Occurrence and distribution of water repellency in size fractionated coastal dune sand in Sri Lanka under Casuarina shelterbelt

- Occurrence of water repellency in dune sand under Casuarina shelterbelt was verified.
- The top layer of dune sand was extremely water repellent.
- Water repellency decreased with increasing soil depth.
- The highest water repellency level was observed in the finest fraction.
- Water repellency showed positive linear correlation to soil organic matter content.

The occurrence of water repellency in Sri Lankan soils has not been extensively studied and reported so far. Although most Sri Lankan soils are readily wettable, some soils show water repellent conditions. In this study, we examined a dune sand under Casuarina equisetifolia shelterbelt in Sri Lanka. The objectives were to ascertain the occurrence and the distribution of water repellency along the particle size fractions and to perceive the role of intermixed organic material in producing water repellency. The soil type is locally known to be sandy Regosols (USDA classification: Ustic Quartzipsamments). Bulk soil samples were taken from 0-5, 5-10, and 10-15 cm depths. Soil samples were separated into six size fractions by passing through a set of sieves and further separated into two sets as washed (with water) and untreated. Soil organic matter (SOM) content and the water repellency were discretely determined in untreated and washed samples. Water drop penetration time (WDPT) test and modified sessile drop method were used to estimate the water repellency. Despite the low SOM content (< 2%), water repellency was extreme (contact angle > 118°; WDPT > 3600 s) on the topmost layer (0-5 cm) and decreased with increasing soil depth. All the size fractions of the topmost layer showed significantly high repellency compared with those of the other two layers. Both the water repellency and the SOM content showed the highest values in the finest fraction and the lowest in the coarsest fraction. Washing samples with water decreased the WDPT in > 96% of all size fractions in the 0-5 cm layer, and by lower proportions in the lower layers. Washing samples significantly decreased the contact angle of most size fractions although the decline did not exceed 15%. Washing with water removed a considerable portion of organic material. Despite the obvious declining through washing, both the SOM content and the water repellency showed the highest values in the smallest particle size fraction. Water repellency in both washed and untreated samples showed a considerably strong positive linear correlation with the SOM content suggesting that water-insoluble hydrophobic organic coatings might not be the dominant factor triggering water repellency in the tested sandy soils.