Effect of Soil Properties and Field Management on the Degree of Preferential Flow.

Detection of chemical compounds in the tile drains and groundwater is related to preferential flow and colloid-facilitated transport. Previous studies have found good correlation between the preferential flow through soil columns and the bulk density, texture, and organic carbon content. These studies, however, looks at the leaching patterns within a narrow texture range. The aim of this study is therefore to look at a larger leaching perspective from sand to clayey soils, and investigate whether it is possible to derive a general trend for the preferential flow based on the fairly easy measurable properties of the soils, such as bulk density, clay and organic carbon. Over 200 cylindrical soil columns of 20 cm height and 20 cm diameter sampled from the top soil at different agricultural fields in Denmark, were used for the analysis. The soils ranged in clay content and organic carbon from 0.05 to 0.42 kg kg^-1and 0.01 to 0.08 kg kg^-1, respectively. The experiments were carried out the same way for all the columns, where an irrigation system and a non-sorbing tracer, were used to obtain among others breakthrough curves. The results showed that the 5% tracer arrival time was fairly well correlated to the bulk density, organic carbon and clay content, when taken the management type into account. For soils with clay content above 0.07 kg kg^-1 increasing bulk density decreased the tracer arrival time, where increasing bulk density for the soils with clay contents less than 0.07 kg kg^-1 did not significantly influence the tracer arrival time. A proxy model was introduced to predict the tracer arrival time and thereby the risk of preferential flow. Additionally, a three dimensional image was created from X-ray computed tomography scanning for each soil and management type to visualize the impact of soil properties to the macro-porosity.