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Study Evaluates Risk to Birds from Cadmium
Date Posted: October 9, 2001Phosphate is mined primarily to be used in fertilizer, but is also used in animal feed supplements as well as a variety of consumer products, primarily as purified phosphoric acid for food and beverages, metal treatment, detergents and electronics. At a single mine in eastern North Carolina, phosphate-containing ore is strip mined at a rate of about 13 million tons per year. Left behind each year are millions of tons of sand tailings, clay tailings and gypsum. To dispose of tailings and reduce the visible impacts of the mining, tailings and gypsum are mixed and used to fill-in previously mined lands. However, this has created a pollution problem. Cadmium, a naturally-occurring metal found in all soils and rocks, is concentrated in the phosphate ore (which is normally deep underground); it is brought to the surface via mining and becomes more concentrated in the tailings from phosphate extraction and processing. Cadmium is not required for the growth and development of either plants or animals and can be toxic to both. Excessive amounts of this naturally-occurring metal can have a harmful affect on the nervous, respiratory, excretory, and reproductive systems of animals, as well as affect their development and feeding habits. The Service’s Raleigh, North Carolina, Field Office recently finished an evaluation of risks to birds exposed to this cadmium-enriched replacement “soil” (that is, the tailings used as fill). The Service found that replaced “soils” in the reclamation areas contain cadmium (20 to 30 mg/kg dry weight), at concentrations 300 to 500-times those found in surrounding, un-mined soils. Earthworms grown in the replaced “soils” were found to be accumulating cadmium in their bodies (30 to 90 mg/kg dry weight). The cadmium concentrations found in these earthworms exceed dietary levels known to be harmful to birds, indicating the possibility that birds eating these worms may be harmed. Based on these results, the Service recommended covering high cadmium content soils to isolate them from wildlife. This recommendation was embraced by the company responsible for the mine and will ultimately improve thousands of acres of wildlife habitat. The Service is currently discussing the depth and composition of cover material with company officials to identify an acceptable remedy for all parties.
Last updated: February 13, 2013