Is soil an appropriate dumping ground for our wastes?

Abstract

New Zealand and Australia generate large quantities of agricultural, industrial, and municipal wastes. As authorities move to protect the environment by regulating waste disposal practices, environmentally sound methods of waste disposal are being sought. In particular, land application of wastes as a means of disposal, nutrient re-cycling, and water conservation is becoming increasingly popular. This paper provides an overview of the types, quantities, and characteristics of wastes generated in New Zealand and Australia, and highlights the problems with current waste disposal practices, including landfilling, incineration, and discharging into waters. This is followed by a detailed review of the beneficial effects and adverse impacts of land application of wastes on plant production and soil and environmental quality, and possible hazards to human health. The management of waste application on land is a challenging task and requires rigorous scientific input. Sludges and euents contain significant concentrations of plant nutrients, particularly nitrogen, phosphorus, and organic matter. Their application on land has been shown, in many cases, to result in significant increases in plant yields and improvements in soil physical conditions and chemical fertility. The constraints with some wastes, particularly those of industrial and municipal origin, are that they contain undesirable constituents, e.g. heavy metals, toxic organics, pathogens, and salts, or have extremely high or low pH. High concentrations of nitrate and phosphate derived from wastes are also of concern for ground and surface water contamination. The processes that control the fate of wastes in the soil are complex and many of them are poorly understood, e.g. rate of release of nutrients and other chemicals; leaching of nutrients, metals, and organics through macropores and as suspended solids; emission of greenhouse gases; impact of solvents, surfactants, and sludge organic matter on the sorption, degradation, and leaching of hydrophobic organics; and the long-term bioavailability and fate of metals and organics fixed by soil organic matter. More research is urgently required to develop a sound understanding of waste characteristics and the processes affecting their fate in the soil in order to ensure that land application of wastes is safe.