Spatial variability of potential water repellency in a lignitic mine soil afforested with Pinus nigra

Reclaimed mine soils in Lusatia/Germany are heterogeneous sandy sediment mixtures, which often contain significant portions of lignite (brown coal). Water-repellent aliphatic organic compounds may cause preferential finger-type flow in mine soils. The objective of this study was to analyse the significance, spatial scales, and spatial variability of potential water repellency in a lignitic mine soil. The mine site was located northeast of the city of Cottbus, Germany. A soil block of 2.5-m length, 1.25-m width, and 1.5-m depth was divided into squares of 0.25-m edge length from where 25 soil cores of 636 cm3 were sampled at each 0.1-m depth increment. After drying the samples at 60°C for 4 days, the Water Drop Penetration Time (WDPT) test was carried out using 30 water drops per sample. The frequency distributions of the classified potential water repellency were analyzed (i) locally, within a sample, (ii) horizontally, within a layer, and (iii) vertically, within the soil block. The WDPT values of most samples were either smaller than 5–10 s or larger than 10 min. Such ‘bipolar’ distribution occurred for the whole block, as well as locally, i.e., the WDPT varied between repellent and not repellent from drop to drop at distances of about 1 cm. The persistence of potential water repellency seems somewhat larger in the subsoil (0.5–1.5-m depth) than in the ameliorated topsoil (0–40-m depth). However, hardly any spatial structures are obvious since severely repellent, as well as not repellent, soil material occurs throughout the soil profile and intra- was as large as inter-sample variability. To a large degree, the spatial variability of water repellency seems to reflect a situation that resulted from partially mixing of different overburden sediments. The small-scale variability of the potential water repellency may probably be correlated with the spatial distribution of lignite, minerals, and more-or-less lignite-coated sand particles. These spatial patterns of repellency may, in particular, affect water and solute movement in mine soils.