Treated wastewater effects on water repellency and soil hydraulic properties of soil aquifer treatment infiltration basins

SummaryIrrigation with treated wastewater (TWW) has been reported to induce low to moderate hydrophobicity in soils of different texture with or without vegetation cover. Additional potential sites for adverse effects of TWW are infiltration basins, where TWW undergoes tertiary treatment, known as soil aquifer treatment (SAT). The annual organic matter (OM) loads in SAT are a hundred fold higher than those applied in irrigation with TWW. Therefore, we initially presumed that in the sandy soils of SAT infiltration basins, hydrophobicity would be expressed to a higher extent and would further affect the hydraulic properties of these soils. This hypothesis was tested in the Dan Region Wastewater Reclamation Project, Tel-Aviv, Israel. In terms of water drop penetration time (WDPT) test, the results obtained exhibited similar hydrophobicity levels (0 ⩽ WDPT ⩽ 238 s) to those obtained in the TWW irrigated fields and in a similar manner, soil hydrophobicity was only exhibited in the surface soil layer. It is suggested that the low to moderate hydrophobicity obtained under TWW irrigation and recharge into the SAT basins, may primarily stem from the drying and wetting cycle regimes employed. Under these conditions, hydrophobic compounds, which may impart hydrophobicity to the soil particles, are likely to detach and dissolve into the soil solution at the subsequent irrigation or recharge event. Furthermore, the frequent displacement of the resident TWW derived OM by repeatedly flooding events, prevents maturation and humification of the resident OM. Further investigations on the hydraulic properties of the basin soils were conducted in conjunction with the solid–liquid contact angle (ω) and the surface tension (γL) of the soil solution. The results obtained imply that the dependence of the hydraulic properties on ω and/or γL, can be attributed to the hydrophobic nature of both the solid and dissolved OM originating from the recharged TWW.

Research highlights
► Treated wastewater induced soil hydrophobicity in soil aquifer treatment basins.
► The hydrophobicity obtained ranged from low to moderate levels.
► Solid and dissolved organic matter altered the original soil hydraulic properties.
► The governing mechanism is the net effect of contact angle and surface tension.