Determination of repellency distribution using soil organic matter and water content

The aim of this study is to investigate the effect of soil moisture and soil organic matter content on the water repellency of a former sewage field. A topsoil block (40×80×30 cm) and a soil transect (300×100 cm) from a former wastewater infiltration site near Berlin were sampled with a high spatial resolution for this purpose. Actual and potential water repellency were measured using the Water Drop Penetration Time (WDPT) test. Gravimetric water content and soil organic matter (SOM) were determined for each sample; a total of 864 soil samples were analysed. The investigations were carried out in January 2003. Water repellency occurred in the top soil even during this winter period. The spatial distribution of water content and water repellency clearly showed preferential flow paths. Water repellency was measured at field moist samples (actual repellency) and at dried samples (potential repellency). The transition zone for distinguishing between water repellent and wettable regions was determined to be 0.15 g g−1. Therefore, a calculation of the wettability by water content alone is not possible. We suggest a new approach for calculating a so-called 'critical water content' (Θcrit) as a function of the amount of soil organic matter (SOM). The function Θcrit (g g−1)=1.12 SOM (g g−1)+0.037 g g−1 leads to a better prediction of wettable and non-wettable soil regions for the entire soil profile. The distinction between these regions is necessary for the calculation of the water and solute transport in a two- or three-dimensional numeric model.