Regulation function of the unsaturated zone on discharge and the export of inorganic agrochemicals from arable lands in south Germany

A representative experimental side has been equipped in Bavaria/Germany to study scale related flow and contamination pathways in the unsaturated zone of Tertiary sands/gravels and Quaternary loess. In these sediments both preferential- and matrix-flow occur and preferential-flow produces interflow. Deuterium tracer experiments show that preferential-flow velocities exceed 0.5 m/day, in contrast matrix-flow was 0.7 m/year in Loess, 1.2 m/year in Tertiary sands/gravels and 2 m/year in Quaternary gravels. Preferential-flow penetrates to depths of less than 1.5 m b.g. in fine grained sands and more than 3 m b.g. in gravels before it either incorporates into matrix-flow or generates interflow in hilly areas. Preferential-flow depends upon geologic, biologic, pedologic, and in arable lands, also on anthropogenic factors; since these factors underlie seasonal variations. Interflow also has a seasonal characteristic with low and high mean residence times during the year. In hilly terrains, preferential- and inter-flow favours the export of agrochemicals especially from the effective root zone; in contrast, in flat lands preferential-flow accelerates the input of agrochemicals to groundwater. From hydrograph, chemical and environmental isotope analysis, as well as from tracing percolation water in the study area, an average of 21% of infiltration produces inter-flow and about 75% groundwater recharge; only 4% discharges as overland-flow. In the study area, the partitioning of overland- and inter-flow is significantly influenced by ploughing techniques and the field sizes. Groundwater recharge is not measurably changed by these factors. Interflow pathways also transport DOC, which strongly favours the export of agrochemicals. Since DOC forms large 'molecules', it is subject to mechanical filtering, and thus follows wide preferential flow paths during interflow. Because DOC has a high sorption capacity, it can promote exportation of many agrochemicals during storm events through direct discharge to surface water. Hence, while interflow and DOC may reduce the potential for groundwater contamination, it may also cause contamination pulses to reach surface waters. (author)