By Alan Broughton
Arsenic levels in rice periodically hit the news. Arsenic is one of several toxic heavy metals found in foods – cadmium, lead, mercury and chromium are others. While these are naturally occurring elements in soil and rock, natural sources are not the main cause of contamination of agricultural produce. Principal sources include fertilisers, pesticides, mining, industrial waste and air pollution. This article focuses on the two most prevalent heavy metal contaminants – cadmium and arsenic.
Origins
There are two major sources of heavy metal contaminants in fertilisers. One is naturally occurring cadmium in rock phosphate, which is mined and processed to produce superphosphate and other soluble phosphatic fertilisers, or used directly. The other arises from the practice of adding industrial waste to fertilisers as a means of disposal, material containing mercury, arsenic, nickel, copper, zinc, uranium, lead, chromium and cadmium.
In the US any material that has some qualities as a fertiliser can be used on fields in the name of recycling, even low level radioactive waste from uranium processing. The California Public Interest Research Group identified 22 toxic elements in chemical fertilisers in 1996, all of which contained industrial waste from steel works, cement factories, paper making and electronic plants. The practice of adding industrial waste to fertilisers was revealed by Duff Wilson in Fateful Harvest (Wilson 2001). Until at least 2002 this practice was totally unregulated. Indeed, the US EPA stated (1997): “EPA’s longstanding policy encourages the beneficial reuse and recycling of industrial waste, including hazardous wastes, when such wastes can be used as safe and effective substitutes for virgin raw materials” (Asokakumar 2017). Few states in the US have any regulations for heavy metal content of fertilisers, only California, Washington and Oregon; nationally there are recommended maximum levels but they are not mandated (McLaughlin 2004).
