Extent and significance of water repellency in dunes along the Dutch coast

Depth, degree and spatial variability of water repellency were examined in the surface layers of dune sands along the coast of the Netherlands. Soil samples were collected at six depths of up to 50 cm at 865 dune sand sites in nature reserves. The potential water repellency was measured on dried samples using the water drop penetration time (WDPT) test. The vegetation at the sites consisted of marram grass, buckthorn, grey hair grass, pine, oak, other grasses and heather. The 5190 samples were dried at the laboratory, after which the potential water repellency was measured using the WDPT test. About 60-70% of the samples taken at several depths in the young dunes with a sparse vegetation of marram grass were wettable, whereas the other samples were slightly to strongly water repellent. The samples taken at a depth of 0-5 cm in the surface layer at the sites with different vegetations were all strongly to extremely water repellent. At all of these sites, the severity of water repellency decreased with depth. The decrease was most evident at the grey hair grass sites. No significant differences in severity of water repellency were found between the samples taken under a cover of buckthorn, pine and oak, any of the grasses and heather. The large variability over short distances in the water repellency and water content of the soil in the dune sands is shown by the intensive sampling of soil blocks at the Ouddorp, Westduinen, Schoorl and Zwanenwater sites. Drier as well as wetter soil areas were visualized in contour plots of the soil water content distributions in transects from the blocks. Large differences in wetting capacity between samples taken at several depths at the Ouddorp site were assessed by measurements of the wetting rate. In all cases, wetter samples wetted faster than their drier counterparts.