Effect of Soil Properties on the Degree of Preferential Flow Across Differently Textured Agricultural Fields.

Detections of chemical compounds in tile drains and groundwater is often related to preferential flow and colloid-facilitated transport. Previous studies have found good correlation between preferential flow and the bulk density, texture, and organic carbon content. These studies, however, investigate leaching patterns within a fairly narrow texture range, or they compare data from different experimental setups. The aim of this study is to look at leaching patterns from sandy to clayey soils using soils with the same laboratory conditions. We investigate whether it is possible to find a general trend for preferential flow based on fairly easy measurable properties of 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 7 different agricultural fields in Denmark were used for the analysis. The soils ranged in clay content, bulk density, and organic carbon content from 0.05 to 0.42 kg kg^-1, 1000 to 1700 kg m^-3, and 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 breakthrough curves and transport parameters. The results showed that the 5% tracer arrival time was correlated to the clay content (r>0.7), the bulk density (r>0.4), and the organic carbon content (r <0.3). 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.