The age of preferential flow paths

Preferential flow is a common phenomenon in soils, but the temporal stability of the heterogeneous flow pattern is largely unknown. We give evidence that preferential flow paths in a structured forest soil are persistent for decades. After staining preferential flow paths in a fine-loamy Dystric Cambisol with a dye tracer, we sampled the soil from the preferential flow paths and from the unstained matrix at five sampling times during I year. In preferential flow paths, the activities of Cs-137, Pb-210 and Pu-239,Pu-240, as well as concentrations of soil organic carbon (SOC), were enriched by a factor of up to 3.5 relative to those of the matrix. The Cs-137 originates mainly from the Chernobyl accident in 1986, the Pb-210 from a continuous 'natural' atmospheric deposition and the Pu-239,Pu-240 from nuclear weapon tests in the 1950s and 1960s. Since all of these radionuclides are only mobile in the soil immediately after deposition, the increased activities of radionuclides in the recent flow paths sampled during our experiments indicate that these flow paths were stable for decades. This is supported by the total enrichment of SOC in preferential flow paths ranging between 740 and 960 g C m(-2), which is high in comparison to published accumulation rates of SOC in forest bulk soils from 20 to 60 g C m(-2) year(-1). The gradient of radionuclide activity and of SOC concentrations between the two flow regions was relatively constant during the I-year experiment. In all of the five sampling times, concentrations of SOC were larger by 37-46% and the activities of Cs-137 were larger by 83-150% in the preferential flow paths than in the matrix down to a depth of 50 cm. This means that despite the differing boundary conditions at the different sampling times, the pathways of infiltrating water were persistent with time. (C) 2002 Published by Elsevier Science B.V.