ESTIMATING SLUDGE APPLICATION RATES TO LAND-BASED ON HEAVY-METAL AND PHOSPHORUS SORPTION CHARACTERISTICS OF SOIL

This paper details the results of research which examined the sorption characteristics of a forest soil profile for phosphorus (P) and the h eavy metals arsenic (As), cadmium (Cd), copper (Cu), nickel (Ni) and z inc (Zn). It is suggested that these sorption measurements may allow a n estimate of metal and P loadings and subsequently sludge application rates which are environmentally acceptable, particularly in terms of minimal off-site movement. The extent of sorption of an element in a p articular soil horizon was characterised by sorption curves which show ed the relationship between the amount of element sorbed by the soil a nd the concentration of that element remaining in the supernatant solu tion after an equilibrating period. Buffer capacity, defined by the sl ope of the sorption curve, was calculated for each element in each soi l horizon. Sorption behaviour of heavy metals and phosphorus varied gr eatly within the soil profile. Cadmium (Cd), nickel (Ni) and copper (C u) were retained mainly in the surface All horizon. This was indicated by their relatively higher buffer capacities in this horizon and may be due to their affinity for organic matter which was present in great est concentrations in this part of the soil profile. On the other hand , the greatest sorption capacity for arsenic (As) and phosphorus (P) o ccurred deep in the profile in the B2 horizon where there was a predom inance of clay and iron (Fe) and aluminium (Al) hydrous oxides. Metal and phosphorus sorption equations were used to estimate sludge-borne e lemental loadings to soil at an ''allowable'' output elemental concent ration of soil leachate to the environment. These loadings, together w ith soil bulk density and extractabilities of elements in the sludge, were incorporated in a ''first approximation'' model to estimate sludg e application rates which were environmentally acceptable in terms of minimal off-site movement. These rates, along with other factors such as health risk and contamination aspects, should be taken into account when calculating ecologically sustainable sludge application rates at a particular disposal site.