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

The effects of wastewater irrigation on hydraulic properties of a highly permeable, volcanic-ash soil are presented. Ponded infiltration rate, water content, and drainage were measured by the instantaneous profile 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 of no irrigation, a third set of measurements was made. Ponded infiltration rate decreased by 50% from 8.3 x 10(-5) to 4.1 x 10(-5) m s-1 after 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 resulted 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 depths less than 1 m and may be due to restricted wetting of the soil. An identical decrease was found in the amount of water that drained from 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 appear to be caused by clogging of soil pores.