There is a great paucity of knowledge on the persistence and long-term effects of metal accumulation in the soil resulting from applications of sewage sludge. McGrath (1978) reports the results of a long-term experiment which received metal-contaminated sewage sludge from 1942 to 1961 and inorganic fertilizers thereafter. Addi­tions of sewage sludge and sludge-straw compost increased considerably the concentrations of Zn, Cu, Ni, Cd, Cr and Pb in the plough-layer soil, and these have persisted for over 4 decades now.

Metal concentrations in the soil started falling since 1941 when the sludge applications were stopped. The removal of metals in crops harvested over 20 years was fairly small and ranged from 0.57 to 0.03% for zinc and chromium respectively (McGrath, 1987).

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In several countries, regulations limit the concentrations of the following metals in the mining effluents: arsenic, copper, lead, nickel, zinc and radium- 226. Mines are taken as all metal mining and milling facilities and associated smelters, pelletizing plants, sinter plants, refineries, acid plants and any similar operations where the effluent from several such operations is com­bined with effluents from mining and milling. The limits in the metal mining effluent regulations are based on the concentrations of harmful substances in the effluent, not on the production rate of the mine. The maximum authorized monthly average concentration prescribed in Canada is 0.5 mg/litre for arsenic, nickel and zinc; 0.3 mg/1 for copper, 0.

2 mg/1 for lead, and 10 pCi/ litre for radium-226.