Use of treated sewage effluent in irrigated agriculture in semi-arid and arid lands, or for land disposal, is on the rise. However, applying effluent to arable lands also involves certain environmental and agricultural risks. In this study, we found that prolonged irrigation with secondary treated sewage effluent caused the development of water repellency in sandy orchard soils.

Soil surface layer (0-5 cm, in 1 cm intervals) and soil profile (0-50 cm) transects were sampled for repellency, organic matter content, and moisture content at a high resolution at the close of the irrigation season and rainy winter season. Extreme to severe soil water repellency in the 0-5 cm soil surface layer persisted throughout the two year study period in the effluent-irrigated Shamouti orange (citrus sinensis (L.) Osbeck c.v. Shamouti) orchard plot. Nearby Shamouti orange plots irrigated with tap water were either non-repellent or only somewhat repellent. Repellency was very variable spatially and with depth, appearing in vertically-oriented “repellency tongues”. Temporal and spatial variability in repellency in the uppermost 5 cm soil surface layer was not related to seasonality, soil moisture content, or soil organic matter content. Non-uniform distribution of soil moisture and fingered flow were observed in the soil profile after both seasons, demonstrating that the repellent layer influenced water flow in the underlying wettable soil profile. A lack of correlation between bulk density and volumetric water content in the soil profile demonstrates that the observed non-uniform spatial distribution of moisture results from preferential flow and not heterogeneity in soil properties. Soil water repellency can adversely affect agricultural production, cause contamination of underlying groundwater resources, and result in excessive runoff and soil erosion.