CHANGES IN INFILTRATION AND DRAINAGE DURING WASTE-WATER IRRIGATION OFA HIGHLY PERMEABLE SOIL

The effects of wastewater irrigation on hydraulic properties of a high ly permeable, volcanic-ash soil are presented. Ponded infiltration rat e, water content, and drainage were measured by the instantaneous prof ile technique in a 5-m3 , 2.3 m deep isolated soil monolith. This was done both before and after 32 mo of irrigation with secondary-treated wastewater. Irrigation then continued for a further 4 mo. After 8 mo o f no irrigation, a third set of measurements was made. Ponded infiltra tion rate decreased by 50% from 8.3 x 10(-5) to 4.1 x 10(-5) m s-1 aft er 32 mo of wastewater irrigation. A further decrease to 1.7 x 10(-5) m s-1 was found 1 yr later. Three years of wastewater irrigation resul ted in a 25-mm decrease in the amount of water stored by the soil to a depth of 2.3 m after 20 d of drainage. This decrease was mainly at de pths less than 1 m and may be due to restricted wetting of the soil. A n identical decrease was found in the amount of water that drained fro m the soil during these 20 d. However, the water content at depth 0.1 m after 20 d of drainage was increased by 0.1 m3 m-3. This increase in water content and the concomitant decrease in infiltration rate appea r to be caused by clogging of soil pores.