EVALUATION OF THE POTENTIAL TO DISPOSE OF SEWAGE-SLUDGE .2. POTENTIALFOR OFF-SITE MOVEMENTS OF SOLIDS AND SOLUTES

This paper reports a study of the potential for off-site movements of pollutants from sewage sludge broadcast onto the soil surface in Pinus plantations established on the coastal lowlands of south-east Queensl and. Laboratory studies of size and settling velocity distributions of rainfall-wetted sludge showed that it is relatively coarse and non-er odible. Field rainfall simulation studies at three sites in pine fores ts near Beerburrum found no significant increase in interrill erosion due to broadcasting of sludge. (Most or all of the sediment was observ ed to come from mineral soil exposed when a gutter to collect runoff w as installed at the downslope ends of the plots.) However, there was c onsiderable movement of solutes in runoff from rainfall simulator plot s that had received sludge-either freshly applied or 'consolidated' sl udge (that had been broadcast on the plots 6 months prior to the rainf all simulation study and exposed to rain and weathering). Electrical c onductivity (EC) of runoff from freshly applied sludge was initially h igh but decreased steadily during the 30 min rainfall event. Runoff fr om consolidated sludge had lower ECs, though significantly (P < 0.05) higher than those of runoff from control plots. Concentrations of nitr ate-N and ammonium-N were initially high in runoff from freshly applie d sludge, but decreased rapidly during the rainfall event. In contrast , concentrations of mineral N in runoff from consolidated sludge were low throughout the rainfall event. These results indicate a significan t risk of off-site N pollution if runoff occurs during the first rains following broadcasting of sludge. By contrast, concentrations of tota l phosphorus, copper, and zinc in runoff from both fresh and consolida ted sludge were relatively high, and showed only small decreases durin g the rainfall event applied. This suggests that sludge will contribut e significant quantities of these elements to runoff for extended peri ods after broadcasting. The implications of these findings become impo rtant in terms of the timing and method of sludge application to the s oil.