Water Repellency Induced by Organic Matter in Treated Wastewater Irrigation.

A soil is defined as water repellent (hydrophobic) when the time required for a drop of water placed on its surface, to infiltrate, exceeds 5 seconds. This inhibition occurs due to surface coating of soil particles by dry organic matter (OM). The source of the OM could be decomposition residues of natural vegetation or anthropogenic OM originating from treated wastewater (TWW) irrigation. In addition to the initial water repellency, water repellent soils may exhibit lower hydraulic conductivity, lower infiltration rates, higher run-off and preferential flow patterns. Our aim was to characterize water repellency in TWW irrigated soils focusing on the OM inducing it. Water repellence characteristics were found in a clayey soil irrigated with TWW in an avocado orchard. In situ measurements showed differences in water drop penetration time (WDPT) values and a reduction in the wetted surface area around drippers in the TWW irrigated plots. Water content measurements near the drippers (surface and profile) revealed dryer areas on in the TWW irrigated plots. The nature and quantity of the OM extracted from TWW irrigated soils differed from the OM obtained from soils subjected to fresh water (FW) irrigation. An experiment examining effects of water quality and soil texture on the degree of water repellency was conducted. Peanut plants were grown in lysimeters in three types of loessial soils: heavy, medium and light texture. The crop was irrigated with FW and TWW of three qualities:  secondary, tertiary, and filtered. At the end of the experiment, the highest repellency was found for the combination of the sandy (light) soil and the secondary TWW, and in contrast the lowest repellency was recordrd for the combination of the heavy soil and the filtered TWW. No water repellent characteristics were measured for any of the FW/soil combinations, as expected.