow, and matrix-flow ranges between 0.7 m year−1 (Loess) and 1.2 m year−1 (Tertiary gravels and sands). In these unconsolidated rocks, bypass-flow seems to (1) be strongest under wet and dry conditions at the soil surface; (2) be more dominant in coarse than fine-grained sediments; (3) be more pronounced in terrestrial than in marine sediments; and (4) penetrate to an average depth of less than 1 m in fine-grained sands and silts, and deeper than 3 m in gravels before it either finally incorporates into matrix-flow or generates interflow. Hydrographic analysis shows that more than 21% infiltration produces interflow by the transformation of bypass-flow into lateral flow, and about 75% infiltration groundwater recharge; 4% of bypass-flow incorporates into matrix-flow. In the study area, plowing techniques and field size influence significantly and proportionally the distribution of overland- and interflow-flow. However, groundwater recharge is not significantly changed. As matrix-flow is too slow an indicator to clarify how changes in land use affect groundwater quality over time, the analysis of direct discharge may be considered a good early indicator to assess land use changes on the export of agrochemicals.