PERCOLATION AND LEACHATE COMPOSITION IN A DISTURBED SOIL LAYER MULCHED WITH SEWAGE BIOSOLIDS

The effect of sludge and sludge compost as a mulch for dissipating rai ndrop impact and maintaining a relatively high percolation rate was st udied using a Paleudalf from South Africa. Three kg soil were packed i n perforated boxes in a making a two cm layer, and subjected to five c onsecutive simulated rainstorms during a period of 26 days. A 4-day in cubation was allowed between the first 4 storms and 10 days before the 5th storm. The soil was amended with either a dry waste activated slu dge or a sludge compost at a rate equivalent to 45 Mg ha(-1). The amen dments (< 2-mm) were applied either on the surface or mixed with the w hole soil. The application of both sludge and sludge compost reduced t he final percolation rate (FPR) compared with the unamended control. T he effect of the consecutive rainstorms on the average FPR from all th e treatments was as follows: FPR after Ist storm >> 2nd congruent to 3 rd congruent to 4th > 5th. The effect of amendment on average FPR from all storms was: control > sludge mixed with the soil > surface-applie d sludge > sludge compost mixed with the soil greater than or equal to surface-applied sludge compost. The decrease of FPR was not related t o the electrical conductivity or clay concentration of the leachates. The main chemical species that appeared in the leachates from the trea ted soils, and persisted throughout the five rainstorms, were ammonium and sulfate. Other ions, such as calcium and magnesium, were present in high concentrations in the leachate during the first storm but at l ow concentrations in subsequent storms. Nitrate concentrations were hi gh during the first and fifth storms. It is suggested that the adverse effect of the amendments resulted from mechanical and microbial clogg ing of soil pores. (C) 1998 Elsevier Science B.V. All rights reserved.